volumetric efficiency

Volumetric efficiency is the ratio of the actual number of cubic meter of free air (at 1.03 kscm abs and 15° C) compressed per unit of time to the number of cubic meter of piston displacement during that time. It is is the ratio of the capacity of the compressor to displacement of the compressor. The term does not apply to centrifugal compressors.

It represents the efficiency of a compressor cylinder to compress gas. It may be defined as the ratio of the volume of gas actually delivered to the piston displacement, corrected to suction temperature and pressure. The principal reasons that the cylinder will not deliver the piston displacement capacity are wire-drawing, a throttling effect on the valves; heating of the gas during admission to the cylinder; leakage past valves and piston rings; and re-expansion of the gas trapped in the clearance-volume space from the previous stroke. Re-expansion has by far the greatest effect on volumetric efficiency.

The theoretical formula for volumetric efficiency is:

Ev = volumetric efficiency, fraction
r    = cylinder compression ratio
Cl  = clearance, fraction.

It has been common for people to measure the Ev of the pressure volume card. This is only adequate as a way to measure capacity in a compressor with no leakage or heating effects. Consider an example with discharge valve leakage. From the pressure volume card it would appear that the volumetric efficiency is increased by the leakage, whereas in fact the capacity is decreased by 40%. In contrast, the power required can be calculated accurately from the pressure volume card.

In practice, adjustments are made to the theoretical formula in computing compressor performance:

where
zs = gas deviation factor at suction of the cylinder
zd = gas deviation factor at discharge of the cylinder
ev = correction factor.

In this equation, the constant 0.97 is a reduction of 1 to correct for minor inefficiencies such as incomplete filling of the cylinder during the intake stroke. The correction factor ev is to correct for the conditions in a particular application that affect the Ev and for which the theoretical formula is inadequate.

Regardless of the mechanics involved, all compressors need volumetric efficiency. Compressors are designed to increase the pressure of a gaseous substance by reducing its volume. It is essential to have high volumetric efficiency so that the maximum gas enters the cylinder and remains in the system to be used.

Note that a cylinder with no losses will have a volumetric efficiency less than 100%. It only relates the actual capacity to the capacity of a cylinder with no fixed clearance, and gives no information on the efficiency of the cylinder.

Discharge Volumetric Efficiency is defined as the actual volume of gas discharged from the cylinder each stroke divided by the swept volume.

 

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