3 replies to this topic

[go-fish]

I have few inter-related queries about shell and tube type heat exchangers:-

1) What are the factors (other than ease of tube cleaning) to be considered while deciding whether a liquid fluid to be heated should be on the shell side or the tube side in a heater using steam?

2) I did two different Pro II simulations where I used a high viscosity crude oil, once on the shell side and once on tube side. I kept heater shell ID same to compare the heat exchanger datasheet from both cases. I found that the transfer rate when the crude is on the shell side is higher than that when it is on the tube side? Why is it so and what is the significance of this number?

3) Also the transfer rate increased when I reduced the shell ID ( which leads to less number of tubes, less material cost), however, the pressure drop across the shell side increased (which leads to higher pumping power requirements, more operating cost). What is the general optimization method since the transfer rate and pressure drop are inversely proportional?

[Zauberberg]

1. Service conditions: higher-pressure fluid usually goes on the tube side which results in a lower-cost exchanger. The application is also important: if it is a vertical reboiler, steam as a heating medium must be on the shell side, for horizontal reboilers is the opposite. More corrosive fluids also go on the tube-side.

2. That must be related to different film coefficients in both applications - obviously the software has calculated higher overall heat transfer coefficients based on higher individual film coefficients. Remember: the overall U is always determined by the smaller film coefficient. If you can boost it up, it enhances heat transfer. However: when handling dirty viscous fluids such is crude oil, I would always think three times before I decide to put it on the shell side.

3. The best philosophy in such applications is: always utilize as much DP as practically feasible (available) and by maintaining velocities below erosion limits. This stands in particular for crude oil/residue exchangers. There is one excellent story in one of Norm Lieberman's books about relying on low DP-high surface area in vacuum residue exchanger application that has resulted in complete failure. If you can't save the money by following the logic of not buying a pump, then don't try to do so.

Good luck,

[daryon]

Be careful putting the viscous crude oil which is likley to be heavily fouling on the shell side. This may require tube bundle removal for cleaning on a regualar basis.

Viscosity can have a dramatic effect on the film heat-transfer coefficient (HTC), and because the crude oil has a high viscosity it's film HTC will be small and the resistance to heat transfer will be large. Whether the crude oil is on the shell or on the tube side it's film coefficient will be controlling the overal HTC, and thus the rate of heat transfer in the exchanager.

If you put the crude oil on the shell side then the HTC may be improved by adjusting the baffle spacing and shell diameter to increase the shell-side velocity and therefore the Reynolds number. Also, for a given tube O.D., the pitch will play a role in influencing the film HTC. If the crude oil is on the tube side the number of tubes and passes and the tube thickness (and therefore I.D.) can be varied to influence the Reynolds number and thereby the film HTC.

You need to treat the Pro II calculated HTC will caution, it is not a heat exchanager thermal design software. The most accurate methods for calculating film HTC are proprietary computer based methods:

• Heat Transfer Research, Inc. (HTRI), College Station, TX;
• Heat Transfer and Fluid Flow Services (HTFS), Harwell, U.K.;
• B-JAC

Hope this helps.

[jcazenave]

BTW both
Heat Transfer and Fluid Flow Services (HTFS) and B-JAC, are now available under AspenTech Exchanger Design & Rating.

Kind Regards