AHU Wrap Around Coils

[Scott_More]

I have a system in a office building each floor has a AHU with a wrap around Chilled water coil that goes through a refrigerant wrap around coil. these units are feed pre conditioned outside air from a DOAS already do the really need to use a wrap around coil in the interior AHU's or is this just reheating the air for nothing. Located in Houston TX so we do have high temperature and high humidity but it seems like because we are not introducing humid OA it is not needed. Would removing the R134A refrigerant out of the wrap around CHW coil help lower the discharge air temperature?

 

[MintJulep]

Please make a sketch of the system.

 

[HVAC-Novice]

Provide more detail. Do you mean a wraparound heatpipe? Learn how it works and how it impacts the air. If you remove the refrigerant, it basically will just be a piece of air resistance.

 

[Scott_More]

Here is a heat pipe system same concept as a wrap around coil. Now I read they add a liquid line solenoid to stop the refrigerant flow when you don't need all that dehumidification
Scott

 

[HVAC-Novice]

That's what I assumed. Yes, if you stop the refrigerant flow or remove the refrigerant, the heatpipe will do nothing besides adding friction.

Generally speaking, disabling the heatpipe would slightly lower DAT and allow more moisture. You would gain nothing in comfort since at higher humidity ratio, people need lower DB. Look at a ASHRAE 55 comfort chart. Unless people complain about dry air, I don't see a benefit. Especially since you still pay the fan energy penalty for having that coil in the air stream.
Did you talk to the designer? I assume they put that wraparound coil in there for a reason. I doubt anyone in a humid climate is wishing for more humidity.... but you know the building and occupants better than us Internet randos

 

[SAK9]

Since the ventilation air is preconditioned(usually to room temperature) the on coil temperature for the wrap around coil is the return air temperature.Assuming the off coil from the cooling coil is 13C there is roughly 9C((25-13)0.8) of reheat available from the wrap around coil if it is about 80% efficient.It is not clear what kind of an AHU is there on each floor VAV/CAV.It appears that the wrap around coil is providing reheat to maintain RH in the office areas.If you disable the wrap around coil AHU discharge temperature will go down to 13C or whatever it is designed to.With the current design you do not have any control over the AHU discharge temperature as the control system has no influence on the wrap around coil.If you have a solenoid valve on the wrap around coil you can enable/disable it based on average RH in the office.You might want to disable the valve for a season and see how the office RH holds up

 

[Scott_More]

I can't turn it off because this is the first-generation of
Wraparound no solenoids yet,. I would have to recover the refrigerant to test this.
Thank You!