RE: Heat Gain In a Chilled Water Circuit
17 Feb 03 10:55
you should reference a heat transfer text or a mechanical engineering handbook such as Mark or Kent for the appropriate formulae on insulation. Thermodynamic texts do not deal with this type of problem. Bear in mind that too little or too much insulation will be incorrect therfore particularly check on the subject of the economic thickness of insulation which will require knowledge about the thermal conductivities of the three layers of the piping and heat transfer coefficient the latter of which will require knowledge about wind effects on the insuated piping. There may also be reflection effects from nearby objects such as ducting, air handling, aluminized coated roofing that you must also evaluated. RE: Heat Gain In a Chilled Water Circuit 25362 (Chemical) 21 Feb 03 08:17 Don't take the following very seriously: there is one theoretical interesting point sometimes raised in respect of pipe thermal insulation with conductivity k, in a convection environment with h as coefficient of heat transfer. Both when the pipe fluid and the environment are at fixed temperatures, there is a critical radius of the insulation called r o = k/h .
If the outer radius of insulation r o is smaller than k/h, heat transfer will increase by adding more insulation .
As normally expected, for outer radii greater than the critical value an increase in insulation thickness will cause a decrease in heat transfer.
From my own experience r o is generally small enough so that any practical insulation is well above this critical value.
RE: Heat Gain In a Chilled Water Circuit rjw57 (Mechanical) 3 Mar 03 09:58 Excellent reply 23562! Most people take the critical insulation thickness thing to the extreme based on a poor understanding of the phenomena. I get the feeling that many people assume that, in any instance, you might be able to add insulation and increase heat transfer from the surface, which is just not so (as you so ably pointed out). Great treatment of this is found in "Heat Transfer, Professional Edition" by Thomas. Anyone involved in learning about insulating surfaces should read up on this topic. RE: Heat Gain In a Chilled Water Circuit vpl (Nuclear) 7 Mar 03 09:07 25362
An interesting aside about the "critical value of insulation". I have seen a case where a company had a problem with calcium carbonate (lime) plateout in their heat exchangers. They would clean them out, and then get real good heat transfer for a couple of cycles before it dropped off sharply - because they'd exceeded the "critical value" of the calcium carbonate and it started acting like an insulator. They were having a dickens of a time trying to explain why there was such a sharp drop - they expected there would be a gradual decrease in heat transfer. Patricia Lougheed
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