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Q & A: metal layer to reduce heat transfer

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Most recent answer: 05/25/2016
Q:
I've got a BME from Ga Tech, but that was back in 1973. My senior engineering project was to invent the wheel. 😊I'm wondering about the double-wall, vacuum insulated stainless steel tumblers that are so popular. The outer surface of the inner wall is coated with copper. The object of the tumbler is to keep hot drinks hot, and cold drinks cold. First, assume a cold beverage, and an ambient temperature of about 70�. Heat will propagate from hot to cold, so in this case we want to prevent heat from outside the tumbler from reaching the interior of the tumbler. The outer wall will absorb heat from the ambient. Disregarding conduction and convection for the moment, heat from the the inner surface of the outer wall will radiate toward the inner wall. But the radiant heat will largely reflect off the copper coating and reduce the amount of radiant heat reaching the inner wall and, ultimately, conducted to the cold drink through the inner wall. Now consider a hot drink. Heat will tend to move from the hot fluid toward the outside of the tumbler. First the inner wall will heat up by conduction, and the copper layer will also heat by conduction. The copper is ineffective in reflecting radiant energy, because no radiant heat is striking the copper surface�it's all conduction. So intuitively, it seems the copper layer would have no effect on radiant heat traveling across the vacuum to the outer wall. Here's my question. Polished copper has a low emmisivity. So will that characteristic reduce the amount of radiant energy propagating to the outer wall? Again, intuitively, it doesn't seem like a copper coating on only one side of a sheet would be equally effective in minimizing radiant heat loss regardless of whether the radiant heat strikes the coated or uncoated side of the wall. So, IS a steel wall plated with copper on only one side equally effective in reducing radiant heat transfer from a hot liquid and to a cold liquid? Or is it only effective in keeping the cold drink cold and of no benefit in keeping a hot drink hot?Thanks.
- Larry (age 65)
Marietta, GA
A:

It does work to keep heat in, due to the low emissivity that you mention. If the material that it coats has a higher emissivity, then removing the copper coating would increase the heat flow.  

It sounds like you're thinking of a thermos with an inner steel wall. Nicely polished steel is pretty reflective/non-emissive itself, so I'm a little surprised that anyone bothered to coat it. A metal film coating for glass makes more sense. I thought aluminum was usually used for the coating, but that may not be right.

Mike W.


(published on 05/25/2016)

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