Air Receiver Question 1

[kelleyl]

Can someone check my work here... I'm trying to size an air receiver for a special fixture that is served by a house compressed air system and the fixture needs constant pressure at 90psi min, no drops.

I want to put a secondary receiver before the special fixture, trying to size it to hold 1 hour of storage, to account for demand spikes in rest of the air system.

The special fixture demand is 6cfm at 90psi, constant. The house air through system is capable of 99.1cfm at 100psi, max. There are system demands of 104cfm occasionally. There is a 240gal wet and dry air receiver at the source system to buffer that. This receiver needs to maintain the 90psi to the special fixture for 1-minute system pressure losses that may occur, and want it to hold 1 hour of storage.

I think 1 hour of storage for a 6cfm flow would mean a 360cf storage tank is necessary (360=6cfm*60mins)? So if I only had a 26cf storage tank on hand, this would hold storage for 4.5mins (4.5mins=26cf/6cfm)?

Or, using the equation from here:

https://www.airbestpractices.com/industries/bulk/dealing-high-volume-intermittent-demands

Which is: V=T(C-S)Pa/(P1-P2)
Where:
V=solving for receiver volume
C=intermittent spike=104cfm
S=supply flow=99cfm
P1=100psig
P2=90psig
Pa=14.7psia
I'm not sure what T ought to be:
T=time in minutes for pressure to drop... I don't want to allow any pressure drop... I want the storage to last an hour.. it makes sense to me T should be 60mins.
So from this equation:
V=441cf

So anyway I am looking at a 360cf-441cf tank to store an hour of air at 90psi for this lab.

Is this right? This is very big. Is there anything I can do to get more storage out of a smaller tank? Is it right if I use the 200gal/26cf tank I have, it will give us a 4.5min buffer?

Thanks for any help

 

[LittleInch]

Lets try and understand what your system is (a sketch would help)

So when you say "storage" I think what you mean is that the compressor is still supplying air all the time, but at what pressure and flow??

When you say 6cfm, at what pressure is this 6? "Atmospheric?", standard?

How is the 90 psi delivered? I can only assume you have some sort of regulator there? What is it's pressure drop?

It seems to me that your system is barely able to supply your 90 psi therefore any device to store air to supply the 90 psi device will end up being quite big as you have very little air supply to use above the 90 psi.

where do you want to place this vessel? Only for use by the special instrument or for all the systems.

You need to think clearly about the inputs and how the system works and what you want it to do.

Sizing a vessel to supply min 90 psi at 6cfm when your pressurizing supply is only 100 psi will be very big as you only have max 10 psi to play with.

Your current calcualtions don't make sense until you work these things out.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zeusfaber]

What happens if you put a non-return valve between your new reservoir and the rest of the system? Would that let you size the reservoir for a 6 cfm flow, rather than the 104 cfm peak?

A.

 

[zdas04]

There simply is no way to answer this question if you insist on using ambiguous terms like "cfm". Really, the air industry does itself a horrible disservice by using "inlet CFM" as a standard of measurement. I cannot add a cubic foot of air at atmospheric pressure to a cubic foot of air at 90 psig and get a number that means anything at all. A vessel with a physical volume of 100 ft[sup]3[/sup] in Denver will hold 100 cubic ft of air at local conditions, but 82 SCF at local conditions and 694 SCF at 90 psig or 2122 SCF at 300 psig.

It should be clear from the above that you have to specify if your "6 cfm" demand is "6 SCF/min" or "6 cfm at local atmospheric conditions" or "6 cfm at 90 psig" like you said. You would need 42 cfm at suction conditions to supply 6 cfm at 90 psig. You either have to do this kind of problem in mass units or some reasonable surrogate for mass like SCF. Doing it on a volume basis is just not going to yield reasonable results.

Once you have your head around "a cubic foot at one pressure holds less air than a cubic foot at a higher pressure", then you can "size" your receiver to hold an appropriate amount of air either by increasing the vessel size or by increasing the air pressure in the vessel. If your device requires 6 SCF/min at 90 psig, then you know that a ten minute usage would remove 60 SCF from the receiver, which would lower the pressure of a 100 ft[sup]3[/sup] receiver from 300 psig to 291 psig, but would lower the same vessel from 100 psig to 91.2 psig.

[bold]David Simpson, PE[/bold]
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