Power required for compression
Power required for compression
(OP)
Hello
I have been trying to calculate the theoretical power required for compression of a small portable 2 stage air compressor with the following spec.
Volume flow rate Q = 2x10-5 m3/s
Inlet pressure P1 = 1 Bar (105Pa)
Outlet pressure = 200 Bar
interstage pressure Px = 14 Bar
k = 1.4
N = number of stages
Motor power = 400 w
Using the equation for power during adiabatic compression I get a theoretical power of 16 watts. Why the huge discrepancy between the theoretical and the actual power required? Is my calculation correct? I am aware there are mechanical losses and inefficiency's in the pump but are they so great as to require 25 x the power?
Thanks very much for any help.
I have been trying to calculate the theoretical power required for compression of a small portable 2 stage air compressor with the following spec.
Volume flow rate Q = 2x10-5 m3/s
Inlet pressure P1 = 1 Bar (105Pa)
Outlet pressure = 200 Bar
interstage pressure Px = 14 Bar
k = 1.4
N = number of stages
Motor power = 400 w
Using the equation for power during adiabatic compression I get a theoretical power of 16 watts. Why the huge discrepancy between the theoretical and the actual power required? Is my calculation correct? I am aware there are mechanical losses and inefficiency's in the pump but are they so great as to require 25 x the power?
Thanks very much for any help.





RE: Power required for compression
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Power required for compression
RE: Power required for compression
secondly, a pump and compressor are not the same device and suggest you not confuse the two.
thirdly, homework assignments are not permitted in this website. please read the site policies.
good luck!
-pmover
RE: Power required for compression
the discharge temps on your homework are exceeding 1000F
RE: Power required for compression
RE: Power required for compression
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Power required for compression
I sit with bated breath awaiting your pearls of wisdom without price.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Power required for compression
Each stage is intercooled. I was just trying to figure out the power required assuming adiabatic compression, given the flow rate stated.
Thanks again.
RE: Power required for compression
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Power required for compression
What is the application of this compressor? The flow rate is negligible for any real compressor that can be built. As others have mentioned, check your input parameters.
The problem statement clearly indicates it to be school homework. You first talk of compressor and then hand pumps for such high pressures, get your fundamentals right even to do well in your school.
RE: Power required for compression
RE: Power required for compression
The machine you have sounds a bit like the Rix Microboost. Inlet pressure 0.2 to 1.7 barg, discharge 150 barg, 3 stages, 0.37 kW, 2x10-5 m3/s which I'm assuming is the 1'st stage volumetric displacement.
http://compressors.rixindustries.com/item/all-cate...
Given your requirements of 7 barg inlet, 200 barg outlet, 3 stages and I'm assuming air or similar diatomic gas, the power required is around 0.1 kW. Even the original 1 barg inlet, 200 barg discharge with 2 stages isn't completely ridiculous. Most of the machines I work with have compression ratios of 8:1. They include metal diaphragm machines and hydraulic intensifier designs such as the Hydro Pac. I also design cryogenic reciprocating machinery in the 100 hp range with 2 stages that can easily do 8:1 compression ratios and often slightly higher. I'm not so surprised by the high compression ratio.
To determine power required, I would suggest using a thermophysical properties database such as Refprop. You can then pull enthalpy values for your gas directly from the database and use those to determine power required for each stage.
To answer your question, "Why the huge discrepancy between the theoretical and the actual power required?" I don't think there's such a large discrepancy. Perhaps your volumetric displacement is off? I'm assuming you don't mean standard cubic meters per second which would be an even smaller machine. I've assumed the first stage displacement is 2x10-5 m3/s.
You'll find that manufacturers who make reciprocating machines that are as small as this one (and even much larger) have to consider a trade between motor size and adding relatively large flywheels since most of the power required for a recip is only needed for a short period of time at the very end of a stroke when the gas pressure has risen close to the discharge pressure and must be expelled from the cylinder. On larger machines, manufacturers will install flywheels, but for smaller machines those flywheels aren't as cost effective as simply increasing motor size by a factor of 3 or more. It's very common to find small, reciprocating machines like this with relatively large motors for that reason.
RE: Power required for compression
Thanks again for the helpful post and happy new year!