The power needed to compress air was discussed in the chapter on Compressor Perform­ ance. It is instructive to compare the equivalent power for some alternative energy forms, in particular for hydraulic and electrical energy.

Energy balance in compressed air systems

Consider as a theoretical concept a system in which it were possible to use the compressed air at the temperature at which it came out of the delivery port of an isentropic compressor, with all the pipework thermally well insulated and with all the energy possessed by the air regainable at the point of use. One would then have a perfectly efficient cycle, which would need the same power to drive it as would an efficient hydraulic or electric system. This can be seen to be so by remembering that isentropic compression is, by definition, reversible, which means that all the energy put into it can be regained. Unfortunately, a practical pneumatic system falls far short of this ideal and is much worse overall than its hydraulic equivalent.

In a simple pneumatic system consisting of a compressor directly connected to a power tool, much of the energy supplied to drive the compressor is rejected as heat into the cooling system and eventually into the surroundings. Only if one can use this heat for space or water heating or for processing will it not be wasted.

For most purposes compressed air cannot be used if its temperature is much higher than ambient. Portable compressors which have a minimum amount of cooling sufficient to supply usable air will have a discharge temperature about 60°C above ambient (where ambient has a maximum of 40°C). Stationary compressors will be equipped with the appropriate size aftercooler to meet the temperature and humidity requirements of its use in the factory.

At the other end of the power chain, ie at the point of use, there will be a rotary or percussive power tool, a pneumatic cylinder, a rotary motor, a paint spray gun or one of the many other ways of using air. If the output device produces a measurable form of mechanical power, it becomes possible to calculate an overall energy balance. This is an instructive exercise for demonstrating the overall efficiency of compressed air systems.

Consider an isentropic compressor, working under theoretically ideal conditions, producing air. The energy required has been previously calculated in the chapter on Compressor Performance and can be obtained from Figure 3 of that chapter. At a pressure ratio of 7:1, ie at a delivery pressure of 6 bar gauge, the power required is 0.26 kW per 1/ s FAD, or 0.26 x 8 (= 2.08) kW per 1/s of compressed air. If an hydraulic pump were able to produce the same compressed volume flow at the same pressure, the power required would be 0.6 kW.

This calculation shows that the ratio of hydraulic power to pneumatic power is 0.6/2.08 = 0.288 or 29%.

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