MARINE ENGINES.

429

water simply finds its way into the ship, and the working of theengines is not sensibly affected. Looking, however, at all sides ofthe question, we may conclude that the advantages are in favour ofinside condensation,—provided that the circulation of the water isproperly attended to and uniformly distributed all round theexterior surfaces. When the surface System of condensation wasfirst introduced into side lever engines for the Royal Navy , 2800square inches of tube surface was adopted for the condensation of60,000 cubic inches of steam per minute, the quantity of cold waterinjected being 10 gallons. To compare these quantities with pre-sent practice, let us take an example of an engine of 400 nominalhorse-power, having 3170 square inches of area in each cylinder,with a piston speed of 300 feet or 3600 inches per minute:

3170 X 2 X 3ÖOO_ 3S0 X 2800 _

60000 — 144

We thus have 7389 square feet of tube surface, equal to i 8'4 squarefeet per nominal horse-power. It will be observed that this resultis about the same as that given for total heating surface of boilerper nominal horse-power, although it is in excess of present prac-tice, 15 to 16 square feet of condensing tubes being now consideredsufficient. To find the quantity of water required for condensation:a cylindrical foot of water equals 5 gallons, 10 gallons will be con-tained in 3456 circular inches, and as 380 times 60,000 cubic inchesof steam passes the engine per minute, we have

3456 x 3 80 = 364 4- 2 - 1S23600

circular inches of area for each pump, when two pumps of thedouble-action type are fitted—or a diameter of say 14 inches will beenough for each of the two circulating pumps. (See pages 508-510.)

The air pumps are generally made of the same capacity as forplain injection condensers, and when one circulating pump is fitted,it is of the same capacity as the air pump; one set of patterns thusserves for both, and in the event of using the condenser with plaininjection, valves are so arranged that the circulating pump can beused for an air pump. The circulating pump should be fitted witha valve for turning on the bilge water, in case of great leakage inthe ship; a valve must also be placed on the pipe for shutting offthe sea-water. Should both of the pumps be used as air pumps(in cases of failure), a bilge injection valve should be fitted. When