Cost Savings - Reducing Condenser Approach Temperature

[TECMSC]

We're considering some work to reduce the condenser approach temp. e.g 6°F to 1.6°F. In order to move forward I need to calculate the cost savings.

The system has a water-cooled centrifugal chiller. Can the cost savings be calculated by using site data e.g. gpm, temperatures, power cost? or, is there rule of thumb i.e. each 1°F change reduces power consumption xx% etc. Not quite sure how to approach this issue.

Can someone assist in calculating cost savings? Thanks.

 

[MechEngNCPE]

It all comes down to the BTU's/h your chiller is consuming. Q=mdotCpdeltaT then compare that with the other temperature, and then convert both to kW, and then multiply by dollers per kW.

 

[MechEngNCPE]

Or you can get the kW/ton from the manufacturer and do equivalent full load hours. Assume .59 kW/ton for centrifugal chillers 300-500tons and .58 for 1000tons.

 

[quark]

Heat rejection by chiller remains same irrespective of the condenser approach temperature, so the equation mcpdT is of not much use to you. Since you are going for lower approach, the discharge pressure of the compressor reduces and thus the power consumption. Check the saturation pressure of refrigerant at new condensing temperature and get energy data of compressor for this pressure.

Deduct the extra energy you require for lowering the condenser approach.

 

[TECMSC]

sorry for the late response, I was away for awhile.

quark - i would think the mcpdt equation can still describe an aspect of the situaion - the heat rejection stays the same then the mass flow increases(corresponding pressure increase)and refer DT decreases due to higher refregerant velocity.

Once the system discharge pressure increases the compressor lift becomes excessive and the centrifugal compressor goes into surge mode. At this point the heat rejection is at its max (not to mention the potential impellar damage). Make sense or do I have it backwards....?