Heat-pump mass charge determination

[desA]

Below is a picture of a typical mass charge determination run for an a/c system.

I have superimposed on the evaporator power & compressor power, the condenser output power curve (in red).

For a heat-pump system, which mass charge would you select, & why?

http://i50.tinypic.com/2cge8oz.jpg

[desA]

The original test data.

http://i50.tinypic.com/2mmgso6.jpg

[desA]

Taking this a step further.

There are, for an a/c system, two 'logical' points to consider:
1. At COP maximum;
2. At maximum evaporator performance (Q'e,max).

For a heat-pump, the condenser output performance (Q'c) seems to continue rising as refrigerant mass charge rises.

Where should the heat-pump cut-off come?

[Gary]

Taking this a step further.

There are, for an a/c system, two 'logical' points to consider:
1. At COP maximum;
2. At maximum evaporator performance (Q'e,max).

For a heat-pump, the condenser output performance (Q'c) seems to continue rising as refrigerant mass charge rises.

Where should the heat-pump cut-off come?

Surely there must be some mass charge point at which Q'c tops out?

[desA]

Thanks Gary.


Surely there must be some mass charge point at which Q'c tops out?

Not according to the chart shown (or in practice, for that matter). It seems that the decision for a heat-pump may not be to cut-&-dried as for an a/c. This is what makes it more difficult.

Things like condenser SC & evap SH seem to become limiting factors, for consideration, well before a Q'c peak can be observed.

What makes matters even more complicated is that the performance of the system changes across the heating range. So, practically, a set of these curves needs to be developed at each stage along the heating cycle. For the purposes of this thread, we can take it that the curve shown represents the hottest condition for the heat-pump.

[Gary]

It looks as if the condenser output, power input, discharge pressure and subcooling are all going up in unison.

It would seem there must be some point at which they part ways. In particular I would expect the subcooling to cause a reversal in condenser output as the liquid backs up into the condenser.

[desA]

It looks as if the condenser output, power input, discharge pressure and subcooling are all going up in unison.

It would seem there must be some point at which they part ways. In particular I would expect the subcooling to cause a reversal in condenser output as the liquid backs up into the condenser.

What I've observed practically, is that towards the top end of this mass-charge scale, the condenser pushes back by raising approach & SC as well. There must obviously be some limit, somewhere, but this seems to be moving towards an unsafe region in terms of compressor operation, for no obvious benefit, as it is already past the point of maximum COP.

What also happens is that Te,sat rises at the same time - releasing more mass-flow, further feeding into the problem.

Its like these systems don't have a self-regulating mechanism within reasonable pressure bounds.

[Gary]

Where is the point of maximum COP?

[desA]

Where is the point of maximum COP?

On the charts above, the point of max EER seems to be at 75% final charge.