The corridor air-handling unit (AHU) in a high-rise was reported to be inoperative. The resident manager indicated the duct traversed a long shaft between the 40th floor and the eighth floor. He suspected some dampers were closed. The shaft was inaccessible. He pointed to a noisy duct on the 35th floor. A look through an access door indicated a closed damper to the toilet-exhaust or kitchen-exhaust system. Adjusting the damper reduced the noise. Obviously, the main duct trunk was not serving the corridor.
Air velocity was measured at the AHU discharge duct. It was too low. Immediately, I pointed to a faulty impeller fan or motor on the AHU. A catalog cut from the vendor showed the diameter of the centrifugal fan was correct. Tightening the fan-belt pulley had no measurable impact on air velocity. The motor was supposed to be 10 hp. The technician who accompanied me indicated the motor was replaced a few months earlier and pointed to the old motor on the floor. A closer look at the motor indicated it was 10 hp at 208 v. The available power was 460 v. Thus, the motor acted as if it were 7½ hp.
The moral of the story: Get submittal sheets ahead of site visits. Chat with the technician who services equipment. His or her input can be key to solving a construction defect.
Joseph Menasce, PE
JEM Consulting Engineer PC
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