It is a common practice in the electric utility industry to utilize multiple stages of shell and tube heaters to preheat feedwater going to the boiler. The challenge has always been transferring as much heat as possible from the steam inside the shell to the water in the tubes. The mechanical design of the heaters helps this by providing baffles to prevent steam from exiting the heater rather than condensing. Heaters cascade from the highest shell pressure being the last in the train to the lowest being the first. There is also one heater called a deaerator that is an open heater between the low-pressure and high-pressure closed heaters where steam mixes with feedwater and any entrained air is removed. The steam for heating feedwater comes from extraction stages of the turbine and from the drain of the next higher-pressure heater.
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