Condensing Surface Required. In the early days of the surface condenser it was thought necessary to provide a cooling surface in the condenser equal to the heating surface in the boilers, the idea being that it would take as much surface to transfer the heat from a pound of steam to the cooling water and condense the steam as it would to transfer the heat from the hot gases to the water in the boiler and convert it into steam. The difference in temperature, too, between the hot gases and the water in the boiler is considerably greater than that between the steam in the condenser and the cooling water.
Note.—The following list gives the numbers with the corresponding names of the parts of the surface condenser, shown in the above outline sketch: 1, condenser shell; 2, outside heads; 3, exhaust inlet; 4, exhaust outlet; 5, water inlet; 6, water outlet; 7, peep holes; 8, tube heads; 9, partition; 10, rib; 11, tubes; 12, stuffing-boxes.
Note.—The numbers and names of parts in the above figure, representing in outline a jet condenser, are as follows: 1, condenser body; 2, exhaust inlet; 3, discharge; 4, injection valve; 5, spray pipe; 6, spraying device.
Steam, however, gives up its heat to a relatively cool surface much more readily than do the hot furnace gases, and the positively circulated cooling water takes up that heat and keeps the temperature of the surface down, while in a boiler the absorption depends in a great measure upon the ability of the water by natural circulation to get into contact with the surface and take up the heat by evaporization. It has been found, therefore, that a much smaller surface will suffice in a condenser than in the boilers which it serves.
The Wheeler Condenser and Engineering Company, who make a specialty of surface condensers, say that one square foot of cooling surface is usually allowed to each 10 pounds of steam to be condensed per hour, with the condensing water at a normal temperature not exceeding 75°. This figure seems to be generally used for average conditions. Special cases require special treatment.
For service in the tropics the cooling surface should be at least ten per cent. greater than this estimate. Where there is an abundance of circulating water the surface may be much less, as with a keel condenser, where 50 pounds of steam is sometimes condensed per hour per square foot of surface; or a water works engine, where all the water pumped is discharged through the condenser and not appreciably raised in temperature, probably condensing 20 to 40 pounds of steam per hour per square foot of surface.