Heat Pump Heat Exchanger Sizing

Evening all,
Just have a question about the sizing of heat exchangers in water/water heat pumps.
If a system is 'cooling only' and the cooling capacity was 6000BTU then I would select a 6000BTU Evaporator, 6000BTU compressor and a 7500BTU Condenser (cooling capacity +heat of compression).

But what happens when it switches over to heating mode? I imagine this sized indoor coil would then be too small and the system pressures would increase.

So is it better to:
A. Size the coils based on cooling only mode and just let the pressure increase in heating mode

OR

B. Size both of the heat exchangers to the condenser capacity (in cooling mode) and therefore a 6000BTU unit would have 2x 7500BTU heat exchangers. Note that they are fixed speed seawater pumps so the flow rate through the condenser doesnt change and neither does the chilled water flow rate.


Also,
Is there any software that you can recommend to make the component selection process easier.
ie you have a cooling capacity of 6000BTU therefore you need this size compressor, condenser, evaporator etc. Im using Danfoss, Copeland and Bitzer software but was wondering if there is something more specific to reverse cycle air conditioning.

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Are both heat exchangers water?

Water to water normally has the evaporator and an air cooled condenser.
If the condenser is air cooled then when it reverses cycle the fan can be capacity controlled
and can regulate the evaporator temperature?

It gets a whole lot more complicated if both heat exchangers are water.

Rob

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Thanks for the reply Rob.

Im actually looking at reverse cycle Fan Coil Units so these would be aircooled indoor coil and coaxial water cooled outdoor coils.

The other type is for small reverse cycle chiller units so they would be BPHE indoor coils and coaxial condenser coils.

I had a reply today from a coil manufacture who seemed to think just size them as a cooling only circuits and let the pressures increase in heating mode because the rise wouldnt actually be that much anyway and the coils would be able to handle it.

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I suppose because the water heat exchangers will have far better heat exchange capabilities
all the load will be placed on the compressor so as long as the comp can deal with the load
it should be ok. The only other options would be fan speed controller and or can you add flow
control to the water side?

Have an inverter controlled compressor that is capacity controlled to regulate the pressures?

Rob

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