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?
_A._--Mr. Watt found that the most beneficial temperature of the hot well of his engines was 100 degrees. If, therefore, the temperature of the steam be 212°, and the latent heat 1,000°, then 1,212° may be taken to represent the heat contained in the steam, or 1,112° if we deduct the temperature of the hot well. If the temperature of the injection water be 50°, then 50 degrees of cold are available for the abstraction of heat; and as the total quantity of heat to be abstracted is that requisite to raise the quantity of water in the steam 1,112 degrees, or 1,112 times that quantity one degree, it would raise one fiftieth of this, or 22.24 times the quantity of water in the steam, 50 degrees. A cubic inch of water therefore raised into steam will require 22.24 cubic inches of water at 50 degrees for its condensation, and will form therewith 23.24 cubic inches of hot water at 100 degrees. Mr. Watt's practice was to allow about a wine pint (28.9 cubic inches) of injection water, for every cubic inch of water evaporated from the boiler.