Heat Loss from a Closed Tank

[BatesEngineer]

I have a large fire protection tank, 34ft tall x 29ft dia, holds 175000 gallons of water. I have a Lowest One Day Mean Temp of 25F and the min target temp is 42F. Tank is made of carbon steel.

Using the NFPA 22 figure 16.1.4, heat loss = square area x (heat difference) / R, I get a heat loss of 8,164kW, or 24 million BTU/hr.
Assumed: R=0.0031 ft2 F hr/BTU, A=4400 ft2, deltaT = 17

However using Q=hA(deltaT), I get a heat transfer rate of 12.4kW or 42308 BTU/hr
Assumed: h = 3.231 W/m2 K, deltaT = 9.44 K, A = 408m2

These are wildly different results and I can not seem to find the reason.

Thanks in advanced.

 

[Latexman]

If I use h = 1/R, and convert your h = 3.231 W/m2 K. I get R = 1.757 ft2 F hr/BTU.

4400 x 17 / 1.757 = 42,562 BTU/hr.

Conversion error?

Good Luck,
Latexman

 

[BatesEngineer]

Ever just stared at something for the longest time and couldn't see it... Thanks Latexman!

 

[IRstuff]

Your h is a bit low, since it only accounts for ideal natural convection. somewhere between 5 and 7 W/m^2-K accounts for some wind and some radiation.

Your R, inverted, comes out to 1831.6979 W/m^2-K, so a factor of ~600 larger than your h; this magnitude is more for convection in a water-cooled application.

TTFN (ta ta for now)
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