1 reply to this topic

[Art Montemayor]

Amit:

You have good cause for doubting that compressor power requirements are lessened by a drop in suction air temperature. If anything, the power to a compressor will increase should the suction temperature be lowered. This is assuming what you failed to state: the compressor is a reciprocating type with fixed capacity and at constant speed. This is logically so and easily explained by the fact that the air density increases with a lower temperature, thereby allowing the machine to suck in and deliver more air - and thereby necessitating more horsepower.

Bear in mind that many engineers often make the mistake of not understanding that the air delivered is often measured and dealt with in volumetric (as opposed to mass) units. Therefore, these same engineers make the mistake that because many books state that the % horsepower saved per volume of air (@ stated conditions) is increased as the temperature is decreased, then they believe that in a given compressor, the horsepower is less if the suction temperature is lower -- which is not true and is not what the books state. The error here is that most books on compression (especially university text books and thermodynamics books) deal with theoretical situations - not the real life. For example, the books don't state it outright, but they are assuming that you can vary the capacity of a given compressor at will -- and differentially at that too! This is not the usual case in practice. That's why, if you state the specific case of sucking in and delivering a given quantity of air (identified at a specific condition - such as Scfm, 70 oF & 14.696 psia), then it is true that you can "save" horsepower by lowering the suction temperature - but based on volumetric capacity - not the empirical mass capacity that a compressor realizes. The fact of the matter is that horsepower represents work done at a certain rate on a specific mass of gas - not volume. Therefore, if you suck in more mass (due to a density increase and a constant, positive volume displacement), you will require more horsepower. There is no alternative to this fact.

I hope this explanation helps you understand what seems to be perlexing you. By the way, who has "assumed that if intake air temerature reduces by 4 degree, the power consumption of compressor is reduces by 1%"? Is it you or someone else?

Art Montemayor
Spring, TX