POWER STATIONS
Prof. Farouk Okasha
Department of Mechanical Power Engineering -Mansoura University
Regeneration
A great part of external irreversibility in the X
economizer section (note the large temperature
difference between 4 and b in Fig. X) can be avoided if
the working fluid is added to the steam generator at B
instead of 4. This may be performed through the
regeneration process where heat is internally
exchanged between the expanding fluid in turbine and
the pumped liquid before economizer, see Fig. Y.
Unfortunately, this procedure is not practically
possible. The heat transfer area is not sufficient and
the effectiveness of such heat exchanger would be low. Y
Moreover, the vapor leaving the turbine would have
an unacceptably high moisture content for proper
turbine operation and efficiency.
Feedwater Heating
A compromise that would reduce rather than eliminate the economizer can be realized by
using feedwater heating. Feedwater heating involves normal adiabatic expansion in the turbine.
The compressed liquid at 4 is heated in a number of finite steps, rather than continuously, by
vapor bled from turbine at selected stages. Heating of the liquid occurs in heat exchangers called
feedwater heaters. There are three types of feedwater heaters in application. These are:
1. Open or direct-contact type
2. Closed type with drains cascaded backward
3. Closed type with drains pumped forward
Modern large steam powerplants use five to eight feedwater
heating stage. None is built without feedwater heating.
2-7 Open or Direct Contact of Feed Water Heats
In case of the open feedwater heater the extraction steam is directly
mixed with the subcooled water at extraction pressure to yield
saturated water. Hence the amounts of bled steam ṁ3 and ṁ2 is
principally equal to that would saturate the subcooled water at 6 and
8. If it were much less, it will result in a much lower temperature than
saturation, which would partially contradict the advantages of
feedwater heating. If it were more, it would lead to unnecessary loss
of work turbine and in a two-phase mixture that would be difficult to
pump. Two other pumps must be used to pressurize the saturated
water from 7 and 9 to subcooled conditions at 8 and 10. The
extracted steam, at 2 or 3, loses its latent heat of vaporization, while
water, at 6 and 8, gains sensible heat, therefore; the amount of
extracted steam ṁ2 or ṁ3 is only small fraction of the steam passing
through the turbine.
In general only one open type feedwater heaters are used as a
deaerating heater, followed by a pump called the boiler feed pump.
Energy balance on high pressure and low pressure feedwater
heaters are:
𝑚ሶ 2 (ℎ2 − ℎ9 )=)1-𝑚ሶ 2 ()ℎ9 − ℎ8 )
𝑚ሶ ሶ 3 )ℎ3 − ℎ7 )=)1-𝑚ሶ 2 -𝑚ሶ 3 )(ℎ7 − ℎ6 )
𝑞𝑎 = (ℎ1 − ℎ10 )
𝑤𝑇 = )ℎ1 − ℎ2 )+( 1-𝑚ሶ 2 ()ℎ2 − ℎ3 )
+(1-𝑚ሶ 2 -𝑚ሶ 3 )(ℎ3 − ℎ4 )
𝑤𝑝 =(1-𝑚ሶ 2-𝑚ሶ 3 )(ℎ6 − ℎ5 ) +( 1-𝑚ሶ 2 ()ℎ8 − ℎ7 )
+(ℎ10 −ℎ9 )
𝑞𝑅 =(1-𝑚ሶ 2 -𝑚ሶ 3 )(ℎ4 − ℎ5 )
𝑤𝑛𝑒𝑡 = 𝑤𝑇 -𝑤𝑝
Cycle thermal efficiency, η = 𝑤𝑛𝑒𝑡 /𝑞𝐴
Work Ratio, 𝑤𝑛𝑒𝑡 = 𝑤𝑛𝑒𝑡 /𝑤𝑇
Closed-Type Feedwater Heaters with Drains Cascaded Backward
Closed type feedwater heater is the simplest and most commonly used in
powerplants although it results in a greater loss of availability than other types.
Close-type FWH is a shell-tubes heat exchanger where feedwater passes
through the tubes while the bled steam passes on shell side. As the condensate
does not mix with bled steam it can be pressurized once to the pressure of
steam generator or to the pressure of deaerator if one is used in the plant. The
condensate, then, goes through successive closed FWH. After the bled steam
condensates it is throttled and fed back to the next lower-pressure FWH. The
condensate of the lowest-pressure FWH is (though not always) led back the
main condenser.
𝑚ሶ 2 (ℎ2 − ℎ11 )=(ℎ8 − ℎ7 ) ℎ12 = ℎ11
𝑚ሶ ሶ 3 )ℎ9 − ℎ3 )+𝑚ሶ ሶ 2)ℎ12 − ℎ9 )= (ℎ7 − ℎ6 ) ℎ10 = ℎ9
𝑤𝑝 = (ℎ6 − ℎ5 )
𝑞𝑅 =(1-𝑚ሶ 2 -𝑚ሶ 3 )(ℎ4 − ℎ5 )+(𝑚ሶ 2 +𝑚ሶ 3 )(ℎ10 − ℎ5 )
Temp-enthalpy diagrams
(a), (b) low-pressure and
(c) High pressure
TTD = terminal temp. diff.
DS = desuperheater
C = condenser
DC = drain cooler
The events in the closed FWH may be presented by the temperature-length diagram as shown,
counter flow. A terminal temperature difference (TTD) is defined as
TTD= saturation temperature of bled steam- exit water temperature
The value of TTD is positive and of order 2.8 OC when the bled steam is saturated or wet steam as
shown in case a. Too small value appears good for plant efficiency but would require a larger heater
than can be justified economically. Too large a value would hurt cycle efficiency. On other hand, TTD
could be negative when bled steam is superheated, at relatively high pressure, as shown in case c.
There are four physical possibilities of closed FWH composed of the following sections:
1. Condenser 2. Condenser, drain cooler
3. Desuperheater, Condenser, drain cooler 4. Desuperheater, Condenser
Closed-Type Feedwater Heaters with Drains Pumped Forward
This type of closed FWH avoids throttling but at the expense of added
complexity due to inclusion of small pump that pumps the drain forward
to the full feadwater line pressure. It appears of more advantageous
when used as the lowest- pressure FWH, where it prevents throttling of
the all accumulated cascaded flows to the condenser where the energy
should transfer and lose the surrounding environment.
The following relation should be considered
𝑡9 = 𝑡11 − 𝑇𝑇𝐷 hp heater
𝑡7 = 𝑡13 − 𝑇𝑇𝐷 lp heater
𝑚ሶ 2 (ℎ2 − ℎ11 )=(1-𝑚ሶ 2 )(ℎ9 − ℎ8 )
𝑚ሶ ሶ 3 )ℎ3 − ℎ13 )=(1−𝑚ሶ 2ሶ −𝑚ሶ 3 )(ℎ7 − ℎ6 )
𝑤𝑝 = (1−𝑚ሶ 2 −𝑚ሶ 3 )(ℎ6 − ℎ5 )+𝑚ሶ 3 (ℎ14 − ℎ13 )+𝑚ሶ 2 (ℎ12
− ℎ11 )
𝑞𝑅 =(1-𝑚ሶ 2 -𝑚ሶ 3 )(ℎ4 − ℎ5 )
The Choice of Feedwater Heaters
1. One open feedwater heater is used in fossil fuel powerplants which acts as a deaerator as well. It is
placed near the middle of feedwater system, where the temperature is most conductive to release
non-condensable gases.
2. The closed-type feedwater heater with drains cascaded backward is the most common type (4-7).
3. One closed FWH with drains pumped forward is often used as the lowest-pressure feedwater heater
to pump all accumulating drains back into the feedwater line. Occasionally another one of this FWH
type may be encountered at a higher-pressure stage after the deaerator.
