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The most efficient compressor will be the one with less electricity consumption and more air flow. Very good, but normally we measure volumetric flow, that is, m3/h or liters/min. The problem is that air at a temperature of 20ºC has more volume than at 0ºC. And that the air at higher pressure is also compressed and the volume decreases.

So if I have two 30 Hp compressors, the first one gives 3.6 m3/min and the second one 4.1 m3/min, at first glance the second one will be more efficient, but is it really like that?

No, we need to know the reference pressure and temperature. If the first is a 10 bar compressor and the second is 8 bar, this difference in flow may be perfectly normal. Because compressing the air at 10 bar requires more power than at 8 bar, and since the rated power of the motor is the same, the delivery flow must be limited.

The same thing happens with the temperature, if the flow value is at 0ºC, and we are at a temperature of 30ºC, the compressor gives less air than advertised.

The correct thing would be to measure mass flow, that is, Kg/min or Kg/h, so there would be no doubts, because temperature or pressure do not influence.

But all electric motors with a nominal power of 30Hp consume the same amount?

Well no, it depends on the efficiency of the motor, the mechanical load, the refrigeration, etc.

In addition, manufacturers have some tricks up their sleeves, such as Power Factor, for example, a power factor of 1.1 means that the motor can work at 110% continuously. For practical purposes, it is a more powerful engine in a smaller size. There are motors that can go up to 1.2, 120%…

There are also other consumptions, such as the cooling fan and the electronics itself.

In other words, an end customer, with two catalogs of different machines, cannot really determine if one compressor is more efficient than another, because, although the flow rate is determined at the same temperature and pressure, they will not know the total absorbed power of compressor.

You should always ask for the model data sheet, where the power absorbed in the terminals is specified, not the power in the shaft (which I have seen many times) and the flow according to the corresponding regulations.

In some countries, such as the US, the compressor has to go through an OCA, so that its efficiency is audited, and it is posted on web pages, so that the end customer can compare.

Normally the efficiency is measured in Kw/m3/min, that is, how many KW do I need to compress 1 m3 in one minute. And this value should be added in all catalogs.