Changing flow rate and effect on U value in T&S HX

[MortenA]

Does anyboy have a rule of thumb that links changes in flowrate to a design U and flowrate.

Im specifically thinking of a gas heater with hot water on the shell side (so im assuming that all thermal resistance is on the gas side). The flow rate to vary is the gas flowrate both with regards to Sm3/hr but also a different pressure and temperature.

Best regards

Morten

 

[25362]

Above Re=10,000 the product (Nu)(Pr^-1/3)(visc ratio)^-0.14 in forced convection is only a linear function of Re on a log-log graph and independent from L/D, as in the classical Sieder and Tate graph. As follows, read with my old eyes:

Re (Nu)(Pr^-1/3)(visc ratio)^-0.14

10000 40
20000 70
30000 95
100000 250
200000 450
500000 940

The variations are about a function of Re^0.8.
So, if you change only the resistance of the gas side, you can estimate the new HTC on this side, and then recalculate the OHTC. J.P's Heat Transfer McGraw-Hill, tabulates the values of the thermal properties of the most common gases and liquids to facilitate the estimation.

 

[25362]

I should have written the full name of the author: J.P. Holman. Sorry.