Relation Between SCFM and FAD for reciprocating compressor 2

[Montemayor]

I think that by “fresher” you mean you are a recent engineering graduate. If so, I can understand your situation.

Acronyms can mean a lot of different things. I have always understood that FAD means “Free Air Delivery” – as opposed to “displacement”. The formal definition of FAD is that it is the ACTUAL rate of compressed air discharged at the discharge port of an air compressor, measured in terms of “standard” conditions. And that is where everyone starts to get either confused because of the use of the term “standard”. This term is strictly a specific or personal definition. Generally, engineers will select 32 oF, 60 oF, 70 oF as their preference for “standard” temperature. Standard pressure values can be 14.4, 14.696, or 14.7 psia. But the values are not limited to just these. You are free to define any values you wish – or decide upon. However, it is smart to conform to general, accepted values – such as 14.696 psia and 60 oF as defined by the GPSA. Nevertheless, it is smart and practical engineering to ALWAYS state what you consider as “standard conditions”.

SCFM, of course, stands for Standard cubic feet per minute. Again, note the use of the general word, “standard”.

Assuming that you have a compressor that delivers all the air at its final discharge port that it sucks in at its initial suction port, then the “standard” conditions applied to either the suction or discharge should yield the same value of air flow rate. In my experience, the normal method of rating a compressor’s capacity is at the suction port, so FAD is not usually even mentioned in most compressor capacity ratings.

 

[zdas04]

We often think of "Standard" as being defined in a gas-sales contract, but with air Montemayor is exactly right that the standard is whatever you want it to be.

One caution that I always give about natural gas that applies to air is that you cannot use flow at standard conditions to directly calculate fluid velocity directly. The "standard" is really an imaginary state (i.e., the volume that the air would occupy if were at your standard conditions) and imaginary velocity is a pretty useless concept. You have to convert the SCF to an ACF number before calculating velocity.

David

 

[dasumit]

Thanks Montemayor and David. Yes, I graduated around 2 months back and am just trying to get an idea of this field. Will look froward to some valuable help on this forum.

Sumit

 

[BigInch]

It's measured at the suction port because the first limiting factor is how much air you can actually get your hands on to take into the compressor to do work on it and compress it. The higher the suction pressure and the lower the temperature, the more air you have available to work with. You can name all the capacity numbers you want at the discharge end and it describes nothing, if you can't actually intake that capacity into the compressor to begin with.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky