Overall Heat Transfer Coefficient (Exhaust Gas to Refrigerant)

[TheBigBiscuit]

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

 

[SteamKing]

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.

 

[TheBigBiscuit]

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

 

[SteamKing]

It depends on the material used to construct the heat exchanger and the physical layout of the heat exchanger itself.