May's Engineer's Notebook column, "Backward-Curved Fans" by Steve Kavanaugh, provides a refreshing analysis for an out-of-the-box approach to energy efficiency. However, some clarity is needed to understand the first section. Two different types of blowers, given a free run, consume 400 W and 375 W respectively; that does not mean they delivered the same air volumes on a free run. Does the author need to tell us if readings were taken with an adjustable tail piece load adjusted to the same air volumes? If not, then Equations 1 and 2 need to work out efficiencies at different air volumes.
Another aspect is the cost element. It is likely that the installed cost of the mechanical element (or even the electromechanical combination) will go up while achieving a lower installed kW.
Kishore Bhattacharya, Member ASHRAE, Calcutta, India
The Author Responds
Obtaining rated performance of smaller fans is difficult in that the Air Movement and Control Association standards do not cover devices less than one horsepower (AMCA Standard 205-10). Therefore, the single input power values listed in the article are all that were available and are more than most fractional horsepower manufacturers provide.
Normally, all that is given is the rated horsepower of the fan motor. The curve for the standard blower was based on static pressure. This was converted to total pressure by adding the velocity pressure. The backward-curve fan ratings did not include the housing, so no adjustments were made. Computation of more precise efficiency values will not be possible until fan rating standards are applied to smaller fans. Therefore, no minimum efficiency levels can be established for the substantial number of fractional horsepower fans being delivered, and the industry will continue to live with devices that are 25% to 30% efficient.
The industrial equipment catalog in my office lists the cost of a 1,285 cfm, direct drive blower with a PSC motor as $325.50; the cost of a 1,330 cfm PSC motorized backward-curve impeller as $366; and a 1,124 cfm ECM motorized impeller as $760. The cost of 0.5 to 1 hp electronically commutated motor alone is $437. So it appears the added cost for ECMs over PSC motors is much greater than the cost increase for a backward-curve impeller. So if motors that are over twice the cost of conventional fan motors (and provide only a modest efficiency improvement) can be justified, why are blower wheels with modest cost premiums not a logical choice given they double fan efficiency? Steve Kavanaugh, Ph.D., Fellow ASHRAE, Tuscaloosa, Ala.
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|Article Type:||Letter to the editor|
|Date:||Aug 1, 2012|
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