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A Refrigerator Compressor Vacuum Pump [2010-02-06]

A Refrigerator Compressor Vacuum Pump


A simple but effective system can be had by modifying the compressor unit from a used refrigerator. In most communities operable units can be purchased from appliance dealers at prices ranging from $5 to $10. F. B. Lee, a member of the faculty at the Erie County Technical Institute in Buffalo, N.Y., has investigated a number of makes and reports that three lend themselves to vacuum work: Frigidaire, Norge and Coldspot.


"Of these," he writes, "the Norge Rollator belt-driven unit rates best for vacuum produced. Frigidaire rates best for availability and second best for two-stage service below .020 millimeter of mercury. (Atmospheric pressure at sea level supports a column of mercury 760 millimeters high in a tube closed at the top.) The Coldspot, though unsuitable for pressures below .5 millimeter, is superior to the Frigidaire as a single unit.


"The modifications are not difficult. Those required to convert the Frigidaire 'Meter-Miser,' the unit that has been standard on this company's domestic refrigerators since 1936, are illustrative. The smaller refrigerators contain split-phase motors rated at less than 1/7 horsepower. The Imperial or Cold Wall series and all refrigerators larger than 135 cubic feet contain capacitor motors. The purchaser is advised to procure the capacitor as well as the compressor. The motor is not self-starting; it is therefore advisable to procure the starting relay as well. If this is not available, one may improvise a starter from a push-button switch. The motor is started by applying power to terminals 1 and 3 in the accompanying drawing [Figure 2] and short-circuiting terminals 2 and 3 (by means of the relay or push button) for a period of about four seconds.


"The pump is modified in three respects. The bypass line which runs between the housing and the check valve must be cut off and the ends sealed. The pump will then produce a vacuum of one millimeter if the check valve is open and the strainer is not wet with oil. The check valve opens automatically when the pressure in the system is above three millimeters. The pressure will not drop below 10 millimeters if the strainer is wet with oil. Oil can be removed from the strainer and the check valve opened by permitting air to flow through the pump for a few minutes prior to connecting the unit to the vacuum system. The screen may be removed, but great care must be exercised thereafter to prevent dirt or foreign material from entering the pump. To make this modification, cut the intake line about an inch away from the housing. Use a tube cutter, not a hacksaw, or particles from the saw will almost certainly find their way into the pump and cause it to stall. Bend the cut tube out of the way, then dig the strainer from the opening by means of a small hook made from a nail or steel wire. Inspect the opening carefully and remove all stray wires of the screen by means of tweezers. Cap the opening with a short length of rubber tubing and close the end with a pinch clamp.


"The pump operates best when tilted at an angle of 10 or 15 degrees, as shown in the drawing of the system [Figure 3]. The line to the oil trap should be pitched upward away from the pump to prevent the formation of oil pockets that would impede the free flow of air. The trap can be made from a quart milk-bottle.


"In addition to the compressor and oil trap, the system includes a dirt trap made from a half-gallon glass jug, and a pair of vacuum reservoirs, each a gallon glass jug. As a safety measure all the jugs are housed in wooden boxes to catch fragments in the event that atmospheric pressure shatters the glass. The various units of the system are interconnected by 3/8-inch copper tubing, perforated rubber stoppers and couplings of rubber hose. Five of the hose couplings are equipped with pinch clamps and act as valves as shown.


"To operate the system, first connect the vessel to be evacuated and close the clamp between the exhaust port [knife cut in rubber tube in illustration in Figure 2] and the rest of the system. Then open all the other clamps, and start the pump. This will reduce the pressure of the entire system, including that in the vacuum reservoirs, to about one millimeter. Now the clamp between the two reservoirs is closed, and operation is continued for about five minutes with the clamp between the pump exhaust and the reservoirs open. This has the effect of connecting the input of a second compressor to the exhaust port of the first, one vacuum reservoir serving as the added compressor. The clamp between this reservoir and the exhaust port of the pump, and the clamp between the reservoirs and the line leading to the oil trap, are now closed. The clamp between the reservoirs and the exhaust port of the pump is opened. This permits the system to exhaust into the second reservoir, now the one of lower pressure. With continued operation the pressure will then fall to the limit of the system's capacity. The compressor can operate for a whole day without increasing the pressure in the reservoir more than one or two millimeters."