Compressed Air Primary Storage (Air Tank Sizing & Placement)

Storage can be used to control demand events (peak demand periods) in the system by reducing both the amount of pressure drop and the rate of decay. Receivers are used to provide compressed air storage capacity to meet peak demand events and help control system pressure.

Receivers are especially effective for systems with widely varying compressed air demand. Where peaks are intermittent, a large air receiver may allow a smaller air compressor to be used, and allow the capacity control system to operate more effectively.

A receiver must have a pressure band if it is to be effective. If production requires 100 psig and the compressor controls are set at exactly 100 psig, there is no storage. Any event makes the pressure drop below the 100 psig requirement until the compressor controls respond by increasing the volume compressed. If the compressors operate at 119 psig then the difference between 110 and 100 psig accounts for air stored in the receiver. If an event suddenly occurs, the pressure can drop 10 psig before the minimum requirement is met. Pressure/flow controllers may provide additional benefit by stabilizing downstream pressure and flattening demand peaks

There are two differing points of view on the location of a primary air receiver in a plant air system

Air Receiver before the Dryer
If the receiver is located soon after the compressor discharge and the compressor(s is a piston type, the receiver dampens pressure pulsations. If the receiver is located before the compressed air dryer, the receive will provide additional radiant cooling and drop out some of the condensate and entrained lubricant, benefiting the dryer. However, the receiver will be filled with saturated air and if there is a sudden demand which exceeds the capacity rating of the compressor and matching dryer, the dryer can be overloaded, resulting in a higher pressure dew point.

Air Receiver after the Dryer
If the air receiver is located after the compressed air dryer, some of the above advantages ar lost. However, the receiver is filled with compressed air which has been dried, so a sudden demand in excess of the compressor and dryer capacity rating will be met with dried air. The dryer is not overloaded, since it is seeing only the output of the compressor, so the pressure dew point is not affected. There must be a receiver located after the dryer is a pressure/flow controller is used in the system. Some facilities have primary storage before and after the dryer.

Minimum Requirement of Sizing of Primary Storage
· In the past, mainly with reciprocating compressors, rules of thumb for sizing a primary air receiver have been from1-3 gallons of cfm of compressor capacity
· A rule of thumb for effective demand side control is 3-5 gallons per each 1-cfm of compressor capacity, but primary receiver size requirement will vary with the demand profile and type of compressor capacity control used, and can be difficult to determine.
· Storage should be sized so that the large demand events do not require additional compressors to be turned on.

Secondary Storage
Secondary storage refers to using air receivers in the distribution system in additional to the primary air receivers in the supply side.

Sometimes this storage is dedicated to a certain end-use application, while other times it is located to provide additional general storage at the end of a distribution line or loop.

Secondary storage can:
· Maintain more stable pressures at points of use
· Improve the speed, thrust of torque of an application
· Be used to reduce the rate of pressure drop in the system during demand events
· Be used to control demand events (peak demand periods) in the system by reducing both the amount of pressure drop and the rate of decay

For some systems, it is important to control the rate of supply to replenish a secondary receiver. If the rate is not controlled, filling receiver will be come a demand event.