pp lab final)

8/6/2019 Pp Lab Final)

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2007-MECH-130 Power Plant Lab Report Section

1

Lab session # 01

To determine the Condenser heat exchange rate

Theory

CondenserIt is an apparatus used to exchange heat between exhaust of turbine and the fluid which is

at low temperature as compared to the exhaust of gas turbine.

Condensers are typically heat exchangers which have various designs and come in many

sizes ranging from rather small (hand-held) to very large industrial-scale units used in plant

processes. Condensers are used in air conditioning, industrial chemical processes such as

distillation, steam power plants and other heat-exchange systems. Use of cooling water or

surrounding air as the coolant is

common in many condensers.

In this power plant lab we

use shell and tube type condenser.

Two fluids, of different starting

temperatures, flow through the

heat exchanger. One flows

through the tubes (the tube side)

and the other flows outside the

tubes but inside the shell (the shell

side). Heat is transferred from one

fluid to the other through the tube

walls, either from tube side to

shell side or vice versa. The fluids

can be either liquids or gases on either the shell or the tube side. In order to transfer heat

efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this

way, waste heat can be put to use. This is an efficient way to conserve energy.

Heat exchange rate of condenser is found simply by noting down the temperature difference at

inlet and outlet points of working fluid i.e. water. Thus

Q=[ ]

Q= Heat Exchange rate, = mass flow rate of water,

C= Specific heat capacity of water= 4.18 kJ per kg = Temperature difference
http://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Air_conditioninghttp://en.wikipedia.org/wiki/Chemical_processhttp://en.wikipedia.org/wiki/Distillationhttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Distillationhttp://en.wikipedia.org/wiki/Chemical_processhttp://en.wikipedia.org/wiki/Air_conditioninghttp://en.wikipedia.org/wiki/Heat_exchanger


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Lab session # 02

Performance test of Steam Turbine

Theory

Steam TurbineA steam turbine is a mechanical device that extracts thermal energy from pressurized

steam, and converts it into rotary motion.

It has almost completely replaced the reciprocating piston steam engine primarily

because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine

generates rotary motion, it is particularly suited to be used to drive an electrical generator.About

80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a

form ofheat engine that derives much of its improvement in thermodynamic efficiency through

the use of multiple stages in the expansion of the steam, which results in a closer approach to the

ideal reversible process.

Principle of OperationAn ideal steam turbine is considered to be an isentropic process, or constant entropy

process, in which the entropy of the steam entering the turbine is equal to the entropy of the

steam leaving the turbine. No steam turbine is truly isentropic, however, with typical

isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The

interior of a turbine comprises several sets of blades, or buckets as they are more commonlyreferred to. One set of stationary blades is connected to the casing and one set of rotating blades

is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and

configuration of sets varying to efficiently exploit the expansion of steam at each stage.

Thermodynamics of steam turbinesThe steam turbine operates on basic principles of thermodynamics using a part of the

Rankine cycle. Superheated vapor (or dry saturated vapor, depending on application) enters theturbine, after it exited the boiler, at high temperature and high pressure. The high heat/pressure

steam is converted into kinetic energy using a nozzle. A force is created on the blades due to the

pressure of the vapor on the blades causing them to move. A generator or other such device canbe placed on the shaft, and the energy that was in the vapor can now be stored and used. The gasexits the turbine as a saturated vapor (or liquid-vapor mix depending on application) at a lower

temperature and pressure than it entered with and is sent to the condenser to be cooled. If we

look at the first law we can find an equation comparing the rate at which work is developed perunit mass.
http://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Reciprocating_enginehttp://en.wikipedia.org/wiki/Steam_enginehttp://en.wikipedia.org/wiki/Power-to-weight_ratiohttp://en.wikipedia.org/wiki/Rotational_motionhttp://en.wikipedia.org/wiki/Electric_generatorhttp://en.wikipedia.org/wiki/Heat_enginehttp://en.wikipedia.org/wiki/Thermodynamic_efficiencyhttp://en.wikipedia.org/wiki/Reversible_process_%28thermodynamics%29http://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Thermodynamicshttp://en.wikipedia.org/wiki/Rankine_cyclehttp://en.wikipedia.org/wiki/Superheatedhttp://en.wikipedia.org/wiki/Superheatedhttp://en.wikipedia.org/wiki/Rankine_cyclehttp://en.wikipedia.org/wiki/Thermodynamicshttp://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Reversible_process_%28thermodynamics%29http://en.wikipedia.org/wiki/Thermodynamic_efficiencyhttp://en.wikipedia.org/wiki/Heat_enginehttp://en.wikipedia.org/wiki/Electric_generatorhttp://en.wikipedia.org/wiki/Rotational_motionhttp://en.wikipedia.org/wiki/Power-to-weight_ratiohttp://en.wikipedia.org/wiki/Steam_enginehttp://en.wikipedia.org/wiki/Reciprocating_enginehttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Thermal_energy


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Assuming there is no heat transfer to the surrounding environment and that the change in

kinetic and potential energy is negligible when compared to the change in specific entropy wecome up with the following equation

t is the rate at which work is developed per unit time is the rate of mass flow through the turbine

Isentropic Steam Turbine efficiencyTo measure how well a turbine is performing we can look at the isentropic efficiency.

Isentropic efficiencies involve a comparison between the actual performance of a device and

the performance that would be achieved under idealized circumstances.

When calculating the isentropic efficiency, heat to the surroundings is assumed to be

zero. The starting pressure and temperature is the same for both the isentropic and actual

efficiency. The specific entropy for the isentropic process is greater than the specific entropy forthe actual process due to irreversibility in the process. The specific entropy is evaluated at the

same pressure for the actual and isentropic processes in order to give a good comparison between

the two.

The isentropic efficiency is given to us as the actual work divided by the maximum

work that could be achieved if there were no irreversibly in the process.

.

h1 is the specific enthalpy at inlet to steam turbine h2 is the specific enthalpy at exit to steam turbine for an actual process h2s is the specific enthalpy at exit to steam turbine for an isentropic process

The efficiency of the steam turbine can be calculated by using the Kelvin statement of theSecond law of Thermodynamics.

Wcycle is the Work done during one cycle QH is the Heat transfer received from the heat source
http://en.wikipedia.org/wiki/Entropyhttp://en.wikipedia.org/wiki/Isentropichttp://en.wikipedia.org/wiki/Second_law_of_thermodynamics#Kelvin_statementhttp://en.wikipedia.org/wiki/Second_law_of_thermodynamics#Kelvin_statementhttp://en.wikipedia.org/wiki/Isentropichttp://en.wikipedia.org/wiki/Entropy


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Lab session # 03

To determine Boiler efficiency, Equivalent evaporation &

evaporating rate

Theory

Boiler Efficiency may be indicated by

Combustion Efficiency - indicates a burners ability to burn fuel measured by unburnedfuel and excess air in the exhaust

Thermal Efficiency - indicates the heat exchangers effectiveness to transfer heat from thecombustion process to the water or steam in the boiler, exclusive radiation and

convection losses

Fuel to Fluid Efficiency - indicates the overall efficiency of the boiler inclusive thermalefficiency of the heat exchanger, radiation and convection losses - output divided by

input.

Boiler Efficiency is in general indicated by either Thermal Efficiency or Fuel to Fluid

efficiency depending on the context.

Boiler EfficiencyBoiler Efficiency related to the boilers energy output to the boilers energy input can be

expressed as:

Boiler efficiency (%) = heat exported by the fluid (water or steam) / heat provided by the fuel x 100

boiler = ( )

= Steam mass flow rate LCV= Lower Calorific value=43000 kJ/kgk

= Fuel flow rate =

Fuel used is kerosene oil and value of its density is 820 kg/m3.

= Enthalpy value at pressure and temperature value at outlet

= Enthalpy value at pressure and temperature value at inlet


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Evaporating RateIt is defined as The amount of steam generated per specific area of boiler. It is denoted

by .

=

=

Where; A= area = 3.06 m

Equivalent EvaporationIt is the quantity of water evaporates from and at 100'C to produce dry saturated steam at

100C by absorbing the same amount of heat as used in the boiler under actual operating

conditions.


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Lab session # 04

To determine Super heater Efficiency

Theory

Super HeaterSuper heater is a device used to convert saturated steam or wet steam into dry

steam used for power generation or processes. There are three types of super heaters namely:

Radiant Convection Separately fired

A super heater can vary in size from a few tens of feet to several hundred feet (a few

meters or some hundred meters).

Radiant super heater is placed directly in the combustion chamber.

Convection super heater is located in the path of the hot gases.

Separately fired super heater, as its name implies, is totally separated from the boiler.

Super heaters are heat exchangers placed in the path of hot gases. They are generally

located in any suitable free space in the neighborhood of the boiler tubes. They receive the

saturated or slightly wet steam coming from the boiler drum and deliver it in a superheated state

the general steam main of the factory. They are generally formed of tubes of small diameter, all

of the same shape with several bends, interposed between two. The transfer of heat from the

gases to the steam is predominantly by convection; figure shows a super heater of such type.

Super Heater Efficiency(sh) = ( )

The efficiency of super heater tells us about the performance of it, means that how

efficiently heat is being transferred from one fluid to another fluid.


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Lab session # 05

To determine Feed water pump work input

Theory

In order to calculate feed pump work we first assume steady flow and steady flow conditions.

Thus,

dQ + dW = dh + d K.E + dP.E

dQ = dU - dW

Tds = dU -dp

= Specific Volume = 1/

.=. ( )

Where

. = Work required for running the feed pump

. = Feed water mass flow rate

=Boiler inlet pressure =Atmospheric pressure

In practical operation of plant, the work required to run the feed pump is calculated and is

subtracted from the turbine output work to calculate the net work produced by the plant.


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Lab session # 06

Determine for a Steam power plant

(a) Net work output (b) Work ratio (c) Overall Efficiency

(d) Specific Steam Consumption (SSC)

Theory

Net Work DoneIt is defined as the difference of total work done by the steam turbine and work done on

the feed pump. Net work done gives us the value of total work that can be produced from a plant

setup.

=

= . .

Work RatioIt is defined as the ratio of net work done by the plant to the Gross work done by the

turbine.

. =

=

Overall EfficiencyIt is defined as the ratio of net work done to the total heat supplied to the fluid.

=

(. +

. )

Specific Steam Consumption (SSC)It is define as the amount of steam consumed per unit time per kilo watt of power output. It is

mathematically written as follows;


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Lab session # 07

To determine the Pressure difference across an orifice using

orifice meter

Theory

Orifice Meter An orifice meter is a device used for measuring thevolumetric flow rate. It uses the

same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a

relationship between the pressure of the fluid and the velocity of the fluid. When the velocity

increases, the pressure decreases and vice versa.

As long as the fluid speed is sufficiently subsonic (V< mach 0.3), the incompressible

Bernoulli's equation describes the flow reasonably well. Applying this equation to a streamline

traveling down the axis of the horizontal tube gives,

Where, location 1 is upstream of the orifice, and location 2 is slightly behind the orifice.It is recommended that location 1 be positioned one pipe diameter upstream of the orifice, and

location 2 be positioned one-half pipe diameter downstream of the orifice. Since, pressure at 1 is

higher than pressure at 2 (for fluid moving from 1 to 2), the pressure difference as defined will

be a positive quantity.

From continuity, the velocities can be replaced by cross-sectional areas of the flow and the

volumetric flow rate Q,
http://en.wikipedia.org/wiki/Volumetric_flow_ratehttp://en.wikipedia.org/wiki/Venturi_effecthttp://en.wikipedia.org/wiki/Bernoulli%27s_principlehttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=incomp#incomphttp://www.efunda.com/formulae/fluids/bernoulli.cfmhttp://www.efunda.com/formulae/fluids/navier_stokes.cfm#continuityhttp://www.efunda.com/formulae/fluids/navier_stokes.cfm#continuityhttp://www.efunda.com/formulae/fluids/bernoulli.cfmhttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=incomp#incomphttp://en.wikipedia.org/wiki/Bernoulli%27s_principlehttp://en.wikipedia.org/wiki/Venturi_effecthttp://en.wikipedia.org/wiki/Volumetric_flow_rate


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Solving for the volumetric flow rate Q gives,

The above equation applies only to perfectly laminar, in-viscid flows. For real flows

(such as water or air), viscosity and turbulence are present and act to convert kinetic flow energy

into heat. To account for this effect, a discharge coefficientCd is introduced into the above

equation to marginally reduce the flow rate Q,

Since the actual flow profile at location 2 downstream of the orifice is quite complex,

thereby making the effective value ofA2 uncertain, the following substitution introducing a flow

coefficient Cfis made,

Where,Ao is the area of the orifice. As a result, the volumetric flow rate Q for real flows is given

by the equation,

The flow coefficient Cfis found from experiments and is tabulated in reference books; it

ranges from 0.6 to 0.9 for most orifices. Since it depends on the orifice and pipe diameters (as

well as the Reynolds Number), one will often find Cftabulated versus the ratio of orifice

diameter to inlet diameter, sometimes defined as ,
http://www.efunda.com/formulae/fluids/glossary.cfm?ref=lam#lamhttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=invis#invishttp://www.efunda.com/formulae/fluids/overview.cfm#reynoldshttp://www.efunda.com/formulae/fluids/overview.cfm#reynoldshttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=invis#invishttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=lam#lam


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Lab session # 08

Determine the Dryness Fraction of steam generated in the

Boiler

Theory

Dryness FractionDryness fraction or Quality is denoted by x and it is the ratio of mass of vapors to the

total mass of the mixture.

X =

Where

mtotal = mliquid + mvapors = mf+ mg

The measurement of dryness fraction is very important to further analyze the steam. Dryness

fraction is usually determined by following few methods;

Throttling Calorimeter Mechanical Calorimeter Electrical Calorimeter


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CalorimeterIt is a device used for calorimetry, the science of measuring the heat ofchemical

reactions, physical changes as well as heat capacity.

Throttling Calorimeter An instrument utilizing the principle of constant enthalpy expansion for the

measurement of the moisture content of steam; steam drawn from a steam pipe through

sampling nozzles enters the calorimeter through a throttling orifice and moves into a well-

insulated expansion chamber in which its temperature is measured. It is also known as steam

calorimeter.

Mechanical CalorimeterIt consists of two

concentric chambers, the inner

chamber and the outer chamber,

which communicates with each

other through an opening at the

top. As the steam discharges

through the metal basket, which

has a large number of holes, the

water particles due to their

heavier momentum get separated
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from the steam and collect in the chamber.

Electrical CalorimeterThe quality of wet steam can also be measured by an electric Calorimeter. The sample of

steam is passed in steady flow through an electric heater, as shown. The electrical energy input Qshould be sufficient to take

the steam to the superheated

region where pressure P2

and temperature T2 are

measured. If I is the current

flowing through the heater

in amperes and V is the

voltage across the coil, then

at steady state

Q = VI x 10-3 kW.

If m is the mass of steam taken in t seconds under steady flow condition, then the steady

flow energy equation for the heater (as control volume) gives

w1 h1 +Q = w1h2

Where, w1 = steam flow rate in kg/s

Therefore, h1+Q/w1 =h2

h1 = hfp1+x1hfgp1, hence x1 can be calculated


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Lab session # 09

Study of Steam Generating Unit

Theory

BoilerIt is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid

exits the boiler for use in various processes or heating applications.

Classification of BoilersBoilers are classified on different basis.

By Application(Utility boiler, Marine boiler, Industrial Boiler)

By Pressure(Low to medium pressure for process industry, High pressure, Super-critical for power

generation)

By Construction(Field erected, Shop assembled or Package boilers)

By location of water and hot gases(Water Tube boiler, Fire tube boiler)

By Fuel used(Coal, Gas, Waste heat)

By Firing method(Burners, Stokers, Fluidized bed)

By Circulation(Natural circulation due to density difference, Forced or pump circulation)

Fire Tube Boiler In fire tube boiler, hot gases pass through the

tubes and boiler feed water in the shell side is converted

into steam. Fire tube boilers are generally used for

relatively small steam capacities and low to medium

steam pressures. As a guideline, fire tube boilers arecompetitive for steam rates up to 12,000 kg/hour and

pressures up to 18 kg/cm2. Fire tube boilers are available

for operation with oil, gas or solid fuels. For economic

reasons, most fire tube boilers are nowadays of

packaged construction (i.e. manufacturers shop erected)

for all fuels.
http://en.wikipedia.org/wiki/Pressure_vesselhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Pressure_vessel


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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 of Fire tube Boilers include

Not suitable for high pressure applications 250 psig and above Limitation for high capacity steam generation

Water Tube Boiler In water tube boiler, boiler feed water flows through the tubes and enters the boiler

drum. The circulated water is heated by the combustion gases and converted into steam at the

vapor space in the drum. These boilers are selected when the steam demand as well as steampressure requirements are high as in the case of process cum power boiler / power boilers.

Most modern water boiler tube designs are within the capacity range 4,500 120,000

kg/hour of steam, at very high pressures. Many water tube boilers nowadays are of packaged

construction if oil and /or gas are to be used as fuel.

Solid fuel fired water tube designs are available but

packaged designs are less common.

The features of water tube boilers are:

Forced, induced and balanced draft provisionshelp to improve combustion efficiency.

Less tolerance for water quality calls for watertreatment plant.

Higher thermal efficiency levels are possibleAdvantages

Available in sizes that are far greater than thefire tube design. Up to several million pounds

per hour of steam.

Able to handle higher pressures up to 5,000 psig Recover faster than their fire tube cousin Have the ability to reach very high temperatures

Disadvantages of the Water tube design include

High initial capital cost Cleaning is more difficult due to the design


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No commonality between tubes Physical size may be an issue

Packaged BoilerThe packaged boiler is so called because it comes as a complete package. Once

delivered to site, it requires only the steam, water pipe work, fuel supply and electrical

connections to be made for it to become operational. Package boilers are generally of shell type

with fire tube design so as to achieve high

heat transfer rates by both radiation and

convection.

The features of package boilers are:

Small combustion space and high heatrelease rate resulting in faster

evaporation.

Large number of small diameter tubesleading to good convective heat transfer.

Forced or induced draft systems resultingin good combustion efficiency.

Number of passes resulting in better overall heat transfer. Higher thermal efficiency levels compared with other boilers.

These boilers are classified based on the number of passes - the number of times the hot

combustion gases pass through the boiler. The combustion chamber is taken, as the first pass

after which there may be one, two or three sets of fire-tubes. The most common boiler of this

class is a three-pass unit with two sets of fire-tubes and with the exhaust gases exiting through

the rear of the boiler.

Steam DrumA steam drum is a standard feature of a water-tube boiler. It is a reservoir of water/steam

at the top end of the water tubes. The drum stores the steam generated in the water tubes and

acts as a phase-separator for the steam/water mixture. The difference in densities between hotand cold water helps in the accumulation of the "hotter"-water/and saturated-steam into the

steam-drum.

ConstructionMade from high Carbon Steel with high tensile strength and its working involves

temperatures around 390oC and pressures well above 350 psi(2.4MPa). The separated steam is
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EconomizersEconomizers are mechanical devices intended to reduce energy consumption, or to

perform another useful function like preheating a fluid.

Modern-day boilers, such as those in coal-fired power stations, are still fitted with

economizers which are descendants of Green's original design. In this context they are often

referred to as feed water heaters and heat the condensate from turbines before it is pumped to the

boilers. Economizers are commonly used as part of a heat recovery steam generator in

a combined cycle power plant. In an HRSG, water passes through an economizer, then

a boiler and then a super heater. The economizer also prevents flooding of the boiler with liquid

water that is too cold to be boiled given the flow rates and design of the boiler.

A common application of economizers in steam power plants is to capture the waste heat

from boiler stack gases (flue gas) and transfer it to the boiler feed water. This raises the

temperature of the boiler feed water thus lowering the needed energy input, in turn reducing the

firing rates to accomplish the rated boiler output. Economizers lower stack temperatures which

may cause condensation of acidic combustion gases and serious equipment corrosion damage if

care is not taken in their design and material selection.

Air pre-heaterAn air pre-heater (APH) is a general term to describe any device designed to

heat air before another process (for example, combustion in a boiler) with the primary objective

of increasing the thermal efficiency of the process. They may be used alone or to replace

a recuperative heat system or to replace a steam coil.
http://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Power_stationhttp://en.wikipedia.org/wiki/Feedwater_heaterhttp://en.wikipedia.org/wiki/Steam_turbinehttp://en.wikipedia.org/wiki/Heat_recovery_steam_generatorhttp://en.wikipedia.org/wiki/Combined_cyclehttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Superheaterhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Flue_gashttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Combustionhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Recuperatorhttp://en.wikipedia.org/wiki/Recuperatorhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Combustionhttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Flue_gashttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Superheaterhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Combined_cyclehttp://en.wikipedia.org/wiki/Heat_recovery_steam_generatorhttp://en.wikipedia.org/wiki/Steam_turbinehttp://en.wikipedia.org/wiki/Feedwater_heaterhttp://en.wikipedia.org/wiki/Power_stationhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Fluid


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The purpose of the air pre-heater is to recover the heat from the boiler flue gas which

increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a

consequence, the flue gases are also sent to the flue gas stack(or chimney) at a lower

temperature, allowing simplified design of the ducting and the flue gas stack. It also allows

control over the temperature of gases leaving the stack to meet emissions regulations.

There are two types of pre-heater are used; Recuperative Type or tubular type Regenerative Type

Regenerative TypeThe rotating-plate design (RAPH) consists of a central rotating-plate element installed within a

casing that is divided into two (bi-sectortype), three (tri-sectortype) or four (quad-sectortype)

sectors containing seals around the element. The seals allow the element to rotate through all the

sectors, but keep gas leakage between sectors to a minimum while providing separate gas air

and flue gas paths through each sector. Flue gases and air flow through different pipe lines.

Recuperative Type or tubular typeIt is a special purpose counter-flow energy recovery heat exchanger positioned within the

supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial

process, in order to recover the waste heat.

Normally the heat transfer between airstreams provided by the device is termed as

'sensible', which is the exchange of energy, or enthalpy, resulting in a change in temperature of

the medium (air in this case), but with no change in moisture content. However, if moisture or
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relative humidity levels in the return air stream are high enough to allow condensation to take

place in the device, then this will cause 'latent' heat to be released and the heat transfer material

will be covered with a film of water. Despite a corresponding absorption of latent heat, as some

of the water film is evaporated in the opposite airstream, the water will reduce the thermal

resistance of the boundary layer of the heat exchanger material and thus improve the heat

transfer coefficient of the device, and hence increase efficiency. The energy exchange of suchdevices now comprises both sensible and latent heat transfer; in addition to a change in

temperature, there is also a change in moisture content of the exhaust air stream.

Flow RegimesTwo kinds of flow regimes are used;

Pool boiling Flow boiling

Pool boilingPool boiling is the process in which the heating surface is submerged in a large body of

stagnant liquid. The relative motion of the vapor produced and the surrounding liquid near the

heating surface is due primarily to the buoyancy effect of the vapor. Nevertheless, the body ofthe liquid as a whole is essentially at rest.

Pool boiling consists of following few steps during boiling;

a) Natural Convection boilingb) Nucleate boilingc) Transitional boilingd) Film boiling
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Flow boilingRelative to pool boiling, heat transfer is enhanced when there is a forced relative motion

between the heater and the fluid, more pronounced in the homogeneous convection zone than in

peak values (e.g. the peak heat flux measured in water increasing from 1.3 MW/m2 to a

maximum of 35 MW/m2). Internal flow boiling presents more variations, since it is a

complicated two-phase flow.

The most-important configurations of flow boiling are: the vertical pipe, the horizontal

pipe, and the micro-channels (e.g. for plate heat exchangers). In the former and commonest case,

a liquid is slowly forced upwards (say at several centimeters per second, since the phase change

multiplies the speed 100 to 1000 times, and chocking must be prevented), inside a pipe (of

internal diameter in the range 5..50 mm) with a hot wall surface at Ts>Tsat(p), and the transition

from liquid to vapor develops along several intermediate stages of two-phase-flow, from single-

phase liquid to single-phase vapor.

Flow regime also consists of following phases;

a) Bubble flow regimeb) Vapor slug regimec) Annular flow regimed) Transitional flow regimee) Mist flow regime


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HRSG systemsHeat Recovery Steam Generators (HRSGs) are typically used in combined cycle electric

power generation. Waste heat from gas turbine exhaust is used to generate steam. The low

temperature of the exhaust gases compared to direct fired units puts less stress on the boiler

tubes. These units are typically constructed of lighter grade materials. One common design is a

three-drum configuration. The low pressure (LP) drum is used as a deaerating feed water heater.

The intermediate pressure (IP) drum is used to generate steam for injection into the gas turbine.

The high pressure (HP) drum is used to generate turbine steam for electrical power generation.

Some units are configuration with duct burners to produce additional power. This can result in

higher heat transfer and boiler system problems.

HRSGs require high-purity water because of the use of the IP drum steam for turbineinjection. Dissolved solids must be kept to a bare minimum in these units. Because many units

are in a cycling mode, start-up, shutdown and lay-up procedures are even more important in

these systems. In a typical HRSG unit, the LP drum is treated with amines and oxygen

scavengers, the HP drum is treated with a coordinated phosphate programme, and the IP drum

uses blow down from the HP drum for some of the feed water.

HRSG systems are available in various configurations and in different sizes to perform

different operation along with waste heat recovery. These configurations are as follows;

Vertical HRSGs Once through HRSGs Water tube HRSGs Horizontal tube HRSGs


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Blow Down heat recoverySteam boilers operate under pressure to produce 1,000s of pounds of steam per hour. To

prevent scale formation on the heating surfaces, which would decrease fuel to steam efficiency, a

volume of the boiler water must be removed on a regular basis.

This process of removing boiler water from the boiler is called blow down. Regulations

specify that for blow down to go to sewer, it must first be depressurized and then cooled to

prevent overheating drains/sewers. A blow down tank or blow-off vessel is typically installedbetween the boiler and the drain to accomplish this task and to mix cool city/well water with the

blow down water to cool it.

pp lab final)

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