Heat saving calculation.Let us first understand how costs / savings are calculated. We know that,

Boiler Heat Input is the energy from fuel, and boiler Heat Output is the energy in steam.

Boiler Heat Input = Qf X GCV

Where, Qf = Quantity of fuel (in kg/hr)

GCV (Gross Calorific Value) =Energy contained in fuel in kcal/kg

Boiler Heat Output = Qs X (Hs – Hw)

Where Qs = Quantity of steam (in kg/hr)

Hs = Heat contained in steam (Enthalpy of Saturated steam hg)

Hw = Heat already present in the water from which steam is raised

To find out the fuel cost for liquid/gas fuels, we need to divide by the Specific gravity (ρ) of that fluid.

This is the basic equation we will work with for allocating fuel cost.

Cost of steam.Example 5.1

Let us take a 5TPH (tons per hour) oil-fired boiler, @10.5 kg/cm2g, using DM water at an ambient temperature of 30°C.

Direct cost

First, we calculate the Direct costs of

• Fuel

• Water

• Water treatment

• Electricity

Fuel

= 7945.66 Rs/hr

Assume the boiler operates for 24 hours a day for 335 days a year which is about 8000 hours per year. So, Cost of FO = 7945.66 x 8000 Rs/year = Rs. 635.6 lakhs/ year (A)

Water

A 5TPH boiler will use 5 tons of water per hour. Assume water cost as Rs. 10/klitre = Rs. 10/ ton of water. So, Cost of water = 5 x 10 x 8000 = Rs. 4 lakhs/year (B)

Water treatment

Assume DM water treatment cost as Rs. 30/Klitre.

So, Cost of treatment = 5 x 30 x 8000 = Rs. 12 Lakhs/year (C)

Electricity

Electricity is used by every boiler for running its feedpumps, blowers, controls, etc. assume electricity consumption @ 40 Kw-hr. Also, take the cost of electricity at Rs. 4.50p per Kw-hr. So,

Cost of electricity = 40 x 4.5 x 8000 = Rs. 14.4 lakhs/year (D)

Indirect costs

These are the costs of

• Capital cost of -Space, Boiler cost, Depreciation

• Manpower

Manpower

Assume 3 IBR boiler Operators (salary Rs. 8000/year) and one boiler Supervisor (salary Rs. 10,000/year).

Total cost = Rs. 4.1 lakhs/year (E)

Capital cost

We will take out the cost of finance of both the space and the boiler. Space cost. 5000 sq. ft of boilerhouse space x Rs. 400/sq.ft. = Rs. 20 lakhs.

Boiler cost. Rs. 50 lakhs

Finance cost of Rs. 70 lakhs = 0.15 x 70 = Rs. 10 lakhs/year

Depreciation of boiler = 0.15 x 50 = 7.5 lakhs/year

A total Capital cost = Rs. 17.5 lakhs/year (F)

Adding A + B + C + D + E + F, we get Rs. 687.6 lakhs for a 5 TPH boiler/year.

Or, Rs. 137 lakhs / ton / year, or, Rs. 1,719 / TPH !!

Cost of leaks.

Cost of Condensate.In the boiler , we heat water by suppling it energy in the form of heat. When the temperature goes above boiling point of water it changes state & converts to steam. Steam is the form in which this heat energy is now carried to the process. Lets say a jacketed vessel containing some liquid which needs to be heated. When steam is supplied to this cooler material to be heated, the energy is transferred from steam which new changes back to liquid state- i.e it condenses. This hot water is called condensate.

Unfortunately, in practice not all of this heat energy is transferred. Only 75% of the total energy carried by steam is transferred. What happens to the rest? That”s right. It is now in the condensate. So, if the total energy produced by the boiler is 100, units only 75 is transferred to the process. 25 units is trapped in the condensate.

This condensate collects in the pipes which are at a high pressure. When condensate is drained out from steam trap to a lower pressure, instantaneously some of the condensate re-evaporates as Flash Steam. About half of the energy carried by the condensate can be cost in this way. So ,out of the 25 units of heat in condensate 12.5 units just evaporates.

What is Condensate Management?

We have already explained condensate and flash steam. When we talk of removing condensate, recovering it and receiving flash steam as well, this is called Condensate Management

It is easy to figure that collecting & recovering both condensate & flash steam is very critical not just in money terms but also to prevent wastage.

How Much can Condensate recovery save the client?

• Money Saving : Of fuel

• Money Saved : Of water Conserved• Money Saved : Of chemicals

Condensate return cuts cost of water

Condensate is after all water, and when returned to the boiler slashes water bills from the government, or, reduces electricity costs if we are pumping it up from a nearby river.

Condensate is ideal Boiler feedwater

As condensate comes from pure steam it is distilled water with no dissolved solids. ( very low TDS). If condensate is returned, the boiler needs lesser blowdown which is a big economic consideration as the boiler loses energy each time it blows down.

For blowdown and TDS talk see Chemistry / TDS section

Not just that, because we are recycling condensate, we save the cost of chemical treatments that we would have to do, were we using fresh make-up water.

Condensate adds to boiler efficiency

When cold water is used to top up the boiler, naturally the boilers efficiency is compromised the lower the feed water temperature, the lesser the steam a boiler produces.

The boilers “From and At rating” falls. Cold water put into the boiler might even lead to thermal shock.

Condensate saves chemical costs

As condensate is pure water, and it has already been treated, it can be used as is in the boiler. This cuts the cost of water treatment, too.

How do we determine how much condensate recovery saves the client in monetary terms? Simple. Condensate returned, means fuel saved. How much this fuel saving is, can be determined from the formula:

Where,

Qc = Quantity of condensate (in kg/hr)

Hc = Heat contained in condensate (in kcal/kg)

Hw = Heat already present in the water at ambient tenmperature (in kcal/kg)

GCV (Gross Calorific Value) =Energy contained in fuel in kcal/kg

η = Boiler efficiency

ρ = Specific gravity of liquid/gas fuel

Example 5.2. Find the cost savings from one ton of condensate at 100°C, 0 kg/cm2g on an oil-fired boiler.

Rs. 183.5/ hour x 800 hrs = Rs. 14.68 lakhs/ year

There is also the added savings from water and water treatment which is calculated similar to Example 5.1.

Cost of Flash.If sufficiently hot condensate from a pressurized system is released to a lower pressure, some of that condensate will have the heat necessary to become steam. This is called flash steam.

‘Flash steam’ is released from hot condensate when its pressure is reduced. Even water at room temperature of 35°C would boil if its pressure were lowered far enough. It may be worth noting that water at 170°C will boil at any pressure below 7 kg/cm2g. The steam released by the flashing process is as useful as steam released from a steam boiler.

Example 5.3

What is the total savings from 1 TPH condensate @ 7barg on oil?

First we use a flash separator to separate out flash at 0.5 barg.

The quantity of flash separated can be found from the formula:

Where,

hf1 = Enthalpy of water at the higher pressure kcal/kg

hf2 = Enthalpy of water at the flashing pressure kcal/kg

hfg2 = Enthalpy of evaporation at the flash steam pressure

% of flash steam generated = hf (7barg) - hf (0.5 barg) / hfg (0.5 barg) = 172 - 111/ 532 x 100 = 11.5 %The above formula, in fact, is used to make the graph below.

Quantity of flash is therefore, 11 % of 1000 kg/hr = 111 kg/hr

= Rs 178.7/hour x 8000 hrs = Rs. 14.29 lakhs/year

After removing flash, we will also recover condensate. The quantity of condensate available is = (1000 - 111 ) kg/hr = 889 kg/hr

The savings from this can be found out similar to Example 5.2. It comes to Rs. 13.05 lakhs/year.

The savings from flash in most cases is as much, if not more than condensate.

Boiler evolution.In the 17th century, the events of the Industrial Revolution, primarily in England, promoted the rapid development of the steam engine by such inventors as Thomas Newcomen and James Watt.

Watt is credited with being the first inventor to separate the steam engine, and the boiler, into two separate units in the latter part of the 18th Century. In these early times, the primary use of the boiler was to generate steam for steam driven engines. As steam driven engines replaced the horse, as a means of motive power, it followed that steam driven engines were rated in ‘Horsepower’.

Boiler design progressed from what was essentially a kettle to a relatively large-diameter flue pipe submerged in water – thus the first fire-tube boiler. As power and pressure requirements increased, boilers became larger and the single-flue pipe became a larger number of smaller diameter flue tubes combined with an external, or internal, furnace for the combustion of the fuel.

Competition between fire-tube boiler manufacturers eventually forced improvements in boiler design and fuel burning equipment. This, together with a broad shift towards liquid and gaseous fuel utilization, resulted in cleaner and more reliable combustion and improved heat transfer within the boiler.

The basic evolution in boiler designs were:

• Plain Cylinder Boiler

• Cornish Boiler

• Lancashire Boiler

• Upright Fire Tube Boilers

• Scotch Boiler

The Plain Cylinder Boiler

The first advancement in boiler design came with the invention of the Plain Cylinder Boiler.

It was a simple design and easily constructed.As its name implies, the Plain Cylinder Boiler is a long metal cylinder with conical (round) ends set horizontally in a brick work. The cylinder was half filled with water and a fire ignited in furnace at one end.

The fire and hot gasses are first channeled from the furnace or fire box along the bottom of the cylinder to the opposite end of the boiler. This channel is called a "flue" and is made of brick on three sides. The other side of the flue is the metal wall of the boiler. The flames and hot gasses touch the bare metal and heat the water inside the boiler.

When the hot gasses get to the end of the first flue, they are channeled back along one side of the cylinder to the front of the boiler. From there they are again channeled back along the other side of the cylinder to the chimney. This would give a boiler 40 feet long 120 feet of heating surface.

The speed with which the fuel burned was controlled by a damper near the chimney. Raising or lowering the damper controlled the draught or amount of air being drawn into the furnace. More air made the fuel burn faster and hotter making more steam. Less air saved fuel and produced less steam.

Although this boiler design was far more efficient than previous boilers, and had been used for more than one-hundred years, it had two major flaws.

1.    The first was dirt. Water contained dirt and as water evaporated, this dirt collected in the bottom of the cylinder and acted like an insulator preventing the heat from reaching the water. Therefore, more fuel was burnt and more cleaning was required.

2.    The second flaw was more dangerous. As the hot gasses traveled along the 120 foot long flue, they cooled a little. The metal of the cylinder was heated unevenly on either of the three sides causing stress leading to frequent explosions.

The Cornish Boiler

The invention of a workable steam boiler and engine made mass transportation possible. However hazardous it might have been. Economics was the driving force behind a new design - the Cornish Boiler.

Until that time, designers had always placed the furnace beneath the water cylinder. Then some genius had the idea of putting the fire where it would do the most good, in with the water. Not actually "IN" the water but literally inside the cylinder containing the water.

The Cornish Boiler made several design changes

1.    First the 3 feet dia furnace was placed inside a metal tube in the boiler. This greatly increased the amount of heat transferred to the water.

2.    The order in which the gasses moved was changed. After leaving flue #1 (the main furnace) the hot gasses were divided at the back into two streams - flue #2 which brings the gas back to the front.

3.    At the front of the boiler the hot gasses were directed downward into flue #3 and traveled back beneath the boiler to the chimney. This helped reduce the amount of mud that accumulated in the bottom of the boiler and that increased the boilers efficiency even more.

4.    The cylinder now had flat ends because of above design changes.

Unfortunately, the internal tube with its furnace and fierce heat was constantly changing length. It would bulge and then contract as the temp dropped even slightly leading to stress. The results were the same. An explosion.

The Lancashire Boiler

The need for smaller more powerful, to say nothing of safer, steam boilers led to the Lancashire Boiler design. It had many advantages:

1. First, each boiler had two completely separate furnaces sitting side by side. And each furnace had a separate flue system. This idea was outstanding. Why?

2. Everything that burns contains some water. This water must be evaporated for fuel to burn efficiently. Doing this, the furnace cools somewhat and that in turn lowers the amount of air being drawn into the furnace. The less air drawn in the less heat created in the furnace. This slight cooling placed a heavy strain on the ends of the boiler. It also slowed the heating of the water inside the boiler reducing the amount of steam available.

3. With the Lancashire Boiler each furnace is stoked at a different time. This means that one furnace is always producing maximum heat and that heat creates a powerful draught in both furnaces speeding up the ignition process. Something like blowing on a campfire. It heats things up real fast.

4. It also means that more air is drawn into the system which allows combustion (burning) of the smoke created by the "low" burning furnace. This combustion takes place in flue #2 thereby increasing the amount of heating of the sides of the cylinder.

5. 2.    Like the boilers we have already seen, the Lancashire Boiler has three flues. But the Lancashire Boiler is designed with two separate flue systems. Gasses from the right side furnace remain on the right side of the boiler while hot gasses from the left furnace remain on the left side. They do not combine until they reach the base of the chimney connection. This system provided a very powerful, even and constant draught in both furnaces.

3.    Another major improvement in heat transfer and fuel efficiency was the addition of "Galloway Tubes". Hollow metal tubes which traverse (connect both sides of ) the main flue #1. Water in the boiler flowed through these tubes which are subject to heating by the hottest fire and gasses which pass around them.

4.    The increased efficiency of Lancashire style boilers also allowed them to be smaller. Commonly only seven feet in diameter (side to side) and twenty-seven feet long. A great saving of both space and weight.

5.    There were extensive number of internal braces designed to keep the cylinder from rupturing. The braces, and stays kept the ends from bulging and added much to the overall strength of the boiler.

6.    These boilers were the first to feature the "low water safety valve" shown in the illustration. Should the water level drop below the top of the internal flues, the intense heat of the furnace would quickly burn through the metal. A float rides up and down with the water level in the boiler. As the water dropped below a predetermined level, the valve would gradually open and release steam pressure. The noise this valve made when it opened would also get the attention of the engineer very, very, quickly.

7.    A second "Pop Off Safety Valve" was also installed at the front of the boiler. These valves operated on steam pressure alone. When the pressure in the boiler exceeded a specified amount, these valves suddenly "popped" wide open and stayed wide open until they were reset or replaced.

They can still be seen in some old paper plants. The concept of riveting came from these boilers (in the past, the welding quantity was not good enough for higher pressures)

The Upright Fire Tube Boilers

We take a side track and explain something about the vertical (upright) boiler.

• The furnace is located inside the water tank. It has a number of brass tubes which extend through the boiler to the chimney carrying the hot gases allowing an extremely high rate of heat transfer to the water. In other words, the fire passed through the tubes, hence the name, Fire Tube Boiler.

• The water tank or boiler is a vertical tank not a horizontal cylinder.

• Their compactness and the speed with which they developed a working steam pressure was of major importance.

• The relatively low pressures developed by this style of boiler made it perfect for many jobs. From heating a home to powering small steamboats.

Disadvantages:

• The extremely hot gasses made only one pass through the boiler so this design is not as efficient as those that route the heat back and forth. Much heat was wasted going straight up the chimney.

The Scotch Boiler (2-pass)

Engineers and designers of steam boilers had long understood the relationship between the amount of heat generated in a furnace and the ability of water to absorb that heat. Yet, unlike the Lancashire boiler, the Scotch boiler does

• The water tank is made from corrugated plates and does not utilize Galloway tubes. Because of the larger water surface exposed to the heat more heat is transferred to the water.

• The end plates are reinforced by heavy through bolts. This combination of through bolts and corrugated plates provided an extremely strong boiler.

• The Scotch Boiler has “fire tubes” arranged above the furnaces, but below the water surface.

• At the back end of the boiler the hot gasses entered a chamber, or Dry Back which allowed the end plate to be heated and also directed the gasses into the fire tubes. From there the hot gasses moved forward through the numerous tubes to the chimney.

• The Scotch Boiler was quite versatile. Designs were built to deliver anywhere from 6 to 300 BHP (boiler horse power).

Disadvantages:

Water circulation within the boiler was poor. Cooler water was settling at the bottom of the boiler, acting like an insulator and decreasing the efficiency of the boiler.

• It also allowed mud and scales to be deposited on the outside of the main flues.

• The metal tubes could not transfer heat effectively to the water. Eventually the insulation effect would allow the metal to heat to a point where it would bend.

Boiler classification.There are two approaches in boiler design: fire tube and water tube. The goal in all cases is to maximize the heat transfer between the water and the hot gases heating it.

Fire-tube boilers

The name firetube is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes that are surrounded by the fluid to be heated. The tubes in a fire-tube boiler are made of carbon steel. The body of the boiler is the pressure vessel and contains the fluid. In most cases this fluid is water that will be circulated for heating purposes or converted to steam for process use.

Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass". So a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A 4-pass will have four sets and the stack outlet at the front. A fire-tube boiler was more common in the 1800s.It consists of a tank of water perforated with pipes. The hot gases from a coal or wood fire run through the pipes to heat the water in the tank, as shown here:

Advantages:

• Relatively inexpensive

• Easy to clean

• Compact in size

• Available in sizes from 600,000 btu/hr to 50,000,000 btu/hr

• Easy to replace tubes

• Well suited for space heating and industrial process applications

Disadvantages:

• Not suitable for high pressure applications 250 psig and above

• Limitation for high capacity steam generation

• In a fire-tube boiler, the entire tank is under pressure, so if the tank bursts it creates a major explosion.

For example, steam locomotives have fire-tube boilers, where the fire is inside the tube and the water on the outside. These usually take the form of a set of straight tubes passing through the boiler through which hot combustion gases flows.

Water-tube boilers

A Watertube design is the exact opposite of a fire tube. Here the water flows through a rack of narrow tubes that are encased in a furnace in which the burner fires into. The tubes frequently have a large number of bends and sometimes fins to maximize the surface area. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum.

These boilers are more common today. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can contain the pressure with a thinner wall.

Large steam users are also better suited for the Water tube design. The industrial watertube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications.

Advantages:

• Available in sizes that are far greater than the firetube design. Up to several million pounds per hour of steam.

• Able to handle higher pressures up to 5,000 psig

• Recover faster than their firetube cousin

• Have the ability to reach very high temperatures

Disadvantages:

• High initial capital cost

• Cleaning is more difficult due to the design

• No commonality between tubes

• Physical size may be an issue

The following simplified diagram shows you a typical layout for a water-tube boiler:

In a real boiler, things would be much more complicated because the goal of the boiler is to extract every possible bit of heat from the burning fuel to improve efficiency.

The older fire-tube boiler design—in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space—is a much weaker structure and is rarely used for pressures above 350 psi (2.4 MPa). A significant advantage of the water tube boiler is that there is less chance of a catastrophic failure: There is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

Here are some other ways to classify a boiler.

Table: Classification of boilers

Types of fuels.Sources of heat for the boiler can be the combustion of fuels such as wood, bagasse, coal, oil or natural gas. Electric boilers use resistance or immersion type heating elements. Nuclear fission is also used as a heat source for generating steam. Waste-heat boilers, or HRSGs use the heat rejected from other processes such as gas turbines.

Boilers are now specifically designed to utilize a wide range of standard and alternative fuels.

Solids:                          Coals like Bitumen and Anthracite

Liquids:                        Oils like FO/LDO/ LSHS / HSD

Gas:                              Natural gas/ LPG

Alternative fuels:         Agricultural waste like bagasse, husk, shells / wood / shavings

FO (Furnace Oil) This fuel is a heavy, unrefined fuel. It is less than half the cost of diesel.

LDO (Light Diesel Oil)

LSHS (Low Sulphur Heavy Stock means it is less polluting)

HSD (High Speed Diesel)

Natural gas This is piped to the plant.

With the development of fluidised bed combustion, the solid fuel boiler's efficiencies have gone up considerably. Therefore, a lot of plants are now looking at solid fuels.

Waste Heat There are some application which are exothermic in nature - like, DG sets, reactors. Their waste water is generally cooled before being released into rivers or sewage pipes. Instead, we now use this waste heat to pre-heat water for steam.

Fuel Selection

Fuel is selected based on the following parameters:

• GCV (Gross Calorific Value)

• Availability

• Cost

• Manageability

• Emissions

• Byproducts

• Boiler Design

• Boiler Efficiency (η) Fuels

GCV (Gross Calorific Value). This tells us how much heat we get by burning the fuel.

Availability and cost. It is important that the fuel is available easily and at a reasonable cost.

Manageability. Fuels like coal or bagasse, are more difficult to transport, store and feed.ustion gases flows.

Emmissions from boilers have become a cause for concern in Mumbai. The eastern side of Mumbai is switching to natural gas-fired boilers as there are residences coming up in Mulund.

Coal has a lot of by-products, especially coal dust. These are undesirable by-products especially for food or pharma plants.

Boiler design. Obviously the choice of design determines what type of boiler is used. It is the fuel which is decided first, as the

cost of fuel used per year is always more than the boiler cost

Efficiency (η) based on GCV

Lancashire – (50-60)%

2 P – 70%

3 P W B (oil) – 85%

FBC - (70 -75)%

Chain grate - (60-70)%

Fig. Chain grate fro a coal Boiler

Modern Boilers.The modern boilers are far superior to these old giants. They are smaller, cheaper, with high efficiencies and the heat release rates are higher, so smaller surface areas are needed for heat transfer.

Reverse flame (eg : Revotherm from Thermax)

The combustion chamber is shaped like a cylinder but narrow at the far end. This way, when the burner fires down the centre (1st pass), the flame doubles back on itself within the combustion chamber (2nd pass) to come out at the front of the boiler. The flue gases now enter the fire tubes surrounding combustion chamber, and go to the rear (3rd pass) of the boiler and the chimney. It has an economizer to pre-heat the boiler water where flue gases are used to pre-heat the water.

Water-tube boilers

The water-tube boiler was patented in 1867 by American inventors George Herman Babcock and Stephen Wilcox. In the water-tube boiler, water flowed through tubes heated externally by combustion gases, and steam was collected above in a drum. Water tube boilers are very huge and their water holding capacity is enormous. The water-tube boiler became the standard for all large boilers as they allowed for higher

pressures than earlier boilers, higher than 30 bar. Example, Babcock & Wilcox boiler manufactured at Thermax Boilers Ltd., Pune.

It is a horizontal, externally fired, stationary, high pressure, water tube boiler with a super heater as shown below.

The coal is fed from hopper on to the grate where it is burnt. The flue gases are deflected by the fire brick baffles so that they pass across the left side of the tubes in a beneficial path transferring heat to water in the tubes and to the steam in the super heater and finally they escape into the atmosphere through the chimney. The drought is regulated by a damper placed at the back chamber.

The position of water tubes near the furnace is heated to a higher temper than the rest. Owing to higher temperature, the density of water decreases and hence the water rises through the uptake header and short tube to the drum. The water at the back end, which is at a lesser temperature now travels down through the long tube and the downtake header. Thus, a continuous circulation of water called as natural circulation is established between the water tubes and the drum. The steam produced gets collected above the water in the drum.Here, saturated steam is drawn off the top of the drum.

Since water droplets can severely damage turbine blades, dry steam from the steamdrum is again heated to generate superheated steam at 730°F (390°C) or higher in order to ensure that there is no water entrained in the steam. Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it helps pre-heat the feedwater supply. To increase the economy of the boiler, the exhaust gasses are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such water-tube boilers in thermal power station are also called steam generating units.

NIBR Boilers

In many small plants, the use of Non IBR Coil type boilers has proliferated, mainly due to the following perceived benefits:

• Lower initial cost

• Equivalent efficiency as IBR boilers

• No IBR approved boiler operators are required

• No “hassle” of annual boiler inspection by IBR

• No IBR piping required

Maintenance

A coil boiler requires very stringent water quality to be maintained. The coil requires frequent maintenance/replacements in view of water quality normally not being maintained. Manufacturers of coil-type non-IBR boilers do a roaring business in spare coils.

Safety

In absence of any proper guidelines/inspecting authority/control (such as IBR inspection), quality of fabrication itself is suspect. Shell (IBR) boilers have much better controls. IBR boilers have controls in their manufacturing process which automatically bring with them a higher level of safety.

3PWB package

We will study this in detail, as this is the most common boiler we will come across.

The 3PWB packaged boiler

The 3PWB packaged boiler is the most common type of boiler in most installations. The main components of the 3PWB boiler are shown as under.

1.    Manufacturers name plate This has the MCR - Maximum Continuous Rating – of the boiler mentioned on it, besides the F&A rating.

2.    Boiler shell. This stores water for heating. It is cylindrical in shape with both ends closed.

3.    Front and rear doors are closed using studs with lugs and brass nuts for ease of opening and closing. These doors allow for total access to the return tubes.

4    Burner. An equipment which burns the fuel. Burners mix air with fuel to provide oxygen in the combustion process. Burners are specifically matched with the furnace diameter and lenght for complete/efficient combustion. Burners utilise gas, light/ heavy oil or pulverized (finely ground) coal in combination with air or pressure atomization of the oil.

5.    Air louver, linkage and setting is visuable for ease of monitoring, adjustment and cleaning.

6.    Control panel. This is used to control the boiler and is either boiler mounted or on a separate skid.

7.    Control circuit junction box with terminal strips to permit checking individual control operations.

8.    Fuel oil heating and filtering systems.

9.    Structural steel skid type base supports the boiler and protects burner. This is bolted to the foundation.

10.    Furnace. Space in a boiler where a burner burns liquid or gas fuel. (A fire grate burns solid fuel). The hot flue gases travel from furnace to the chimney. A large diameter furnace will provide for complete combustion and maximum heat transfer.

11.    Wetback turnaround eliminates the need for refractory lining, bafles, gaskets and provides additional primary heating surface for incrased efficiency.

12.    Split/hinged rear doors provide access to the rear tube sheet and third pass tubes.

13.    Round rear smoke outlet.

14.    Heavy steel lifting eyes for ease of handling.

15.    Mineral fiber insulation reduces heat loss through the jacket and provides jacket support.

16.    Hard enamel paint finish to the galvanized coated steel jacket.

17.    3 Pass design for optimum efficiency and economical operation.

18.    Front smoke box and doors lined with ceramic fiber blanket.

Chimney. This is a system for venting hot gases and smoke from a boiler to the outside atmosphere. They are typically almost vertical to ensure the hot gases flow smoothly, drawing air into the combustion through convection. The space inside a chimney is called a flue. Chimneys are tall to disperse pollutants in the exhaust over a greater area reducing the concentration of toxins to a safe level and to increase the draw.

On and around a boiler.The heart of any modern steam system is the boiler house. Here we need to achieve

• maximum Safety via Boiler mountings

• high Efficiency and economical running via Boiler auxillaries

• excellent Controls on pressure and temp (see Boiler controls)

Boiler mountings - for maximum safety

All the following are mounted on the boile r shell and are a must for every boiler. All of them are provided for the safe working of boiler.

• Feed pumps

• Feed check valve

• Main steam stop valve(MSSV) or Crown valve

• Mobrey - water level indicator

• Safety Valve

• Gauge glass

• Fusible plug

Feed check valve

The boiler feed check valve is designed specifically for use on boiler feedwater systems. The valve is opened by the boiler feedwater pressure and is closed by its spring as soon as the flow ceases, preventing reverse flow. The strong spring supports the head of water in an elevated feedtank when there is no pressure in the boiler, preventing the boiler from flooding. It is a normally a stainless steel disc check valve to ensure tight shut-off against boiler pressure, even under poor water conditions.

Main Steam Stop valve (MSSV) or Crown valve

This valve isolates the boiler and its steam pressure from the process or plant. Always open this slowly to prevent a drastic increase in downstream pressure. Other dangers of opening the Main steam stop valve quickly is priming, where slugs of water may enter the distribution system or waterhammer, where collected condensate slugs in the distribution pipe picks up sudden velocity. It should not be used as a pressure reducing valve to throttle steam flow, but should be in fully open or closed mode.

Mobrey / Water level indicator

This is a level switch. This is an extremely important safety mechanism which maintains water level above the fire-tubes at all times. If the water level falls below the tubes, heat cannot be dissipated to water. The temperatures within the fire tube build up and the tubes can melt.

Most accidents happen because of a fall in water level in the boiler !!

In fact in most cases there is a second low water level alarm interlocked to the burner. This shuts off the burner as soon as the water level falls below a specified limit.

When we want to shut down a boiler, we cut off the burner and then start the feed pumps to cool down the water in the boiler.

Safety Valve

A safety relief valve is one of the most critical safety devices on any boiler. It is the boiler’s last measure of protection against overpressure. It must be adequately sized and of the correct pressure rating for the boiler.

But getting a safe installation is only the beginning. The safety relief valve also must be inspected and tested regularly. Mud and scale from the boiler can interfere with the operation of the relief valve. Plugged discharge lines can prevent proper operation or allow discharged water and steam to come in contact with equipment or operating personnel.

Lifting the test lever while the boiler is operating will confirm its proper operation. At no time should technicians test the valve by increasing the pressure of the boiler to

a level higher than the safety-valve setting. They should exercise caution when testing relief valves, as steam or hot water will be discharged through the valve at the operating pressure of the boiler. Valves should be tested every time a boiler is started and at the interval recommended by the manufacturer.

A deadweight safety valve on top of a boiler. The valve lifting pressure is set by the movement of the weight to the left of the arm. The further the weight from the valve, the higher the pressure in the boiler required to release the excess steam.

Gauge glass

The glass tube, or pair of flat glass plates, fitted to a water-gauge to provide a visual indication of the water-level in a boiler or a tank.

A simple clear glass tube is mounted between two shut off valves which are connected to the rear of the boiler by pipes. When both valves are opened, the tube fills with water and steam. The surface of the water in the tube is precisely at the same level as the water in the boiler. You know in an instant how much water the boiler is holding.

Feed pumps

A pump that supplies water to a boiler. In most modern-age boilers, these are vertical, multi-stage pumps to provide the high pressures required.

Fusible plug

A hollowed threaded plug having the hollowed portion filled with a low melting point material used, for example, in the crown of a boiler firebox. If the water level falls below them the metal in the plug melts and steam is dumped into the firebox to prevent serious overheating of the plates.

Boiler auxillaries - for high efficiency and economical running These are used to improve the efficiency of the boiler. An economiser, super heater and air pre-heater are the main accessories. These are not a must for any boiler but are highly desirable.

• Economisers

• Super-heaters

• Air pre-heaters

Economisers

An economiser is a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to pre-heat the cold water used the fill it (the feed water).

Flue gases from large boilers are typically 250 - 350°C. Stack Economizers recover some of this heat for pre-heating water. The water is most often used for boiler make-up water or some other need that coincides with boiler operation. Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the boiler or large amounts of live steam are used in the process so there is no condensate to return.)

It consists of an array of vertical cast iron tubes connected to a tank of water above and below, between which the boiler's exhaust gases are passed. This is the reverse arrangement to that of fire tubes in a boiler itself; there the hot gases pass through tubes immersed in water, whereas in an economiser the water passes through tubes surrounded by hot gases. For good efficiencies, the tubes must be free of deposits of soot.

They are often referred to as feedwater heaters and heat the condensate from turbines before it is pumped to the boilers.

The savings potential is based on the existing stack temperature, the volume of make-up water needed, and the hours of operation. Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers.

Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the boiler or large amounts of live steam is used in the process so there is no condensate to return) or there is a simultaneous need for large volumes of hot water.

Super-heaters

A superheater is a device that heats the steam generated by the boiler again, increasing its thermal energy and decreasing the likelihood that it will condense inside the engine.

Superheat refers to the process of increasing the temperature of steam above saturation temperatures to produce a very "dry" steam with absolutely no water vapor. This feature is most common in very large power plant boilers of watertube construction. Power plants use superheated steam to run the turbine blades. Turbine blades are very vulnerable to water damage. Super heated steam being absolutely dry, is much more suited for this expensive equipment, increasing its life and reducing replacement costs.

Air pre-heaters

An air preheater is designed to heat air before combustion in a boiler. The purpose of the air preheater is to recover the heat from the flue gas from the boiler to improve boiler efficiency by burning warm air which increases combustion efficiency, and reducing useful heat lost from the flue. As a consequence, the gases are also sent to the chimney or stack at a lower temperature, allowing simplified design of the ducting and stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).

It uses waste heat to pre-heat air for combustion in boilers. Better combustion can be achieved as the fuel can be atomized better after pre-heating of air.

Efficiency and losses.A modern steam boiler will generally operate at an efficiency of between 80 and 85%. Some distribution losses will be incurred in the pipework between the boiler and the process plant equipment, but for a system insulated to current standards, this loss should not exceed 5% of the total heat content of the steam. Heat can be recovered from blowdown, flash steam can be used for low pressure applications, and condensate is returned to the boiler feedtank. If an economiser is fitted in the boiler flue, the overall efficiency of a centralised steam plant will be around 87%.

There are two methods to calculate boiler efficiency.

Direct Method

Where,

Qs = Quantity of steam (in kg/hr)

Hs = Heat contained in steam at operating pressure (in kcal/kg)

Hw = Heat of feedwater (in kcal/kg)

Qf = Quantity of fuel (in kg/hr)

GCV (Gross Calorific Value) =Energy contained in fuel in kcal/kg

Indirect Method

We calculate boiler efficiency by the Indirect Method using the BS 845 / ASME PTC 4.1 standards.

Boiler η =100 – losses,               =100 – (L1 + L2 + L3 + -------------+ Ln)

Here L1, L2, etc are the various losses.

Boiler losses

Let us take a look at these losses from and around the boiler.

• GCV vs NCV

• Stack loss

• Radiation loss

GCV Loss

There is a difference in the gross calorific valve (GCV) - theoretical ; and Nett Calorific Valve (NCV) - actual of fuel. This is because of two reasons.

• The fuel contain moisture and when burnt, first the heat,makes the moisture in fuel evaporate and then the heat is used to heat water.

• The fuel contains hydrogen, which also has to be burnt

In furnace oil (FO) the difference (loss) between GCV and NCV is 6.25% (typically).

Stack Loss

The flue gases lost to the atmosphere from chimney is called the stack loss. (The chimney is called stack as earlier bricks were stacked to make a chimney).

It is a function of the ΔT between the temperature of the flue gases (Tg) and the ambient temperature (Ta).

Stack loss is dependent on the difference(Tg – Ta). Stack losses can be as high as 10%, or be optimised to about 5%.

Radiation Loss

Even though the boiler surface is insulated with 3''- 4'' of insulation material, there is still a radiation loss taking place. In a lot of older boilers, in fact, the insulations may be old & not working. Typically, based on boiler loading, radiation loss is 1-4%.

Boiler controls.We must have an excellent control on pressure and temperature as well as other parameters in the boiler.

• Sequence control

• Feedwater level control\

• Pressure (firing) control

• Trim control

• Blowdown control

Sequence control

Every boiler has a sequence of events to be executed before start-up and after shut down. To illustrate, a

simple start-up sequence for an oil-fired boiler is shown.

Feedwater level control

The purpose of feedwater control system is to maintain the corect water level in the boiler under all load conditions.

Feedwater level control must be able to regulate water level under very dynamic conditions when the heat rate changes in the boiler cause the boiler level to shrink and swell, due to steam bubble volume changes in response to iring rate changes.

It also has to respond to momentary changes in steam demand replacing steam that has left the boiler with feedwater.

If the feedwater control system fails we are looking at serious problems. A high water level can cause serious damage to the distribution system and to m/c such as turbines. A low water level can expose the boiler tubes allowing the high flame temperatures to weaken and melt the boiler steel causing a catastrophic high energy release of steam from inside the boiler.

A level control system usually employs probes which sense the level of water in the boiler. At a certain level, a controller will send a signal to the feedpump which will operate to restore the water level, switching off when a predetermined level is reached. The probe will incorporate levels at which the pump is switched on and off, and at which low or high level alarms are activated. Alternative systems use floats.

Pressure (firing) control

Energy is supplied to the boiler via a combustion process and the combustion control system regulates the firing rate by controlling the amount of air and fuel delivered to the burners.

Combustion control systems are regulated to maintain the desired steam pressure and they must be able to respond to the many fluctuating conditions of the burner, fuel and air control sub-systems in a co-ordinated way to maintain the steam pressure set point in spite of varying process demands.

Trim control or, Air-fuel ratio control

Air is comprised of approximately 21% oxygen and 79% nitrogen. When air is delivered for combustion, the nitrogen absorbs heat and is carried up the stack, resulting in energy losses. If there is excess air, the result is unused oxygen as well as even more nitrogen to absorb heat that is carried up the stack.

Boiler efficiency can be improved by incorporating an excess air trim loop into the boiler controls. It is easy to detect and monitor excess air, as oxygen not used for combustion is heated and discharged with the exhaust gases. A stack gas oxygen analyzer can be installed to continuously monitor excess air and adjust the boiler fuel-to-air ratio for optimum efficiency.

Performance/Costs: An often-stated rule of thumb is that boiler efficiency can be increased by 1% for each 15% reduction in excess air or 20°C reduction in stack gas temperature. An annual fuel savings of 5% is often obtained with tighter excess air control.

You can periodically “tune” your boiler and manually optimize fuel-to-air ratios after measuring the oxygen in the flue gas with an inexpensive test kit. More expensive hand held computer-based analyzers display percent oxygen, stack gas temperature, and boiler combustion efficiency. An automatic oxygen trim control system minimizes operating costs through ensuring that the proper fuel-to-air mixture is maintained at all boiler loads.

Chemistry Excess Air

Blowdown control

Rather than control blowdown manually, Continuous blowdown, sometimes called surface or skimmer blowdown, is more effective in controlling the concentration in boiler water. Where continuous blowdown systems are used, bottom blowdown is used for removal of precipitated impurities, especially those that tend to settle in the lower parts of the boiler.

Blowdown is also controlled to eliminate energy wastage - which is possible if more water than is necessary is dumped from the operating boiler - quite possible in case of manual blowdown.

Heat exchangers can be used with continuous blowdown, to recover energy from this expelled boiler water.

F&A rating.The boiler is rated to work at a certain pressure and at that pressure it can generate a defined quantity of steam. This can also be written as the F & A rating of the boiler.

This is used as a measure of the boiler ability to produce steam. It gives us the amount of steam (in kgs) that a boiler will produce if supplied with water at 100ºC. eg. A 2 ton boiler gives us 2000 kgs of steam per hour. It is written as 2 TPH F&A 100.

This means that when the water inside the boiler is at 100ºC and the steam take-off is also at 100ºC, the boiler will gives us 2 tons/hour of steam.

F&A Rating X 540 = Actual Rating X ( hg – hfFW)

Where,

hg = enthalpy of steam at generation pressure

hfFW = Feed water enthalpy

Example 6.1. Take a 10 TPH boiler, @ 10.54 barg, 75°C

= 9151 kg/hr nettSo, a 10TPH boiler will never give you 10,000 kg/hr of steam.This disparity is because the feed is not at 100 deg C. Water at 100ºC has 100 kcal /kg latent heat. The F&A rating assumes an ideal condition:                          + 540 kcal/kg100 kcal/k ----------------------------> 640 kcal/kg(water at 100°C)        (+heat)       (steam's latent heat)

But actually, the feedwater is at ambient temperature. So we need more heat to first raise the temperature to saturation (100°C). This is what gives rise to the difference in boiler output.

                   + 610 kcal/kg

30 kcal/k ----------------------------> 640 kcal/kg

(water at 30°C)     (+heat)         (steam's latent heat)

FAQs.Question 1: What is the steam space / water holding of a boiler ?

Any boiler must be designed so that it can carry the largest volume of water possible keeping adequate steam space.

If there is a larger body of water at close to saturation, any sudden loading of the boiler (switching on a new process in the plant) will not impact the pressure. It can give off huge steam loads intermittently. On the other hand, if there is a smaller quantity of water in the boiler, sudden load increase could drop the steam pressure.

A minimum steam space of 25-30% of diameter of boiler is needed. This is because, the closer you bring water to the top of the boiler, the greater the chances of carry over inside a boiler as there is a lot of turbulence at the separation surface. Bubbles of steam are rising up and bursting, so water droplets might enter the system along with steam. That means if the steam space is too small, dryness fraction decreases.

For a similar reason, a larger furnace is often fitted to a boiler. It can withstand thermal shocks which occur when there is a sudden rise in steam demand.

Question 2: Shouldn't we do a 4th pass to get higher efficiency ?

In fire tube boilers, the temperatures of the various passes are :

1st Pass - 700 to 1100

2nd Pass - 400 to 700

3rd Pass - 200 to 300Normally, we generate steam at 10 bar (187ºC) so in a 4th pass the water will heat the fuel. Instead of a 4th pass, we can use the fuel gases to pre-heat the fuel (economizer).

Question 3: Why should stack temperature rise? Is it because excess air blows out unburnt fuel and gases?

No. in fact, excess air will only decrease the temperature even if it is blowing out unburnt fuel and gases.

Stack temperatures rise mostly when the tube in the boiler cannot provie efficient heat transfer because of soot on tubes, or oil inside the tubes. The water cannot therefore, absorb all the heat being generated in the firetubes and the end result is a lot of heat lost via the stack . Hence the rise in stack temperatures. Inadequate heat transfer area within the boilers will also result in elevated stack temperature levels.

Water salts and scale deposits on tube from outside also impede heat transfer. Boilers using water from DM plants need less cleaning as compared to boilers using well water.

Fouling of the heat transfer area also occurs because of unclean fuel.

Question 4: Why should dampers for excess air levels be watched ?

As the seasons change, the air density also changes. In summer, for example, the air is less dense as compared to winters. This is because when it gets hot, the volume increases. So, we may need to ramp up excess air levels during the summers as compared to winters.

Also, the damper is a mechanical movable part. So it may lose its calibration. A pocket O2 analyzers will help us set the damper to a correct level. (On line O2 analyzers are also available but they are expensive).

Typical steam circuit.Steam runs in a closed circuit. Steam starts from the boiler, is utilised by the process and returns back via the condensate line.

Generation. Heat is applied to water in the boiler. We convert water to gas, steam. The resultant expansion, pressurizes the system. Steam is forced out of the boiler by its own pressure. It moves into the second stage ....

Distribution. Steam is carried by piping to various equipment that heat or process material. Again, the steam is carried along because of pressure changes within the system. Steam has now arrived at its point of use.

Utilisation. Heat from the steam is now put to work. Special devices absorb heat from steam to do different types of jobs. As the steam gives up its heat through heat transfer or use, it condenses or changes its state - this time from a gas back into a liquid. This is called condensate. As condensate can lead to various problems in the steam system, it is drained via steam traps almost immediately.

Condensate Return. Condensate is that is already treated before entering the steam circuit. If this is returned to the boiler, it can replace an equal quantity of cold make-up water. This is not only energy-wise but also helps save fuel.

Generation - the boiler house and its controls.The boiler

Our journey starts at the boiler house. Our boiler has to be safe, efficient, its pressure and temperature must be well controlled. Also, it has to be economical to run.

We explain boilers in full in the "Boilogy" section.

Boiler Feedtanks

One of the most important factors in keeping your boiler on-line is to keep enough water in it. Otherwise the boiler will shutdown on a low water condition. This is especially true with firetube boilers that are fired automatically. That is why it is so important to size a feedwater system so that it has the capability of maintaining the proper water level in your boiler.

A properly sized feedwater system will have a tank adequately sized to feed your boiler and pumps selected to deliver that water at the correct rate and pressure. Typical FWT size is 2.5 to 3 times of generation.

Furnace

Space in a boiler where a burner burns oil, gas or pulverized (finely ground) coal.

Burners mix air with fuel to provide oxygen in the combustion process. A burner sends heat into the boiler tubes and it is set to maintain the correct pressure in the boiler. If the boiler pressure falls because of growing steam demand, the burner switches on to produce more steam from the boiler. As long as the amount of steam being produced in the boiler is as great as that leaving the boiler, the boiler will remain pressurised. This maintains correct pressure. If correct pressure is maintained, correct temperature is also maintained as they are interlinked.

Combustion, stack losses etc are covered in Chemistry.

Boiler mountings - for maximum safety

These are provided for the safe working of boilers. A feed check valve, a main steam stop valve, a safety valve, water level indicator, a fusible plug are some of the mountings. They are mounted on the boiler shell and are a must for every boiler.

• Feed check valve

• Main steam stop valve

• Mobrey - water level indicator

• Safety Valve

• Gauge glass

• Feed pumps

• Fusible plug

• MSSV

Boiler auxillaries - for high efficiency and economical running

These are used to improve the efficiency of the boiler. An economiser, super heater and air pre-heater are the main accessories. These are not a must for any boiler but are highly desirable.

• Economisers

• Super-heaters

• Air pre-heaters

Boiler controls - for regulating boiler parameters

We must have an excellent control on pressure and temperature as well as other parameters in the boiler.

• Combustion control

• Air control

• Feedwater level control

• Blowdown control

• Furnace Pressure control

• Steam temperature control

• Cold end temperature control

• Soot blower control

Distribution – piping steam to the plant.The boiler has one or two main steam pipes – called steam mains. These branch outwards to smaller pipes which distribute steam to various processes.

Boilers generate pressurised steam, as it occupies less space. So, more steam can be produced by a smaller high pressure boiler and transferred to the point of use using small bore pipework. Steam pressure is then reduced at the point of use.

The steam flows through the pipes losing heat via radiation. As steam condenses to water, the pressure drops, suctioning the steam forward. This pressure drop creates the flow of steam through the pipes.

Condensate and Air in the distribution system

Knowing that it is virtually impossible to keep air, oxygen and carbon dioxide from getting into a system, lets deal with getting them out of a system. These gases become free when the steam condenses.

We must drain condensate out of our distribution system because it can

• reduce heat transfer , and

• cause water hammer

We also should evacuate air and other non-condensible gases because they

• can reduce heat transfer by reducing steam temperature and insulating the system

• foster destructive corrosion

For this, we use a device called a steam trap, which is simply an automatic valve that opens for condensate , air and CO2 and closes for steam. For economic reasons , the steam trap should do its work for long periods with minimum attention.

Water hammer, corrosion due to gases, etc. are fully explained in Chemistry.

Piping details and steam traps will be taken up in detail in Training Level 2.

Once the steam has been employed in the process, the resulting condensate needs to be drained from the plant and returned to the boiler house. This loop is called the condensate loop and is talked about later in this Module.

Utilisation – steam and the process.Steam is generated, distributed and now it reaches the point of use. At the point of use, steam gives up its energy to the process, ie, a heat transfer takes place. Steam could be utilised for example, by any of the following processes:

• Jacketed vessels

• Heat exchanger

• Autoclave

• Heater battery

• Process tank heating

Pressure reduction

We control steam to the process on start-up and also during normal working. Why?

• On startup, a gradually increasing flow of steam will be needed to deliver slow heat build-up in the plant. As the process reaches the desired temperature, the flow must be reduced.

• More important, steam is usually generated at high pressure, and the pressure may have to be reduced at the point of use, either because of the pressure limitations of the plant, or the temperature limitations of the process.

We therefore need a way to control the flow of steam. A PRV is a special Pressure Reducing Valve which functions as a safety device to keep the low pressure header from gaining more low pressure steam than it can distribute. A variety of pressure control options exist, from the simplest to the more complicated and accurate pressure reducers.

1. Pilot operated:

2. Ex: Spirax DP 17 / DP 1433. Self-acting diaphragm-type:

It is comparatively a low cost valve, and is easy to maintain. It is a mechanical device and can be easily looked after by the normal maintenance crew.

4. Ex: Steamline Samson 41-23

3.    Pneumatically actuated valves: Compressed air is applied to a diaphragm in the "actuator" to open or close the valve. The process has a sensor which is relaying process conditions to the controller. Depending on the set values, the controller compares the process condition with the set value and sends a corrective signal to the actuator, which adjusts the valve setting.

The CV is an electro-mechanical device and is highly specialized. Its accuracy is very high, but it is expensive. It needs trained personnel for maintenance.

Ex: Samson 241-1, Arca

Distribution end: On the steam mains and distribution lines, we reduce pressure using a simple direct-acting pressure reducing valve or a pilot operated valve. ( 1 or 2 above )

Utilisation end: This is the process end, which is a more critical area, and here control valves are used to control the flow of steam. ( 3 or 4 above )

The need to drain the heat transfer unit

When steam comes in contact with condensate cooled below the temperature of steam, it can produce another kind of water hammer known as thermal shock. Steam occupies a much greater volume than condensate, and when it collapses suddenly, it can send shock waves throughout the system. This form of water hammer can damage equipment, and it signals that condensate is not being drained from the system. Obviously, condensate in the heat transfer unit takes up space and reduces the physical size and capacity of the equipment. Removing it quickly keeps the unit full of steam.

Fig. Coil half-full of condensate can't work at full capacity.

As steam condenses, it forms a film of water on the inside of the heat exchanger. Non-condensable gases do not change into liquid and flow away by gravity. Instead, they accumulate as a thin film on the surface of the heat exchanger - along with dirt and scale. All are potential barriers to heat transfer.

Fig. Potential barriers to heat transfer: steam heat and

temperature must penetrate these potential barriers to do their work.

The need for Steam traps

All steam pipes and heat exchangers are drained by steam traps placed at strategic locations. The job of the steam trap is to get condensate, air and CO2 out of the system as quickly as they accumulate.

• Condensate does not transmit heat effectively. A film of condensate inside plant will reduce the efficiency with which heat is transferred. Condensate also causes water hammer.

• Dissolved air causes corrosion.

In addition, for overfall efficiency and economy , the trap must also provide.

• Minimal steam loss. Unattended steam leaks can be very costly.

• Long life and dependable service. Rapid wear of parts quickly brings a trap to the point of undependability. An efficient trap saves money by minimizing trap testing, repair, cleaning, downtime and associated losses.

• Corrosion resistance. Working trap parts should be corrosion -resistant in order to combat the damaging effects of acidic or oxygen -laden condensate.

• Air venting. Air can be present in steam at any time and especially on start -up. Air must be vented for efficient heat transfer and to prevent system binding.

• CO2 venting. Venting CO2 at steam temperature will prevent the formation of carbonic acid. Therefore, the steam trap must function at or near steam temperature since CO2 dissolves in condensate that has cooled below steam temperature.

• Operation against back pressure. Pressurized return lines can occur both design and unintentionally. A steam trap should be able to operate against the actual back pressure in its return system.

• Freedom from dirt problems . Dirt is an ever-present concern since traps are located at low points in the steam system. Condensate picks up dirt and scale in the piping, and solids may carry over from the boiler. Even particles passing through strainer screens are corrosive and, therefore, the steam trap must be able to operate in the presence of dirt.

A trap delivering anything less than all these desirable operating /design features will reduce the efficiency of the system and increase costs. When a trap delivers all these features the system can achieve.

• Fast heat-up of heat transfer equipment.

• Maximum equipment temperature for enhanced steam heat transfer.

• Maximum equipments capacity.

• Maximum fuel economy.

• Reduced labour per unit of output.

• Minimum maintenance and a long trouble-free service life.

Sometimes an applications may demand a trap without these design features, but in the vast majority of applications the trap which meets all the requirement will deliver the best results.

Types of traps

There are three primary categories of steam traps:

1. Mechanical: This trap is made up of mechanical apparatus that are driven by the density of the condensate to operate a float or a bucket.

Float traps

In the float steam trap a valve is connected to a float in such a way that a valve opens when the float rises. As condensate enters the trap, a float is raised and the float lever mechanism opens the main valve to allow condensate to drain. When the condensate flow reduces the float falls and closes the main valve, thus preventing the escape of steam. The valve is positioned so that when the float is at rest the valve is seated in the outlet of the trap, ie, it is closed.

2.    Thermodynamic: In addition to downstream flash steam assist, this type of trap operates on the difference in velocity or kinetic energy between steam and condensate passing through a fixed or modulating orifice. These are mostly used for mainline applications. They are comparatively cheap. This is a blast discharge trap, not a continuous discharge type. There is a build -up of condensate which is then discharged at one go. The float inside this trap is mechanically coupled to a valve.

Thermodynamic traps contain a disc which opens to condensate and closes to steam. This trap is reliable, effective and has a long life. The float is made of pressed SS on Titanium and the body is generally CS or cast iron. There are 10 times as many TD traps compared to float traps.

2. Thermostatic: This type of trap operates on the principle of expanding liquids and metals used to drive a valve into or back it away from a seat. There are two basic designs for the thermostatic steam trap, a bimetallic and a balanced pressure design. Both designs use the difference in temperature between live steam and condensate or air to control the release of condensate and air from the steam line.

3. These are mostly used for process applications. These traps are used in 10% of total applications. They are reliable but expensive, so only used in critical applications.

Which trap is preferred depends on the application. A steam trap prime missions is to remove condensate and air preventing escape of live steam from the distribution system. The steam trap must adapt to the application. A disc thermodynamic steam trap should never be used together with a modulating heat exchanger - and a floating ball steam trap is overkill for draining steam pipes.

In most cases, when start-up occurs, we bypass the trap. The cost of traps rise exponentially with increasing pipe size. This way , when the normal load comes on and condensate reduces, we can function with a much smaller trap.

Keeping cost in mind, we can also decide to use cheaper (and efficient ) traps for non- critical applications like the steam lines, and more expensive ones for critical areas like process.

Practically, we need to install traps every 30 M in a stream line. But, if we are using cheaper traps, we can even reduce this distance to 25 M for increased reliability of trapping and replace as & when required.

Good design practice

In terms of configuration, this should include, among other things, the following: proper slope, the elimination of pockets, proper trapping of condensate when pockets do occur, strategic location of steam traps and a configuration that integrates flexibility to keep the system piping itself within allowable stress ranges during expansion and contraction cycles.

Condensate return.Condensate is the by -product of heat transfer in a steam system. It forms in the distribution system due to unavoidable radiation. It also forms in heating and process equipment as a result of desirable heat transfer from the steam to the substance heated. Onces the steam has condensed and given up its valuable latent heat, the hot condensate must be removed immediately.

Often, the condensate which forms will drain easily out of the plant through a steam trap. The condensate enters the condensate drainage system. If it is contaminated, it will probably be drained.

Although the available heat in a kg of condensate is small as compared to a kg of steam, condensate is still valuable hot water and should be returned to the boiler.

If not, the valuable heat energy it contains can be retained by returning it to the boiler feedtank. This also saves on water and water treatment costs.

Sometimes a vacuum may form inside the steam using plant. This hinders condensate drainage, but proper drainage from the steam space maintains the effectiveness of the plant. The condensate may then have to be pumped out.

Steam powered mechanical pumps are used for this purpose. These, or electric powered pumps, are used to lift the condensate back to the boiler feedtank. Steam and the condensate system represents a continuous loop.

Fig Steamline CRPS50

Once the condensate reaches the feedtank, it becomes available to the boiler for recycling.

Steam consumption

Now almost all clients are very energy conscious and it is common for customers to monitor the steam consumption of their plant.Steam flowmeters measure the steam consumption, and are used to allocate costs to individual departments or items of plant.

..More details in Level 2.

Pipeline accessories.The steam pipeline has many accessories, all designed for special purposes and needs.

• Stop Valves

• Bypass valves

• Non-return Valves (NRV) and Disc Check Valves (DCV)

• Control Valves (CV)

• Pressure Reducing Valves (PRV)

• Strainers

• Moisture Separators (Msep)

• Traps

• Pressure gauges (PG)

• Pressure sensors

• Temperature gauges (TG)

• Temperature sensors

• Vacuum Breakers (VB)

• Safety Valves (SV)

Stop Valves

These are basic valves that shut off or supply steam, water or air supply to the downstream end. These come in three basic types:

• Ball

• Gate

• Globe

Our PRS for example has two stop valves one at the inlet and the other at the outlet.

Bypass Valve

This valve are the normal stop valves, but installed in a bypass line, which can be used to bypass a piece of equipment, or a section of pipe during routine maintenance, when new fittings are to be put online, or removed.They are usually Globe valves.

For example, if the trap needs maintenance, the stop valve before trap is shut, the bypass opened, and the trap taken out.

Non-return Valves (NRV) and Disc Check Valves (DCV)

These are used to ensure flow in a certain direction only. Take a look at this NRV on the steamline pump inle

Control Valves (CV)

These are valves that provide automatic controls in a process plant, especially in the critical process areas. Their job could be any, or all the following:

• Safety - The plant or process must be safe to operate.

Dangerous and complex plants or processes need automatic controls for safety.

• Stability - The plant or processes should work steadily, predictably and repeatably, without fluctuations or unplanned shutdowns.

• Accuracy - This is a primary requirement in processes to prevent spoilage, increase quality and production rates, and maintain comfort.

• Economy, speed, and reliability are other desirable benefits.

There is normally a Sensor, that senses the process Parameter to be Controlled, sends it to a Controller, which matches it with a predetermined Set Point, and Actuates the Control Valve to make the required adjustments

Pressure Reducing Valves (PRV)

These are specialized Control valves that provide steam at the correct pressure for the process.

Strainers

These keep steam clean, and free of dirt, and grit by straining them out of the system.

Ref: Chemistry / Quality of steam/ Clean, clean steam

Moisture Separators (Msep)

This eliminates wet steam from the process which is the cause of corrosion and decreased efficiencies.

Ref: Chemistry / Quality of steam/ Dry steam

Traps

No matter how much we try, the twin enemies of our steam system – air and condensate – will be present in our system in some degree. Steam traps are used to drain condensate and air from steam lines and heat transfer units, and are a must on every equipment to prevent air and water related problems.

Pressure Gauges (PG)

We use a simple Bourdon tube type pressure gauge. It is a tube, one end sealed and the other is open from which the gas or liquid enters. This causes a distortion in the tube proportional to the pressure of the process.

Our dial is normally 150 mm in diameter and, is marked to indicate the normal working pressure and the maximum permissible working pressure / design pressure.

Fig. Pressure gauge

Pressure gauges are connected to the steam space in the PRS and usually have a ring type siphon tube which fills with condensed steam and protects the dial mechanism from high temperatures. All Steamline PRS's, for example are fitted with two pressure gauges. The inlet PG helps the boiler operator to visually monitor the inlet pressure, to check if the steam is being supplied at the correct pressure. The outlet PG is used by him for setting and monitoring the outlet pressure by looking at the gauge, if required. We also fit a pressure gauge to our flash vessels to see at what pressure flash steam is being generated.

Pressure sensors

These sense the process parameter – Pressure, and return a signal to a Controller.

Temperature Gauges (TG)

The TG helps the Utilities staff to visually monitor the temperatures, either of steam or process.

Temperature sensors

These sense the process parameter – Temperature, and output a signal to a Controller.

Sight glass

Through a sight glass, we can see the water levels, water flow and also the colour of the process, if need be. Traps sometimes have a sight glass mounted to check correct working. Also, if a valve or strainer is blocked flow will be affected and that too can be checked visually.

Vacuum Breakers (VB)

Steam, when it condenses, ie, cools, becomes water and shrinks tremendously in volume. What happens? Around it we have suddenly an almost perfect vacuum. This cannot be allowed to occur, otherwise, our plant and expensive machinery may get damaged.

Steam condenses when heat is lost to the atmosphere for example, on the distribution lines while working, or , more regularly when a machine in the process is switched off.

We therefore insatll a device called a vacuum breaker on the steam inlet of expensive machinery, before it enters the process. It is a valve, that basically allows air in as soon as a vacuum starts to form.

Safety Valves (SV)

The safety valve is fitted to protect the process that the PRS is supplying steam to. The SV protects from over pressure and in the worst case, an explosion.

A safety valve must meet the following criteria:

• The total discharge capacity of the safety valve must be at least equal to the flow through the PRS at the set pressure of PRV. This way, the safety valve capacity will always be higher than the actual maximum flow through the PRS.

• The full rated discharge capacity of the safety valve(s) must be achieved within 110% of the PRS set pressure.

• There must be an adequate margin between the normal operating pressure of the PRS and the set pressure of the safety valve, otherwise the safety valve will keep blowing. Typically 10% above set pressure.

Typical steam using equipment.Space Heaters / Hot rooms / Air Steam heater / RadiatorsSteam heaters or steam coils are heat exchangers in which one medium is steam being condensed while the other medium is a gas (air) being heated when forced through the heat exchanger with a fan. The inlet air may be ducted or simply gathered from the room in which the steam heating unit is positioned. The actual heat exchanger is constructed as a matrix of tubes and corrugated or flat fins (aluminium, copper or other materials) with as high thermal conductivity as feasible for the given application.

Process Air heaters

Process air heating with steam coils is one of industry’s toughest jobs. Many steam coils become early victims of mechanical failure and the internal/external corrosion that can be the beginning of the end of efficient heat transfer.

A process air heating solution should deliver the ability to achieve and maintain the temperatures you need to keep production running at optimum speed and efficiency. The coils must have extra-sturdy fins to stand up to high-pressure cleaning and be made out of tough materials as a defense against galvanic action to survive the rigors of high pressures, high temperatures and corrosive conditions.

Driers – Tray and Rotary

Used for drying out products like tobacco and paper with heat.

Tanks – Injection, coil, jacketed

Steam Sparger

Steam sparging is common in open tanks or kettles containing liquid products or water.This is simply a pipe mounted inside the tank, generally at the bottom, with small holes drilled at regular positions spaced along the length of the pipe with the end blanked off. The steam exits the pipe through the holes as small bubbles, which will either condense as intended or reach the surface of the liquid.

Steam Injection Heater

Steam injection heating for products is a direct-contact process in which steam is mixed with a pumpable ingredient. Heating occurs when the steam transfers some of its internal energy to the product. Steam gives up all of its latent heat of vaporization while condensing and, depending upon the system pressure, some of its sensible heat. Since the steam directly contacts the product and the condensate becomes incorporated into it, the steam source must be clean. Typical steam injection units are compact, inexpensive and simple to control. An example is provided in the figure.

A direct steam injector draws in cold liquid and mixes it with steam inside the injector, distributing heated liquid to the tank. It discharges of a series of steam bubbles into a liquid at a lower temperature. The steam bubbles condense and give up their heat to the surrounding liquid. Heat is transferred by direct contact between the steam and the liquid, consequently this method is only used when dilution and an increase in liquid mass is acceptable. Therefore, the liquid being heated is usually water.

Steam Jacketed Kettles and vats

A jacket, used to distribute steam over a wide surface area, consists of a thin space formed between two, parallel, metallic surfaces. Steam jackets are typically used to heat bulk products held in tanks and kettles. An example of a steam-jacketed kettle is shown in figure. Condensing steam, held captive within the jacket, transfers heat to the product in the kettle. A layer of insulation over the jacket protects operators and conserves heat.

Used in Food processing.

Humidifier

Certain drying and curing processes require humidification of the air surrounding the product to control the drying rate. Culinary steam can be injected directly into a drying chamber or into a ventilation air duct.

Steam-In- place (SIP) Culinary steam is used to achieve high temperatures and moisture levels required to sterilize enclosed surfaces (such as closed tanks, pipes and valves) in food processing equipment.

PHE

The basic plate heat exchanger consists of a series of thin, corrugated plates that are gasketed, welded together (or any combination of these) or brazed together depending on application. The plates are then compressed together in a rigid frame to create an arrangement of parallel flow channels. One fluid travels in the odd numbered channels, the other in the even.

Fig. Inside a Heat Exchanger

Shell & Tube Heat Exchanger

Shell and tube heaters are commonly used to heat a flowing liquid by condensing plant steam or a pumped heat transfer media. A thin-tube wall separates the heating media from the product being heated. In the case of a pumped heat-transfer media (such as hot water), steam is often used to heat the media in a separate heat exchanger.

Fixed Tube-Sheet Exchangers

These are the most economical and are used more often than any other type. The tubes sheets are welded to the shell. The tubes can be examined and replaced easily. Expansion joints are used where there is a possibility of excessive stresses due to differential expansion during operation.

U-Tube Heat Exchangers

The tube Bundle, in this case, consists of a stationery tube sheets, "U" tubes (or hair pin tubes). The tube bundle cab be removed from the shell for inspection and cleaning from outside. The design is particularly recommended for high pressure, high temperature applications. The disadvantages of this design are that the tubes cannot be mechanically cleaned from inside, and also that the tubes can not be replaced except for a few outer bands. Variations of this design are used as Tank Suction Heaters and also in Kettle type re-boilers / evaporators etc.

Evaporators

In the field of thermal separation / concentration technology, evaporation plants are widely used for concentration of liquids in the form of solutions, suspensions, and emulsions.

The major requirement in the field of evaporation technology is to maintain the quality of the liquid during evaporation and to avoid damage to the product. This may require the liquid to be exposed to the lowest possible boiling temperature for the shortest period of time.

This and numerous other requirements and limitations have resulted in a wide variation of designs available today. In almost all evaporators the heating medium is steam, which heats a product on the other side of a heat transfer surface.

• Typical evaporator applications

• Product concentration

• Dryer feed pre-concentration

• Volume reduction

• Water / solvent recovery

• Crystallization

Plate Evaporators

Compact and economically efficient, the plate evaporator /condenser replaces conventional large and expensive fallingfilm units. Its deep channels, large ports and laser welding allow vacuum and low pressure evaporation and condensing for both aqueous and organic systems.

Fig. Plate Evaporators

Framed plates are used as heating surface. These plate assemblies are similar to plate heat exchangers, but are equipped with large passages for the vapor flow. In these units a product plate and a steam plate are connected alternately. The product passage is designed for even distribution of liquid on the plate surfaces and low pressure drop in the vapor phase. Used especially in Dairy and Pharmaceutical industries.

Distillation columns

Distillation is a separation process, separating components in a mixture by making use of the fact that some components vaporize more readily than others. When vapours are produced from a mixture, they contain the components of the original mixture, but in proportions which are determined by the relative volatilities of these components. The vapour is richer in some components, those that are more volatile, and so a separation occurs.

In fractional distillation, the vapour is condensed and then re--evaporated when a further separation occurs. It is difficult and sometimes impossible to prepare pure components in this way, but a degree of separation can easily be attained if the volatilities are reasonably different. Where great purity is required, successive distillations may be used.

In traditional distillation, steam or another heat source is indirectly applied through an external reboiler. In contrast, in direct steam distillation, the steam acts as a dilutant, preventing the buildup of undesirable by-products at the bottom of the distillation column.

Used especially in Chemical and Pharmaceutical industries.

Case study: Steam distillation is also the most common method of extracting essential oils. Many old-time distillers favor this method for most oils, and say that none of the newer methods produces better quality oils.

Steam distillation is done in a still. Fresh, or sometimes dried, botanical material is placed in the plant chamber of the still, and pressurized steam is generated in a separate chamber and circulated through the plant material. The heat of the steam forces the tiny intercellular pockets that hold the essential oils to open and release them. The temperature of the steam must be high enough to open the pouches, yet not so high that it destroys the plants or fractures or burns the essential oils.

As they are released, the tiny droplets of essential oil evaporate and, together with the steam molecules, travel through a tube into the still's condensation chamber. As the steam cools,it condenses into water. The essential oil forms a film on the surface of the water. To separate the essential oil from the water, the film is then decanted or skimmed off the top.

Autoclaves and sterilizers

An autoclave is a pressurized device designed to heat aqueous solutions above their boiling point.

The heat generated under pressure is called latent heat and has more penetrative power to squeeze through bacteria and even their dormant, heat-resistant form—the spores. This works just fine on solid objects when we start to talk about hollow objects ( needles, tools etc etc) you need to make sure all the air get sucked out or otherwise it will act as an insulation for the bacteria you want to kill.

Their chambers are usually made of SS316 grade stainless steel chamber, and conform to pressure vessel codes. They have to produce sterile loads repeatedly. They must be very easy to clean and reliable.

Autoclaves are widely used in medicine and metallurgy.

Sterilisation is the elimination of all transmissible agents (such as bacteria, prions and viruses) from a surface, a piece of equipment, food or biological culture medium. This is different from disinfection, where only organisms that can cause disease are removed by a disinfectant.

In general, any instrument that enters an already sterile part of the body (such as the blood, or beneath the skin) should be sterilized. This includes equipment like scalpels, hypodermic needles and artificial pacemakers. This is also essential in the manufacture of many sterile pharmaceuticals.

CSG Clean Steam Generators / PSG Pure steam generators

They are basically heat exchangers in which steam is used to convert ultra-pure water to ultra-pure steam. Used in Pharma, Food industries.

Steam jacketed molding presses

Used in the following industries: Tyre, Rubber, Chocolate, Fibre glass thermocole packaging.

Vapour absorption chillers

The absorption chiller is a machine to produce chilled water by using heat such as steam, hot water, gas, oil. The chilled water is then used for airconditioning plants.

Absorption chillers use heat instead of mechanical energy to provide cooling. A thermal compressor consists of an absorber, a generator, a pump, and a throttling device, and replaces the mechanical vapor compressor.

Chilled water is produced by the principle that liquid, which evaporates easily, absorbs heat from surrounding when it evaporates. In the chiller, refrigerant vapor from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator. There the refrigerant re-vaporizes using a waste steam heat source. The refrigerant-depleted solution then returns to the absorber via a throttling device.

Pure water is used as refrigerant and lithium bromide solution is used as absorbent.

Ejectors

Ejector is the generic name of a jet appliance capable of aspirating different products: gases, liquids and solids (powders, granulates or sludge) and takes different names according to its functions: jet vacuum pump, thermocompressor, gas scrubber, eductor, etc. The operating theory is the same for every type of ejector.

Jet vacuum pump (our main application for the ejector)

Static operating apparatus capable of obtaining a vacuum within a capacity. The vacuum corresponds to the suction pressure of the steam or gas needed by process requirements. The suction pressure is obtained by means of thermodynamic and fluid mechanics laws: A high energy potential motive fluid is relieved through a converging and diverging nozzle and accelerated to velocities that are often supersonic. At the outlet of the nozzle, the potential energy of the motive fluid is transformed into kinetic energy.

At the inlet of the diffuser, the motive fluid gives off part of its kinetic energy to the aspirated fluid so that the mixture of the two fluids goes through inverse transformation in which the velocity is converted into pressure at the diffuser discharge.

1.     Motive fluid inlet

2.     Vacuum – suction

3.     Nozzle

4.     Diffuser

4.1.  Converging mixing cone

4.2.  Diffuser neck

5.     Discharge

Turbines

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. It is operated by highly pressurized steam directed against vanes on a rotor.

It has completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Also, because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator - it doesn't require a linkage mechanism to convert reciprocating to rotary motion.

Pic. Turbine rotors on which high-pressure steam is directed

Steam turbines are made in a variety of sizes ranging from small 1 hp (0.75 kW) units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity.

There are several classifications for modern steam turbines.

Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam is available.

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially saturated state, typically of a quality greater than 90%, at a pressure well below atmospheric to a condenser.

Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feed water heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

Quality of steam.Steam may be at the correct pressure and temperature, but the quality of steam is very important as well. Good quality steam must be dry. Dry steam is steam which has a very high dryness fraction, ie almost no moisture. Unfortunately, in most steam systems we are faced with wet steam. Wet steam is steam containing a degree of water. It can reduce plant productivity and product quality, and can cause damage to most items of plant and equipment. It can cause erosion and affect heat transfer processes.

What is the ideal steam quality?

• Steam must not be wet, but with as high a dryness fraction as possible.

• There should be no air present in steam.

• Steam should not contain any dirt.

Only then do we deliver it to our process.

Dry steam

Dryness fraction achieved from a typical shell boiler where the heat is supplied only to the water and where the steam remains in contact with the water surface, typically contains around 5% water by mass. If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95. This is because bubbles of steam break through the water surface and they cause turbulence and splashing. Therefore the steam space contains a mixture of water droplets and steam. This kind of steam is called wet steam.

The actual enthalpy of evaporation of wet steam therefore is only 95% of the one in the steam table. So, wet steam has lower usable heat energy than dry saturated steam. Also, the specific volume of water is much lower than steam. Therefore, the total specific volume of steam will also reduce by the same factor.

Causes of wet steam

The boiler itself generates saturated steam which is inherently wet. Most shell type steam boilers produce steam with a dryness fraction of between 95 and 98%.

Priming and carryover beacause of foaming and scale within the boiler shell increase wetness still further.

Steam condenses on the way from the boiler to the process as there is always a certain degree of heat loss from the distribution pipe. The condensed water molecules will eventually gravitate towards the bottom of the pipe forming a film of water. Steam flowing over this water can raise ripples that can build up into waves. The tips of the waves tend to break off, throwing droplets of condensate into the steam flow.

Problems caused by wet steam

Water – a heat barrier. Water doesnt allow the heat in the steam to cross over to the process, ie the medium to be heated. You can see the temperature drop because of the layer of moisture in the steam pipe. This hampers not just plant productivity by increasing the cost of fuel, but also product quality.

Waterhammer. Failure of valves and flowmeters due to rapid wear or waterhammer.

Corrosion and Wiredrawing. Water droplets increase the amount of corrosion. Water droplets travelling at high steam velocities will erode valve seats and fittings, a condition known as wiredrawing.

Erratic operation of control valves and flowmeters beacause of the above.

Scaling. Pure steam cannot carry any impurities, but water can. These impurities only increase scaling of pipework and heating surfaces.

Understanding Water hammer

In a lot of steam systems, one can hear a thudding sound and feel the pipes vibrating intermittently. This phenomenon is called water hammer and is caused by water lying in the bottom of steam lines, trapped in the steam system. Water is formed by steam condensing. Steam traveling at up to 100 Kms per hour makes “waves” as it passes over this condensate. (see fig.)

If enough condensate forms, high-speed steam pushes it along, creating a dangerous slug that grows larger and larger as it picks up liquid in front of it. Anything that changes the direction - pipe fittings, regulating valves, tees, elbows, blind flanges - can be destroyed. In addition to damage from this “battering ram”, high velocity water may erode fittings by chipping away at metal surfaces.

Fig. Condensate allowed to collect in pipes isblown into waves by steam passing over it and blocks flow at point A. Condensate in B

causes a pressure differential that allows steam pressure to push the slug of condensate along like a battering ram.

Going back to basics, Momentum = mass X velocity

As water is 1000 times denser than steam, its mass and therefore momentum is very large. At the high speed inside a steam pipe, it doesnt turn at the bends, but crashes into them impacting the steam system. ( Practical: When you walk into factory where the pipes are old, but the bends are new, you know they have problems with moisture and water in their system).

If its not removed by moisture separators or drained by traps, it can cause a lot of damage to the steam system.

Steamline Moisture Separator

How do you know a plant needs a moisture separator? When a plant experiences reduced heat exchanger efficiency, erosion at pipe directional changes, erosion to in-line equipment and water hammer, the installation of a separator is a must. All are possible indicators that the presence of entrained condensate particles and the accumulation of condensate exists in the flow of steam.

Moisture separators remove the moisture that remains suspended in the steam flow, which cannot be removed by either drainage or steam trapping. The separator is designed to work in-line and removes approximately 95 - 98% of the entrained condensate particles.

Water drops have more mass and therefore more inertia than steam. (Water hammer is caused because of this same reason, as, at bends the steam passes easily, but the water slug crahes into the bend causing damage.)

In the Steamline plate type moisture separator we take advantage of this inertia difference between water and steam. The Msep contains intercepting plates in its body. The steam + water mix has to change direction a number of times to go through.

Fig. Inside a Moisture separator

What happens? First, the separator has a larger cross-section than the pipe. As the steam, with the entrained condensate particles, enters the chamber of the separator it suddenly and momentarily looses some of its velocity due to the sudden enlargement of the separator chamber. Some drops just fall to the bottom of the separator as condensate. The mass of most condensate particles propels them forward into the impingement baffles. These drops have too much inertia and mass to change direction when they hit the plates, so they just collect on them or the outer perimeter wall of the separator chamber and collect at a low point in the separator. The dry steam flows around the intercepting plates and comes out on the other side.

The MSep is fitted with a suitable steam trap module from the bottom to ensure the efficient removal of condensate, without the loss of live steam.

There are other types of separators in use like the cyclonic and coalescence types of separator, but the plate-type has an acceptable efficiency for steam velocities which are typically in the range of 10 m/s to 30 m/s.

Steam and air don't mix

Effect of Air on Steam Temperature

When air and other gases enter the steam system, they consume part of the volume that steam would otherwise occupy. The temperature of the air/steam mixture falls below that of pure steam. There are tables available that show the various temperature reductions caused by air at various percentages and pressures.

Effect of Air On Heat Transfer

The normal flow of steam towards the heat exchanger surface carries air and other gases with it. The steam velocity pushes the gases to the walls of the heat exchangers, where they may block heat transfer. This compounds the condensate drainage problem, because these gases must be removed along with the condensate.

Since they do not condense and drain by gravity, these non-condensible gases set up a barrier between the steam and the heat exchanger surface. The excellent insulating properties of air reduce heat transfer. In fact ,under certain conditions as little as ½ of 1% by volume of air in steam can reduce heat transfer efficiency by 50 % as seen in the figure below.

Fig. Steam condensing in a heat transfer unit moves air to the side of the heat transfer area. Here, the air collects or "plates out" to form effective insulation.

When non-condensible gases (primarily air) continue to accumulate and are not removed, they may gradually fill the heat exchanger with gases stop the flow of steam altogether. The unit is then “air bound”.

An air film 1mm thick has the same resistance to heat transfer as water 1" thick or iron 4.3" thick.. As a film it acts as an insulator, in solution with steam it deprives the steam of its full heating potential. In other words, air will assume a part of the total volume or pressure that is available. This is explained by Daltons Law of Partial Pressures.

Daltons Law of Partial Pressures

In gas mixtures, each gas assumes a part of the total volume or pressure. This is referred to as partial pressure. The partial pressure of each gas is dependent upon its proportion of the total mixture.

If we were to have a total steam line pressure of 5 kg/cm2 g consisting of 80% steam and 20% air the effective steam pressure would be:

0.80x 5 = 4.0 kg/cm2 g

And the effective air pressure would be:

0.20x 5 = 1.0 kg/cm2 g

As a result the steam would effectively be 4 kg/cm2g steam in a 5 kg/cm2g line. In checking a thermometer in the line at that location we would find the temperature to be 151.2° C for the 4 kg/cm2g steam and not 158.2° C for the 5 kg/cm2g steam we would expect to find. That is a 7°C difference between the two pressures. In addition there will also be a change in kcal's.(Although the 4 kg/cm2g steam has more enthalpy of latent heat per weight than does the 5 kg/cm2g steam it has less by volume. Latent heat of evaporation at the 5 kg/cm2g is 498.3 kcal/kg while for 4 kg/cm2g is 509.5 kcal/kg. It shows a difference of 11.2 kcal/kg.)

Fig.Chamber containing air and steam delivers only the heat of the partial pressure of the steam, not total pressure.

In effect the air has displaced a portion of the enthalpy needed, by displacing a portion of the steam.

In order to make the distribution system as efficient as possible it becomes necessary to remove any air before it can effect heat transfer by filming or becoming mixed with the steam.

Corrosion

Because boiler systems are constructed primarily of carbon steel and the heat transfer medium is water, the potential for corrosion is high. Iron is carried into the boiler in various forms of chemical composition and physical state. Most of the iron found in the boiler enters as iron oxide or hydroxide. Any soluble iron in the feed water is converted to the insoluble hydroxide when exposed to the high alkalinity and temperature in the boiler.

These iron compounds are divided roughly into two types, red iron oxide (Fe2O3) and black magnetic oxide (Fe3O4). The red oxide (hematite) is formed under oxidizing conditions that exist, for example, in the condensate system or in a boiler that is out of service. The black oxides (magnetite) are formed under reducing conditions that typically exist in an operating boiler.

The deposition of these metallic oxides in the boiler is frequently more troublesome than the actual damage caused by the corrosion. Deposition is not only harmful in itself, but it offers an opening for further corrosion mechanisms as well.

Contaminant products in the feed water cycle up and concentrate in the boiler. As a result, deposition takes place on internal surfaces, particularly in high heat transfer areas, where it can be least tolerated. Metallic deposits act as insulators, which can cause local overheating and failure. Deposits can also restrict boiler water circulation. Reduced circulation can contribute to overheating, film boiling and accelerated deposition.

Localized attack on metal can result in a forced shutdown. The prevention of a forced shutdown is the true aim of corrosion control.

Clean, clean steam

Strainers

They are used to provide clean steam to the process. Strainers are installed in the steam pipes to ensure that no dirt gets through with the steam. Often, pipeline debris such as dirt, metal burrs from welding, scale, rust and other solids find their way into the steam pipes leading to more maintainance hassles and plant shutdowns. Strainers are an important pipeline accessory that literally 'strains out' these solids in flowing liquids or gases, and protects steam equipment. Every important equipment like a PRV or trap has to have a strainer fitted upstream, ie just before it.

There is a fine mesh provided in the strainer which effectively filters out solids from the system. During routine maintenance strainers must be cleaned regularly otherwise unclean steam will go through and damage the plant pipework and fittings. It may contaminate the product as well.

Steamline makes two strainers. The CRPS and trap modules have the Y-strainers and the PRS has a 'bucket' strainer.

Y-type Strainers.These strainers are manufactured in-house for the CRPS and the Trap modules. These are standard strainers and can be used for steam, any other gas, or even liquids. The body is a cylindrical pipe and has a mesh pocket attached to it which contains the strainer mesh (80microns). This filters out all the solids. It can handle pressures upto 10 kg/cm2g and is available in line sizes upto 2". Generally used for condensate or small steam lines.

As the Y-type strainers are more compact than the pot strainers, the surface area of the mesh available for straining is less and therefore, the dirt accumalates faster. This means more frequent cleaning. This is only a problem during commissioning when the plant is new and a lot of welding grit is present in the pipes.Installation of Y-type strainers.

• For Steam : Horizontal mounting with drain pocket in the horizontal plane prevents water collection, which prevents carryover.

• For Liquids : For eg. in our CRPS systems, the strainer is mounted with the mesh pocket facing vertically down. If this is not done, the water may draw back the dirt upstream if the flow reduces.

If you install the strainer with the mesh pocket pointing up, all the debris will fall into the pipe!!

Bucket strainers.

The Steamline pot strainer has a bucket type structure. This is a vertical cylinder. It's chamber is larger than a typical Y-type strainer. The straining area is therefore much larger and can go without cleaning longer. This also reduces the pressure drop across the pot strainer as the flow is not hampered by that much debris. The bucket strainer can be used on bigger dia pipes. Strainers may have accumalated debris in the bottom of the bucket, which is removed via the drain plug.

Bucket strainers have to be installed in a horizontal position only.

Feed water quality.Feed water purity requirements can vary widely. A low pressure firetube boiler can usually tolerate a high level of water hardness, with proper chemical treatment, while virtually all impurities must be removed from the feedwater of most modern high pressure water-tube boilers. Most plants use one or more of the following processes.

• Clarification

• Filtration

• Softening

• Dealkalization

• Demineralization

• Deaeration

• Heating

What is ideal feedwater like? Ideally feedwater should conform to the following:

• We must supply boiler feedwater at not less than 75 – 85°C.

• We must remove dissolved gases, ie, de-aerate feedwater. Dissolved gases like oxygen lead to corrosion inside the boiler.

• Feedwater needs chemical treatment to avoid excessive foaming and scaling in the boiler. This can result in other problems like carryover, priming, dirty and wet steam which hamper efficiency and cause untold damage to the boiler.

Heating feedwater

Cold make-up water and returned Condensate usually mix in the feedtank. Conventionally, both make-up water and condensate are fed to the feedtank above the water surface.

We heat feedwater because of three reasons.

• Cold feed will result in thermal shock to the boiler. When we feed at optimum temperatures the life of boiler is also prolonged

• When feedwater is at the highest temperature for injection to the boiler, the boiler efficiency increases drastically. Return of condensate further boosts η.

• All water sources have a certain amount of dissolved gases mixed in them at ambient temperature. Cold water absorbs free oxygen and other gases. As the condensate heats the make-up water, the temperature of the make-up water rises. At high temperatures undesirable gases have minimum solubility and are liberated when heated.

Deaeration

Air is always present during equipment start-up and in the boiler feed-water. The most common source of corrosion in boiler systems is dissolved gas: oxygen, carbon dioxide and ammonia. Of these, oxygen is the most aggressive. The importance of eliminating oxygen as a source of pitting and iron deposition cannot be over-emphasized. Even small concentrations of this gas can cause serious corrosion problems.

Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water system from the condensate return system. Possible return line sources are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate receiving tanks and leakage of nondeaerated water used for condensate pump seal and/or quench water. With all of these sources, good housekeeping is an essential part of the preventive program.

One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can produce failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH and the temperature of the water.

The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.

It is essential to remove the dissolved gases – "deaerate" – before it can be released in the boiler or the feedtank, to prevent corrosion of the tank, the boiler and the steam system.

The feedwater used in generating steam will, of course, contain oxygen. It can also contain bicarbonate and carbonate alkalinities which, when broken down due to high temperatures, will produce C02. These two gases, O2 and CO2, alone or combined, when disolved in condensate are very corrosive. The oxygen causes oxygen pitting while the carbon dioxide, in solution with the condensate, forms carbonic acid. When combined, the oxygen accelerates the corrosive effects of the acid.

Deaerating the feedwater removes almost all of these gases. In general, as the feedwater enters the deaerator low pressure steam, typically 0.35 kg/cm2 g, is used to break up the water into a spray continuing across the spray carrying off the gases.

All modern boilers have some form of deaeration arrangement. The removal of this oxygen can be done by three ways – thermal, mechanical and chemical.

• In chemical removal of oxygen, an oxygen scavenger like Sodium Sulphite is dosed to the feedtank, which absorbs the oxygen. However, this is detrimental because the addition of any chemical to the boiler water increases its TDS (explained later in this section), again causing problems.

• In mechanical de-aeration, water is stirred or sprayed, causing removal of oxygen from the feed water.

• Thermal de-aeration uses the property of water shown in the graph . As is seen, the amount of dissolved oxygen in water is proportional to its temperature. So if we can heat the make-up water before it enters the feedtank, it will liberate the oxygen, thus preventing corrosion of the tank. Further, if the system used to preheat the make-up is made of Stainless Steel, corrosion will be negligible.

What is the job of a deaerator?

• Deaerators remove oxygen, carbon dioxide and other noncondensable gases from feed water. Oxygen and carbon dioxide are very harmful to boiler systems. Deaerators are designed to remove dissolved

gases from boiler feedwater. They are effective and oxygen can be reduced to trace levels, about 0.005 ppm.

• Along with temperature control systems, an effective deaeration system can heat the incoming cold makeup water and mix it with available return condensate.

Fig. Cutaway of a pressurized deaerator tank with live steam sparging

Steamline deaerators – how do they work?

On larger boiler plants, pressurised de-aerators like the one shown above are installed. Live steam is used to bring feed water temp above 100ºC to “drive off” the oxygen content. This action is normally enhanced by the steam “scrubbing” the feedwater. The make-up water enters the deaerator and is broken into a spray or mist, and scrubbed with steam to force out the dissolved gases. At the elevated temperature the solubility of oxygen is extremely low. Steam and other non-condensibles flow upwards into the vent condensing section where the steam is condensed. Freed oxygen and other gases are vented to the atmosphere through the vent outlet. However, these are pressure vessels and are therefore expensive.

This type of deaerator usually consists of a heating and a deaerating section. The storage section of these units typically have a residual deaerated feedwater storage tank often designed to hold about 10 mins of rated capacity of boiler fedwater.

Pic. Pressurized Steamline deaerator at Fresenius Kabi, Ranjangaon

Accordingly, the Deaerator Head was developed – a compromise for fitting to any feedtank to drive off as much oxygen as possible at atmospheric pressure. The Steamline De-aerator Head uses a combination of thermal de-aeration and mechanical de-aeration. It has three restrictions to the flow – a nozzle in the make-up line, a baffle plate between the mixing head and the immersion tube, and a sparger in the immersion tube. Therefore it ensures that the oxygen in the make-up water is driven off by using the heat in the condensate which it is mixed with, and all the dissolved gases are released in the De-aerator Head before it enters the feed tank and the boiler. These are vented out by the automatic Air Vent provided on top of the De-aerator Head.

Pic. A Steamline Flash condensate Deaertor Head has an all SS assembly and is fitted to the top of the feedtankwith inlet for condensate, make up water and flash steam.

In addition, sometimes Flash Steam is generated from high pressure Condensate. This flash steam will escape to the atmosphere and the heat will be lost. A third inlet is sometimes provided in a De-aerator Head to mix flash steam with make-up water, thus condensing the flash steam and saving its heat. This type of unit is called the Flash Condensing De-aerator Head.

Deaerators are typically elevated in boiler rooms to help create head pressure on pumps located lower. This allows hotter water to be pumped without vapor locking should some steam get into the pump.

Chemical dosing of feedwater

The basic contaminants are reduced to a minimum, consistent with boiler design and operation parameters. That is - calcium and magnesium hardness, migratory iron, migratory copper, colloidal silica, etc.

Ion exchange systems

Ion exchange systems range from light commercial water softeners and filters to specially designed industrial equipment. Also known as deionizations (DI) systems. These systems are considered high-end where the highest quality of water treatment is needed, such as with steam turbines.

Reverse Osmosis (RO)

Reverse Osmosis systems are available for tap water, brackish water or seawater.  These systems are considered high-end where the highest quality of water treatment is needed, such as with steam turbines

TDS control.Water that is fit for human consumption is not necessarily fit for a boiler. This is because water can have many dissolved solids that lead to " hardening " of water. These cause scaling which in turn leads to '' hot spots" in the boiler which lead to corrosion.

Boiler feed water contains dissolved solids, both from raw water and water treatment chemicals. As steam is raised from a boiler, the level of concentration of Total Dissolved Solids (TDS) in the boiler water increases.

The maximum allowable TDS is 3500 ppm for any boiler. This seems very small. How can such a small value affect the working of such a large body of water inside the boiler?

The salt truck story

Suppose the TDS in a boiler is 200 ppm (Parts Per Million). This gives us a percentage value of

200 / 1,000,000 X 100 = 0.002 %

This salt does not evaporate. So, over time the TDS valves keep rising. 0.002% is seeming such a small number, but look what happens inside the boiler.

We assume a 10 ton boiler working 3 shifts ( 24 hours) for 30 days. The feed water has 200 ppm TDS.

We will have 14.4 kgs of salt in 30 days. So, in a few months we will have a small truck-load of salt in the boiler!

Lets see if the TDS can be reduced in a water softening plant.

Water softening and TDS

Total Dissolved solids (TDS) is the sum of both Hard and Soft salts.

A water softener basically uses chemicals to remove (precipitate) the hand salts and substitutes it with a soft salt. It does this by a reaction called base exchange softening.

Ca SO4 + Na OH -------- Na 2 SO4 + Ca OH

Soft salt    + Hard salt precipitate

Before softening,

Hard salts + Soft salts = TDS

10               +  90             = 100

After softening,

Hard salts + Soft salts = TDS

2                  + 98             = 100

So, unfortunately, while a water softening plant reduces the “hardness” (i.e. the presence of scale forming salts), it does not reduce the TDS of the feed water. In practice we may in fact, find a slight rise in TDS as brine (NaOH) gets added to precipitate the hard salts. So, in a boiler the foaming and carryover problems remain as the TDS has not decreased. The only issue we have dealt with is reduced the scaling because the hard salts are precipitated.

Even if we use RO water, the Boiler water TDS keeps rising, albeit slowly.

Normally, plants periodically drain 10-15% of the boiler water and this is called blowdown.

High TDS, big problems

Foaming & Carryover

Pure water does not foam when it boils. However, as the amount of impurities rise, a foam layer is formed at the steam separation surface. The amount of foaming is directly proportional to the TDS level in the boiler.

Foaming (or “priming”) causes carryover of water, or wet contaminated steam, which may be carried over into the steam system and depending on the conditions at the steam water interface, can even cause surges of water into the steam system. The products of carryover would be deposited on heat transfer surfaces and ancillary equipment, reducing steam system efficiency and plant productivity. This is what causes fouling of heat exchangers, malfunctioning of control valves and steam traps etc.

Formation of foam at the separation layer is a matter of great concern for any boiler operator. Level controls in a boiler recognize liquids and gas. But, they start malfunctioning when confronted with foam in the boiler. We know that the water must never drop below the fire tube level. If the water level is falling, but the foam prevents a level control from sensing low level water the feed pumps do not switch on and a potentially dangerous situation could develop.

Fig. Foaming and carryover in a boiler

Scale Deposition

If the TDS is too high, scale will deposit on the boiler tubes and furnace (water side), all the heat transfer surfaces. This has the effect of reducing heat transfer with its subsequent effect on fuel consumption. When scaling goes up exponentially, as seen in the graph, tube failure can occur.

Scale only 1mm thick on the water side could increase fuel consumption by 5 to 8%. (Source: PCRA Handbook No. 3 - Efficient Generation of Steam)

Tube surfaces underneath the scale may become overheated leading to tube damage or tube failure. High TDS levels in a boiler also shows up in the steam system -valves get while deposits strainers need to be cleaned very often, etc.

In order to prevent these problems, the TDS needs to be controlled within a certain specified maximum limit. The chart below shows the recommended water characteristics for shell boilers in accordance with IS: 10392-1982 and BS: 2486-1964, for pressures up to 25 bar g:

The Dissolved solids, PPM (parts per million) is 3500. This is the Set point of TDS that no boiler should be allowed to across because of scale, foam and carryover problems. This set point can be measured by a conductivity meter dipped in a sample of boiler water. (Conductivity is directly proportional to TDS levels.)

Blowdown

All steam boilers need to be "blown down" to control their TDS level.

The main purpose of blowdown is to maintain the solids content of the boiler water within prescribed limits. This would be under normal steaming conditions. However, in the event contamination is introduced in the boiler, high continuous and manual blowdown rates are used to reduce the contamination as quickly as possible.

Because each boiler and plant operation is different, maximum levels should be determined on an individual basis.

Bottom Blowdown

Conventionally, this is done through a manual slide valve. By definition, bottom blowdown is intermittent and designed to remove sludge or sediment from the bottom of the boiler where it settles. The frequency of bottom blowdown is a function of experience and plant operation. Bottom blowdown can be accomplished manually or electronically using automatic blowdown controllers. The control is a large (usually 25 to 50 mm) key operated valve. This valve might normally be opened for a period of about 1 - 2 minutes, once a shift.

Disadvantages

• A large quantity of hot, saturated water is drained and the same amount of relatively cold make-up water is added. This leads to thermal shock in the boiler.

• As we now have to provide more heat to bring this extra water to 100ºC, we are decreasing the overfall efficiency of the boiler

The formula for amount of blowdown required is

Where,

F = Feedwater TDS in ppm

B = Boiler water set point in ppm

S = Steam generation in kg/hr.

Continuous Blowdown

Frequently used in conjunction with manual blowdown, continuous blowdown constantly removes concentrated water from the boiler.

Continuous blowdown allows for better control over boiler water solids. In addition, it can remove significant levels of suspended solids. Another advantage is that the continuous blowdown can be passed through heat recovery equipment.

How much blowdown is enough?

Too little, and your TDS could rise above limits specified for your boiler, giving rise to foaming, scaling and wet steam. Too much, and you're draining water that you've paid to heat.

The need for Automatic Blowdown Controller (ABCO)

Blowdown of the boiler can keep TDS within the required limits. Blowdown is achieved either by manual or automatic methods. In the manual method, blowdown is achieved by opening a large bore valve at the bottom of the drum (or on the side of the drum in case of continuous blowdown). However, this practice can be highly wasteful. As the period of blowdown is not related with either boiler steam load or feedwater purity, the TDS level in manual methods can vary greatly, causing an average TDS level much lower than the allowable limit, and leading to excess blowdown.

On the other hand, an automatic blowdown control system, based on TDS measurement and subsequent corrective action, can maintain a TDS level much closer to the set point, resulting in considerable fuel savings.

Fig. Blowdown quantity is reduced with an automatic system

As seen in the graphs above, the automatic control of TDS results in an average TDS level much closer to the set point. This means that the actual quantity of blowdown over a period of time gets reduced compared to the manual method. Blowdown water is water that has been heated to the saturation temperature of the boiler, so it contains a lot of heat. At a boiler pressure of 10.5 Kg/cm2 g, each kg of blowdown water contains almost 190 kcal of heat energy. If an automatic boiler controller can reduce the blowdown of a 10 TPH boiler from 6% to 3%, i.e. a saving of 3%, the blowdown quantity would reduce by 300 Kg/hr, or 7200 kg/day. This would mean a saving of 1368000 kcal/day. This would mean a fuel saving of approximately 180 litres of oil, if the boiler was fired with furnace oil. The cost benefit of preventing corrosion in the boiler and the steam system, though it cannot be quantified exactly, would be in addition.

Combustion.

Combustion is a process of oxidation in which combusinble products such as fuel is burned in the presence of oxygen. Combustion liberates heat and by products such as ash.

Burner

For liquid or gaseous fuels, we use a Burner. A burner sends heat into the boiler tubes and it is set to maintain the correct pressure in the boiler. If the boiler pressure falls because of growing steam demand, the burner switches on to produce more steam from the boiler. As long as the amount of steam being produced in the boiler is as great as that leaving the boiler, the boiler will remain pressurised. This maintains correct pressure. If correct pressure is maintained, correct temperature is also maintained as they are interlinked.

Proper atomised fuel gives better combusion efficiencies. Modulating control gives optimum fuel handling. Also provision of appropriate excess air for combusion gives good results.

In case of gaseous fuels, minimum excess air is required and there is no extra cost incurred in ha

Fire grate

In case of solid fuels, there is no burner required. Fuel is supplied over a grate which is fire pulverise, and excess air is supplied to acheive better combusion efficiencies. Ash handling is main problem incase of solid fuels.

Boiler Turndown

Turndown tells us the amount of control we have over the fuel.

In a simple on-off control, the boiler either fires x amount of fuel or is switched off.

A boiler turndown of 1.2 means we can fire less fuel on more fuel.

A 1:2 turndown means a 2 step control is possible.

1:4 turndown looks something like this.

It has 4 step control. Modern boilers can also provide a stepless control from 25-100%.

As fuel load increase or decreases, the amount of fuel fired into the boiler should proportionately be increased or decreased. But when modulation is not possible, (as in a simple on-off control) , the boiler burns less fuel at low loads and the rest of the unburnt fuel is blown out through the stack .

The O in the left side of the equation is the exact amount of air required to burn a given amount of fuel, also called stoichiometric air quantity.

Practically though, the amount of air needed is in excess of times, to achieve complete combustion of fuel. Therefore we provide "excess air". Excess air from 20-30% is normally used. We have to carefully control this excess air quantity as we risk blowing out unburnt fuel.

else the losses can really add up over time.

The boiler furnace has a punker plate (similar to our home gas burners ) through which a pressurized mixture of liquid fuel and air is sprayed. This atomizes the fuel and a proper mixing of fuel with air takes place. This ensures the highest combustion efficiency.

Solid fuels too are crushed or broken down (there is always a coal crusher at power plants) to increase the combination efficiency.

Excess Air

Perfect combustion is attained when the flue gas analysis shows no carbon monoxide or oxygen. This means that every available fuel molecule and every available oxygen molecule came into contact with each other.

Ideally, the Combustion Equation (stoichiometric air) should look something like this:

Cn Hn + O2 ------> CO2 + H2O + Heat (fuel) (air)

Such perfect mixing is not possible, even with the most advanced burner. Given complete mixing, a precise amount of air (stoichiometric) is required to completely react with a given quantity of fuel. In practice, if only stoichiometric amount of air is provided, then the combustion looks something like this:

Cn Hn + O2 ------> CO2 + H2O + CO + Heat

Fuel Stoich. Air

As seen above, CO (unburnt fuel) is present in the flue gas as the stoichiometric air quantity is insufficient to ensure complete combustion.

Therefore, additional or “excess air” must be supplied for complete combustion to occur. The presence of excess air means that more air is available for combustion than is actually required. For efficiency reasons,

“excess air” is always provided to assure that all fuel is burned inside the boiler. There is no CO component anymore. By operating your boiler with a minimum amount of excess air, you can decrease stack heat losses and increase combustion efficiency.

In addition, air has other components besides O2, and fuel may have impurities like sulphur. Hence the real world combustion equation, looks like this:

Cn Hn + S2 + N + O2 ------> CO2 + H2O + Heat + O2 + SOx + NOx Fuel                 Excess Undesirable by-products

However, the excess air passing through the boiler is heated by the fuel, and vented out of the chimney. So, the more the excess air, the lower the boiler efficiency. Hence, excess air must be optimized such that it is just enough to burn all the fuel while not removing too much heat.

Large, unnecessary amounts of excess air can occur because of:

♦ Burner/control system imperfections

♦ Variations in boiler room temperature, pressure, and relative humidity

♦ Need for burner maintenance

♦ Changes in fuel composition

In modern history, excess air is controlled using a feedback system in which a sensor (generally O2) is used to provide feedback on flue gas constituents and controls the amount of combustion air provided to the boiler. This is called Trim Control, or Combustion Control.