Subsection 7.2.3 Condensation
As stated previously, a large pressure drop across the turbine is required to get maximum energy extraction from the steam. The boiler supplies the high pressure at the inlet to the turbine, it is the job of the condenser to create a low pressure condition at the turbine outlet.
The condenser is a shell-and-tube heat exchanger cooled by seawater. It consists of a large, rectangular or cylindrical sealed shell containing thousands of tubes. A circulating pump forces cold seawater in at one end, through the tubes, and it exits overboard at the other end. Steam exiting from the turbine enters the condenser at the top, condenses on the tube surfaces and drains out the bottom. That is, the steam gives up its latent heat of condensation to the seawater, and returns to the liquid water state. Once the steam passes across the condenser tubes, it is no longer steam. It has condensed to liquid water, and from this point in the cycle is referred to as condensate. Condensate leaves the condenser as a low temperature, low pressure liquid.
Because the condensation is occurring at approximately the temperature of the seawater, the pressure in the condenser will be approximately equal to the saturation pressure of steam at this temperature, assuming that no air or other gases are present. For example, if the seawater temperature is 80 °F, the corresponding saturation pressure would be 0.5073 psia. This is a very high vacuum, and the greater the vacuum, the better the plant efficiency.
The latent heat removed from the steam by the seawater raises the temperature and increases the energy contained by the seawater, and this cooling water is discharged overboard.
It is unfortunate that some of the energy once contained by the fuel, and then passed on to the steam in the boiler, is eventually thrown away with the seawater. Wouldn’t it be better to somehow use this energy to power the ship? Absolutely, but there is nothing we can do about it! The only way to extract more energy from the steam is by having the turbine discharge to an even lower absolute pressure than we do. The only way to do that is to condense the steam at even lower temperatures than we are. But we can’t do that, because aboard ship there’s nothing colder than the surrounding ocean available to reject the heat to. If we decided not to reject any heat to the ocean at all, the pressure in the condenser would build up to the point where it would equal the steam pressure entering the turbine, and without a pressure drop, the steam would stop flowing altogether. There’s no win, but we’re doing the best we can.
