SOUNDING BOARD

Sept. 1, 2011
I read the article on the use of variable-frequency drives on forced-draft fans at the University of Rochester ("Variable-Frequency Drive Improves Boiler Operation," January 2011, http://bit.ly/Eisenhauer_0111).

Motor Amps

I read the article on the use of variable-frequency drives on forced-draft fans at the University of Rochester ("Variable-Frequency Drive Improves Boiler Operation," January 2011, http://bit.ly/Eisenhauer_0111).

Why do the amps on the motor not change significantly when the demand on the boiler changes with the damper-controlled airflow?

I put a monitor on a motor, and the amps did not change significantly.
James B. Jones
NutraSweet/GreenWood
Augusta, Ga.

Co-author's response:
I do not know the type of air handler you have or where the dampers are positioned. Therefore, a rough explanation of why your amps do not drop significantly when you throttle the flow via a damper on a centrifugal air handler basically is based off of this equation:

Fan hp = (cfm × psi) ÷ (229 × fan efficiency)

When you drop your cfm with an outlet damper, for example, your pressure (inside your fan) goes up, and your fan’s efficiency drops severely. Amp reduction is minimal.

In the case of inlet dampers, flow, pressure, and fan efficiency drop. Therefore, inlet dampers are more electrically efficient than outlet dampers. However, they don’t compare to reducing the speed of the blower and taking advantage of the affinity laws:

hp = speed3

Following are links to documents providing a much more thorough explanation:

Dean Williams
EMA of NY Inc.
Cortland, N.Y
.

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About the Author

Scott Arnold | Executive Editor

Described by a colleague as "a cyborg ... requir(ing) virtually no sleep, no time off, and bland nourishment that can be consumed while at his desk" who was sent "back from the future not to terminate anyone, but with the prime directive 'to edit dry technical copy' in order to save the world at a later date," Scott Arnold joined the editorial staff of HPAC Engineering in 1999. Prior to that, he worked as an editor for daily newspapers and a specialty-publications company. He has a bachelor's degree in journalism from Kent State University.