Another form of coupling used by the Spencer Turbine Cleaner Company is the all-rubber male and female end, as illustrated in [Fig. 47]. This has the advantage over the metal-slip couplings and the coupling with pure gum ends in that when it is properly locked it cannot be pulled apart. It is absolutely air tight, which is true of no other coupling. But it does not permit the reversal of the hose and is, therefore, recommended for use only with hose of 1¹⁄₄-in. diameter or larger, where there is less liability of stoppage, and where the ball-bearing swivel is used at the connection to the stem, preventing excessive wear at this point. The pure gum ends, with the internal-slip coupler, is considered to be the most satisfactory for use in all cases, except as above stated.
Hose Friction.
—Hose friction plays an important part in the action of any vacuum cleaning system. In fact, where 1-in. hose is used, it becomes a limiting factor in the capacity of the system to perform some kinds of cleaning.
There are several tables of hose friction published by the manufacturers of vacuum cleaning systems, all of which appear to have been based on a constant velocity within the hose equal to that which would be obtained if the air were at atmospheric pressure throughout the entire length of the hose. But in practice the air is admitted to the hose from the renovator at a considerably lower absolute pressure of from 25 in. to 27 in. of mercury, and is, therefore, moving at a higher velocity. As the pressure is decreased by the friction loss in the hose, the velocity constantly increases with the expansion of the air.
The results of many tests made by the author during the past seven years, with hose ranging from 1-in. to 2-in. diameter and with an entering vacuum ranging from 0 to 7 in. of mercury and a friction loss of from 1 in. to 25 in. of mercury, indicate a close agreement with the formula given in Prof. William Kent’s “Mechanical Engineer’s Pocketbook,” which is based on the formula:
Q = c √ pd⁵wL
Q = free air in cubic feet per minute.
c = a constant which was determined by D’Arcy as approximately 60.
p = the loss of pressure in pounds per square inch.
d = the diameter of pipe in inches.