Insulation of cold pipes

[scorer]

Hi

We currently pump a slurry from a tank at 3 degC through a bank of tubular filters. Product returns to the feed tank, which has glycol on the tank jacket. Product increases in temp over time despite this cooling. Recently the bank of filters was insulated - the idea being that this would reduce the rate of increase in temp, which is undesirable. The lagging was intended to reduce heat pick up from the air Lagging seems to have made things worse. The temperature now rises more quickly. Is this possible? Any ideas?

 

[cdvk]

Although I don't have a clear picture of your situation in my head - I can suggest maybe you have a thickness of insulation where the increase in convection caused by the extra surface results in an increase in the heat transfer? see critical thickness of insulation in a heat transfer book if this sounds like something that may apply.

cdvk

 

[IRstuff]

Didn't get a clear idea of the temperature of the glycol, which can get somewhat sluggish below 3°C, depending on the dilution.

TTFN

 

[scorer]

Glycol is supplied to the jacket at -5 degC. I'm not sure of the dilution. A crust builds up on the inside wall of the tank which reduces the heat transfer. It seems that the amount of heat put in by the recirc pumps is more than can be removed by the jacket. However, will lagging the filter bank (formerly stainless steel tubes) have made things worse.

 

[25362]

It seems as absorbed moisture is the culprit. If the low-temperature insulating material absorbs atmospheric moisture or vapour (condensing below its dew point) its thermal conductivity will dramatically increase. One should use coating vapour barriers of low moisture permeability to avoid this occurrence. Ice, if forming at the prevailing temperatures, has an intensification factor of about 4 !!
Good luck.
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