H (heat transfer coefficient) problem

  • #1
I have a cuboid shaped box. It is being used as a chilling tank in my refrigeration system based on Evan Perkins cycle. For heat load calculation I tried to calculate the heat leakage in the sysyem. So I considered conduction and natural convection. I am in preliminary stage, so no use of softwares and simulations. Can anyone help me using approximations and comparing with simple geometric shapes like flat plates or any kind of characteristics length concept.
 

Answers and Replies

  • #2
Tom.G
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In English (American) units:

For a metal box in still air:

1BTU / Hr / Square Foot / oF

For a metal box with forced air circulation (either inside or outside): 2BTU / Hr / Square Foot / oF

(Sorry about the English units, that's the only one I've memorized.)
 
  • #3
In English (American) units:

For a metal box in still air:

1BTU / Hr / Square Foot / oF

For a metal box with forced air circulation (either inside or outside): 2BTU / Hr / Square Foot / oF

(Sorry about the English units, that's the only one I've memorized.)
It is a wooden box and also I need to know the method to get there. Analytical or approximate.
 
  • #4
Tom.G
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Try this Google search: thermal resistance of wood

One result that goes into detail is:
http://web.ornl.gov/info/reports/1988/3445602823407.pdf [Broken]

There are many more.

One thing to consider is that when below 0oC, the moisture in wood freezes and greatly increases the thermal conductivity.
 
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  • #5
Try this Google search: thermal resistance of wood

One result that goes into detail is:
http://web.ornl.gov/info/reports/1988/3445602823407.pdf [Broken]

There are many more.

One thing to consider is that when below 0oC, the moisture in wood freezes and greatly increases the thermal conductivity.
Can I consider wooden box having four vertical plates and one horizontal plate at top...
 
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  • #6
Tom.G
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Can I consider metal box having four vertical plates and one horizontal plate at top...
Yes, if that is what you are using... assuming the bottom is closed.
 
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  • #7
Actually I need to calculate the cooling load of the chilling tank. I calculated cooling load of brine placed inside the box but I also want to calculate the heat leakage from the system and I want to consider the conduction and convection.
 
  • #8
Yes, if that is what you are using... assuming the bottom is closed.
Let me give you the dimensions of the tank 600mmx450mmx300mm it is a model in laboratory. I was doubting about hydraulic diameter concept for the characteristic length of whole box instead of treating as individual walls.
 
  • #9
Tom.G
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Calculate the outside surface area of the box.

Look up the thermal conductivity of the wood you are using.
(Conductivity is often specified as Watts / (meter oC))

Multiply the conductivity by the surface area and by the temperature difference, then divide by the wall thickness.
{( conductivity x surface area x temp difference ) / wall thickness }

For a more complete description see: http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html
 

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