Vacuum Pumps

Vacuum pumps often do not get much attention, yet they are a vital component of the lyophilizer -critical to the freeze drying process- and are a necessary maintenance tool as well.

To help maintain proper vacuum pump operation:
1. Determine the condition of the vacuum pump oil. Taking a small oil sample from the pump drain in a clean container (white Styrofoam or plastic cup) will allow you to see if there is any contamination in the oil.
2. Maintain a proper level of oil in the reservoir. If there is too much oil the vacuum pump has no place to exhaust the vapors. If there is too little oil in the reservoir there is no oil for sealing.
3. Before using any vacuum pump, the pump should be preheated to at least 140 °F to 160 °F. That means the pump should be started at least 30 minutes before it is opened to the system. If the pump is not pre-heated, any contamination will stay in the pump and oil and not be exhausted.
4. A good procedure is to verify the blank-off pressure of the pump before it is needed. The blank-off pressure is read with the pump isolated from the system and should be at least 50 millitorr below the required operating pressure. To do this a vacuum sensor will have to be installed on the pump.
5. Every permanently mounted pump should have an exhaust piping drop leg that can be drained. Any contamination exhausting from the pump will collect in the drop leg rather than recontaminating the oil.
6. Any visible oil around the vacuum pump is a sign that oil is leaking out, so atmosphere could be stealing efficiency.
7. A big source of vacuum pump contamination is water:
   • 1 Drop of water is equal to 1.3 cubic feet of vapor
   • 25 Drops of water in one teaspoon of water
   • 1 Teaspoon of water is equal to 33 cubic feet of vapor

Vacuum pumps are also used in freeze dryer maintenance to evacuate refrigeration systems before charging. This evacuation is to remove any contamination, such as moisture or noncondensables, that might be in the refrigerant loop. The same attention to vacuum pump performance is required for this evacuation step to be successful.

The pumping rate and blankoff pressure of a vacuum pump used for refrigerant system evacuation is critical. Any pump with a rate less than 4 cfm, or a blankoff pressure higher than 500 microns, is not going to do an adequate job.

For more on refrigerant contamination, see our October, 2011, Tech Tip.