Chillers Require Cold-Eyed Review

Readers suggest ways to avoid temperatures swings and trips.

By Chemical Processing Readers

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Depending on the age of these units, it may be more practical to scrap them. Newer systems are easier to connect to plant controls and may be more energy efficient.

You also might consider some equipment troubleshooting and revamping: 1) look at the entire system — chillers, cooling tower, pumps, compressors, etc., but focus particularly on the compressors, which represent 60% of operating cost; 2) check the compressor inner cooler and all the heat exchangers for fouling; 3) verify the capacity of the cooling tower; 4) adjust load by matching requirements with smaller units and larger ones; 5) compare the cost of speed controls for pumps versus impellers sized for operation at the best efficiency point — don't ignore the possibility of matching small pumps with large pumps; and 6) consider replacing the shell-and-tube exchangers with plate-and-frame ones, as these are superior for compactness and efficiency in clean service.
 Dirk Willard, senior engineer
Ambitech Engineering, Downers Grove, Ill.

JANUARY'S PUZZLER
Our refinery has a project to fully enclose all API separators and other equipment to meet stringent EPA regulations. This includes a concrete sump, 15-ft long, 10-ft wide, and 7-ft 2-in. deep, built in 1944 and buried in earth. We reckon the slop oil in the sump consists of a mixture of 70 mol-% gasoline and 30 mol-% water. The 8,000-gal sump has two pumps, each eight feet below ground level, operating at 100 gpm each. A structural engineer, with some trepidation, set the pressure limit at 15 inches of water column (IWC) and the vacuum at 1.5 IWC. The normal vent exits the roof, which is 15 ft above the sump, and then must go to an activated carbon bed (ACB) located 60-ft away; the ACBs are on the ground. The bed has a pressure drop of 0.5 IWC at 100 scfm air (60oF). A flame arrestor normally is installed upstream of the ACB and likely would add a pressure drop of 35 IWC at 1,000 scfh air. The emergency vent goes direct to the roof. How can we design these vents?

Send us your comments, suggestions or solutions for this question by December 9, 2011. We'll include as many of them as possible in the January 2012 issue and all on CP.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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