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Adeaerating feed tank (DFT), often found in steam plants that propel ships, is located after the main condensate pump and before the main feed booster pump. It has these three purposes:
Based on the relevant theoretical Rankine cycle diagram, there are four main processes, or stages:
In the practical implementation of a Rankine cycle, it is common to break the pump process (stages 1→2) into three pumps: (in water flow order: condensate pump, feed booster pump and then feedwater pump).
The dearating feed tank's surge volume allows the ship to change "bells" (steam turbine power output) and change the ship's speed without running the feed pump dry or flooding the turbines with liquid water.
When the officer in chargeonship's bridge orders an increased bell, the steam turbine power output is demanded, using more steam and requiring an increased feed rate. This draws more water from the condenser, potentially to the point of going dry and starving the boilerorsteam generator resulting in a loss of propulsion. This is until the water, converted to steam, provides its energy to the turbine and then is condensed in the condenser.
When the bell is decreased, turbine power output is reduced and the feed rate drops. Without the DFT's surge volume, less water is drawn from the condenser, the condensate level rises, potentially covering condenser tubes and reducing the ability of the condenser to maintain vacuum. If the level is allowed to go high enough, vacuum could be lost and/or water could impinge (and damage) the turbine blades as the turbine normally sits directly above the condenser. The feed tank serves as a surge volume to take this excess condensate and avoid losing vacuum.