There are three types of feed water heaters in use.
They are: 1. Open or Direct Contact Feed Water Heaters 2. Closed Type Feed Water Heaters with Drains Cascaded Backward 3. Closed Type Feed Water Heaters with Drains Pumped Forward.
Type # 1. Open or Direct Contact Feed Water Heaters:
In open or direct contact feed water heater, the extraction steam is mixed directly with incoming sub-cooled feed water; to produce saturated water at extraction steam pressure Fig. 22.3 (a) and (b) shows a ‘schematic flow diagram and corresponding T-s diagram for a Rankine cycle using two such feed water heaters.
Condensate water leaves the condenser at 5 and is pumped to 6 to a pressure equal to that of the extraction steam at 3. The now-subcooled water at 6 and wet steam at 3 mix in the low pressure feed water heater to produce saturated water at 7.
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Amount of bled steam m3 should essentially be such that subcooled water at 6, should be raised close to its saturation point. If it were much less, the water will be heated to below its saturation temperature and advantage of feed water heating would be partially lost. If it were more there will be surplus steam at point 7. This would mean loss of turbine work and presence of steam-water mixture at 7 which is difficult to pump.
The pressure at 6-7 should be less than extraction steam pressure at 3 or else reverse flow of condensate water would enter the turbine at 3. The second pump pressurises saturated water at 7 to a subcooled condition at 8, which is at the pressure of extraction steam at 2. In the high pressure feed water heater superheated steam at 2, mixes with subcooled water at 8 to produce saturated water at 9. This is pressurised to 10 by a pump to enter the steam generator at its working pressure.
Steam at 2 or 3 loses its latent heat of vaporisation which is much larger than the gain of sensible heat by water at 8 or 6. Therefore the quantity of extracted steam ṁ2 and ṁ3 is only a small fraction of the steam passing through the turbine.
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In addition to a condensate pump, one pump for each open feed water heater is required. This is a disadvantage.
Open feed water heaters because of vigorous action of mixing remove non-condensable gases like Air, O2, H2, CO2 from the feed water. They are therefore known as de-aerating heaters or DA.
Mass Flow Rates and Energy Balance:
This is done on the basis of unit mass flow rate at the turbine inlet (point 1).
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The mass balance therefore is as follows:
For energy balance, consider the low and high pressure feed water heaters. At low pressure feed water heater, heat lost by condensing steam equals heat gained by feed water heater, or,
Type # 2. Closed Type Feed Water Heaters with Drains Cascaded Backward:
This is the simplest and most commonly used type in power plants. It is a shell and tube type heat exchanger. The feed water passes through the tubes, and the bled steam, on the shell side transfers its heat energy to it and condenses. Thus, they are like surface condensers, operating at higher pressures than those in the main condenser.
Since feed water does not mix with the steam and passes through the tubes of successive close feed water heaters, only one condensate pump is sufficient to pressurise the water from condenser to the boiler. However, to reduce the pressure rise in one pump, one condensate pump before the feed water heaters and one feed water pump after the feed water heaters are used. If deareater heater is used, one boiler feed pump is automatically required after the same.
Figure 22.4 shows a simplified flow diagram and corresponding T-s diagram of a non-ideal superheat Rankine cycle. There are two feed water healers. One pump 5-6 pressurises the condensate, sufficient to pass through the two feed water heaters and enters the steam generator at 8.
The bled steam condensed at each FWH is fed back to the next lower pressure FWH. The condensate from the last fwh is led back to the main condenser. One can imagine, then, a cascade from high pressure to lower-pressure heaters. Hence, the name.
Wet steam bled at 3 at the rate of ṁ3 enters the low pressure FWH and heats the subcooled water at 6. The temperature of water at 7 is less than saturation temperature at 3. This difference is called the Terminal Temperature Difference (TTD) and is defined for all closed FWHS.
TTD or TD = saturation temperature of bled steam – exit water temperature
The value of TTD for low pressure heaters is positive and is of the order of 3°C. For high pressure heaters it may be negative and lies between 0 and – 3°C. In case no data is available, temperature at 7 is taken equal to saturation temperature at 3.
The drain from low pressure FWH is led to the main condenser and enters it as two phase mixture at 10. The process 9 – 10 is throttling process from the pressure corresponding to 9 and the main condenser pressure. Due to throttling there is some loss of availability. Also enthalpy at 9 and 10 are equal, as the process is constant enthalpy one.
The high pressure FWH receives superheated steam bled from the turbine at 2 that flows on the shell side at the rate of ṁ2 and heats the water entering the tubes at 7. The inlet steam at 2 is superheated, hence temperature at 8 may be higher than saturation temperature of that steam and TTD may be negative. The drain at 11 may be slightly subcooled; and is throttled to the low pressure fwh which it enters at 12 as a two phase mixture. There it joins with the steam bled at 3 and aids the heating of water in low pressure FWH. The drain from low pressure fwh is thus ṁ2 + ṁ3, which is throttled to the main condenser at 10. The higher pressure exit water at 8 is led to the steam generator.
Mass Flow Rate and Energy Balance:
A mass balance based on unit flow rate at turbine inlet point 1, is given clockwise by –
From mass flow rates, energy balance equations and enthalpy values, we can determine the amounts of steam extracted i.e., ṁ2 and ṁ3, and then the pertinent cycle parameters. We assume mass flow rate of 1 kg/s at turbine inlet (point 1).
3. Closed Type Feed Water Heaters with Drains Pumped Forward:
This is a second closed type FWH. It avoids throttling and the irreversibilities therein but has to use a small additional pump. Like previous closed type FWH, it is also a shell and tube type heat exchanger. The feed water passes through the tubes the steam passes over them; transfers heat and condenses. They do not mix and only one pump is sufficient to carry water from the condenser to the boiler. If, however, DA is used one more pump would be necessary.
The drain from these FWHS is pumped forward into the main feed water line. Figure 22.5 shows a simplified flow diagram and corresponding T-s diagram for a non-ideal superheat Rankine cycle with two such FWHS. Like open FWH these heaters also require an additional pump per heater. But the pump is small and carry only fractional flows corresponding to ṁ2 and ṁ3 while pumps in open FWH carry full steam and water flow.
The drain of low pressure FWH at 13 is pumped forward to the main feed water line and enters it at 14. It mixes with exit water from l.p. heater at 7 and forms mixture at 8. Point 8 is closer to 7 than 1.4 on the T-S diagram because flow at 7 is much greater than the drain flow ṁ3.
The water at 8 enters h.p. heater and is heated to 9. The drain leaves the heater at 11, is pumped to 12. It mixes with the feed water at 9 to form full feed water flow at 10, which crow goes to boiler.
Mass Balance and Energy Balance:
A mass balance based on a mass flow rate of 1 kg/s at turbine inlet (point 1) is given clockwise on T-S diagram by –
