When we install some instrument on a hot process line, we cautiously
provide a stub of some length to allow for cooling since the electronic
boffins in Silicon Valley haven't come up with decent electronic
components capable of withstanding reasonable temperatures such as 450
°C (and I have seen the odd pressure gauge mounted directly on a
steam line looking decidedly most peculiar whilst its polycarbonate
shield was slowly melting down).
Our standard approach is usually trial and error, "I used 6 inches last
time and it did not work, let's make it 200mm this time" (Please note my
versatility, switching effortlessly between metric and imperial
units,... actually all I am trying to do is confuse the reader. Give me
some credit for trying!).
And there's nothing wrong with trial and error except it does not
satisfy this new breed of inquisitors called QA auditors. If , unlike
me, you can't deal with them in a subtle and elegant manner (I usually
kick the s..t out of them the first time we meet and they leave me well
alone thereafter), try doing it the scientific way, the suckers will
fall for it every time.
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This is the formula giving L once To, Ta and
TL (the temperature at the end of the
tube) are specified. You need to know k, the conductivity of the tube (including correction for the fluid conductivity inside the tube), and h, the heat transfer to the ambient air. |
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Here are a few figures for thermal conductivities. I have not included materials such as silver or gold (though I have the values) but if you contact me I shall be delighted to Email them to you in exchange for a tube sample. |
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If the cooling tube is mounted horizontally, the natural convection heat transfer, h, to the surrounding air is given by this formula. |
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Same units as above, L in metres | If the cooling tube is mounted vertically, the natural convection heat transfer, h, is given by this one. |
The formula has been calculated by assuming the heat transfer coefficient, h, is constant over the total length of the stub, the doubting Thomases who won't take my word for it can retrieve a zip file containing the source document (MSWord 6.0 format).
Whilst every care was taken when calculating the above formula, the
author (who is undergoing shock treatment therapy at the very moment)
assumes no responsibility for the information contained herein.
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