Copyright Goodheart-Willcox Co., Inc. 402 Fluid Power A receiver-sizing formula suggested by the Compressed Air and Gas Institute is: V = T × C × P A P 1 – P 2 where: V = Capacity of the receiver (ft3) T = Time to reduce receiver pressure from P 1 to P 2 (min) C = Air requirement (ft3/min) P A = Atmospheric air pressure (psia) P 1 = Initial receiver air pressure (psig) P 2 = Final receiver air pressure (psig) This formula assumes a constant air temperature in the receiver and a standard air pressure of 14.7 psia. The formula also assumes that no additional air is being supplied to the system. In other words, the compressor is stopped or unloaded using a start-stop or inlet valve capacity control system. If air is being added to the system, that additional volume needs to be subtracted from variable C of the formula. Sizing the Receiver An air receiver must supply air to a pneumatic system for 10 minutes while the compressor is unloaded. The system is consuming air at 10 cubic feet per minute (cfm) during this time and the receiver air pressure drops from 100 psig to 50 psig during this time. Temperature is constant and the atmospheric air pressure is 14.7 psia. V = T × C × PA P1 – P2 Substituting in: T = 10 min C = 10 cfm PA = 14.7 psia P1 = 100 psig P2 = 50 psig V = 10 min × 10 cfm × 14.7 psia 100 psig – 50 psig = 1,470 ft3 50 = 29.4 ft3 EXAMPLE 16-1 16.2 Air-Distribution System The purpose of the air-distribution system is to carry high-pressure, conditioned air from the system receiver to workstations. Delivery must be done with output inherent in the operating cycle of reciprocat- ing compressors and some other designs. The receiver also acts to remove additional water vapor from the air as the air temperature continues to drop after passing through the system aftercooler. In addition, the stored volume of air in the receiver reduces the frequency of compressor cycling in capacity-control systems using start-stop operation or inlet valve unloading. Receiver design and construction A receiver is typically cylindrical with domed ends, although other designs exist. The domed, cylindrical con- fi guration provides an easily constructed unit that can withstand the pressures used in consumer and industrial systems. Standardizing organizations provide construc- tion specifi cations and pressure ratings that are often listed as part of consumer and industrial safety codes. Care should be taken to be certain codes are followed. A large volume of highly compressed air can present serious safety issues. CAUTION The receiver is constructed with inlet and out- let ports, fi ttings to allow the connection of various associated components, and, in some designs, a hole to facilitate cleaning. An internal baffl e may also be incorporated to ensure liquefi ed water, pipe scale, and rust particles that enter from the connecting compres- sor line fall out of the airstream. Receivers are also fi t- ted with feet for vertical or horizontal installation. In smaller units, they may include fi ttings to allow for the easy attachment of compressors and prime movers. Accessory components often directly connected to the receiver are an electrical pressure switch for operat- ing the prime mover, pressure gauges, liquefi ed-water drain valve, and maximum-pressure safety valve for the system. The size and complexity of the pneumatic system has a great deal to do with the included acces- sories and if they are located on the receiver or in some other location in the system. Sizing the receiver An adequately sized receiver must be provided if a pneumatic system is to operate effi ciently. Formulas are available that can be used to provide a reasonable estimate of the needed size of receiver for a system. The variables involved in the operation of a pneumatic system make the strict application of a receiver-sizing formula questionable. It is often suggested that a fac- tor based on operating experience be used as a part of a recommended mathematical formula. For example, one major manufacturer suggests a factor of 1.5 to 3.0 this is based on experience alone.