mark.c
10-10-2011, 03:01 PM
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
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