Regenerative feed heating

For a comprehensive discussion on feed heating in a CCGT plant, readers may refer to Kehlhofer s excellent practical book on CCGTs [2] a summary of this discussion is given below. 

Kehlhofer explains that the pre-heating loop must be designed so that the heat extracted is. sufficient to raise the temperature of the feed water flow from condenser temperature T to Ta (see Fig. 7.6). The available heat increases with live steam pressure Ipf), for selected 7 b(= Ta) and given gas turbine conditions, but the heat required to preheat the feed water is set by (Ta — T. ). The live steam pressure is thus determined from the heat balance in the pre-heater if the heating of the feed water by bled steam is to be avoided but the optimum (low) live steam pressure may not be achievable because of the requirement. set by this heat balance. 

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E3 CONDENSING LOSSES BOILER LOSSES r-i REGENERATIVE FEED HEATING 03 AIR PREHEATER... 

BOILER LOSSES n REGENERATIVE U fEED HEATING HU AIR PREHEATER... 

Determine the efficiency and power output of a regenerative Rankine cycle using steam as the working fluid and a condenser pressure of 80 kPa. The boiler pressure is 3 MPa. The steam leaves the boiler at 400° C. The mass rate of steam flow is 1 kg/sec. The pump efficiency is 85% and the turbine efficiency is 88%. After expansion in the high-pressure turbine to 400 kPa, some of the steam is extracted from the turbine exit for the purpose of heating the feed-water in an open feed-water heater, the rest of the steam is reheated to 400°C and then expanded in the low-pressure turbine to the condenser. The water leaves the open feed-water heater at 400 kPa as saturated liquid. Determine the steam fraction extracted from the turbine exit, cycle efficiency, and net power output of the cycle. 

Figure 2. Simplified schematic drawing of the extraction plant (1 regenerative pump, 2 fluid cyclone, 3 storage tank, 4 gear pump, 5 circulation gas condensator, 6 diaphragm pump, 7 heat recovery, 8 preheater, 9 heat exchanger, 10 cyclone separator, 11 feed pump, 12 feed preheater, 13 thermostatic chamber)...

In this ideal regenerative Rankine cycle, the steam extracted from the turbine heats the water from the condenser, and the water is pumped to the boiler. Sometimes, this occurs in several stages. The condensate from the feed water heaters is throttled to the next heater at lower pressure. The condensate of the final heater is flashed into the condenser... 

For large-capacity plants with subautogenic feeds, regenerative heat recovery is very important to minimize the need for supplementary fuel and/or additional oxidant. 

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