Hot Water Single or Multi Pass?

[AndyHX]

Hi all heat pump enthusiasts !
I have a few questions about design of hotwater heatpumps for domestic hot water . I was thinking of modifying an A/C unit , i have 2 PHE which match the units heating capacity. If i use the first one as condenser and have a water regulating valve to control head pressure and temperature out , and then liquid going into second phe and the coldest water from cylinder subcooling this further.
The other way is no regulating valve and circulate water through system at high flow and gradually pick up temperature. this would seem to produce a system that would be unstable , i am unsure about the effect of evaporating pressure if condensing pressure is only just above the temp of water out? but i quess you would be getting a higher cop for a start ?
Is one way more efficent in the full cycle of heating up complete cylinder.
If water is gradually getting warmer i will lose my subcooling effect in my sc phe.
So that is why i am favouring the regulated system at moment.
I would appreciate all views on this
cheers Andy

[desA]

Chat to MadFridgie - he's the resident heat-pump guru in your neck of the woods.

[mad fridgie]

Thanks DesA.
Multipass by design has to be more efficient, as long as you have a wide operating range expansion device.
Econergy unit is a single pass and is well done, because it has excellent thermal length heat exchanger, testing standards tend to benefit top down heating systems (one pass).
If your unit has a capillary then one pass, if it has a variable expansion device TEV, EEV, then multipass, just let the system reach equalibrium, your SST drops so your heat transfer increase at the evap.

[AndyHX]

Hi Mad , if i am using 2 plate exchangers, would it be best to use one pass and have the benefits of subcooling? You mentioned the econergy with the long heat exchanger , i guess that they get capacity with good subcooling?
I will be using a txv.
thanks Andy

[mad fridgie]

Hi Mad , if i am using 2 plate exchangers, would it be best to use one pass and have the benefits of subcooling? You mentioned the econergy with the long heat exchanger , i guess that they get capacity with good subcooling?
I will be using a txv.
thanks Andy

Using a TXV I would go multi-pass, have avergared lower discharge pressure, PHE are not the best for single pass systems, but good for multipass.
if you want to use 2, "because you have them" just but them in parallel, reduces both water and refrigerant pressure drop.
Install a sparge or diffussion pipe in the bottom of the cylinder (for the return water)
draw water out of the bottom, re-introduce into the bottom (via sparge). You are looking at a very small split 2-6C
Temp control sensor about 1/3 to 1/2 up the cylinder, unless a very big user of water then have at the bottom (slight loss of efficiency, but better usability) I do have a better method than any other on the market "SFT", but this SFT system has a patent, and i would hate to have to sue you LOL

[AndyHX]

Ok thanks for your advice MF.