Heat loss calculations

[Pfluid]

Hi All:
I'm designing a 5000 gallon liquid wax holding vessel. The vessel will jacketed with hot oil flowing through it to maintain 200 Deg C inside the vessel. The tank will also be insulated. I'm trying to figure out the heat load consumption on hot oil and hot oil flowrate through the jacket. I'm thinking that, if I can calculate heat loss through the tank (assuming no hot oil jacket) then the amount of heat loss will be hot oil load to maintain the temperature of 200 Deg C
I've used the simple formula Q = U x A x delta T where U is the heat transfer rate in Btu/hr-ft2-Deg F and I've assumed it to be 2 (rate of heat transfer through the vessel in air) The surface area is 347 ft2 and the delta T is 315 deg F
The above formula gave me a big number 218,000 btu/hr
Does this sounds right?
Is there another way to verify this number?
Thanks
Pfluid

 

[racookpe1978]

Decide on some temperature and design units, and stick with them!
You have listed ANSI-Standard US units (deg F, BTU/Hr-ft^2, deg C, gallon (US or Imperial)...

What is your actual calculation for area (inside tank to oil, and outside tank and insulation to air)?
Why a 2 BTU/Hr ?
Thickness of oil blanket around the tank holding the hot wax?
Is the hot wax being circulated?
What are ALL of the heat transfer properties of the oil and hot (melted ?) wax?

 

[racookpe1978]

I think also you are missing an important point:
The inner tank - the hot wax tank being heated by the oil bath surrounding it - is only part of the heat transfer problem.

The wax in the hot wax tank must be heated from ambient up to your desired 200 deg C. That is one part of the dynamic heat transfer problem: Whatever your mass wax transfer rate is out of this tank must be replaced by new cold wax re-heated from its original ambient temperature (or from the inlet transfer/transportation/wax creation temperature) back up to the stored 200 deg C holding temperature.

But the stored wax must be kept at 200 deg C while stored. Yet the hot oil (at 210, 220, 250 deg C ??) must be between the wax tank and the ambient air outside the wax tank. (Is the wax tank inside at 20-30 deg C and still air maybe? Or outside in blowing winds at -20 deg C)? Your heat loss is not only inside the oil blanket surface to the wax, but outside from the oil blanket to ambient through the insulation around the oil bath.

Once the transient heating of fresh cold wax is complete (part one of the problem), then the now-hot wax at 200 deg C and the oil blanket (at 210, 220 or 250 deg C) around it are still losing heat to ambient.

 

[georgeverghese]

There are 2 equations that must match up in terms of the heat leak to the ambient

Q = k.A.dt/dx for the insulation blanket
and
Q= h.A.dt for the loss from the insulation outer cladding to the ambient. Here h represents the combination of the convective and radiative components.

In most cases, where you've got a good thickness of insulation, heat transfer from the wax to the insulation layer (through the vessel wall and hot oil jacket) need not be considered, as it will be a minor resistance to heat transfer in comparison the dominant resistance at the insulation layer.

Presume you know what these terms mean. Q is the same for both these expressions. If the holding tank is a small dia circular vessel , then use the appropriate expression for A ( which involves a log (d1/d2) term ). Start off with some assumed values, and reiterate till all computed values match up with previously assumed values. Look up heat transfer text for appropriate values for the radiative htc component.
Agreed, we are assuming here that heat loss to the ambient is the controlling case for heat transfer, and not some initial warm up case. Beware of high heat loss through parts of the vessel / tank that may not be insulated.