ASHRAE Journal - May 2012 - 82

engineer’s notebook
(2040 m3/h). The fan law for power (W) can be applied to estimate input power assuming constant fan and motor efficiency. W1 = W2 × (RPM1/RPM2)3 = 375 W × (1,450/1,730)3 = 221 W This would result in: • An input power reduction of 7.5% (2,400 to 2,221 W); • An 1.7% increase of net capacity (179 W × 3.412 Btu/W·h = 610 Btu/h); and • A 10% increase in EER from 15 to 16.5 (EER = 36,610 Btu/h ÷ 2,221 W). ficiency of standard blowers when they don’t have to work very hard at lower rotational speeds, and the fan power will have a smaller impact on system efficiency. The “rated EER” can be impressive without an improved fan wheel. Customers can be convinced to be green and pay extra for the more efficient variable speed fan and resulting higher “rated EER.” Hopefully, the installing contractor will take care to avoid 50 ft (15 m) runs of drooping flexible duct and undersized antimicrobial MERV 14 filters since those fantastic ECMs can ramp up in speed (and input power) to shove 1,200 cfm (2040 m3/h) of air through whatever restrictions are placed in its path. Manufacturers who choose to use more efficient fan wheels will be satisfied to some degree with higher rated efficiencies. The full benefit to customers will not be so obvious. It seems an enhanced way to reward those manufacturers is to make a change to the standards, so they more accurately reflect conditions in the field.

Reward Manufacturers That Use Higher Efficiency Fans
One would think manufacturers would jump at the chance to increase equipment efficiency 10% to 11% by upgrading the fan wheels in unitary heat pumps and air conditioners. Currently, the boost in rated equipment efficiency will be nowhere near 11% because the rating standards require so little fan pressure. ANSI/ AHRI Standard 210/240-20083 for air-cooled equipment only requires an external static pressure (ESP) of 0.15 in. w.c. (37 Pa) for a 3 ton (10.6 kW) unit with a filter loss of 0.08 in. w.c. (20 Pa). ANSI/ARI/ASHRAE ISO Standard 13256-1-19984 for waterto-air heat pumps goes even further and requires only 0.0 ESP. Equipment ratings can be boosted using the part-load motor efficiency advances made by those darn electrical engineers with their ECMs. Couple this with the typical increase in ef-

references
1. Fans-Tech. 2009. EC Fan Series. Malvern, Pa. 2. 2008 ASHRAE Handbook—HVAC Systems and Equipment, Chpt. 20. 3. ANSI/AHRI 210/240-2008, Standard for Unitary Air-Conditioning and Air-Source Heat Pump Equipment. 4. ANSI/ARI/ASHRAE ISO Standard 13256-1-1998, Water-Source Heat Pumps-Testing and Rating for Performance—Part 1: Water-to-Air and Brine-to-Air Heat Pumps.

Steve Kavanaugh, Ph.D., is a professor emeritus of mechanical engineering at the University of Alabama, Tuscaloosa, Ala.

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ASHRAE Journal

May 2012

ASHRAE Journal - May 2012

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