AWHP superheat & sub-cooling

[Gary]

I have seen a number of references from some of the manufacturers, with water discharge temps quoted in the range of 70-75'C, using R-134a.

Now, this would at the very best, require Tc,sat temps in excess of 70-75'C, with minor recovery from the de-superheating of the incoming hot gas stream. This operating point is right up hard against the European compressor performance envelope & exceeds the US advise.

Practically, with air approach temps in the range 35'C, it will be an ongoing struggle to maintain Te,sat < 15'C.

As we've found, the two viable options for maintaining Te,sat<15'C are:
1. Control fan speed;
2. Use an MOP setting for the TXV.

The first option gives lots of spare room to optimise further, the MOP option concerns me for a number of reasons (maintenance issues, performance issues at start-up?, price/availability).

And let's not forget:

3. CPR valve.

Of these three, the MOP would be the least suitable. It reduces the Te,sat and compressor load by reducing refrigerant flow through the evaporator, but this raises the superheat and thus reduces the compressor cooling. The reduced compressor load would probably compensate for the high superheat to control the compressor heat.

The CPR would do the job and in fact would be the simplest. The evaporator pressure increases with the load, but the CPR will allow no more than setpoint pressure to enter the compressor.

The fan control strategy responds directly to the compressor heat and I'm thinking will be the best in safeguarding the compressor.