It is difficult to lift hot water by suction, but not everyone can explain the cause; the reason is as follows:
Figs. 673 and 674.
In Fig. [674], let A be a vessel in which a vacuum exists, and let it communicate by a tube, as shown, to the lower vessel containing water. The pressure of the atmosphere upon the surface will force the water up into the pipe until the column is high enough to exert a pressure per square inch equal to that of the atmosphere. A cubic inch of water weighs about 1⁄28 of a pound, so that it will take 28 cubic inches to weigh a pound, or a column 28 inches high to exert a pressure of one pound per square inch. The atmospheric pressure is 14.7 pounds, or to avoid fractions, say, 15 pounds. This pressure would then support a column (15 × 28)/12 = 35 feet high; that is, the column a-c in Fig. [674] would be 35 feet in height. Attach a gauge at or above the top, a, of a column, and it will indicate a perfect vacuum; if the gauge were attached 28 inches below a, it would indicate a pressure of one pound above absolute zero, or a vacuum of 15-1 = 14 pounds; and if the gauge were moved further downward, it would indicate an increasing pressure, that is, a diminishing vacuum, at the rate of one pound for every 28 inches of the water column above it, until at the level of the water in the tank the pressure would be 15 pounds absolute, and the vacuum would be zero.
Now, suppose the pipe to be lowered until the distance from the bottom of the vessel, A, to the water level, c, is 21 feet. In that case we will have the pressure of the atmosphere (15 pounds) forcing the water up into the vessel, and the column 21 feet high, or (21 × 12)/28 = 9 pounds, opposing it. The difference, 15-9 = 6 pounds, is available to force the water into the chamber. This arrangement is shown in Fig. [673], where A is a pump cylinder; then the difference in pressure, 6 pounds, lifts the valve, and the water enters the pump chamber with a velocity due to that pressure. In order to insure smooth and quiet running of the pump, it is necessary to keep the speed of the piston inside of the velocity with which the cylinder would fill under this pressure, reduced by the friction of the water, the pressure required to lift the valves, etc.
But this supposes that there is a perfect vacuum in A, and we cannot realize this in contact with hot water. Water at any temperature will boil unless it is under a pressure equal to or greater than that corresponding with the temperature. Water at 60 degrees F. will boil if the pressure upon its surface is reduced to a quarter of a pound per square inch, and in the case shown in Fig. [674], it would boil and fill the space A with steam at that absolute pressure.
Note.—Water at 170 degrees F. will boil if its pressure is reduced below 6 pounds absolute, and if the water were at this temperature in Fig. [673], the cylinder, A, would be filled with steam at 6 pounds pressure; and this added to the 9 pounds pressure of the column would completely balance the atmospheric pressure, and the water would not rise above the level, A.