Supply Ductwork Insulation

[sooty]

I am currently looking at a project where cooling is to be provided to a suppermarket. The HVAC plant is situated externally, therefore, the supply ductwork runs externally along the back wall of the store.

Has anyone any advice on whether an insulation with a black surface finish should be used or not.

I have been told, that due to heat gains being greater because of the black finish then this could have an affect with the cooled supply air, at present we have an 'off coil' temp of 12 deg C.

The ductwork is 1600mm diameter and approximately 70m long.

 

[Drazen]

Theoretically, absorption of black color is much greater and you'll have adverse radiation heat gain.

In practice, there could be even more problems. I have no idea of your insulation setup, but often it is predicted to resist only air humidity entrance, not rain showers and similar. You can design more ruggred isulation or choose some available setup, but I saw several times how does it look like only year later.

Of course it depends on overall climate condition, but my suggestion is to install plastic or galvanized steel hood above and on outward side of ductwork. It will not cost more then alternative sophisticated insulation setup, but is proved much more age-resistant and will greatly reduce heat radiation gain.

My modest practice and hearing from colleagues is not all the world knows, but maybe it could help you somehow.

 

[sooty]

Thanks for the info, but the insulation is to be made waterproof, this hopefully shouldn't be a problem.

 

[quark]

Sooty!

Are you referring to Black Neoprene or Nitrile Rubber Insulation? The absorptivity as well as emissivity of black body (?) is more. If you are thinking about the above said material then they are good insulation materials.They are water proof too. (They come in gray colour also, if you want) The moisutre penetration is also low when compared to conventional insulating material.

You can also paint on the insulation. Take enough care that the color of the painting is not too attractive. I found incidents of birds plucking the insulation

Don't go with aluminium cladding on top of insulation. Otherwise you will end up with extra insulation thicknesses.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[aidan]

Where are you, as a matter of interest?

The black finish is probably PIB (polyisobutylene, I think) sheet, which is used to waterproof insulation. The black PIB will absorb solar radiation, but since it will be separated from the duct by the insulation, the heat gains into the supply airstream probably won’t be significant. The only way to accurately assess this is to estimate solar gains, PIB temperature, U values, etc., or measure the surface temperature of a similar bit of PIB on an insulated duct in a similar location. I’d be more concerned about the PIB getting excessively hot. The alternative would be fibre-reinforced aluminium foil.

The PIB surface finish is there to provide water-proofing and a vapour barrier, otherwise, at 12 degC the insulation will become waterlogged with condensation from the external air. I would suggest that you need to consider testing the dust for air leakage before the insulation is applied. Leaks, which would be acceptable elsewhere, will cause the PIB to inflate and may cause it to break loose. The duct supports should be outside the PIB, or they will puncture the vapour barrier.

There is a useful British Standard on insulation.

 

[ChasBean1]

The supply duct work should be insulated with two inches of insulation with each inch having a conductivity of 0.12 BTU/Hr*ft2*°F. Paint at least the top and outer sides of the duct work white with an oil-based paint that adheres to the surface.

One important issue to consider is the duct concavity - we don't want standing water on the top of the insulation. We should ensure the water flows off by having the top of the duct concave upward. Round, ideally (highest point in the center) or angling a rectangular duct which would likely not be popular with the architect.

Just a note. Say painting the duct white recovers ONLY ONE degree (°F), with a 50,000 cfm unit, you can save over $13,000 over the course of a year in energy cost at $.10 per KWH. Divide this by a certain factor assuming evening, shade, and clouds, but my guess is lightening the duct will be an option that pays back itself within a pretty short period.