Standard re cipro in tin-type air compressors do not create a vacuum, which is high ("deep") enough to evacuate service systems. No conventional refrigeration compressors designed to create the necessary deep vacuum. There are two main types of vacuum pumps, single and dual stage.
Single-stage vacuum pumps are used in triple evacuation method works. The two-stage vacuum pumps are used in die deep vacuum (high vacuum) method is used. There are two designs in high-vacuum pumps:
Circulation pump (oil-sealed).
Steam pump (diffusion type).
Circulation pump should be able to pull 50 micron " vacuum-pressure-vacuum. (50 micron vacuum is equal to 0.05 Torr. In Torr equal to 1 mm) Circulation pump uses two rotors in the series (pump connection). Most of refrigeration compressors pull about 50 Torr to 80 Torr (50 000 microns up to 80 000 microns or 50 mm to 80 mm). Evacuate, do the following;
Extracting the air with a low-vacuum pump (50 Torr to 80 Torr).
Switch high vacuum pump.
Repeat the pull-down to 50 microns, growth (in three minutes or more) to 300 microns indicates that the cooling system is dry and evacuated.
The use of copper pipes or special metal hoses for connection of the vacuum pump.
If the standard of synthetic charging and maintenance hose sections may fall on the high vacuum. In addition, a synthetic material of tube passes will allow the passage of gases).
Portable two-stage high-vacuum pumps. Attract less than 1 micron of the mercury column, micron is equal to 1/1000 of a millimeter (0.001 mm). One inch is equal to 25.4 mm Thus, 25 400 micron is 1". One micron is near perfect vacuum.
High vacuum pump produces vacuum lower than the 29" Hg (23 mm, or 23 000 microns, see Fig. 12-98). Vacuum cleaners lower than the 29" are necessary to fully dihydrate (removing moisture from the system.
Industrial 
..