High Pressure Boilers

Prepared by:

Meet Dalwadi 130010119018Mayank Lethwala 130010119048Maharshi Soni 130010119050

Submitted to: proff. Ronak R. ShahSubject of: Power Plant Engineering

Definition

According to Indian Boiler Regulation (I.B.R) Act 2007,”Boiler is a closed pressure vessel in which steam is generated with capacity exceeding 25 liters, gauge pressure greater than or equal to 1 kg/cm^2, and water is heated at 100°C or above”

Steam boiler is a closed vessel in which heat produced by the combustion of fuel is utilized to generate steam from water, at desired temperature and pressure

Introduction

APPLICATIONS OF BOILERS

1. Power generation2. Heating3. Industrial processes

1. Relative position of hot gases and water

• Fire tube boilers (Cochran, Lancashire, Cornish, Locomotive)• Water tube boilers (Babcock and Wilcox boiler)

2. Geometric orientation of boiler

• Vertical boilers (Cochran boiler)• Horizontal boilers (Locomotive boiler)

3. Method of firing

• Internally fired boilers (Lancashire, Locomotive)• Externally fired boilers (Babcock and Wilcox boiler)

CLASSIFICATION OF BOILERS

4. Pressure of steam

• Low pressure boilers (<= 80 bars-Cochran, Lancashire, Cornish, Locomotive)• High pressure boilers (>80 bars-Babcock and Wilcox boiler, Lamont boiler)

5. Method of circulation of water

• Natural circulation boilers (Lancashire, Locomotive, Babcock & Wilcox boilers)• Force circulation boilers (La-mont boiler, Velox boiler, Benson boiler)

6. Nature of service to be performed

• Land boilers ( Lancashire boiler)• Mobile boilers (or) Portable boilers (Locomotive boiler)

7. Number of tubes in boiler

• Single tube boilers (Cornish boiler)• Multi tube boilers (Locomotive, Cochran Babcock and Wilcox boiler)

A boiler is called a high pressure boiler when it operates with a steam pressure above 80bar. The high-pressure boilers are widely used for power generation in thermal power plants.Example: Lamont boiler, Benson boiler, loeffler boiler, Babcock and Wilcox boiler

Features of High pressure Boilers

1. Forced circulation of water2. Large number of small diameter tubes3. Higher steam pressure and temperature4. Improved mode of heat transfer and heating5. Pressurized combustion6. Compactness7. High efficiency8. Once through construction

High Pressure Boilers

La mount boiler:

• A forced circulation boiler was first introduced by La-Mont in the year 1925 which is used in Europe and America. This is a modern high pressure boiler (water tube type steam boilers) working on forced circulation system.

Steam separator drumThe la Mont boiler consists of a steam separator drum which is placed wholly outside the boiler setting . The drum receives a mixture of steam and water from the evaporator tubes and feed water from the economizer. The steam is separated from water in the drum.

Circulating pumpThe water from the drum is then drawn to the circulating (centrifugal) pump through the down-comer. The pump circulates water (“forced circulation”) equal to 8 to 10 times the weight of steam evaporated. This prevents the tubes from being overheated.

Distributing headerThe circulating pump delivers the feed water to the distributing header with orifices at a pressure above the drum pressure.

EvaporatorThe header distributes water through orifices into the evaporator tubes acting in parallel. Orifice in the header controls the flow of water to the evaporator tubes. Here part of the water is evaporated and a mixture of steam and water from these tubes enters the drum.

Working principle of La Mont Boiler

Convection superheater

The steam produced in the boiler is nearly saturated. This steam as such should not be used in the steam turbine. The presence of moisture in it will cause corrosion of turbine blades, etc. to raise the temperature of steam and thereby to increase the turbine efficiency, superheater is used.

The principle of convection superheater is similar to steam generating tubes of the boiler. The hot flue gases at high temperature sweep over convection superheated tubes and raise the temperature of steam. Convection superheater thus receives heat from the flue gases flowing from the combustion chamber, entirely by convective heat transfer. Such a superheater may be more conveniently located since it is not necessary for it to “see” the furnace.

Saturated steam from the top of the drum enters the convection superheater placed in the path of the flue gases and is superheated.

Steam outlet

Superheated steam from the superheater passes out to the steam turbine through the steam outlet.

EconomizerThe quantity of superheated steam thus delivered to turbine is continuously made up in the form of feed water. Feed water supplied by the feed pump is heated in the economizer on its way to the steam separator drum.

Air pre-heaterSince the heat of the exit gases cannot be fully extracted through the economizer, the air pre-heater is employed to recover some of the heat escaping in these gases. These exit gases preheat the air from the blower in the air pre-heater. The preheated air is supplied to the furnace for combustion.

CapacityThe capacity of la-mont boiler is about 50 Tones/hr of superheated steam at a pressure of 170 kgf/sq.cm. and at a temperature of 500’C.

Benson Boiler•The presence of steam bubbles in contact with the surface of tubes seriously impairs heat transmission from the flue gases to water. By rising the boiler pressure to the critical pressure of steam (225 kgf/sq.cm.), this difficulty is overcome, as suggested by Mark Benson in 1922. At the critical pressure water and steam have the same density and no bubbles are formed.

•The first modern high pressure drum less boiler developed by Benson was put into operation in 1927 in west Germany power station.

Working principle of Benson Boiler:This boiler has a unique characteristic of absence of steam separating drum. The entire process of heating, steam generation and superheating is done in a single continuous tube.

EconomizerThe feed water by means of the feed pump is circulated through the economizer tubes. Hot flue gases pass over the economizer tubes and the feed water is preheated.

Radiant evaporatorThe feed water from the economizer flows into the radiant evaporator with radiant parallel tube sections. The radiant evaporator receives heat from the burning fuel through radiation process and majority of water is converted into steam in it.

Convection EvaporatorThe remaining water is evaporated in the convection evaporator, absorbing the heat from the hot gases by convection. Thus the saturated high pressure steam at a pressure of 210 kg/sq.cm is produced.

Convection super-heater

The saturated steam is now passed through the convection super-heater where the saturated steam os superheated to 650’C. The radiant evaporator, the convection evaporater and the convection super-heater are all arranged in the path of the flue gases.

Steam outlet

The superheated steam is supplied to the steam turbine through the steam outlet.

Capacity

Capacity of Benson boiler is about 150 tones/hr at a pressure of 210 kgf/sq.cm. and at a temperature of 650’C. (Efficiency may be improved by running the boiler at a pressure slightly lower than the critical pressure).

Salient features of Benson Boiler

• As there are no drums, the total weight of Benson boiler is 20% less than other boilers. This also reduces the cost of the boilers.

• As no drums are required, the transfer of the Benson parts is easy. Majority of the parts may be carried to the site without pre-assembly.

• Since no drum is used, this is an once-through boiler and the feed water entering at one end is discharged as superheated steam at the other end.

• Circulating pump and down comers are dispensed with.

Loeffler boilerThis is also a modern high pressure water tube boiler using the forced circulation principle and named after Prof. Loeffler.

Salient features of Loeffler BoilerThe novel feature of the Loeffler Boiler is to evaporate water solely by means of superheated steam. The furnace heat is supplied only to economizer and superheater. In other words, steam is used as a heat absorbing medium.

The major difficulty experienced in La-Mont boiler is deposition of salt and sediment on the inner surfaces of water tubes. The deposition reduces the heat transfer, ultimately, the generating capacity.

This difficulty was solved in Loeffler boiler by preventing the flow of water into the boiler tubes. Feed water is evaporated in the drum using part of the superheated steam coming out from the water-heater.

Thus only the dry saturated steam passes through the tubes. Poor feed water can, therefore, be used without any difficulty in the boiler, which is great advantage of this boiler.

EconomizerThe feed water from the feed tank is supplied to the economizer by feed pump. In the economizer the feed water is made to flow through a number of tubes surrounding which the hot gases leaving the furnace pass over. There is a heat exchange from the hot gases to the feed water, which is preheated in the economizer.

Evaporated DrumIt is housed away from the furnace. It contains a mixture of steam and water. The feed water from the economiser tubes enters the evaporator drum into which is also passed two-thirds of the superheated steam generated by the boiler. The superheated steam gives its superheat to the water in the drum and evaporates it to saturated steam.

Mixing NozzlesThe nozzles distribute and mix the superheated steam throughout the water in the evaporator drum.

Steam circulating pumpA steam circulating pump forces this saturated steam from the evaporator drum to the radiant superheater through the tube of the furnace wall.

Working principle of Loeffler Boiler

Radiant superheaterThe radiant superheater is placed in the furnace. The hot gases in the furnace are used for superheating the saturated steam from the drum. The radiant superheater receives heat from the burning fuel through radiation process.

Convection superheaterSteam from the radiant superheater enters the convection superheater where it is finally heated to the desired temperature of 500’C. The convection superheater receives heat from the flue gases entirely by convective heat transfer. Both radiant and convection superheater are arranged in series in the path of the flue gases.

Steam outletAbout one-third of the superheated steam from the convection superheater passes to the steam turbine while the remaining two-thirds is passed on to evaporator drum to evaporated the feed water to saturated steam.

CapacityCapacity of the Loeffler boiler is about 100 Tonnes/Hr of superheated steam generated at a pressure of 140 kgf/sq.cm and at a temperature of 500’C.

Velox Boiler

Velox boiler is a forced circulation water tube boiler. It is mostly used in gas turbine. In this boiler, the velocity of flue gases is greater than the velocity of sound, which causes more heat transfer from gas to the water, which increases the steam generation rate. Due to this, it is most important boiler.

Principle:When the velocity of the gas is greater than the speed of sound, its heat transfer rate is also increases. So more heat is transfer from gas to water as compare when the heat transfer at the subsonic speed. This is the basic principle of of it. This boiler can increase the heat transfer rate or can say steam generation rate without increasing boiler size. This is why; Velox boiler is most successful boiler in the gas turbine industries.

Construction:Velox boiler is a water tube forced circulation boiler. It has a gas turbine driven air compressor, which compresses the air.

This compressed air enters into the vertical combustion chamber, as result, high rate of heat release from the fuel, which increases the flue gases velocity up to the sound velocity.

This is a force circulation boiler, so pump is used to circulate water inside the boiler.

This boiler also consist water and fire tube to maintain the flow of gas and water inside the boiler.

This boiler also consists other necessary mounting and accessories like economizer, super heater, blow off valve, safety valve etc.

The Velox boiler works as a basic heat exchanger. The working of this boiler is as follow.

The air is compressed by air compressor driven by gas a turbine driven. This compressed air passes from the combustion chamber, where more heat release by the fuel which increase the velocity of the flue gases up to sound velocity. From the bottom of combustion chamber, this flue gases pass from the fire tubes. These fire tubes surrounded by the evaporator water tubes.

The water from the economizer passes from the evaporator tube force by a circulating pump. This water passes 15 - 20 time from the evaporator tube at very high speed. Due to this high speed circulation, heat is transfer from the gases to the water at very high rate. The mixture of water and steam is formed which further passes from the water and steam separator.

The steam from the steam separator passes to the super heater and further for process work. The remaining water in the steam separator again passes from the evaporator tube. The flue gases from the fire tubes send to the super heater tubes, where it increases the steam temperature. The gas from the super heater sends to the turbine where it rotates the gas turbine and then passes from the economizer.

Working:

1. This boiler has high heat transfer rate.

2. It has great flexibility.

3. it is compact in design.

4. It is easy to control. It is fully automatic.

5. It has great thermal efficiency of about 90 - 95%.

Advantages:

Construction: -Schmidth-Hartmann boiler consists of air pre-heater to heat the surrounding air which is required for combustion chamber.

It also consist steam drum, superheater, and feed pump.Feed water pre-heater, primary separator is also used in this boiler.

In Schmidth-Hartmann boiler there are two circuits one uses distilled water and other uses impure water.

Schmidth-Hartmann boiler:

The Schmidth-Hartmann boiler is different from other boiler because in this boiler distilled water is used for generation of the high pressure steam which is recalculated without any wastage in the circuit.

This high pressure steam is utilized for generation of low pressure steam from impure water. Distilled water from water drum enters into the primary evaporating tubes by natural circulation. Steam at 95-100 bar pressure is generated in the evaporating tubes with the help of hot flue gases circulated over the tubes from combustion chamber.

This steam enters via primary separator into the tubes submerged in impure water of the evaporator or steam drum. The exchange of heat between high pressure steam and impure water allows the water to be converted into steam at a pressure of 55-60 bar which is further passed into superheater for superheating purposes. The superheated stem is finally supplied to the steam turbine.

The condensate of high pressure steam collected from the drum passes through the feed water pre-heater where it heat the low pressure feed water up to its saturation temperature. The low pressure (L.P.) feed water is pumped into pre-heater with the help of feed pump.

Working: -

1.It can use impure water for generation of steam.

2.As it can use impure water there is no requirement of water softening plant ultimately reduces cost.

3.Any deposits in the evaporator drum due to impure water can easily brushed off by removing the submerged tube from the drum or by blowing off the water.

Advantages:-