consumed power in compressor

[shamanlawrence]

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,

 

[zdas04]

The same mass flow rate (and the same suction/discharge pressure/temperature) of two gases with different molecular weights would require the same hp.

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

 

[chicopee]

Of course you have not indicated if we are dealing with liquids or gases. If dealing with liquids, the type of viscosity can influence the results in horsepower.

 

[georgeverghese]

As mol wt increases, the Cp/Cv of the gas also increases for hydrocarbons, and you'll find the increase in hp due to increase in mass rate through the compressor is offset by the decrease in hp due to the increase in Cp/Cv ( since required polytropic head is lower for higher Cp/Cv to get to the required discharge pressure). So loosely, speaking, there is little to no effect on absorbed power for a change in mol wt of the gas if std volumetric rate is kept constant.

 

[zdas04]

georgeverghese,
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

 

[georgeverghese]

Hi David,

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

 

[NghiaPP]

It depends on the power is calculated for a compressor in design phase or the compressor has been built and now to be operated with a gas with higher MW.
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.

 

[NghiaPP]

In the previous post, I forgot to mention that centrifugal compressor has been assumed. It is not valid for reciprocating compressor. In this case the power will be less.