The contractor was about to start installing new coils to replace coils that froze when the director of engineering called to go over my design.

"Why did you add those strainers?" he asked. "That's just extra cost for nothing."

"Well," I replied, "one possible reason the old coils might have frozen is that debris in the piping could have partially blocked water flow through the coils. There is a pretty long history of dirty water in this system. These are 100-percent-outside-air coils. If debris blocks one of the small tubes, the new coils could freeze, too."

"Waste of money," the engineer growled. "Maybe there were problems before I came here, but I know what I'm doing. I watch the water-treatment program and check the chemicals myself. I've got the system running clean now."

Because the general manager was nervous about another freeze-up, the strainers were installed. The day after the coils went online, I visited the site.

"Something funny is going on," the foreman said. "When I put the coils into operation yesterday, everything looked fine, but now the pressures are all out of whack, and we're getting hardly any heat from the coils."

After a quick look at the pressure gauges, I had a thought.

"Just out of curiosity," I said to the foreman, "could you pull out a couple of strainer baskets so I can take a look?"

The accompanying photos depict what I saw. That is about half of an inch of sludge that built up in about 24 hr of operation. If not for the strainers, the sludge would have been lining the insides of the tubes in the coil, where it would have impeded heat transfer and possibly blocked flow through some of the tubes.
Gary M. Elovitz, PE
Energy Economics Inc.
Newton, Mass.

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