93 % heat loss ? Is that reasonable ? 1

[titwah]

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.

 

[nicovdven]

The U value you're using is extremely high. Have you taken any tank insulation into account in your U value determination?
For an insulated tank I would expect a value that is a factor of 25 to 100 lower.

 

[titwah]

It is a non-insulated steel tank, assuming no scale form in the initial heating. If I only use thermal conductivity of air 0.025 W/mC, and air thickness 0.2 mm. I got a high U value of 125 W/m2C. The tank heat transfer surface area is about 20 m2, which are all 4 walls of the tank 14'x40"x6'. May be my estimation of air thickness is too small, any suggestion of the value ?

 

[magicme]

hello

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

 

[IRstuff]

Your htc is incredibly high, by a factor of 10-ish for unforced convection:

http://www.coolingzone.com/Content/DesignCorner/CZ_Calc/baseframe.htm

TTFN

 

[Montemayor]

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:

http://www.the-engineering-page.com/forms/he/typU.html

Typical Overall Heat Transfer Coefficients

 

[titwah]

Thanks for all your replies. You guys are right, my U value is way too high. And, Montemayor raise a good point that if we keep the air flow arround the tank to minimum that it is reasonable to assume a thicker air film thickness e.g. 1mm, Now, use the thermal conductivity of 0.025 W/mC, I get 25 W/m2-C. I was using 0.2 mm before that would probably happen when it was blowing a strong wind. Thanks guys/gals again.

 

[trashcanman]

Go here: It is a link for sizing swimming pool heaters. Your tank is close enough to a pool to get a good idea of the size of heater required. There is another link on the page to calculate evaporative losses (major heat loss).

http://www.engineeringtoolbox.com/swimming-pool-heating-d_878.html

 

[IRstuff]

Your htc is still too high. Go to the calculator here:

http://www.coolingzone.com/Guest/News/NL_AUG_2001/CZ_tool/August_CZ_2001.html

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

 

[titwah]

Thanks guys/gals, now I have a better idea of what the U value should be for my estimation. Thanks

 

[25362]

From J.P. Holman's Heat Transfer for vertical plates of length L (m), and a temperature difference [Δ]T (^oC), the free convection heat transfer coefficient in laminar flow h (W/m².^oC) would be:

h = 1.42([Δ]T/L)^0.25​

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.

 

[athomas236]

You could try standards like:

ASTM C680
BS5970
BS5422
ISO BS EN 12241

athomas236

 

[JStephen]

I don't see why you'd correlate the heat loss per hour to the heat required to raise the temperature, either. The "93% heat loss" is meaningless since the 1-hour is an arbitrary time period. Take the time period as 1- second or 1-month and results will vary considerably.

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.