Cope With Condenser Constraints

Readers suggest ways to maintain performance on hot days.

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Andrew W. Sloley, principal engineer
CH2M HILL, Bellingham, Wash.

FAN POWER WON'T DO IT
There are few ways to improve the performance of air condensers in your situation. Looking at the difference between how the condenser is supposed to work and how it works now poses a real problem for the water cooler.

Let's assume you've already considered increasing the fan speed to maximize the air flow. In turbulent flow, air heat transfer is proportional to ΔT0.34; in laminar flow or natural convection, it is proportional to ΔT0.25. That's nice theory, but let's consider the practical. The trouble is that changing the air heat-transfer coefficient, hair, is difficult and not rewarding. Working with a few values from Figure 45, p. 225, of Kays and London's "Compact Heat Exchangers," McGraw-Hill, 1984, shows the following: Δhair = (Δfan power)0.173 and ΔP = (Δhair)3.81. This says that increasing the fan power increases hair only modestly with a huge increase in pressure drop. If we assume that heat transfer across a flat plate applies, Nu = 0.0292×Re0.8×Pr0.62, where Nu is the Nusselt number (which indicates heat transfer at a boundary), Re is the Reynolds number and Pr is the Prandtl number. Pr represents only immutable physical properties. Because Reynolds number is the dominant means of improving heat transfer, and this is an expensive approach, it may be possible to change coefficient by changing the fluid: spray the fins of the air condenser with cold water. A fine spray works best; perhaps cooling the shell of the column could help.

Also, it may be a good time to look at the air flow into and out of the fans. Recirculation can reduce exchanger efficiency significantly. Consider erecting walls to direct the air flow as desired.

There is another option: change the feed characteristics. At 260°F, the feed is a superheated vapor. A rough check using McCabe-Thiele for a typical relative volatility shows a couple of possibilities if the feed is changed from superheat to a subcooled or saturated liquid; even reducing the superheat might help. Lowering the feed temperature or changing the state would require raising the feed tray and reducing the condenser recycle rate; the minimum recycle rate decreases by going from a superheated vapor to a subcooled liquid. If this change is possible, it might be another way to deal with the air condenser problem. Unfortunately, this will put the load on another heat exchanger downstream of the debutanizer column. A refinery is a well-balanced machine; reduce the burden in one place and it must be taken up by another exchanger or tower.
Dirk Willard, lead process engineer
Fluor Global Services, Inver Grove Heights, Minn.

FEBRUARY'S PUZZLER
The seals of the lean amine pump serving our fluid catalytic cracker unit suffer a very short mean time between failure (MTBF). We are using tandem seals with an API Plan 52. The buffer fluid is a light lube oil recommended by our pump salesman. The amine is 21% by weight monoethanolamine. The 3×2 centrifugal pump was designed for 190 gpm at about 160 psig. The impeller is a mixed-flow type. The nominal suction specific speed is about 7,500 rpm; the suction diameter is 4 in., schedule 40. The difference between the net positive suction head available and required is about 6 ft water at nominal. Unfortunately, the pump usually runs above the nominal rate, at around 205 gpm at about 145 psi head. Inspection after failure shows severe crystallization and scoring of the shaft seal. In addition, the pump seems to run rough even when it's operating at 60–70% of the best efficiency point, which is at around 185 gpm. The cavitation grows worse over time so we pull the pump after six months to avoid potential failure. We followed the pump manufacturer's recommendation and installed a minimum flow loop that operates continuously with an orifice; the flow is about 60% of the nominal flow. Any suggestions on how we can improve the MTBF?

Send us your comments, suggestions or solutions for this question by January 11, 2013. We'll include as many of them as possible in the February 2013 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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