please check my scfm vs acfm calcs!

[RV]

Can someone verify that I am calculating my scfm to acfm conversion correctly?

Problem: The goal is to achieve 35 scfm at 100 psig. The conditions are 6500 ft elevation (assume 11.56 psi absolute atmospheric pressure) and I am assuming 80° F as the compressors discharge temperature based on vendor-supplied information . What flow will I see discharged from this compressor under these actual conditions?

I have calculated that the acfm (actual flow) will equal only about 5 cfm, but the product vendor tells me more like 30 cfm. Can anyone verify the correct answer for me? Thanks.

 

[TD2K]

Well, it obviously can't be 30 cfm if you only need 35 scfm simply based on your 100 psig operating pressure versus 14.7 psia for standard cubic feet. The compressor manufacturer doesn't understand what you want or is thinking of something else.

At 100 psig discharge pressure, your absolute pressure is 111.6 psia. Your discharge temperature is 80F.

Therefore 35 scfm = 35*(14.7/111.6)*(80 + 460)/(60 + 460)
= 4.8 actual cfm at 100 psig and 80F

Same as you got. When you talk to him, make you your differentiate between scfm and acfm, I always include the 'a' to help make the actual stand out.

 

[tad123]

The calculation above is assumed compressibility Zactual=Zstandard = 1, which is OK for the discharge conditions. I'm curious of what type of gas you compress and what is the compression ratio. The discharge temperature of 80 F at 100 psig seems to be low to me.
TAD123

 

[RV]

The gas is air and the compressor is outfitted with an aftercooler that the vendor tells me allows a discharge temperature of 25° F above intake gas temperature. Does this seem unrealistic?

 

[saxon]

RV, It sounds as if you're caught up in the client/compressor vendor bugaboo. Some vendors will rate their equipment based on inlet conditions (either acfm, scfm, or mass flow) others rate equipment on outlet conditions (either acfm, scfm or mass flow). It sounds as if you and the vendor haven't got together and determined whether the rating is too be inlet based or outlet based and in what units. Once this determination is made, the conversion is straight forward and will usually work out to within plus or minus 5% of each others calc. Check with the vendor to see where they are rating the equipment at, either inlet or outlet and in what units.

saxon

 

[arte]

The compressors I have purchased were rated based at sea level. A compressor used at a higher elevation will have to have a higher rating. In your case (6500ft) you will need a compressor about 25% bigger than the same service at sea level.