Compressed air consumption convention
Compressed air consumption convention
(OP)
Hi
In the industry, is it normal to give compressed air consumption figures as free air (i.e. "normal" or "standard" - atmospheric pressure) volumes rather than an actual volume at the given pressure?
For example, if an equipment data sheet states that the air pressure required is 4 barg, and the air consumption is 1300 l/min, without specifically knowing, would you expect that to be:
a) 1300 l/min of air at atmospheric pressure then compressed to 4 barg
or
b) 1300 l/min of air at 4 barg (equivalent to 6500 l/min of air at atmospheric pressure)?
As far as I know air compressors are specified using the volume of air at the inlet (i.e. generally atmospheric pressure(, so I'm thinking a) would be most likely.
Many thanks
In the industry, is it normal to give compressed air consumption figures as free air (i.e. "normal" or "standard" - atmospheric pressure) volumes rather than an actual volume at the given pressure?
For example, if an equipment data sheet states that the air pressure required is 4 barg, and the air consumption is 1300 l/min, without specifically knowing, would you expect that to be:
a) 1300 l/min of air at atmospheric pressure then compressed to 4 barg
or
b) 1300 l/min of air at 4 barg (equivalent to 6500 l/min of air at atmospheric pressure)?
As far as I know air compressors are specified using the volume of air at the inlet (i.e. generally atmospheric pressure(, so I'm thinking a) would be most likely.
Many thanks





RE: Compressed air consumption convention
Dejan IVANOVIC
Process Engineer, MSChE
RE: Compressed air consumption convention
For this particular example however, the units are l/min not Nl/min so possibly b) is more likely.
RE: Compressed air consumption convention
Dejan IVANOVIC
Process Engineer, MSChE
RE: Compressed air consumption convention
RE: Compressed air consumption convention
Compressors tend to be standard, but again can vary as they don't always know what pressure you will use, but the equipment wants it at a certain pressure and a certain flow.
If you go for b and it's a then you just have a compressor or air pipe that's a bit too big. Do it the other way around and your equipment won't work.
It's easier and cheaper to call the vendor...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Compressed air consumption convention
Pneumatic equipment manufacturer's are kind of worse. In Oil & Gas (where the equipment is more likely to have a compressed natural gas actuation than air) environmental regulations have driven manufacturer's toward SCF or mass units. Devices that are targeted at other industries still tend to be all over the map between mass, SCF, and ACF and often just dump a number without units.
If I couldn't call the vendor, I'd assume SCF of air (SG=1.0), but I'd be uneasy about it.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Compressed air consumption convention
Much appreciated.
RE: Compressed air consumption convention
RE: Compressed air consumption convention
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Compressed air consumption convention
Given the fact that you mention "air consumption" it could be (note I am cautious...) that the 1300 l/min are required for the downstream consumers at 4 barg.
If this is the case you would have to calculate backward what would be the actual flow at the inlet of compressor, specifically at the flange if you have in mind the sizing, and possibly including for the losses of the inlet filter - an element which is normally applied, assuming the site conditions are known. Not needed to precise that for sizing the maximum site temperature shall be considered (lets skip a bit any debate of what is max and what is design temperature).
It is also sometimes tricky to fix the design as the discharge temperature depends upon the efficiency so there might be some itérations which need to be performed before to reach a certain set up : inlet flow and conditions, outlet flow and condition for certain frame and sizing. I am more familiar with centrifugals so excuse if other considerations apply with other machine typologies.
Surely I am NOT saying this is how it should be interpreted...but it is something I would really envisage and therefore it becomes even more clear to me how critical it is to get clarifications on the data sheet (whenever possible of course);
RE: Compressed air consumption convention