Vacuum pumps are component of one of the final procedures in the repair process. After you’ve diagnosed and repaired any leaks or other issues in the system it’s time to recharge. Before recharging you need to make sure the system is free from any unwanted gasses, air, or water. If these were to remain in the machine, it could turn the oil it runs on into sludge and keep behind damaging impurities.
The draw on a vacuum pump is usually divided into its “CFM,” or Cubic Feet each and every minute. To estimate what your CFM requirements, consider the tonnage of the Ac Vacuum Pump systems you work on. Take the square root of that number and you’ll obtain the approximate CFM you need to 
have to work with that system. Typical home systems need 4-5 CFM, while commercial and rooftop systems generally require just as much as 6-8 CFM.