Target Specific Power



Target system specific power

Specific power is the ratio of power to the compressed air flow rate at a given pressure. The lower the specific power the more efficient the compressor is at a given compressed air load. For compressors, the measure is usually given at full load. For variable speed compressors, this measure is usually given for a number of points along with the load range of the compressor. Typical units for specific power are kW/100 cfm or kW/m3/min. A related performance measure is specific energy which has typical units kWh/cfm or kWh/m3

While it is true that are many actions one could take to try to decrease this measure, the greatest effect can be expected from either upgrading the trim compressor from an ON/OFF regulated compressor to an ASD ( Adjustable Speed Drive) or using a sophisticated control system to manage the compressor station. Of these two options, the first usually has larger applicability and larger gains than the latter.

Deciding on a good target specific power for your particular system is difficult as it involves many variables including the pressure required, ISO class of air required, the average and deviation of consumption of the plant and so on.

A quick check of the feasibility of improving your system specific power can be found here:

it assumes a target specific power of 7.3kW/m3/min or 20.5kW/100cfm. Depending on your specific plant this may be unbelievably low or even too high but for most plants using running at about 100 psi(g) or 7 bar with an installed power of between 50 and 500 kW, this is a good target.

This calculator assumes that the most expensive and probably most effective way to reduce system specific power is to replace the trim compressor. The trim compressor should have a capacity of at least 125% of your largest baseload increment. In smaller compressed air stations this is usually 125% of the largest compressor in the system.

Another assumption is the estimate of the procurement and installation costs of such a compressor. A conservative estimate is 750€/kW.

Estimation formula

As the formula is simple you can easily change these estimates to suit your particular case.

The result returns the simple return of investment in years. Anything below 3 years is a strong indicator that this is a feasible investment.

For further reading and a discussion on the nomenclature used :