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Compressed air consumption convention

Compressed air consumption convention


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
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

I wouldn't take any of the two without confirmation from the designer/supplier, unless there is a note somewhere in the datasheet specifying the conditions at which the flow is measured/calculated.

Process Engineer, MSChE

RE: Compressed air consumption convention

Thanks for the reply. There are some cases where it's not possible for me to confirm, so I was just wondering if I had to guess, which would be more likely, based on common practice in the industry.

For this particular example however, the units are l/min not Nl/min so possibly b) is more likely.

RE: Compressed air consumption convention

You see? You've changed your mind twice already. ponder

Process Engineer, MSChE

RE: Compressed air consumption convention

Exactly, that's why I'm asking for the opinion of people that know better than me!

RE: Compressed air consumption convention

For a piece of equipment which is consuming air, I would tend to go with the actual volume at the required pressure if not specified, but as Emmanuel Top states, there is no fixed and firm convention. The absence of S or N in front of the volume flow figure would tend to make you believe it is actual they are referring to, but only the vendor will know unless you can find a footnote in micro writing on the data sheet.

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

The pneumatic industry clings really hard to their (inconsistently applied) conventions. If you ask an air compressor manufacturer what their stated capacity is based on they will say "actual cubic feet per minute at suction conditions of 14.7 psia and 60F". That statement is amazingly ambiguous since actual suction conditions are almost never 14.7 psia and/or 60F. Their assumed suction conditions are within shooting distance of a version of STP that is very common. I've found that I get decent results if I just replace the word "actual" with "standard" when I'm looking at air compressor specifications. At elevated altitudes, converting actual conditions to SCF will properly derate the machine. I will never know why they cling to the word "actual" when it adds no value.

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

Thanks for the replies, really useful stuff.

Much appreciated.

RE: Compressed air consumption convention

The conventional units for Free Air Delivery are 1.0atm, 20degC, zero% RH.

RE: Compressed air consumption convention

Except when its not.

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

If I try to see from a different perspective;

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

I have installed many equipments that have to be feed with compressed air and the compressed air requirement was always state like example b)

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