Ductwork condensation
Ductwork condensation
(OP)
Hi Guys,
We want to use an existing uninsulated fresh air ductwork hidden in ceiling to cool a post office by adding a DX cooling coil at the supply. Should we be concerned about the condensation on the ductwork exterior surface hidden in ceiling which is not a return air plenum (no air circulation).
The new DX would be 4000 cfm / 2 stages.
Most of the cooling load is sensible (light and sun) but we are located in Montreal where summer is humid.
Thanks
Alex
We want to use an existing uninsulated fresh air ductwork hidden in ceiling to cool a post office by adding a DX cooling coil at the supply. Should we be concerned about the condensation on the ductwork exterior surface hidden in ceiling which is not a return air plenum (no air circulation).
The new DX would be 4000 cfm / 2 stages.
Most of the cooling load is sensible (light and sun) but we are located in Montreal where summer is humid.
Thanks
Alex





RE: Ductwork condensation
As the cooling continues, humidity should lessen during the hours of operation and less sweating should occur on the duct surface.
RE: Ductwork condensation
More so, you should do something to prevent it. Like insulating the duct.
RE: Ductwork condensation
Since we don't have much moisture generation in this place (beside outside air which is dehumidified at cooling coil), do you think we can take the risk of adding a DX coil without having major condensation problem on ductwork.
Does increasing cooling air temperature supply (e.g. 58f rather than 55F) would reduce significantly the risk of condensation on uninsulated ductwork.
Thanks
Alex
RE: Ductwork condensation
If the return condition is 72F and 60% RH then you have trouble as the dew point is 57.2F. The absolute humidity difference is 1.8 g/kg approximately and at your flowrate, this will be 2.77 kg/hr.
RE: Ductwork condensation
RE: Ductwork condensation
Nobody said insulation is cheap.
RE: Ductwork condensation
I would not be surprised that ceiling area can reach 70-80% r.h, and with interior temperature assumed, you can calculate dew point temperature, which makes lower limit of your supply air temperature.
You can slightly beat that with increasing air flow, but that directly affects your energy costs over the whole life cycle.
That is issue of costs and benefits, but in no case you can simply neglect condensation problem, that would become an issue the moment you start the system (and would shut it down at once).
RE: Ductwork condensation
I am thinking now to create an air return in the closed ceiling by adding transfer grills and return ducts. This would bring the air around the ductwork at the same temperature than the return air (75F, 55%)
RE: Ductwork condensation
RE: Ductwork condensation
How much duct is this? Cutting and patching a few holes in sheetrock isn't that much of a deal if there is space to move around above. I would ask a good contractor about options before going too much further.
RE: Ductwork condensation
You will need careful specs on treating exposed insulation edges, etc. and you will have to deal with moisture carryover from the DX coil unless your velocities are less than 500 FPM. Food for thought, perhaps?