Overall Heat Transfer Coefficient (Exhaust Gas to Refrigerant)

In summary: Some common materials used for heat exchangers are copper, aluminum, and stainless steel. The type of heat exchanger, whether it is a plate or shell and tube, also affects the heat transfer coefficient. You can refer to heat exchanger design handbooks or consult with an expert in the field to determine a more accurate value for your specific system. In summary, the overall heat transfer coefficient ("U") for a heat exchanger is not a physical property of the fluids, but rather a property of the material and geometry of the heat exchanger. To determine a more accurate value, consult heat exchanger design handbooks or seek advice from an expert in the field.
  • #1
TheBigBiscuit
4
0
Hi there,

I am doing a project modelling and simulating a gas engine waste heat recovery system using an organic rankine cycle.
As part of this project I would like to size the surface area of a heat exchanger (if only roughly). I have found the appropriate formula using Q = U.A.LMTD and all the variables are fine and I believe I am calculating correctly but I'm assuming an overall heat transfer coefficient ("U") of ~400 W/m^2.K. I have no idea if this is accurate
The two fluids exchanging heat are an exhaust gas (heat source) and various refrigerants (R134a, R245fa), I was hoping someone might be able to suggest an accurate "U" for these fluids and if possible something to reference.
I don't have a fantastic knowledge of heat exchangers by any means, do I assume the heat exchange surface to be made from metal and is it relevant whether i specify plate or shell and tube etc?
Any help would be greatly appreciated, thanks advance for your time.

Ronan
 
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  • #2
The U in heat transfer is a property of the material used to fabricate the heat exchanger and the geometry of the unit. It is not a physical property of the fluids passing thru the unit.
 
  • #3
oh ok thank you, I was confused because any information I have searched for contained tables with both the hot and the cold liquid for example: gas to water and a corresponding "U" value...
Would you know how to obtain a relatively accurate value for my predicament or an educated guess perhaps?
Thanks again for your post
 
  • #4
It depends on the material used to construct the heat exchanger and the physical layout of the heat exchanger itself.
 

1. What is the definition of overall heat transfer coefficient (U)?

The overall heat transfer coefficient, or U-value, is a measure of the overall rate of heat transfer between two fluids, such as exhaust gas and refrigerant. It takes into account the individual heat transfer coefficients of the two fluids as well as the thermal resistance of any material separating them.

2. How is the overall heat transfer coefficient calculated?

The overall heat transfer coefficient is calculated by taking the reciprocal of the total thermal resistance between the two fluids. This includes the thermal resistances of the individual fluids and any material separating them, such as a heat exchanger or pipe wall. The formula for U is: U = 1 / (1/h1 + R + 1/h2).

3. What factors affect the overall heat transfer coefficient?

The overall heat transfer coefficient is affected by several factors, including the flow rate and physical properties of the two fluids, the geometry and design of the heat transfer surface, and the thermal conductivity and thickness of any material separating the fluids. Other factors such as surface roughness, fouling, and temperature differences between the two fluids can also impact U.

4. How can the overall heat transfer coefficient be improved?

The overall heat transfer coefficient can be improved by increasing the flow rates of the two fluids or using more efficient heat transfer surfaces, such as fins or turbulators. Additionally, using materials with higher thermal conductivity, reducing the thickness of any separating material, and maintaining clean heat transfer surfaces can also help improve U.

5. What are some common applications of overall heat transfer coefficient calculations?

The overall heat transfer coefficient is a crucial parameter in many engineering applications, including heat exchangers, refrigeration systems, and power plants. It is used to design and optimize heat transfer processes, determine the required size and materials for heat transfer equipment, and assess the overall efficiency of a system.

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