Effect of intercooler temperature on efficiency

[ClicketyClack]

A colleague of mine is evaluating the benefits of installing a 200 kW, two stage, oil free, rotary screw air compressor with energy recovery. The energy recovery option recovers the heat of compression and released latent heat of water in the compressed air stream. This recovered energy would then be used to heat process water elsewhere in the plant.

The argument is that under the right conditions, the air compressor has a carbon zero footprint. I suspect the ambient reference conditions in this scenario would yield a heat index of 200 degrees F but that is beside the point.

I noticed in the energy recovery documentation that the cooling water loop within the compressor is "rearranged". Normally the cooling water encounters the intercooler first. With the energy recovery option the cooling water hits the intercooler last. The result is an intercooler cooling water inlet temperature increase from 80 degrees F (standard offering) to 130 degrees F (energy recovery offering).

If you hold the ambient inlet conditions constant, wouldn't the efficiency of the system be reduced with higher cooling water temperature in the intercooler? If so, where could we expect to "see" this reduction in efficiency? It's a positive deplacement machine so I can't comprehend significant reduced flow. However neither can I illustrate an increase in power draw. Is my intution misleading me?

 

[dcasto]

A zero carbon foot print should be your first clue that something is wrong.

Next, the thermaldynamic efficency of a screw compressor is lower than that of a reciprocating one.

raise the interstage temp on a multi stage unit does increase power requirements to raise the gas presssure. BUT, it does make for a higher discharge temp and therfore hotter water in the heat recovery system, so in effect they are recovering energy that they wasted on purpose.

 

[ClicketyClack]

Thanks for the response dcasto.

I agree about the carbon footprint. However the vendor produced a certificate from a third party testing facility certifying the claim (TUV). They claim the additional energy to exceed efficiency and heat losses lies within the latent heat of water. Therefore I suspect ambient conditions are unrealistic, maybe a 200 degree F heat index for example.

I just want to make sure the increase in power draw is accounted for in any calculation. I ran the numbers assuming an isentropic compressor and yielded a 6% increase in BTU/lb of air. In reality, given the high temperatures involved, I assume this percentage will be much higher. I suspect this loss in efficiency will be divided and "seen" with an increase in power draw and slight reduction in flow.

 

[dcasto]

OK, now I see what the vendor is doing. He is making a classical "heat pump". the heat you can put into your home with a heat pump is more than the energy you put into the refrigerant compressor. This difference is called COP of the heat pump. For example, you can run a 100 kw/hr compressor and reject into the process 400 kw/hr of heat.

the best way to simulate it is to use and 80% efficient compressor on each stage and watch how the energy input changes with the change of interstage temperatures.