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Slowing Heat Transfer

A tea cozy keeps a pot of tea warmer longer by reducing heat transfer, primarily conductive transfer.

My blog last week was changed at the last minute to talk about Hurricane Irene. Although she “cooled” compared to what was predicted, the damage was significant, especially here in Vermont where 7 inches of rain in our steep valleys resulted in lots of flooding.

Let’s get back to the image I showed two weeks ago of a “tea cozy” covering a pot of hot tea. This simple device—essentially a piece of decorated insulating cotton or wool fabric—originated well over a hundred years ago as a means of keeping tea warm longer, therefore extending the social networking that accompanied the traditional “afternoon tea.”

Tea Cozies Today

Cozies are still widely used in the UK, in particular, where tea is popular and interior temperatures during the heating season tend to be cool enough to speed heat transfer. In a pinch, they might even double as a hat (for example, Dobby the house-elf, showed the world in one of the Harry Potter films)!

In the image of the cozy in place we can see the temperature in the warmest area of the surface of the cozy is 90.4°F (32.4°C). If heat transfer has reached a steady state condition—that is, heat is being transferred on one direction at a steady rate—it becomes obvious the teapot must be warmer still.

Second Law of Thermodynamics

The Second Law of Thermodynamics, can be stated simply, without doing work, heat is always transferred from areas of higher energy (warmer) to those of lower energy (cooler). Stated another way, the tea and the teapot and the tea cozy will all, in turn, eventually cool to the temperature of the surroundings.

The same image of the warm cozy with level and span adjusted to more clearly show the relationship with the warm teapot and the hot tea. Heat transfer is reduced by both the teapot and the cozy, as suggested by the drop in temperature at each surface.

I’ve redone the image of the teapot with the cozy in place using a wider temperature span so we can more easily compare two additional images. These show the teapot immediately after the cozy has been removed and, quickly thereafter, the tea itself in the pot. We can see the tea, with a peak temperature of 173°F (78°C) has transferred some of its heat to the surface of the pot. With the cozy in place the surface of pot reached a temperature of 159.5°F (71°C). Intuitively we can say the teapot itself and the cozy have both reduced heat transfer as evidenced by reduced temperatures at each interface.

We know heat transfer is from hotter to cooler but it also becomes clear temperatures by themselves can mislead us in several ways. Unfortunately many thermographers don’t understand this and, as a result, make mistakes, some serious. I should know as I’ve been among that group! Next week I’d like to talk more about transitional heat transfer and how we can avoid problem caused by not understanding where we are in the thermal cycle.

I’d be amiss to not give a big “thank you” to our friends in organized labor who work so hard to keep the world going. I’ve been fortunate over the years to work with many of them as they implement this remarkable technology of thermography. Labor Day may have come and gone but my appreciation remains for all organized labor does.

Thinking Thermally,
John Snell—The Snell Group, a
Fluke Thermal Imaging Blog content partner

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