# compression ratio

• 01-12-2013, 08:02 PM
tpodany88
compression ratio
For my 19DK chiller: to find compression ratio of this system which operates in a vacuum (R-11), typically 14"hgVAC with 7 psig head, can I use this formula: comp ratio=head pressure +14.7 divided by suction pressure + 14.7?
• 01-12-2013, 08:52 PM
hvacrmedic
Quote:

Originally Posted by tpodany88
For my 19DK chiller: to find compression ratio of this system which operates in a vacuum (R-11), typically 14"hgVAC with 7 psig head, can I use this formula: comp ratio=head pressure +14.7 divided by suction pressure + 14.7?

21.7psia/6.9 psia=3.14 compression ratio. If you have digital gauges then just switch to absolute pressure units, such as psia, then you can enter those directly in the equation. Compression ratio is the ratio of absolute pressures. HTH.
• 01-12-2013, 09:05 PM
Tech Rob
You first have to convert inches of mercury to PSIG by multiplying by .491. Then, you can add 14.7 to both discharge and suction to obtain PSIA for both. Divide suction into discharge and you have your compression ratio.
• 01-12-2013, 11:26 PM
hvacrmedic
Quote:

Originally Posted by hvacrmedic
21.7psia/6.9 psia=3.14 compression ratio. If you have digital gauges then just switch to absolute pressure units, such as psia, then you can enter those directly in the equation. Compression ratio is the ratio of absolute pressures. HTH.

Sorry, I forgot to subtract my converted low side pressure from the atmospheric pressure . 14 in hg = 6.9 psi, not 6.9 psia, and is measured relative to 0 psig, not 0 psia. 0 psig in turn is equal to 14.7 psia. So the low side absolute pressure is given by 14.7 psia - 6.9 psi = 7.8 psia.

So the compression ratio was actually 21.7/7.8 = 2.78
• 01-12-2013, 11:31 PM
tpodany88
Thanks!
• 02-12-2013, 09:54 PM
muskey
hu compression ratio on a centrifugal compressor? Thought it had more to do with stroke and volume.Or maybe not but got me thinking:grin2:
• 02-12-2013, 09:57 PM
muskey
O Ya its more to do with impeller lift and gas velocity