H (heat transfer coefficient) problem

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Discussion Overview

The discussion revolves around calculating the heat transfer coefficient (H) for a chilling tank used in a refrigeration system, specifically focusing on heat leakage through conduction and convection. Participants explore various methods and approximations for determining heat transfer characteristics in both metal and wooden boxes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant describes a chilling tank shaped as a cuboid and seeks help with heat load calculations, specifically regarding heat leakage through conduction and natural convection.
  • Another participant provides specific heat transfer coefficients for a metal box in still air and with forced air circulation, noting the use of English units.
  • A later post clarifies that the box is wooden and requests methods for calculating heat transfer, whether analytical or approximate.
  • One participant suggests searching for the thermal resistance of wood and notes that moisture in wood can freeze below 0°C, affecting thermal conductivity.
  • Another participant confirms that a metal box can be treated as having four vertical plates and one horizontal plate at the top, assuming the bottom is closed.
  • A participant expresses the need to calculate the cooling load of the chilling tank and mentions having already calculated the cooling load of brine inside the box.
  • One participant suggests calculating the outside surface area of the box and provides a formula for conductive heat transfer, emphasizing the importance of thermal conductivity and temperature difference.

Areas of Agreement / Disagreement

Participants present multiple approaches and considerations for calculating heat transfer, but no consensus is reached on a specific method or model. The discussion remains unresolved with various viewpoints on the characteristics of the materials and methods to use.

Contextual Notes

Participants mention specific conditions such as the impact of moisture in wood and the dimensions of the chilling tank, which may affect calculations. There is also uncertainty regarding the appropriate characteristic length for the box.

Ravi Singh choudhary
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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.
 
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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.)
 
Tom.G said:
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.
 
Try this Google search: thermal resistance of wood

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

There are many more.

One thing to consider is that when below 0oC, the moisture in wood freezes and greatly increases the thermal conductivity.
 
Last edited by a moderator:
Tom.G said:
Try this Google search: thermal resistance of wood

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

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...
 
Last edited by a moderator:
Ravi Singh choudhary said:
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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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.
 
Tom.G said:
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.
 
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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