As people always say, things change. And nowhere is this more true than in temperature rate and molecular weight.

As per the previous post "Principles of Impeller Sizing", the temperature and molecular weight of a gas affect the sizing of the impeller. However, they also affect performance after the compressor is installed. This may sound odd, because once an impeller has been manufactured, the head is fixed — the diameter determined by the specified temperature, molecular weight and pressure ratio.

But, as we said, things change. Especially in the world of air compressors, where the inlet temperature varies from not only morning to night, but from winter to summer. In some locations the ambient temperature can vary from 0°F (-18°C) to 100°F (37°C). As we saw in Compressor Primer #1, the impeller must be sized for 100°F (37°C).

But what happens when the temperature falls to 0°F (-18°C)? What happens is that the discharge pressure and power both increase. Remembering the Head equation:

HeadPr × R × T

Since metal has been cut, the head is now fixed. Assuming the molecular weight is constant, the temperature decreases and then the pressure ratio has to increase.


The same principle applies to gas streams, which have varying molecular weights. Assuming the inlet temperature remains constant, the pressure ratio has to increase.


Note that R=1545/MW, so increasing the molecular weight has the inverse effect. The decreasing temperature and increasing molecular weight both have the effect of increasing power.

The equation is:

PowerP1 × Q × b

where: P1 = inlet pressure