The controllers that have been devised for maintaining a constant vacuum without the introduction of air into the system operate on one of three principles:
1. Closing the suction of the vacuum producer.
2. Opening the suction of the vacuum producer and holding vacuum in the system.
3. Varying the speed of the vacuum producer.
FIG. 99. FIRST TYPE OF CONTROLLER INTRODUCED BY THE SANITARY DEVICES MANUFACTURING COMPANY, KNOWN AS THE “UNLOADING VALVE.”
The first type of controller was introduced in the vacuum cleaning field by the Sanitary Devices Manufacturing Company, and was known as the “unloading valve.” It was similar to the unloader which had been used for some time in connection with air compressors. The detail of construction is shown in [Fig. 99], and consists of a balanced valve, which is connected to a weighted piston, operating in a chamber communicating with the separators by a pilot valve. The pilot valve is operated by an auxiliary piston which is weighted to overcome the lifting effort due to the vacuum desired. When the vacuum in the cylinder becomes great enough to overcome the weights attached to the auxiliary piston, it rises, allowing vacuum to reach the main piston, which is drawn up and the suction valve closed. When this valve is closed the vacuum in the pump at once starts to build up to the maximum possible for the pump to produce, and if the pump used is of the piston type the vacuum will run up to nearly 28 in., resulting in the pump’s taking the least power on which it can be operated. As soon as the vacuum in the separators falls below that which will sustain the weight on the auxiliary piston the valve falls open and the pump again draws air through the system. In actual practice this valve will operate at more or less frequent intervals. The author timed the action of one of these valves connected to the suction of an eight-sweeper piston pump, and its time varied from ²⁄₅ second to 65 seconds. The current taken by the pump when the suction was open was 100 amperes at 220 volts. When the valve was closed for but ²⁄₅ second the current dropped to 75 amperes, there not being sufficient elapsed time for the pump to produce a perfect vacuum. When the valve was closed for 2¹⁄₅ seconds, the vacuum reached its maximum value and the current fell to 32 amperes.
[Fig. 100] is a curve plotted from the results of this test and shows an increase in the power above that necessary to overcome the friction in the moving parts of the pump in direct proportion to the percentage of full load that the pump was serving.
FIG. 100. TEST OF CONTROLLER CONNECTED TO SUCTION OF 8-SWEEPER PISTON PUMP.