Compressed air storage basics
Primary storage volume requirements
Traditionally, air receivers were sized for reciprocating air compressors. The general guideline was that every cfm of air produced required one gallon of storage. Today, with lubricant-injected rotary screw compressors using load/unload controls, the suggested minimum size is between three and five gallons of storage per cfm produced. Air receiver size requirements can also vary by the type of compressor and the type of compressor capacity control. When determining the volume of system storage required for multiple compressor systems, only the capacity of the trim compressor needs to be considered for reducing part-load energy requirements.
A larger air receiver will also provide other benefits such as stabilizing pressure or to prevent another compressor from coming online to meet large intermittent air demand requirements. To satisfy high volume intermittent demands air receiver sizing must address the total cubic feet of air required for the duration of transient demand events and the interval between them. Further consideration must be given to having ample storage to cover the time required to bring an additional compressor on line, should the size and duration of a demand event require it.
Pressure differential and the effects on storage
For storage to work there must be a pressure differential and an allowable pressure band. The usable pressure differential and the air receiver size determine the available storage. A receiver has a given volume, normally measured in gallons. Larger sizes are generally expressed, or measured, in cubic feet.
There are 7.48 gallons in a cubic foot. The amount of free air in the receiver depends on the size and on the pressure. At sea level, the atmospheric pressure is 14.7 psia (0 psig). If compressed air in the receiver is at 100 psig, the absolute pressure is 114.7 psia (100 psig + 14.7).
A 1,000 gallon receiver at 100 psig will hold the equivalent of: (1,000 *114.7)/(7.48 * 14.7) = 1,043 cubic feet of free air. The amount of usable compressed air would depend on the pressure differential. For example: with the 1000 gallon air receiver and an allowable pressure differential of 10 psi (100 to 90 psig) the available compressed air in storage would be:
1000 gal. x (100 - 90)/(7.48 * 14.7) = 91 cu. ft.
Be careful not to raise compressor operating pressure to increase storage. For positive displacement compressors at a nominal 100 psig, a 2 psi increase in operating pressure increases input power one percent. For example, the increase in energy cost for raising a 100-hp air compressor’s operating pressure 10 psi is five percent, which may also overload the motor. For a 24/7 operation having a $0.10 per kWh energy rate, this is $3,800 per year in additional energy costs.
Purchasing additional storage is a one-time cost and can often provide a rapid payback compared to raising compressor operating pressure.