The relationship between phase change and COP in refrigeration cycle

[yfuo28]

Hey all!

For some research I'm doing on liquid cooling, I'm trying to figure out how changing the phase, physically, is related to the efficiency of the refrigeration cycle.
That I look mathematically at the formulas of cooling cycle efficiency I understand why the efficiency is greater than 1.
But understanding is only mathematically and not physically.


I tried searching on media about it and didt find it.
If someone can explain or direct where to look for it will greatly help me.

thx!

 

[chicopee]

whether you are calculating COP or the efficiency of a refrigeration cycle, you need to know the work required for that cycle which is the work from the compressor. The compressor can not have any saturated liquid during compression of the refrigerant therefore the refrigerant is slightly superheated, around 1 dF or 1 dC during entry into the cylinder. If saturated liquid enters the compressor cylinder, the piston head will crack because the liquid can not be compressed as a vapor; and since the clearance at top dead end of the compression is a few thousandth of an inch or a few millimeters, with that small clearance any liquid will crack the piston head.
Therefore to avoid the catastrophe mentioned above, the saturated liquid entering the cooling coil(or evaporator) must change phase, being a vapor phase, before compression.

 

[yfuo28]

hey,

thanks for your reply.
I know this reason for the change in phase.

I try to understand if the phase change has an effect on COP.
Is the change in phase cause to the COP greater than 1 in cooling systems?

 

[IRstuff]

Is this for school?

A phase change is efficient, because the conversion from liquid to vapor, or vice-versa, comes with the heat of fusion, which is often substantially larger than just the raw heat capacity.

[edit] sorry too early. Nevertheless, liquid to solid also applies, specifically for passive cooling applications where a solid is melted to maintain temperature of a system that may be constrained from using conventional cooling methods.

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[yfuo28]

not for school, work.

But in a cooling circuit the phase change is between liquid and gas and vice versa.
Or it's the same like liquid-solid.

 

[MintJulep]

I guess you need to look at:
Pumping cost
Mass flow of the refrigerant
Change of enthalpy of refrigerant at condenser and evaporator


Simplification:
No phase change --> change of enthaply is basically a function of mass flow only. Pumping cost is a function of mass flow only.
Phase change --> change of enthalpy is a function of mass flow and fluid properties. Pumping cost is a function of mass flow and pressure change.