Vacuum Pump Speeds

Question: My question relates to vacuum pumps and pumping speeds. I have always read that a vacuum pump with a higher CFM rating will evacuate a system quicker.

I believe that the CFM ratings for vacuum pumps are usually given in “free air” CFM. When evacuating a system, the pumps should spend very little time actually pumping in “free air” conditions. I would think that the evacuation time for different size pumps should be similar and would be more dependant on system conditions, the pump's ultimate blank-off-pressure, and its ability to maintain a pressure differential between itself and the system being evacuated. What am I missing here?

-Robert Taylor CM, Sunnyvale, CA

Answer: All things being equal, larger CFM pumps will have a faster pumping speed. However, your assumptions on vacuum pump performance are fairly accurate. Many factors affect “pumping speed.” Among them are ultimate vacuum rating, condition of the pump oil, and all the other equipment used for an evacuation.

The ultimate vacuum achievable by the pump has a direct effect on pumping speed. During an evacuation, “free air” pumping time is very short. Most of the pumping time is spent in the micron range. As a result, pumping speed in the micron range is more important than free air capacity. To illustrate, a two-stage pump will achieve a deeper micron reading than a single-stage pump. Compared to a single-stage pump, a two-stage pump will maintain a greater pressure differential in the micron range, resulting in faster pump down. This is true even if the single-stage pump has a larger CFM rating.

Pumping speed is influenced by the quality and condition of the vacuum pump oil. Low quality oil will greatly reduce the pumping speed in the micron range. Contaminated oil may prevent complete evacuation of the system. Clean, high quality vacuum pump oil is required to achieve the best blank-off performance, which will result in the maximum pumping speed.

All the equipment used to evacuate a refrigeration or air conditioning system has an effect on pumping speed. In practice, the speed advantage of larger CFM pumps is greatly reduced if the charging equipment is undersize. The size and length of hoses, valve orifices, fittings, and Shrader valves are equal to or more important than the CFM rating of the vacuum pump. If the hoses are too small or too long, the flow is impeded. Shrader valve access ports, with the valve core in place, have the flow characteristics of a 1/16” hole, roughly equal to the pumping speed of a 1 CFM pump. The best way to assure maximum pumping speed is to make sure that the smallest orifice of all the evacuation equipment is equal in size to the pump intake fitting. While this is not always possible, larger equipment will result in faster pump down times.

The above factors are some of the elements that dictate overall pumping speed. The use of a larger CFM pump will not overcome poor ultimate vacuum pump performance, low quality, contaminated vacuum pump oil, or undersize plumbing.

-George Marvin, Marvac Scientific