When specifying a compressed air system for optimum operation and energy efficiency, proper selection of the compressed air receiver tank(s) is one of the most critical decisions one can make. There are several different “rules of thumb” and formulas that will assist you, but finding straightforward guidance that says “use this size air receiver” is difficult if not non-existent.
Air receivers can be used in several different ways in a compressed air system:
- “Primary” receiver between the supply side (air compressor and ancillary equipment), and the demand side (your plant). Today’s air compressor controls (on/ off-line, modulation, and variable frequency) strive to maximize energy efficiency and smooth compressor operation by responding to demand side pressure changes sensed at the discharge of the package.
A properly sized air receiver acts as a “buffer” and minimizes the effect of dynamic demand side pressure changes, allowing the compressor controls to operate smoothly and consistently. The end result is less energy used, longer component life, and consistency in plant air pressure.
- “Secondary” receiver typically on the demand side, at the point of use to minimize the effect large intermittent air demands have on the overall compressed air system.
Sizing a PRIMARY receiver for general FIXED SPEED APPLICATIONS:
- The air compressor industry has widely accepted the general rule of thumb that a properly sized air receiver for a fixed speed compressor should be between 1-2 gallons per CFM output of the compressor.
Example: a 100 CFM fixed speed air compressor should have an air receiver between 100-200 gallons sitting next to it. Err on the high side if your budget permits.
Sizing a PRIMARY receiver for general VARIABLE SPEED APPLICATIONS:
- When considering air receiver sizing on a variable speed drive application, the general rule of thumb is between 2-4 gallons per CFM output of the compressor.
Example: a 100 CFM variable speed air compressor should have an air receiver between 200-400 gallons sitting next to it. Err on the high side if your budget permits.
Useful Air Receiver Sizing Formulas (Primary and Secondary):
Use this formula when you have an existing air receiver
and need to know how long you can draw CFM greater than the output of the air compressor, from the receiver, while still maintaining system pressure:
- T = R x P1 – P2 Qr – Qc 7
Use this formula to determine what receiver size to use to supply pressure for a given period of time, not allowing the system to drop below a minimum pressure. The demand of air is greater than the CFM output of the air compressor:
- R = 7 x (Qr – Qc) x T
P1 – P2
Use this formula to determine how long it will take to recharge an air receiver to P1, after demand goes back to being below CFM output of the air compressor:
- T = R x (P2 – P1)
Qr – Qc x 14.7
T = Time in minutes
R = Receiver in cubic feet Qr = CFM removed
Qc = Compressor output in CFM
P1 = Maximum air receiver pressure P2 = Minimum air receiver pressure
14.7 = Atmospheric pressure in PSI (sea level)
CUBIC FT to GALLON CONVERSION: 7.48 gallons in a cubic foot