Heat Transfer in HRSG

Heat transfer is the transfer of energy that occurs due to temperature differences between objects or materials. In thermodynamics, the moving energy is heat. Analysis on heat transfer rate should be considered. Generally, heat transfer in HRSG that occurs there are three kinds:

1. Conduction heat transfer
2. Convection heat transfer
3. Radiation heat transfer

In this case, the heat transfer which is used is conduction heat transfer, convection and combination of conduction and convection. In the HRSG boiler is assumed does not use additional fuel and heat sources only from exhaust gases of gas turbine. So it does not analyze the radiation heat transfer in this HRSG boiler.

1. Conduction Heat Transfer

Conduction heat transfer is the transfer of heat from one part of solid objects to other parts of same solid object without any movement of solid molecules itself. Generally, equation used in the conduction heat transfer is:

2. Convection Heat Transfer

Convection heat transfer is the heat transfer which is performed by molecules of fluid (liquid or gas). Generally, the equation of convection heat transfer is:

where Tw is temperature of solid object while T∞ is temperature of fluid.

Convection heat transfer consists of two types of heat transfer. I.e., free convection heat transfer in which the air velocity is assumed does not exist. As for air that has flow is type of forced convection where the effect occurred at Nuselt number for every condition is different.

2.a. Free Convection Heat Transfer

Free convection heat transfer in HRSG as follow:

-          On horizontal cylinder:

Multiplication between the Grashof number and Prandtl numbers is called Rayleigh number:

Ra = Gr . Pr

Where the Rayleigh number can be calculated as follow:

where:

Tw = temperature of surface

T∞ = ambient air temperature

υ = kinetic viscosity

g = gravity (9.8 m/s2)

δ = D = characteristic dimension

β = coefficient of volume expansion

-          On the ball

The equation above can be changed by entering Prandalt number, thus can be obtained following formula:

Then the value of free convection can be obtained as follow:

2.b. Forced Convection Heat Transfer

-          Through inner cylinder

-          Through bank of tubes

where:

C = flow coefficient across bank of tubes

Re = Reynolds Number

Pr = Prandlt Number

do = outside diameter of cylinder / tubes

di = inside diameter of cylinder

3. Combination of Conduction and Convection Heat Transfer

Heat transfer that occurs in HRSG is a combination of conduction and convection, such as the following figure where on one side there is hot fluid A and on the other side fluid B has cooler temperature.

Figure 1: Heat Transfer on Flat Plane
(Source: Heat Transfer Book - Holman JP)

Heat transfer can be expressed as:

Heat transfer can be described in the network above, so that the overall heat transfer is calculated by dividing overall temperature difference by the amount of thermal resistance:

Overall heat flow as a result of the combined conduction and convection can be expressed by thorough heat transfer coefficient (U), formulated in relationship:

where A is the area of ​​heat flow field, in accordance with the above equation then the overall heat transfer coefficient is:

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Combined Cycle Power Plant

Combined cycle power plant is a cycle that utilizes exhaust gases from gas turbine (power plant) to heat water in the boiler, in this case is called HRSG (Heat Recovery Steam Generator) and the steam that is produced by HRSG is used to drive steam turbine.

Exhaust gases from gas turbines coming out at pressure and temperature above 500 ^oC. Exhaust gas cannot be utilized as working fluid if it has low pressure and high temperature (high enthalpy). Regenerator can be used to utilize this exhaust gas by heating gas out of compressor before entering combustion chamber.

Several constraints of regenerator usage as follow:

1. Regenerator resulting in pressure drop between compressor outlet and combustion chamber inlet which causes the increase in compressor work due to certain turbine inlet pressure. Compressor outlet pressure should be higher.
2. Regenerator cause rise in back pressure turbine that cause drop in turbine work.
3. Regenerator difficult to serve high flow rate.

To avoid the matters above, utilization exhaust gases from gas turbine is with HRSG boiler. This can  be clearly understood, where the exhaust gases from gas turbine are  still contain relatively high energy, which can be utilized as energy source for steam cycle. Therefore, the two cycles can complement each other thermodynamically. Thus it can be combined into one combined cycle power plant consisting of gas turbine and steam turbine that drive each generator separately.

Figure 1: Combined Cycle Power Plant

Combined cycle power plant as shown in Figure 1 above, in addition to producing high efficiency and greater power output, combined cycle is flexible, easily ignited with no full load, suitable for base load operation and turbine has cycle and has high efficiency in wide load area. The disadvantage is associated to is complexity because basically of installation of two combined technologies in complex power plant.

By using combination of recycled gas, two main advantages can be obtained; it can add power and save fuel costs. The addition of electric power without increasing fuel also means it will increase thermal efficiency of system and can be raised from about 24% to 42%. The magnitude of this efficiency improvement depends on temperature of cooling water which is used in the plant and the size of flue gas temperature in power plant. Colder cooling water temperature so will make higher temperature of exhaust gas, furthermore efficiency will also greater.

Another reason election of combined cycle power plant is fast construction time so that when surge in electricity demand will be met within short time, combined cycle power plant can be built gradually. The first stage was built gas generator power plant to meet surging demand, while HRSG and steam generator power plant is built and operated later when the electricity demand has increased. Combined cycle power plant can be operated as generator for peak load and for base load.

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HRSG Components

Heat Recovery Steam Generator (HRSG) consists of several components such as superheater, evaporator and economizer which each component has different functions. HRSG components can be seen as following below:

1. Superheater

Superheater is one of HRSG components that serves to raise temperature of saturated steam to be superheated steam. Superheated steam when used to perform work by way of expansion in the turbine or steam engine will not condense, thus reducing the possibility of danger which is caused by back stroke caused by steam that condenses yet in time, so it will cause vacuum in undue area of expansion.

2. Evaporator

Evaporator is one of HRSG components which serve to convert water into saturated steam. Evaporator pipes in steam boiler are usually located on the floor (water floor) and also on the wall (water wall). In this pipes, quality of saturated steam at 0.80 to 0.98, so most are still shaped in liquid phase. Evaporator will heat water that falls from steam drum which still in liquid phase to form the saturated steam so it can be forwarded to superheater.

Figure 1: HRSG Components

3. Economizer

Economizer is one of HRSG components that consist of water pipes that are placed on the track of flue gases after evaporator pipes. Economizer pipes are made ​​of steel or cast iron materials that are able to withstand high heat and pressure. Economizer serves to heat feed water before it enters steam drum and evaporator. Furthermore the evaporation process can be easier by using high temperature flue gas of HRSG so increase the efficiency of HRSG because it can reduce heat loss in the Heat Recovery Steam Generator (HRSG). Water which enters evaporator is at high temperature so evaporator pipes are not easily damaged due to difference in temperature is not too high.

Figure 2: Arrangement of Economizer and Evaporator

4. Preheater

Preheater is one of HRSG components that serves as initial heater of water which is pumped from condenser before enter into feed water tanks. In the HRSG system, preheater serves to raise temperature before enter feed water tanks which later will be forwarded to economizer.

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Heat Recovery Steam Generator (HRSG)

Heat Recovery Steam Generator (HRSG) is a boiler that utilizes heat energy of residual exhaust gases from gas turbine unit to heat water and convert it into steam, and steam is then used to drive steam turbines. In general, Heat Recovery Steam Generator (HRSG) boiler is not equipped with burners and do not consume fuel, so there is not radiation heat transfer process. The heat transfer process that occurs only convection and conduction from exhaust gas of gas turbine into water to be processed into steam through heating elements inside HRSG boiler room.

Figure 1:  Combined Cycle Power Plant (CCPP)

Heat Recovery Steam Generator (HRSG) boiler is very useful to improve efficiency of fuel that is used in gas turbine unit, which will further drive steam turbine unit. Power generation systems that take advantage of this process is called Combined Cycle Power Plant (CCPP). Combined Cycle Power Plant (CCPP) is a combined gas turbine Brayton cycle and steam turbine Rankine cycle. HRSG boiler is part of the Rankine cycle.

TS diagram illustrates the overall process of Combined Cycle Power Plant (CCPP) which is shown in Figure 2. Diagram I expresses Brayton cycle for gas turbine and diagram II states Rankine cycle for steam turbine.

Figure 2: Diagram CCPP and HRSG Single Pressure

Figure 3: Diagram T-S Combined Cycle Power Plant (CCPP)

Steam production capacity that can be generated by Heat Recovery Steam Generator (HRSG)depend on the capacity of heat energy that is still contained in exhaust gas from gas turbine unit, which means depend on the load of gas turbine unit. Basically, gas turbines operate at fixed rotation, the flow of incoming air compressor also fixed but it may occurs changes in load of turbine although the fuel flow is constant, so that the temperature of exhaust gas  will also changes following the change of gas turbine load.

Exhaust gas temperature from gas turbine unit can maintained constant by regulating the opening of Inlet Guide Vane (IGV) to regulate the flow rate of air into compressor in which the exhaust gas temperature act as feedback.

Figure 4: Flow Diagram of Heat Recovery Steam Generator (HRSG)

Some Heat Recovery Steam Generator (HRSG) boilers has some components and can be equipped with additional burner to increase steam production capacity and some production of steam can be used for heating purposes of other applications (cogeneration). With this additional combustion,  the stability of steam production HRSG can be maintained. Furthermore the stability of steam turbine that uses steam can be maintained too, although the gas turbine load changing and also temperature exhaust gas of gas turbine (air flow into the compressor) should not be kept constant (IGV setting is not required).

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