heat loss through an insulated vessel

[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

 

[IRstuff]

No. Your heat transfer coefficient is a tad high, but tolerable; your biggest error is the temperature difference. Consider placing your hand on an insulated tank, would you feel 315F on the surface?

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[chicopee]

This is a big uninsulated tank and I image that U has taken into account the convective heat transfer ccefficient on the inside and outside surfaces of the tank. Conductivity value of a thin wall tank would not affect substantially the U value so your 218,000 btu/hr may seem reasonable. Just realize that answers to heat ransfer problems in the real world will be much different than the text book answers.

 

[IRstuff]

The tank will also be insulated

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[JStephen]

"I've assumed it to be 2"
You've basically assumed your answer there, right or wrong.
You'll have conduction through the vessel wall, and through the insulation.
You'll have convection from the vessel to the ambient air.
Radiation is likely to be negligible.
If you're actually heating the tank contents (as opposed to just maintaining the temperature), you'll have some heat consumption there as well.

 

[IRstuff]

The heat transfer coefficient's magnitude is secondary to the fact that the temperature difference shouldn't be more than around 5 degC.

Assuming 4 inches of insulation with 0.035W/m*K thermal conductivity and heat transfer coefficient of 10 W/m^2*K, I get about 5.83 degC surface temperature rise and 58.3 W/m^2 heat flow. More, or better, insulation can drop that even further.

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[chicopee]

Yeah, right about the insulation, but it seems to me that his math did not take into account the K value of the insulation to determine his U value.

 

[IRstuff]

The OP used the "simple" formula, which does not account for insulation at all. Once insulation is included, one needs to solve the conservation of heat flow equations.

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