COP and Superheat - does is it help?

[Chef]

Without do the numbers your results look correct!

Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.

My SH is useful as it comes from the evaporator but just because it useful in terms of taking heat away from the box/product does mean it is useful in a cycle as the COP falls.

The more you increase the SH inthe the TXV the further down the evaporator pressure gets depressed and so COP falls off.

My initial comment still stands - I have not seen a COP increase with SH.

[Peter_1]

Solcane
with 50% compressor effficiency
Troom = -20°C, Te = -21°C SH suction line is 3K -> COP 1.95
Troom = -20°C, Te = -21°C SH suction line is 8K -> COP 1.91
This seems normal although numbesr are a little different .

[Chef]

Solcane
with 50% compressor effficiency
Troom = -20°C, Te = -21°C SH suction line is 3K -> COP 1.95
Troom = -20°C, Te = -21°C SH suction line is 8K -> COP 1.91
This seems normal although numbesr are a little different .

The SH must be taken from the Troom and at your conditions this does not seem feasible.

So you have Troom at -20C and Te at -21C but how does Troom add 8C to the SH - it is not possible unless Troom was at -11C. My guess is that both Coolpack and Solcane (do not know this program) both allow you to vary the SH outside of practical limits.

If you want an 8C SH from your room at Troom=-20C then Te has to be -29C. That will indeed reduce the COP.

Your results show a drop in COP but not nearly enough for the extra 5C.

Also I get 1.776 and you get 1.95 so a difference here as well.

[Peter_1]

So you have Troom at -20C and Te at -21C but how does Troom add 8C to the SH - it is not possible unless Troom was at -11C. My guess is that both Coolpack and Solcane (do not know this program) both allow you to vary the SH outside of practical limits.

If you want an 8C SH from your room at Troom=-20C then Te has to be -29C. That will indeed reduce the COP.

Your results show a drop in COP but not nearly enough for the extra 5C.

Also I get 1.776 and you get 1.95 so a difference here as well.

Of course Chef, in a room at -20°C, you can't add 8SH in a normal, practical way. But you can do it of course with an in-line heat exchanger p.e. But I thought you guys were more discussing theoretical values to prove that Coolpack calculates something wrong


I also noticed the different values for the COP both program gives you. Solkane is from Solvay and can be downloaded for free at their website. It has a nice graphical interface, better than Coolpack but less features than Coolpack.
Honeywell also made something similar and I tested it some time (in SI units) but found very strange results when calculating cycles. Seems that the authors of this program doesn't care about there mistakes although it was nice made.

[Peter_1]

....
The more you increase the SH inthe the TXV the further down the evaporator pressure gets depressed and so COP falls off.
..

Chef, the SH isn't created in the TXV but at the end of the evaporator and then further not usefully outside the cold room. If it comes from the room, even from the suction line inside the room, then energy that needed to be removed from the room was removed, so this energy was useful.
So with the energy needed to evaporate the refrigerant, we add some additional energy also taken from the room to become a total energy retracted versus needed energy. If we add some more energy,then TE will also slightly increase, TC as well but influence of Te is much bigger resulting in a COP increase.

In fact, I guess this explanation wasn't usefull :-) for you at all and it's me not following for 100% due to a 'language-misunderstanding-barrier' from my side?

[Chef]

Chef, the SH isn't created in the TXV but at the end of the evaporator ?

I got this and the bulb is positioned at the end of the evaporator and is set to 8C so the TXV and the bulb act together to control the SH. That is how it was input to the program. It seemed to understand.

So if my temperature is set to -20C then the system keeps on running till the Te is -29C and the SH is 8C.