Coiling and heating coils and humidity - Brisbane

[geopat69]

Hi all

I am a not a hvac engineer, but would appreciate some help in understanding something

This is in regard to a typical Brisbane 20 storey building with AHU on every floor (each floor has a small mech plant zone and the AHU is inside that room)

My question: Are the AHU cooling coils meant to be ON even in winter? I.E to remove humidity? Or is it only the heating coils that are ON? Or do they both get activated?

I am getting confused with mixed opinions. I have been told that in winter within the AHU, the cool outside air is first preheated in the AHU, then cooled inside AHU, then reheated in the AHU, before being discharged tonthe office space.

Would love a laymans explaination if somebody could kindly asssit.

 

[nuuvox000]

There's not enough information to tell for sure. They could be fresh air units; is the intake ducted to the outdoors for 100% outside air? Not sure why they would preheat just to cool a second later.

Any plans would be helpful.

 

[BronYrAur]

I don't know anything about the climate in Brisbane, but "cold" air can't hold nearly as much moisture as "warm" air. So you probably do not need to dehumidify in the winter. The air is probably dry enough. So you are probably only preheating it in the AHU and then you may have "reheat" coils in the ductwork. You are not running the cooling coil, so the term "reheat" isn't really appropriate in the winter. The duct coils would just further heat up the air to the desired condition.

If you don't have reheat coils in the ductwork, the AHU reheat coil would serve the same purpose. You are not cooling, you are just heating up the air more than the preheat coil can provide. This is just my guess.

It would not make sense to preheat and then cool immediately after. It would make sense to cool in the summer and then reheat, but not preheat.

 

[geopat69]

Appreciate the responses to you both. Thanks.

If its any help...the office space has egg crate grilles in ceiling tiles and i assume thats for the return air ... so maybe not 100 percent fresh air unit??

To be honest, being a novice at this.. i did not know 100 percent fresh air was even around for ahu...due to high energy cost to cool down 100percent outside air.

 

[MintJulep]

According to Bard AI the relative humidity in Brisbane during the winter is pretty low, so there is likely no reason to dehumidify.

There could be exhaust heat recovery in the AHUs. If so, that might explain the "the cool outside air is first preheated in the AHU" statement.

 

[SAK9]

In Brisbane the winter day temperature is 21-22C so there is no need of any heating during the day. The only heating you may need is a morning warm up. From what I have heard most buildings in Brisbane do not have any heating. I have come across similar weather in the Middle East winter as well and I have not seen any heating being employed there too.