consumed power in compressor
consumed power in compressor
(OP)
Dears,
I have an important question about consumed power in compressor, I read in some articles that the mentioned power increases with molecular weight i.e higher molecular weight needs higher power, but when I tried different composition in Hysys, I found that higher molecular weight needs lower power. Of course with the same feed conditions (pressure and temperature) and same molar flow rate.
Are my calculations right or not, and please explain the reason of any result.
I know that mass flow rate will increase with molecular weight, yet I have noticed that the consumed power in compressor depends on volume flow rate not mass flow rate.
If somebody helps me in this issue, I will appreciate it so much.
Best Regards,
I have an important question about consumed power in compressor, I read in some articles that the mentioned power increases with molecular weight i.e higher molecular weight needs higher power, but when I tried different composition in Hysys, I found that higher molecular weight needs lower power. Of course with the same feed conditions (pressure and temperature) and same molar flow rate.
Are my calculations right or not, and please explain the reason of any result.
I know that mass flow rate will increase with molecular weight, yet I have noticed that the consumed power in compressor depends on volume flow rate not mass flow rate.
If somebody helps me in this issue, I will appreciate it so much.
Best Regards,





RE: consumed power in compressor
The only way that increasing molecular weight increases (or decreases) hp requirements is if you are holding volume flow rate at actual conditions constant between the cases. The equation I use expects volume flow rate at standard conditions so I get the same hp for different molecular weights.
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: consumed power in compressor
RE: consumed power in compressor
RE: consumed power in compressor
Good catch. Changing from methane Cp/Cv to air gave me an increase in hp of nearly 30%. Heavier defiantly increased hp required for a constant mass flow rate and suction/discharge conditions. I'm assuming positive displacement compressor so I'm using the adiabatic exponent instead of the polytropic exponent.
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: consumed power in compressor
I get a decrease in hp for constant mass rate when changing from methane to air, with all other inlet and exit conditions te same, which matches the observation that the OP has made.
For low pressure range, Cp/Cv for these 2 gases is 1.26 and 1.4 respectively. Yes, for a pd machine where isentropic eff is approaches 1.0, the adiabatic exponent can also be used since at this eff, adiabatic eff = poly eff = 1.0
RE: consumed power in compressor
If in the design phase, the higher MW gas will result in lower power (with the same temperature, pressure and volume flow rate). The reason is given in previous answer. It should be note that the result will be two different compressors. The compressor designed to handle low MW gas will have larger head than the compressor with higher MW gas.
For a compressor, that has been built for low MW gas, if the feed is changed to high MW gas, the power will be larger with the same speed. The main reason is the same head will be produced for the high MW gas. It results in higher discharge pressure thereby higher power.
RE: consumed power in compressor