AHU coils - DX or Chilled Water/LTHW?

[mark.c]

Hi There,

I'm currently designing a ground source heat pump installation for a development and am considering the options for the heating and cooling coils in the AHUs. The main heating/cooling system will be VRF - Water to refrigerant heat pumps configured for ground loop temperatures, with refrigerant connection to local FCUs. In addition to this there will be AHUs. One for tempering (pre-heat/pre-cool) the fresh air load and a couple for areas that require full air-conditioning. There will also be underfloor heating, which is likely to be supplied via a water to water heat pump also running off the ground loops.

The question I have is regarding the AHU coils. In theory it seems like a good idea to use DX coils in the AHUs. We would need 3 of them - frost protection, cooling and heating (also used for reheat). Efficiency at full load should be quite high. However, as I understand it, it is not possible to modulate the output of a DX coil very effectively. This would make it difficult to achieve close control of supply air temperatures or meet part load conditions. It would negate any energy benefit of using DX rather than water. Is this right or is the technology available to get the same level of control as is possible with chilled water or LTHW? Part of the reason to go for refrigerant is the space saving as plant room space is limited. Anyone's thoughts on the pros and cons of DX VS CHW/LTHW would be much appreciated!

Mark

[MikeHolm]

IMHO, if you have multiple zones for the heat use the GSHP to heat a big storage tank and send the water out to the air handlers. I don't know about your building design but some buildings can have a heat load on the north side and the cooling load on the south side and having a properly piped system can allow you to cool one side while heating the other. It gives flexibility.

[mark.c]

The VRF system allows for heat recovery in-between zones for simultaneous heating and cooling. Space is quite limited so we would only allow the minimum buffer tanks for CHW/LTHW systems.

[frank]

Firstly, if you are using fresh air on your AHU, then DX will not be suitable for heating both the frost coil and reheat coil, due to the low dt it can produce across the coils.

DX cooling though would be OK.

Daikin do a condensing unit (ERQ range) that can take a 0-10v input signal and control output accordingly. We have had success using these units on AHU's where either the load varies considerably or the air volume varies in response to the extract volume demands (laboratory fume cupboards etc).

Closer control on AHU's can always be achieved using water based heating and cooling via 3 port modulating valves, although chilled water cooling does not always de-hum as good as DX.

[RSTC]

Climate master do a range of water source AHUs. But I would agree with CM, don't use DX for fresh air (either heating or cooling).

Are you using heat recovery on the AHU's?

there are a number of nifty water cooled reverse cycle chillers with built in vessels which shouldn't be too much bigger than a condensing unit. You may only need one instead of three.

[RSTC]

Sorry, Frank not Chillerman. (But if you want to throw your oar in CM, I'll agree with you too)