Non-condensing or high-pressure engines are less economical than condensing or low-pressure engines, because they use much more steam. When the waste steam is let out of the cylinder, the air rushes in and takes its place. This air presses hard against the piston so that it takes power to drive it down.
After the steam is condensed in the condensing engine there is a vacuum, or an empty space, on one side of the piston, so that but little fresh steam is necessary to drive it. Thus the object of condensing is to do away with the back pressure on the piston and thereby increase the mean effective pressure. There is a gain of 20 to 33 1/3% in economy, depending on the size and type of engine. In small engines the saving is not enough to be considered.
Where fresh water is scarce, it is of great importance to the marine engineer to condense the steam by leading it into a condenser when it has finished moving the piston. In this process the steam as it leaves the cylinder enters a condenser and passes over a number of copper tubes, through which sea water is circulated by means of a pump. The steam is thus condensed into water and a vacuum is created. Since this water is warm, it is pumped into a hot-water well, whence a pipe leads it to a pump, which in turn carries it back to a boiler.