Ductwork condensation

[mech4321]

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

 

[chicopee]

Under high humidity condition, condensation will occur on cold surfaces. Definetely have a drain line at the cooling coil. The trap of the drain line should be accessible for periodic cleaning. As far as the external surfacesof the duct, may be a drainage plate or insulation running some portion of the duct length.
As the cooling continues, humidity should lessen during the hours of operation and less sweating should occur on the duct surface.

 

[MintJulep]

Yes. You should be worried about it.

More so, you should do something to prevent it. Like insulating the duct.

 

[mech4321]

Adding insulation on this existing ductwork hidden above sheetrock would cost a lot of money.
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

 

[quark]

Do a simple check plotting the points on a psychrometric chart. Your main concern will be return air conditions. If you maintain return conditions of 72F and 50% RH, then the corresponding dew point is 52F and there will be minor problems with condensation even if your coil leaving temperature is 55F.

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.

 

[quark]

...but that is the maximum limit.

 

[willard3]

If you have to ask the question, you have to insulate the duct.
Nobody said insulation is cheap.

 

[Drazen]

If there is no circulation in ceiling area, humidity will be signigicantly higher, which means dew point temperature will be higher as well.

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).

 

[mech4321]

I agree with the psychometric chart and the theoritical dew point temperature, but in fact, it is unlikely to see uninslated ductwork sweating in open ceiling altough supply air is at 55F and room temperature (or return air) at 75F 55% r.h.

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%)

 

[mauricestoker]

I would recommend looking at contract criteria first. As this is a post office, assumption would be that ASHRAE 90.1 is included as a federal contract. Table 6.8.2A should be able to give a contractual basis for making a decision (the ASHRAE 90.1-2004 Handbook gives a specific example for Chicago). A suitable alternative may be to insulate the isolation damper (for when unit is not on) at the envelope to meet ASHRAE 90.1 requirements.

 

[KiwiMace]

Converting the ceiling to an RA plenum has it's own issues - combustibles in the ceiling space and adequate partitions to isolate from other spaces both spring to mind. This will require a full fire protection, MEP and architectural review.

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.

 

[RayWK]

Any chance of installing a duct liner instead of using external insulation on the duct? Not sure of your duct sizes (or how small your insulation contractors are.....
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?