Supply Air Duct Condensation Temperature

[jdmz]

We are adding a cooling coil to an existing commercial VAV AHU without supply air duct insulation. I am trying to calculate the minimum duct temperature that is allowable while avoiding condensation forming on the duct surface. The duct is exposed in the space and some of it is in a plenum but the system has plenum return so the air conditions should be close to what the room temperature is.

I understand there will be condensation if the duct surface temperature is at dew point temperature in the space. However, the only sources of outside air coming in the space are infiltration and the air introduced into the room from the duct (for example, near saturated 55 F air).

Thoughts? Raising the supply air temperature of the system also allows more moisture into the space which is why it's a bit confusing.

 

[nuuvox000]

I think a starting point is finding out exactly what temperature (db/wb) the room is kept in the summer. Find the room conditions on the psych chart and then it's easy to follow in a straight line to the left to find the temperature that water will condense in that environment. If not acceptable for any reason, just install the duct with liner; a lot of manufacturers make round duct with insulated liner.

https://www.jm.com/en/hvac/duct-liner/spiracoustic-plus/

 

[MedicineEng]

what are you cooling?
Remember that there are other sources of water vapour in an area besides infiltration and outside air.
And typically plenum areas are warmer than room temeprature.

 

[LittleInch]

I'm struggliong to understand this. Can you draw a sketch with where the air flows are going and what the temperatures are please.

When you say "supply air" do you mean cool air from the AHU?

If it is why is it not insulated?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zar45]

2021 IECC C403.12.1 ... Exception 2 says the duct does not require insulation if the "design temperature difference between the interior and exterior of the duct or plenum is not greater than 15°F". I error on the side of insulation. Especially in moist climate zones.

 

[HVAC-Novice]

You'll need to meet ALL codes and use common sense. IECC likely only is concerned with energy savings. That is why they don't care about insulation if dT is less than 15F. but i bet somewhere in IBC thee is something about not dripping condensate or any other water inside the building (and if not in IBC, then it is in the common sense law).

So, if there is a chance of condensation, insulate it. and a room could have any dewpoint... you don't know about actual use, infiltration, humans sweating etc. Our design just assumes some standard scenarios. Infiltration is a huge unknown and the climate you are in plays a huge role here. Likely not a problem in Arizona, but a huge consideration in Florida.

 

[jdmz]

This is an existing building with an air handler that never had cooling. There are provisions (room in ahu) to add a cooling coil.

The ahu is serving a gym, state, and a cafeteria. The reason why we evaluating not adding insulation is because it is extremely expensive.

 

[HVAC-Novice]

I would check if the existing AHU and trunk ductwork has sufficiently low perm rating. If it never was intended to cool, it may just be insulation for heating (where condensation doesn't matter). New insulation = expensive. Corroded ducts and AHU = more expensive.

 

[ChasBean1]

The cooling coil: DX or chilled water? That matters. Infiltration: the space should be slightly positive. For a negative space, you can count on the duct sweating, especially if temperature control is cyclical.