In the early days of the steam-engine, very low pressure was ordinarily employed for engines with condensers, while, on the contrary, what was considered a very high pressure was adopted for engines that exhausted into the atmosphere. Hence arose the terms high and low pressure engines, the former being engines with, and the latter without, condensers. At present, a high pressure of steam is ordinarily carried in both kinds of engines, so that the terms do not describe the two varieties as well as formerly. Many engineers prefer to class engines as condensing and non-condensing, rather than as high and low pressure; and this classification is generally considered the more correct of the two. One who regards economy puts in a condensing engine, if he has plenty of water in the locality; and many old non-condensing engines are being fitted with condensers, under the more enlightened engineering practice of the present time.

It may be fairly assumed that a non-condensing engine has, on an average, at least 2 lbs. per square inch back pressure on the piston. By the application of a condenser, it might be expected that there would be a negative pressure of 10 lbs. per- sq inch on the back of the piston, so that the piston pressure would be increased by 12 lbs. In this assumption, an allowance is made for the power required to work the air-pump, and the engine is supposed to be at least To horse-power. For an engine smaller than this, it would be better to allow an increase in the positive pressure of not more than 10 lbs. per square inch. As the condenser, by decreasing the back pressure on the piston, adds just as much to the positive pressure, it is plain that a lower pressure of steam can be used, or the steam may be cut off at an earlier point of the stroke. The gain in either case can be approximately calculated. If the gain in positive pressure produced by the reduction in back pressure be multiplied by 100, and divided by the mean effective pressure on the piston, it will give the percentage of gain in pressure due to the condenser.

Thus, if the mean effective pressure on the piston is 30 lbs. per square inch, the gain in pressure will be 100 times 12, or 1200, divided by 30, which is 40 per cent. Now suppose that before the condenser was attached, the steam was cut off in the cylinder at half stroke; under the new conditions the required mean effective pressure can be obtained with a lower boiler pressure than before. Before the condenser was in use, it would be necessary to maintain a pressure in the boiler of about 58 lbs. per square inch by gauge, to give a mean effective pressure of 30 lbs. on the piston; while with an increase of 12 lbs. in the effective pressure, by the application of the condenser, a boiler pressure of about 39 lbs. would suffice. As the weight of steam per cubic foot at 58 lbs. pressure is 0.17481 lbs., and only 0.132 lbs. at 39 lbs. pressure, there would be a saving of about 24.5 per cent in the amount of steam required to run the engine. Instead of reducing the steam pressure after attaching a condenser to an engine, it might be better to maintain the same pressure in the boiler, and cut off the steam at an earlier part of the stroke. In the case under consideration, the increase in 12 lbs. of the effective pressure would permit of closing the steam port a little before the completion of one third of the stroke; and supposing that the clearance space in the cylinder amounts to 5 per cent of the capacity of the cylinder, the quantities of steam required per stroke, before and after the use of the condenser, would be in the ratio of 550 to 363, so that there would be a saving of 34 per cent.

The example given represents a case in ordinary practice. By varying t he data, of course a greater or less amount of saving would result; but with an engine in good condition, it is generally safe to estimate that a saving from 20 to 25 per cent of the amount of steam used, and, consequently, of the consumption of coal, will be realized by the application of a condenser. Indeed, it is not unusual for manufacturers to guarantee this amount of saving in converting a non condensing into a condensing engine.