325. Q.—­Are air pumps now sometimes made double acting?

A.—­Most of the recent direct acting marine engines for driving the screw are fitted with a double acting air pump, and when the air pump is double acting, it need only be about half the size that is necessary when it is single acting.  It is single acting in nearly every case, except the case of direct acting screw engines of recent construction.

326. Q.—­What is the difference between a single and a double acting air pump?

A.—­The single acting air pump expels the air and water from the condenser only in the upward stroke of the pump, whereas a double acting air pump expels the air and water both in the upward and downward stroke.  It has, therefore, to be provided with inlet and outlet valves at both ends, whereas the single acting pump has only to be provided with an inlet or foot valve, as it is termed, at the bottom, and with an outlet or delivery valve, as it is termed, at the top.  The single acting air pump requires to be provided with a valve or valves in the piston or bucket of the pump, to enable the air and water lying below the bucket when it begins to descend, and which have entered from the condenser during the upward stroke, to pass through the bucket into the space above it during the downward stroke, from whence they are expelled into the atmosphere on the upward stroke succeeding.  But in the double acting air pump no valve is required in the piston or bucket of the pump, and all that is necessary is an inlet and outlet valve at each end.

337. Q—­What are the dimensions of the foot and discharge valves of the air pump?

A.—­The area through the foot and discharge valves is usually made equal to one fourth of the area of the air pump, and the diameter of the waste water pipe is made one fourth of the diameter of the cylinder, which gives an area somewhat less than that of the foot and discharge valve passages.  But this proportion only applies in slow engines.  In fast engines, with the air pump bucket moving as fast as the piston, the area through the foot and discharge valves should be equal to the area of the pump itself, and the waste water pipe should be of about the same dimensions.

328. Q.—­You have stated that double acting air pumps need only be of half the size of single acting ones.  Does that relation hold at all speeds?

A.—­It holds at all speeds if the velocity of the pump buckets are in each case the same; but it does not hold if the engine with the single acting pump works slowly, and the engine with the double acting pump moves rapidly, as in the case of direct acting screw engines.  All pumps moving at a high rate of speed lose part of their efficiency, and such pumps should therefore be of extra size.

329. Q.—­How do you estimate the quantity of water requisite for condensation?