CONSTANT SURFACE TEMP VS CONSTANT HEAT FLUX

[dkm0038]

I HAVE A HARD TIME UNDERSTANDING THE APPLICATION OF CONSTANT SURFACE TEMPERATURE AND CONSTANT HEAT FLUX. AND WAS HOPING SOMEONE CAN DUMB IT DOWN FOR ME.

FOR AIR COOLED HEAT EXCHANGERS THE INTERNAL HEAT TRANSFER COEFFICIENT'S NUSSELT NUMBER VALUE FOR THE LAMINAR REGIME DEPENDS ON BEING EITHER SUBJECTED TO CONSTANT SURFACE TEMPERATURE OR CONSTANT HEAT FLUX. I AM CONFUSED IN WHICH IS USED FOR AIR COOLED HEAT EXCHANGERS.

THANK YOU FOR YOUR HELP.

 

[ChrisConley]

Easy on the caps lock button. Most people find the ALL CAPS to be annoying, akin to shouting.

What is your exact application?

 

[dkm0038]

I appologize for the caps, didn't realize it.

As I said it is for an air cooled heat exchanger, hot fluid is going through the tubes and cool ambient air is blown over it to cool the hot fluid.

 

[dkm0038]

Anybody?

 

[mauricestoker]

For laminar tube flow, heat flux is assumed constant. Temperature varies with distance along the tube, as well as along the cross-sectional (radial) area from centerline to wall.

A "bulk" temperature is assumed for any position along the heat tansfer surface to represent the total energy of flow at a location along the tube. Heat transfer coefficient is a function of the wall temperature and the bulk temperature. Nusselt will be constant (Nu=hd/k).

 

[IRstuff]

Constant surface temperature perhaps implies that the material is actively heated and controlled to a particular temperature, in which case, the convective heat loss is added to the heat load of the heater.

In a constant heat flux condition, the convective heat loss is either all of, or the bulk of, the heater heat load, i.e., the heat flux has already done its intended purpose and is now completely waste heat.

TTFN

FAQ731-376

 

[25362]

As an aside, some electrically heated exchangers maintain process temperatures at constant heat fluxes notwithstanding the degree of fouling. Of course, the temperature on the surface of heating element goes up until a allowable limit is reached and shutdown for cleaning is called for.