Compressed Air Storage

The sizing of receivers is important as it has a direct impact on both the overall reliability and the energy efficiency of the compressed air system.

The size of an air receiver will depend on the amount of fluctuation in air demand. In most cases an adequately sized receiver will be able to supply the extra air during a high demand period and then recharge when the demand drops off. This function allows the air compressor to be sized for the average demand, rather than for the maximum demand. In some cases when the fluctuation is too great, a solution can be to have a smaller compressor that can ‘kick in’ as required.

There are a number of formulae for calculating the storage volume required. However, the following empirical rule can provide an approximation for planning purposes, taking into account the compressor(s) output and the pattern of demand.

Sizing

The Air receiver should be sized (in Litres) to be at least 6 – 10 times the compressor free air output (in litres a second).

It is also worth considering the following:

To provide optimum performance, the receiver should be sized for the largest expected air demand event.
An undersized receiver will cause the compressor to cycle frequently in response to small changes in pressure.
An oversized receiver will cost more and will store more air, but it will require the compressor to remain on load for longer periods to recharge the air receiver. This is balanced by the extra time the compressor will have to cool before it must come on load again.
The volume of the pipe work is often significant but is not included in the calculations.
An effective control system will ensure that the receiver volume balances the demand from the system with the supply from the compressor.

Additional local air receivers for intermittent demands

To provide optimal performance, receivers need to be sized to handle the largest demand for air in the system. However, this event may be a process or an item of equipment with a large intermittent air demand. There are certain situations where the demand is not continuous. In this instance it is better to install an air receiver close to the process/equipment rather than to oversize the main air receiver (or to install an additional compressor that would stand idle most of the time).

To determine whether a local (auxiliary) air receiver is needed:

Calculate the total maximum storage for the main receiver as described above.
Then calculate the storage required for the largest event. If this exceeds 10% of the total, then a local air receiver is recommended.

The air receiver should be sized (in litres) to be at least 6–10 times the compressor free air output (in litres/s).

Storage

For example, if the total compressor output is 20 litres a second, then the maximum air storage is 200 litres.

If the single largest event is 2.5 litres a second, then the maximum air storage is 25 litres. In this instance, installation of a local air receiver is recommended. The size of any reserve air capacity is dependent upon the amount of air used per operation and the pressure drop that can be tolerated; it can be calculated as follows:

Required receiver volume = Demand per operation (litres free air) / Acceptable pressure drop (bar).

It is important to check that the compressor is large enough to recharge the air receiver up to the original pressure before the next period of high demand.

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