Air Coolers: Viscous Service Issue and Thermal Shock Issue

[thermmech]

I would appreciate any advice you could give re:

1. On my project, there is an aerial cooler cooling bitumen. Viscosity of bitumen at the outlet > 50 cP. Allowable pressure drop = 600 kPa which is used in full.

API 661 recommends that air coolers do not be employed if fluid outlet viscosity > 50 cP.

I have employed in the design of this unit every possible good design practice I could think of (deep tube bundle, 2 or 3 passes per row, large diameter tubing, dual recirculation ducts, large allowable pressure drop, etc.) - however, I am still worried about the possibility of "unstable flow" phenomenon. What is your particular experience with aerial coolers in high viscosity services?

2. The same project: there is a possibility of an upset condition that could introduce hot (265°C) hydrocarbon fluids into AC and S&T heat exchangers. Ordinarily, these exchangers are operating at temperatures of 80 - 150°C.

Is there a possibility of any adverse effects on the tube-to-tubesheet joints of these exchangers due to introduction of these hot fluids during upset in the plant?

AC joints are 2-grooved & expanded.
S&T joints are 2-grooved & expanded and seal welded.

 

[srfish]

It appears that you have done all of the right things. The most important design feature is the relatively high pressure drop. A low pressure drop gives maldistribution of the process stream. There needs to be a pressure drop above a certain value. I have sucessfully designed this type of oil cooler with pressure drops in the 500 kPA range.

Watch out for a temperature change across the inlet sections of over 200 C. if this occurs, the tubes will probably pull out of the tubesheet. For air coolers, use a split header design.

 

[djack77494]

You might also go with cocurrent flow (hot fluid into bottom of air cooler; cold fluid out at top). Also, consider a means of adding heat (such as a steam coil) to the air.
Doug

 

[thermmech]

djack77494, thanks for input.

Steam coil is provided for start-up only, and it is cost-prohibitive to go with cocurrent-flow configuration.

Regards,
Sean