Why we need a Vapor cycle?
We know that the Carnot cycle is most efficient cycle operating between two specified temperature limits. However; the Carnot cycle is not a suitable model for steam power cycle since:
- The turbine has to handle steam with low quality which will cause erosion and wear in turbine blades.
- It is impractical to design a compressor that handles two phase.
- It is difficult to control the condensation process that precisely as to end up with the desired at point 4.
Other issues include: isentropic compression to extremely high pressure and isothermal heat transfer at variable pressures. Thus, the Carnot cycle cannot be approximated in actual devices and is not a realistic model for vapor power cycles.
Ideal Rankine Cycle
The Rankine cycle is the ideal cycle for vapor power plants; it includes the following four reversible processes:
1-2: Isentropic compression: Water enters the pump as state 1 as saturated liquid and is compressed isentropically to the operating pressure of the boiler.
2-3: Constant Pressure heat addition: Saturated water enters the boiler and leaves it as superheated vapor at state 3.
3-4: Isentropic expansion: Superheated vapor expands isentropically in turbine and produces work.
4-1: Constant Pressure heat rejection: High quality steam is condensed in the condenser.
Energy Analysis for the Cycle
All four components of the Rankine cycle are steady-state steady-flow devices. The potential and kinetic energy effects can be neglected. The first law per unit mass of steam can be written as:
Pump q = 0, w pump,in = h 2 – h 1
Boiler w = 0, q in = h 3 – h 2
Turbine q = 0, w turbine,out = h 3 – h 4
Condenser w = 0, q out = h 4 – h 1
The thermal efficiency of the cycle is determined from:
ηth = Wnet / qin = 1 – qout / qin
Wnet = q in – q out = w turbine , out – w pump , in
If we consider the fluid to be incompressible, the work input to the pump will be:
(h 2 − h 1 ) = v(P 2 − P 1 )
Deviation of Actual Vapor Power Cycle from Ideal Cycle
As a result of irreversibilities in various components such as fluid friction and heat loss to the surroundings, the actual cycle deviates from the ideal Rankine cycle. The deviations of actual pumps and turbines from the isentropic ones can be accounted for by utilizing isentropic efficiencies defined as:
Increasing the Efficiency of Rankine Cycle
We know that the efficiency is proportional to:
That is, to increase the efficiency one should increase the average temperature at which heat is transferred to the working fluid in the boiler, and/or decrease the average temperature at which heat is rejected from the working fluid in the condenser.
- Decreasing the of Condenser Pressure (Lower T L)
Lowering the condenser pressure will increase the area enclosed by the cycle on a T-s diagram which indicates that the net work will increase. Thus, the thermal efficiency of the cycle will be increased.
The condenser pressure cannot be lowered than the saturated pressure corresponding to the temperature of the cooling medium. We are generally limited by the thermal reservoir temperature such as lake, river, etc. Allow a temperature difference of 10°C for effective heat transfer in the condenser. For instance lake @ 15°C + ∆T (10°C) = 25°C.
The steam saturation pressure (or the condenser pressure) then will be ⇒ P sat = 3.2 kPa.
2. Superheating the Steam to High Temperatures (Increase T H)
Superheating the steam will increase the net work output and the efficiency of the cycle. It also decreases the moisture contents of the steam at the turbine exit. The temperature to which steam can be superheated is limited by metallurgical considerations (~ 620°C).
3. Increasing the Boiler Pressure (Increase T H )
Increasing the operating pressure of the boiler leads to an increase in the temperature at which heat is transferred to the steam and thus raises the efficiency of the cycle. Note that for a fixed turbine inlet temperature, the cycle shifts to the left and the moisture content of the steam at the turbine exit increases. This undesirable side effect can be corrected by reheating the steam.
The Ideal Reheat Rankine Cycle
To take advantage of the increased efficiencies at higher boiler pressure without facing the excessive moisture at the final stages of the turbine, reheating is used. In the ideal reheating cycle, the expansion process takes place in two stages, i.e., the high-pressure and low-pressure turbines.
The total heat input and total turbine work output for a reheat cycle become:
q in = q primary + q reheat = ( h 3 – h 2 ) + ( h 5 – h 4 )
w turbine , out = w H P turbine + w L P turbine = ( h 3 – h 4 ) + ( h 5 – h 6 )
The incorporation of the single reheat in a modern power plant improves the cycle efficiency by 4 to 5 percent by increasing the average temperature at which heat is transferred to the steam.
The Ideal Regenerative Rankine Cycle
The regeneration process in steam power plants is accomplished by extracting (or bleeding) steam from turbine at various stages and feed that steam in heat exchanger where the feedwater is heated. These heat exchangers are called regenerator or feedwater heater (FWH). FWH also help removing the air that leaks in at the condenser (deaerating the feedwater).
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There are two types of FWH’s, open and closed.
Open (Direct‐Contact) Feedwater Heaters
An open FWH is basically a mixing chamber where the steam extracted from the turbine mixes with the feedwater exiting the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure.
Using Fig. 8, the heat and work interactions of a regenerative Rankine cycle with one FWH can be expressed per unit mass of steam flowing through the boiler as:
q in =h 5 – h 4
q out = ( 1 – y )( h 7 – h 1 )
w turbine , out = ( h 5 – h 6 ) + ( 1 – y )( h 6 – h 7 )
w pump , in = ( 1 – y ) w Pump I + w Pump II
y = ṁ6 / ṁ5
w Pump I = v 1 ( P 2 – P 1 )
w Pump II = v 3 ( P 4 – P 3 )
Thermal efficiency of the Rankine cycle increases as a result of regeneration since FWH raises the average temperature of the water before it enters the boiler. Many large power plants have as many as 8 FWH’s.
Closed Feedwater Heaters
In closed FWH, heat is transferred from the extracted steam to the feedwater without any mixing taking place. Thus; two streams can be at different pressures, since they don’t mix. In an ideal closed FWH, the feedwater is heated to the exit temperature of the extracted steam, which ideally leaves the heater as a saturated liquid at the extraction pressure.
Many system and industries require energy input in the form of heat, called process heat. Some industries such as chemical, pulp and paper rely heavily on process heat. The process heat is typically supplied by steam at 5 to 7 atm and 150 to 200 C. These plants also require large amount of electric power. Therefore, it makes economical and engineering sense to use the already-existing work potential (in the steam entering the condenser) to use as process heat. This is called cogeneration.
In the cogeneration cycle shown in the above figure, at times of high demands for process heat, all the steam is routed to the process heating unit and none to the condenser.
Combined Gas‐Vapor Power Cycle
Gas-turbine cycles typically operate at considerably higher temperatures than steam cycles. The maximum fluid temperature at the turbine inlet is about 620C for modern steam power plants, but over 1425C for gas-turbine power plants. It is over 1500C at the burner exit of turbojet engines. It makes engineering sense to take advantage of the very desirable characteristics of the gas-turbine cycle at high-temperature and to use the high temperature exhaust gases as the energy source for the bottoming cycle as a steam power cycle. This is called combined cycle. Combined cycles can achieve high thermal efficiencies, some of recent ones have η about 60%.
M. Bahrami ENSC 461 (S 11) Vapor Power Cycles