Refrigeration Service Vacuum Pumps
Vacuum pumps that are sold for the servicing of refrigeration systems offer good performance at reasonable cost.
The full version of this article appeared in Volume 4, Number 1 of the Bell Jar.
While commercially available mechanical pumps provide an optimal solution in terms of speed, reliability, and ultimate performance, they are also relatively expensive even in used or rebuilt condition. For the amateur with modest requirements, or for someone who is just beginning to experiment with vacuum apparatus, there is a good alternative: the type of vacuum pump that is used in the refrigeration service trade for recharging refrigeration systems. (They should not be confused with the vacuum pumps that are incorporated within refrigeration systems.) With some slight modifications, they are well suited to the purposes of the vacuum experimenter and educator. Such pumps may be obtained at relatively low cost, have good vacuum capabilities, are fairly rugged, offer many features of industrial vacuum pumps and can reliably achieve pressures to 20 mTorr. They are also suitable for backing small diffusion pumps.
Two such pumps have been evaluated. One is a two-stage 4 cfm pump manufactured by Robinair. The other is a two-stage 3 cfm pump manufactured by J/B Industries. These pumps represent two of the more popular models and they are commonly available at local distributors who cater to the HVAC and appliance repair trade. Prices generally run in the $350 range but substantially lower prices may occasionally be had when the dealer has made a volume purchase agreement with the manufacturer.
Both pumps are direct drive and incorporate inlet shut-off and gas-ballast valves. Oil drains are conveniently located and the exhausts are directed through the lifting handle. As supplied, these pumps are designed to be used with small diameter refrigeration charging hoses and the inlet fittings are dual flare coupings. Such hoses (typically with an inside diameter of about 3/16") have a very low conductance and the only real modification needed is to make an adapter that can couple the pump to a hose of more reasonable diameter. The next section will describe an adapter that can be used to couple the pump to regular vacuum hose.
While these pumps have reasonably good throughput at the inlet, the hoses that are compatible with the flare fittings are quite effective at choking the pump. Why the manufacturers supply such skinny tubing is a mystery to me. The service tech undoubtedly feels that he is doing a great job because he has a high capacity 2-stage pump and the gauge, which is usually attached to the pump inlet, will read a nice high vacuum. Of course, the system being evacuated, which is what the tech should be caring about, is undoubtedly at a much higher pressure with the innards evolving water vapor like crazy.
A constructive exercise is to compare the conduction characteristics of a standard refrigeration hose (3 foot length, 3/16" id) to something more suitable in a small laboratory setup (a similar length of 5/8" id tubing). If you don't want to bother with the entire calculation, just remember that, in viscous flow and with all other factors being equal, conductance varies with the diameter of the tube to the fourth power. For the tubes we are comparing, the difference in conductivities (again, same pressure, same length) amounts to a factor of about 123. Figure 1 shows a simple fitting that can be added to the stock pump to permit the attachment of standard 5/8" id PVC or thick-wall rubber tubing. The main components are standard brass fittings that are available from any well stocked hardware or plumbing supply store. The required lathe work is non-critical and, lacking a small lathe, the ingenious experimenter can easily figure out some entirely satisfactory alternative.
After turning, join the two pieces with 2% silver-tin solder. The fitting goes on the top (larger) inlet port on the pump. In the case of the Robinaire, this is a 1/2" flare fitting. The J/B has a 3/8" fitting. With the O-rings, there is no need to really crank these fittings onto the pump. A gentle wrench tightening is all that's needed. The O-rings are from the hardware store's faucet fix-it section. Corresponding Moen part numbers are 14611 (1/2" fitting) and 14510 (3/8" fitting). Each pump has a 1/4" side-arm fitting with an O-ring sealed cap. This is useful as a vent valve.
Coping with Water Vapor
Contamination of the oil in any pump by water (or any other high vapor pressure liquid) will undo any attempt to achieve a good vacuum. The pump oil should be changed on a regular basis or after performing any experiments involving water or volatile solvents. Many pumps (these included) have a provision called a gas ballast which is very useful when pumping condensable vapors. The gas ballast is a valved arrangement by which atmospheric air may be admitted to the compressed gas in the exhaust stage of the pump just before the exhaust cycle. Diluting the moisture-laden air in this part of the pump prevents the vapors from condensing. Lowest pressures may not be attained while the gas ballast valve is open. The usual procedure is to start the pumping cycle with the gas ballast operating, then slowly close the valve as the vapors are removed and the pressure plateaus. The pump should then continue pumping to a lower pressure.
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©2008 Stephen P Hansen, the Bell Jar