Heat Transfer Coefficient

[ClydeMule]

I have a question reagrding the use of the heat transfer coeffiecnt for forced convection.

The type of problem I am working on is that of a copper coil immersed in a tank of water. Chilled water is flowing through the coil.

From Mark's Handbook I have found equeations to get the U from the water velocity, temperature, etc for the coil. But how do I relate that to the condition of the fluid I have the coil immersed in?

Let's say I have the coil paramters set, but I want to changed the a tank water from still to flowing. Or, what if the tank water is now 50% glycol? I see some equations for developing a U from the tank water side, but how do you bring the tank water and coil together to develop a single equation?

Thanks for any input.

Jeff Johson

 

[Tgatica]

As with any convection problem, you must first start with the heat transfer coefficient, h. This depends on the fluid type, flow rate, and temperature. This is not always easy to determine and one must usually make many assumptions. Turbulent or laminar flow certainly affect these calculations.

To get h for the fluid your coils are immersed in, I suggest calculating the idealized case for fluid over cylinders for flow parallel to and perpendicular the coil axis. These will probably be different for each case and then it will be up to you to determine what percentage of the coils are parallel and perpendicular to the flow of the watre or glycol mixture.

Tony

 

[whylie]

Dear ClydeMule,

Calculating overall heat transfer coefficients invloves ,many correlations ,such as :

Nu = K Pr^n Re^m ,where Nu is Nusselt number,Pr Prandtl number,and Re reynolds number,n& m are powers for convection heat transfer..This is the usual jargon!!

For engineering calculations,normally based on experience,I use tabulated values in Kern,Perry or Coulson ..for a metallic coil ( copper,aluminium,stainless steel..etc),a good conservative value is U=200 W/sq M,for water -water system.The big influence on U ,is fuilds handled,agitation ..The effect of metal(K,conductivity diminishes by comparison,in the overall U ,equation).

Good design figures ,can be selected from am references,without indulging in the detailed equations,unless you rae using a specific software,or you want to be exact..give it a try..

Regards,

Whylie

 

[mphoza]

I have a prac on modeling thermal conductivity. In my group we have serious arguments regarding the relationship between the heat transfer coefficien and the diameter of the rod. We are working on same rod at different diameters. Is the heat transfer coefficient, h, affected by the diameter of the rod?

 

[gcordoba]

I am working in a model of pyroclastic flows (a lethal flows which are produced by eruptions of some volcanoes). Such flows should heat the soil due its high temperature (almost 1000 Celsius degrees). Please, anyone know a value or an expresion for the heat transfer coefficient of soil which can be used in this case?