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Join Date: Feb 2008
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02/08/2011 4:53 PM

I am seeking guidance on how to address a shelter's environmental control unit capability as a function of solar load specified by the contract.

The specification requires:

The system shall provide shelterized work areas capable of maintaining an effective internal operating temperature between 19 and 29.4 degrees Celsius (66.2 – 84.9F) per MIL-STD-1472.

a. The system environmental control unit's (ECU) shall maintain shelter internal operational temperatures when the external ambient temperatures are between -23.3 degrees Celsius (-10F) and 43.3 degrees Celsius (110F)

b. The system's ECUs shall maintain internal operating temperature while the shelter is exposed to a solar load of 355 BTU/ft2/hr, without suffering physical or functional degradation.

ANALYSIS

It is a closed unmanned shelter: 7.3'W x 7.2'H x 12.3'L (outside dimension). It has aluminum skin painted Woodland Camo. The shelter specification is R value of 3.6

Considering the solar load, I calculated resultant heat load using Stefan Boltzmann constant (radiation energy per unit time is proportional to the 4th power of the abs temperature). I used 2 surfaces for solar load (side and top) Perhaps optimistic, this calculates to a temperature of 342 F. However, the calculation determines how hot it could get without factoring in convection. The convection is discarded and all the heat (conduction) is assumed to go into the shelter in the next calculation (H = (A x T)/R. The point is to calculate a heat load, not the exact temperature of the shelter skin, right?

Can anyone offer any guidance on how to quantify the effects of the specified 355 BTU/ft2/hr? How would you estimate the resultant heat load from this solar load?

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#1

02/08/2011 5:29 PM

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#2

02/08/2011 8:31 PM

Any specification using words like "shelterized" is likely to have been written by bureaucrats rather than engineers. No mention was given of thermal loads within the shelter, such as electrical/electronic/chemical reaction equipment; nor air infiltration. The R3.6 value is surprisingly low, but it may be necessitated by inside vs outside dimensional limits.

At a rule-of-thumb level, solar effect is sometime considered as adding something like 10-15°F to the simple air temperature difference. On that basis, the solar load per ft2 would be (15 + 110 - 85°F)/3.6 ≈ 11.1 Btu/h. I don't yet see where the 355 Bth/h-ft2 figure comes from.

This is so weird that I will try to review it again. (Just in case I've blown a decimal point placement.)

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#3

02/09/2011 4:27 AM

In the tropics (in Bangalore where I live) peak insolation (Mid-day, summer, normal to the surface) is about 800w/sq.mtr. Even this equals only 253 btu/hr.sq.ft. 355 Bth/h-ft2 appears to be high.

bioramani

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#5

02/09/2011 9:21 AM

The 355 Bth/h-ft2 is dictated by the customer in their requirements specification.

There are equipment heat loads (about 15000 BTU) that would presumably be added to the solar load that would contribute to sizing an HVAC system.

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#4

02/09/2011 7:34 AM

To try to add to what others have posted - what are you using the Stefan Boltzmann constant for, the heat lost from the shelter? You don't need it for the heat input from the sun. 355 BTU/ft2/hr = 1.12 kW/m2, is about right for maximum heat from the sun. The solar constant is ~ 1.4 kW/m2, above the atmosphere. 800 watt/m2 in #3 seems a bit low but could be right.

Is the R value 3.6, presumably °F*ft2/(BTU/hr) just based on the conductivity of the walls, or is it an overall factor allowing for convection from inside air to inside wall surface, and outside wall surface to outside air? Assuming the latter (worse case) I make conduction heat loss with ΔT 40K about 28kW.

Cheers.........Codey

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#6

02/10/2011 10:37 AM

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