ASHRAE Journal - April 2010 - 64

Selecting Efficient Fans
By John Murphy, Ph.D., Member ASHRAE or much of the past 50 years, HVAC system design and fan selection decisions were driven more often by first, rather than life-cycle, cost. This was due primarily to the low price of energy. Even when life-cycle cost comparisons were made, the projected energy costs were artificially low by today’s standards. The large number of speculative buildings constructed heightened the trend. Recent events have shown the need to rethink our philosophy about using first cost as the deciding factor for system design and fan selection. Because the need to supply ventilation will not disappear, the required ventilation will need to be supplied at the lowest possible total cost, accounting for the impact of lower velocities on duct (and, therefore, building) size, the initial cost of the fan and associated components and the power consumed over the life of the system. It is clear that there must be significant pressure to use the most efficient components, especially the fans and motors. High efficiency motors are already being required by most building authorities. Improving the efficiency of fans is now the focus of a concerted effort, both in the United States and worldwide. Standards are being written defining fan efficiency grades (FEG) and fan/motor efficiency grades (FMEG). Although these efforts are not yet complete, it is clear that these definitions will soon become available to aid in selecting high efficiency fans. The Air Movement and Control Association International (AMCA) has developed a standard (AMCA 205-10) that defines fan efficiency grades (FEG) based on the peak value of total efficiency of the fan without consideration of the motor/drive. This standard also recommends that any specification or code that sets a minimum acceptable FEG also include a requirement that the efficiency at the actual operating point(s) be within 10 points of the peak value. The next revision of Standard 90.1 is expected to set a minimum FEG grade for fans used in non-residential buildings. It is clear that the current direction is toward setting minimum fan efficiency levels. Before discussing the impact this may have on fan manufacturers, let us look at some performance data. The peak efficiency that can be achieved in any fan is a function of fan type and size. Table 1 shows values of peak total efficiency achievable by production units with diameters of 24 in. (600 mm) or greater. The values in this table were generated from a thorough review of published catalogs from many U.S. and European Union manufacturers. Although the reduction in
64	 ASHRAE	Journal	
Fan Type Airfoil Backward Curved Centrifugal Backward Inclined Forward Curved Vane Axial Axial Tube Axial Propeller Mixed Flow Tangential 80 70 86 75 55 75 25 Peak Total Efficiency % 88 84

F

Table 1: Peak total efficiency by fan type.

efficiency at smaller sizes is significant, the relative ordering by fan type is basically unchanged. There may be fans produced that exceed the values in Table 1 by a small margin (one or two points of efficiency) in any fan type. The variation in peak efficiency with fan type can be understood by considering the differences in the details of the various designs. In general, the more efficient fan types are more expensive to manufacture than the lower efficiency types. The factors that affect efficiency of centrifugal fans are blade profile, wheel width, inlet to wheel clearance and cutoff height. For axial fans, the important factors are blade profile, tip clearance and swirl recovery. Some fan types have inherent advantages or disadvantages due to their configuration. As an example, a forward-curved centrifugal fan cannot achieve peak efficiencies close to those achieved by any backward-oriented blade design because the forward-curved blade guarantees separated flow (locally) downstream of the blade, and this separation causes a loss of energy. The backwardoriented designs rank in efficiency order according to the aerodynamic quality of the airflow passage through the impeller; curved blades are better than flat blades and airfoil shapes are better than single thickness. For axial fan types, propeller fans suffer from larger tip clearances and the absence of turning vanes and tube axial fans suffer from the lack of turning vanes. The values in Table 1 suggest that as minimum FEG grades are specified there will be considerable pressure to restrict the use of (at least) forward-curved and propeller fans.
ashrae.org	 	 April	 2010



ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010

ASHRAE Journal - April 2010
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
The Science of Evaporation is Key to Defense in Murder Trial
Selecting DOAS Equipment with Reserve Capacity
Technology Award Case Studies: Greening Hospitals
Technology Award Case Studies: Sustainable Remedy for Hospital
Building Sciences
Emerging Technologies
Technical Topics: Selecting Efficient Fans
Technical Topics: Dual-Capacity Heat Pumps
IAQ Applications
International Column
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2010 - Intro
ASHRAE Journal - April 2010 - ASHRAE Journal - April 2010
ASHRAE Journal - April 2010 - Cover2
ASHRAE Journal - April 2010 - 1
ASHRAE Journal - April 2010 - 2
ASHRAE Journal - April 2010 - Contents
ASHRAE Journal - April 2010 - Commentary
ASHRAE Journal - April 2010 - 5
ASHRAE Journal - April 2010 - Industry News
ASHRAE Journal - April 2010 - 7
ASHRAE Journal - April 2010 - 8
ASHRAE Journal - April 2010 - 9
ASHRAE Journal - April 2010 - 10
ASHRAE Journal - April 2010 - 11
ASHRAE Journal - April 2010 - 12
ASHRAE Journal - April 2010 - 13
ASHRAE Journal - April 2010 - 14
ASHRAE Journal - April 2010 - 15
ASHRAE Journal - April 2010 - 16
ASHRAE Journal - April 2010 - 17
ASHRAE Journal - April 2010 - Letters
ASHRAE Journal - April 2010 - 19
ASHRAE Journal - April 2010 - Meetings and Shows
ASHRAE Journal - April 2010 - 21
ASHRAE Journal - April 2010 - The Science of Evaporation is Key to Defense in Murder Trial
ASHRAE Journal - April 2010 - 23
ASHRAE Journal - April 2010 - 24
ASHRAE Journal - April 2010 - 25
ASHRAE Journal - April 2010 - 26
ASHRAE Journal - April 2010 - 27
ASHRAE Journal - April 2010 - 28
ASHRAE Journal - April 2010 - 29
ASHRAE Journal - April 2010 - Selecting DOAS Equipment with Reserve Capacity
ASHRAE Journal - April 2010 - 31
ASHRAE Journal - April 2010 - 32
ASHRAE Journal - April 2010 - BRC1
ASHRAE Journal - April 2010 - BRC2
ASHRAE Journal - April 2010 - 33
ASHRAE Journal - April 2010 - 34
ASHRAE Journal - April 2010 - 35
ASHRAE Journal - April 2010 - 36
ASHRAE Journal - April 2010 - 37
ASHRAE Journal - April 2010 - 38
ASHRAE Journal - April 2010 - 39
ASHRAE Journal - April 2010 - 40
ASHRAE Journal - April 2010 - 41
ASHRAE Journal - April 2010 - Technology Award Case Studies: Greening Hospitals
ASHRAE Journal - April 2010 - 43
ASHRAE Journal - April 2010 - 44
ASHRAE Journal - April 2010 - 45
ASHRAE Journal - April 2010 - 46
ASHRAE Journal - April 2010 - 47
ASHRAE Journal - April 2010 - 48
ASHRAE Journal - April 2010 - 49
ASHRAE Journal - April 2010 - Technology Award Case Studies: Sustainable Remedy for Hospital
ASHRAE Journal - April 2010 - 51
ASHRAE Journal - April 2010 - 52
ASHRAE Journal - April 2010 - 53
ASHRAE Journal - April 2010 - Building Sciences
ASHRAE Journal - April 2010 - 55
ASHRAE Journal - April 2010 - 56
ASHRAE Journal - April 2010 - AP1
ASHRAE Journal - April 2010 - AP2
ASHRAE Journal - April 2010 - AP3
ASHRAE Journal - April 2010 - AP4
ASHRAE Journal - April 2010 - 57
ASHRAE Journal - April 2010 - 58
ASHRAE Journal - April 2010 - 59
ASHRAE Journal - April 2010 - Emerging Technologies
ASHRAE Journal - April 2010 - 61
ASHRAE Journal - April 2010 - 62
ASHRAE Journal - April 2010 - 63
ASHRAE Journal - April 2010 - Technical Topics: Selecting Efficient Fans
ASHRAE Journal - April 2010 - 65
ASHRAE Journal - April 2010 - Technical Topics: Dual-Capacity Heat Pumps
ASHRAE Journal - April 2010 - 67
ASHRAE Journal - April 2010 - 68
ASHRAE Journal - April 2010 - 69
ASHRAE Journal - April 2010 - IAQ Applications
ASHRAE Journal - April 2010 - 71
ASHRAE Journal - April 2010 - 72
ASHRAE Journal - April 2010 - 73
ASHRAE Journal - April 2010 - International Column
ASHRAE Journal - April 2010 - 75
ASHRAE Journal - April 2010 - 76
ASHRAE Journal - April 2010 - 77
ASHRAE Journal - April 2010 - 78
ASHRAE Journal - April 2010 - 79
ASHRAE Journal - April 2010 - 80
ASHRAE Journal - April 2010 - 81
ASHRAE Journal - April 2010 - 82
ASHRAE Journal - April 2010 - 83
ASHRAE Journal - April 2010 - 84
ASHRAE Journal - April 2010 - 85
ASHRAE Journal - April 2010 - Classified Advertising
ASHRAE Journal - April 2010 - 87
ASHRAE Journal - April 2010 - Advertisers Index
ASHRAE Journal - April 2010 - Cover3
ASHRAE Journal - April 2010 - Cover4
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