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Finding the Right Equipment

I designed an HVAC system for a large electronics store in midtown Manhattan, a fast-track construction project that was supposed to be completed during mid-summer. I used a large water-cooled packaged air conditioner for the main open-space floor of the store.

The manufacturer's representative came to the store to start the unit. After the unit was started, its main supply fan overloaded because it was drawing too many amps. I looked at the fan curve and saw that the fan was a forward-curve fan, which can overload a motor if the static pressure is too low.

There was not enough resistance in the ductwork because all of the balancing dampers were wide open. I told the contractor to partially close all of the main dampers. At first, the amperage of the fan went down, but it still overloaded. After closing the dampers some more, the fan ran without overloading.

Ever since that project, I have tried to avoid forward-curve fans and instead specify backward or airfoil fans that are non-overloading, even though they can be more expensive.
Jonathan Ottenstein, PE, LEED AP
Kallen & Lemelson, Consulting Engineers LLP
New York, N.Y.

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