The scene: The project engineer visits the job site to conduct a final inspection. The project is an auditorium at a high school. Two 25-ton gas/electric rooftop units (RTUs) are installed and operating on a low-mass roof constructed with steel-bar joists topped with metal decking, rigid roof insulation, and modified bitumastic roofing material.
A loud noise is heard. Not only is each rooftop unit vibrating, but the steel roof decking is vibrating in a resonant manner, amplifying the RTU-vibration noise. Isolation roof curbs had not been specified.
So the engineer says to the contractor: “Get four 5-gal buckets. Bolt two of the buckets securely to the floor of the condensing section within each rooftop unit. Fill each bucket with concrete.”
And the contractor says to the engineer, “Are you nuts?”
The result: The vibration was eliminated because adding the mass of the concrete lowered the resonant frequency of the roof system far enough away from the driving frequency of the RTUs.
Gary Gabriel PE
CDH Partners Inc.
The woodshop dust-collection system we had designed as part of a major addition to a high school was being balanced and commissioned while school was in session. It was getting a real test with a full class of students working on their class projects, generating a good volume of wood cuttings, shavings, and sanding dust.
The shop instructor had been used to a cyclone-type dust collector in the past. The newly installed “bag-house”-style collector was a new piece of equipment to him, so he was a little unsure of how it would perform. After the first couple of days of running the new wood-dust collector, the shop instructor and the school-district maintenance person called to complain that the new dust collector was undersized and not picking up the wood dust and debris from the woodworking equipment.
The location was a two-hour drive from the office, so before leaving, I spent a day checking the original design criteria and all the usual troubleshooting issues that might be causing the problems.
The contractor claimed it was wired and being controlled properly. The balancing agent was not finished with his readings, but informed me that the air volume was about half of what was specified, and the suction pressure was about two-thirds of design. He suspected the ducts either were blocked or the fan was not sized properly.
I checked the calculations, along with the shop drawings. The unit had been selected with extra collection and air-volume capacity to accommodate a planned expansion of the woodshop in the next five years. The shop instructor was adamant that the underfloor dust-collection ducts were not blocked, and he was quite sure that none of his students had been dropping rags or chunks of wood into the suction connections.
At the site, I started at first principles by checking the ductwork in the crawlspace under the shop, tapping on the ducts to detect blockage. Clean-out access ports at the duct elbows allowed a fairly complete inspection of the ducts, which appeared clear. The next step was to check the filter bags. They looked fine, not very loaded at all.
I then climbed to the top of the dust collector to check the fan. The data plate and motor proved fine. “Give it a bump,” I yelled down to the shop instructor. Sure enough, the electrical contractor had cross-wired a three-phase motor, and the fan was running backward.
A quick repair was made and suddenly the instructor said, “Hey, why is it so quiet now?” The floor sweeps were tested with some wood debris, which vanished into the duct in quick order.
The balancing agent and the contractor were duly humbled, and the new dust collector was tested and found to be performing slightly better than specified.
Geoff McDonell, P.Eng., LEED AP
Vancouver, British Columbia, Can.
My daughter used to work on the flight line for a major airline and related this story. A 757 came in fully loaded with passengers. One of the toilets was completely plugged by a diaper, which the mechanics could not get out. The plane was scheduled to depart in an hour. The only way they could get the diaper out was to disassemble the whole lavatory and clean it out pipe by pipe, which meant that it would take hours to get the plane flyable again.
One of the mechanics got a bright idea: With the plane being empty, why not hook up the lavatory truck, pressurize the plane, and blow out the diaper?
It worked; however, the rectangular dump tank on the lavatory truck had only about a 1-in. vent line. When the diaper blew, the sudden increase in pressure inflated the dump tank, so that instead of a nice rectangular box, they had a sheet-metal balloon.
I guess it could have been worse.
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