93 % heat loss ? Is that reasonable ?
93 % heat loss ? Is that reasonable ?
(OP)
I know this probably a chem eng 101 question for you guys, but I need to help so that I can get a right heat exchanger for our processing tanks.
I am currently working on aluminum anodizing line setup. When I calculate the heat loss for the processing tanks I got results, which reaaly puzzled me. Here is an example, we have an alkaline clean tank volume 1750 us gals.
I used the Q= MC dT to get the total heat required to heat up the solution from 15 to 50 C, I required 270,000 BTU/hr is for 4 hours.
I know that is theoretical without heat loss, then I calculate heat loss to the surrounding using Q=UA dt. I know the heat loss rate increase as the temp increase, that probably needs a simulation program to do it, but for now I use the maximum heat loss i.e. 50 to 20 C, my U value is about 115 W/m2C mostly contributed by outer air film.
I finally got 250,000 BTU/hr heat loss, that is 93% loss not including any vapourzation. I am just wondering that is reasonable, or may be I missed something.
I am currently working on aluminum anodizing line setup. When I calculate the heat loss for the processing tanks I got results, which reaaly puzzled me. Here is an example, we have an alkaline clean tank volume 1750 us gals.
I used the Q= MC dT to get the total heat required to heat up the solution from 15 to 50 C, I required 270,000 BTU/hr is for 4 hours.
I know that is theoretical without heat loss, then I calculate heat loss to the surrounding using Q=UA dt. I know the heat loss rate increase as the temp increase, that probably needs a simulation program to do it, but for now I use the maximum heat loss i.e. 50 to 20 C, my U value is about 115 W/m2C mostly contributed by outer air film.
I finally got 250,000 BTU/hr heat loss, that is 93% loss not including any vapourzation. I am just wondering that is reasonable, or may be I missed something.





RE: 93 % heat loss ? Is that reasonable ?
For an insulated tank I would expect a value that is a factor of 25 to 100 lower.
RE: 93 % heat loss ? Is that reasonable ?
RE: 93 % heat loss ? Is that reasonable ?
am i reading this wrong, but .... you are mixing units.... you are quoting both US units and SI units .... do you have those correct in your calculations?
magicme
RE: 93 % heat loss ? Is that reasonable ?
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TTFN
RE: 93 % heat loss ? Is that reasonable ?
Titwah:
Everyone is coming down in the correct direction. The "normal expected" Overall Heat Transfer Coefficient is going to be 5 - 35 W/m2-C. And this is based on forced convection - not natural convection. So your factor is more than TWENTY times the normal expected.
There is no better insulation than static air (except for NO air - which is a vacuum). All thermal insulation depends on its insulating qualities on having static air pockets. That's what makes it work. If you maintain just natural convection (not forced or indirect) of air around your tank (I presume that it is located indoors), you should have no trouble getting around 5-10 W/m2-C for a "U". This is where your calculations are in error or flawed. If you maintain a layer of stagnant air around the tank, you should have negliglible heat loss compared to what you are putting in.
My source for the "U" is:
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Typical Overall Heat Transfer Coefficients
RE: 93 % heat loss ? Is that reasonable ?
RE: 93 % heat loss ? Is that reasonable ?
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RE: 93 % heat loss ? Is that reasonable ?
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a 10 m x 1 m plate with 700 W results in 2 W/m^2-K and 50ºC temp against ambient of 15ºC
A second problem is that you are comparing the Joule load to heat the bath from a dead stop to operating temperature. The amount of power required once the bath is at temperature is drastically lower, probably on the order of 700 W, which would represent 100% of the heat loss.
TTFN
RE: 93 % heat loss ? Is that reasonable ?
RE: 93 % heat loss ? Is that reasonable ?
From J.P. Holman's Heat Transfer for vertical plates of length L (m), and a temperature difference ΔT (oC), the free convection heat transfer coefficient in laminar flow h (W/m2.oC) would be:
For a horizontal plate facing upward the factor drops to 1.32.
Remember the overall U is never higher than the lowest of the participating coefficients.
RE: 93 % heat loss ? Is that reasonable ?
ASTM C680
BS5970
BS5422
ISO BS EN 12241
athomas236
RE: 93 % heat loss ? Is that reasonable ?
In terms of cooling, even if the heat loss was as calculated, it wouldn't cool off that quickly, since the heat loss reduces as the temperature differentical decreases.