Good question. The answer is different for everyone. What type of Business do you have?
Body shops, Mechanics shops, Granite companies, and Machine shops all have different needs.
The first thing you want to do is check your shop air system for leaks. Leaks can contribute to as much air as is being used in some cases. If there are leaks, you shoud fix them before installing a new compressor. If this isn't possible, for whatever reason, you will need to compensate by buying a larger compressor.
Compressor Location is important. You must be sure you locate the compressor in an area where, water, excessive heat, or heavy dust will not affect them.
Sizing is important. An improperly sized compressor may exhibit the following symptoms:
1 Excessive Heat
2. Low Air Pressure
3. Unit never unloads
4. High Temperature shut down
In Mechanics shops one rule of thumb is to take the number of shop employees you have and multiply that number by 5. THis is the average amount of CFM that a shop worker uses throughout the day.
Example- 6 Shop workers x 5= 30CFM
In Body shops that number increases to 8
Example- 6 Shop workers x 8= 48CFM
If there is a lot of sanding and grinding you may want to increase that number to 10
Of course the best way to establish your need is to total the CFM of tools your using.
Example 3 DA sanders- 10CFM 30CFM
3 Impact Guns- 6 CFM 18CFM
2 HVLP Paint Guns- 15 CFM 30CFM
78CFM x 1.3= 102 CFM
We multiply x 1.3 to insure the compressor does not run fully loaded all the time
I hope this was helpful.