Heat exchanger surface temperature

[Lukezo]

In a water vapor condenser HTC there is a 'wall temperature'. How to proceed? I was thinking about guessing it and calculating iterations, but how do I verify it?

 

[georgeverghese]

At any location in the HX, if the shellside fluid bulk temp is Ts, and that for the tubeside, it is Tt, then the wall temp Tw at that location is given by

(Ts-Tw)/(Ts-Tt) = ht/(hs+ht), where the local shellside htc is hs, and the tubeside htc is ht.

You can thus calculate Tw for the HX cold end and the HX hot end, and then get a mean Tw = (Twh + Twc) / 2 if you need a mean Tw for approximate purposes.

Further, at any location, the shellside surface temp Tss = (Tw+Ts)/2, and similarly,
Tst = (Tw+Tt)/2 for the surface temp on the tubeside

The mean or average surface temp values are required for computation of the film htc for each side.

 

[Lukezo]

Thanks for your effort georgeverghese, but I am still missing an equation (Because I don't know Tt)

Maybe from known ht and known (desired) Tw?

 

[goutam_freelance]

Pipe surface temperature will vary between inlet and outlet. Since you are calculating average HTC it should be sufficient to calculate the average pipe temperature between inlet and outlet. Since tube metals have good conductivity, inside tube temperature may be assumed to be same as outside temperature. Average inside tube temperature may be calculated using average inside fluid temperature, average outside fluid temperature as well as rough inside/outside HTCs. If the temperature range is not very high the effect of averaging of temperatures will not have much effect. Another option-Since the condensing steam film coefficients are very high, it may not matter much if you consider outside steam temperature as pipe outside temperature.

 

[georgeverghese]

Well, at least you know the terminal temps for this HX? At the hot end and at the cold end. Compute htcs' and Tw for each terminal, then get an " average" Tw over the entire HX to get you started. Keep reiterating till any guess values match calculated.