What is the Effect of Increased Subcooling on Capacity and Efficiency?

[eric_ul]

Here is the scenario:
A/C system has optimized charge amount for mfgr. specified subcool and superheat.

We then decide to cool the liquid line with water to increase subcooling.

Assuming that there is still sufficient superheat at the compressor, will this increase capacity, efficiency, both, or neither?

Eric

[mad fridgie]

Here is the scenario:
A/C system has optimized charge amount for mfgr. specified subcool and superheat.

We then decide to cool the liquid line with water to increase subcooling.

Assuming that there is still sufficient superheat at the compressor, will this increase capacity, efficiency, both, or neither?

Eric

This can be descussed in great length, so the simple answer "yes" increases capacity and efficiency.
By sub cooling you are reducing flash gas, after the expansion device, which leaves a greater proportion of liquid for net cooling, because you have more capacity, (whilst still requiring superheat), you will have a bigger TD across the evap, so you would expect a slightly lower suction pressure to reach an energy equalibrium. (slightly lower suction refrigerant mass flow), which also means you can have a lower discharge pressure (the two drops in pressure cancel each other out as far as efficiency) Where it becomes complex is how the reduction in flash gas effects the internal heat transfer of the evaporator. (in some case a massive benefit (normally evaps with high internal pressure drops), in others a negative effect)

[eric_ul]

Thanks for the help.

[powell]

Here is a good explanation from my friend "Icemeister" who has posted on other HVACR talk forums.

"The percentage varies with different refrigerants, but for R22 the capacity increase is about .5% per ºF if the subcooling comes from outside the system. So if you gain 10ºF subcooling from an external source, the system capacity gain is about 5%.

In the case of suction/liquid heat exchange, there's no net gain as the gain due to liquid subcooling is offset by the compressor capacity loss due to higher suction superheat."

[Sandro Baptista]

In the case of suction/liquid heat exchange, there's no net gain as the gain due to liquid subcooling is offset by the compressor capacity loss due to higher suction superheat."

Not true for R404A and R507.