Low discharge temp??

[Fnhanha]

Hello everybody.
I am new at this forum!
I am developing a heat pump working on R410a, and i am having some problems that i can't understand with my discharge temperature...
I am measuring a discharge temperature that is lower than my isentropic temperature??? What means that my compressor isentropic efficiency is higher than 1 (not possible)... I have a suction line accumulator so i don't think i am getting liquid at the compressor! an example of values that i am registering is disch temp 55ºc, condensing temp 45ºc(27bar), temp entering the condenser 50ºc, sph=1K, evap Temp 1.53ºc...
Can anybody help me understand what is happening?
best regards

[AlexG]

Maybe your suction accumulator is full of liquid... Check your superheat. Must be 5...10K

[nike123]

Low suction superheat mean low discharge superheat.
You have 1K suction superheat and 10K discharge superheat which, to me, sound OK regarding low suction superheat.

[Fnhanha]

Thanks to your response, it looks like i had too much refrigerant on my system... and i was getting a wet compression...I am trying to optimize my R410a charge now, for best COP and EER at the conditions of EN 14511... Any suggestions?

[AlexG]

To optimize ref charge check subcooling, Must be 5...8K. Good luck.

[Fnhanha]

Thanks AlexG, but should it be 5...8K in heating mode?

[AlexG]

It must be 5...8k in any mode. But cooling/heating mode needs different ref charge. So design of your system must compensate this difference (liquid receiver). If main mode is heating one and cooling is for defrost only, pay more attention for heating...

[Fnhanha]

Thanks for your reply!
I have tried with different charges, but i get a better COP for high subcooling (13K) than for 5...8K! I am confused about this... The higher COP better right? I still think i have under dimensioned my heat exchangers this can be a problem... I am making a review at that now...

[AlexG]

Of course 13K subcooling is better for COP than 5K. But you fill condenser with refrigerant and your cond pressure goes up. If you are able to have 13K SC at normal cond pressure it means your condenser is oversized. For COP it is good but for your budget...

[Fnhanha]

Thanks Alex,
My condenser is really oversized for the compressor and evaporator that i have got right now... I am getting a new compressor with more refrigerant flow... and a bigger evaporator... so i can increase the sistem load...
regards...