ASHRAE Journal - September 2011 - AMCA12

from 24-in. to 36-in., could be considered using their actual brake horsepower (bhp) and other selection criteria. As mentioned earlier, FEG ratings apply only to the fan efﬁciency and not to the motor and drives. Adding motors and drives to the selection process is partially a matter of complying with national energy regulations. The National Electric Manufacturers Assoc. (NEMA) established Premium Efﬁciency speciﬁcations for motors that were adopted into federal regulations via the U.S. Energy Independence and Security Act (EISA). EISA speciﬁes minimum full load motor efﬁciencies; for example, a 7.5 hp open motor on the above fan selections has a minimum motor efﬁciency of 91.7%. Accounting for drives begins with determining whether the fan will be driven directly or through a V-belt. V-belt drive losses vary considerably with power transmitted, type and number of belts, pulley diameters, tension, and so on. AMCA Publication 203, Field Performance Measurement of Fan Systems, has a chart of the typical range of losses for V-belt drives as they relate to power transmitted. The average drive losses range from around 9% for a 1 hp drive to around 4% for 50 hp and higher. Said another way, the drive efﬁciency would range from around 91% for a 1 hp drive to around 96% for 50 hp and higher. Note that direct driven fans do not encounter these drive losses and, therefore, have an effective drive efﬁciency of 100%. Direct drive fans should, therefore, be used whenever possible and especially on smaller fans. For the 7.5 hp drive on the above fan selections, the drive loss is estimated at 6% or 0.45 hp. Variable frequency drives have some internal losses and also cause a slight decrease in motor efﬁciency. The combined effect is small, approximately 3% to 5% loss. However, their advantages in allowing fans to respond to system requirements and run at reduced speeds far outweigh these losses. The following is an example of an energy analysis comparing a 24-in. SWSI fan to a 33-in. fan at 10,000 cfm at 3-in. total pressure. The fan operates 16 hours per day, and the electric rate is $0.10 per kW-hour. For simplicity, a VFD is not considered: Size 24: Total mechanical power = 6.84 fan shaft bhp + 0.45 hp drive loss = 7.29 hp Total kW into motor = 7.29 hp × 0.746 kW/hp/ 91.7% motor efﬁciency = 5.93 kW Total annual cost = 5.93 kW × 16 hours/day x 365 days/year × $0.10/kW-hour = $3446 12
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Size 33: Total mechanical power = 5.67 fan shaft bhp + 0.45 hp drive loss = 6.12 hp Total kW into motor = 6.12 hp × 0.746 kW/hp/ 91.7% motor efﬁciency = 4.98 kW Total annual cost = 4.98 kW × 16 hours/day × 365 days/year × $0.10/kW-hour = $2,908 Annual Savings = $3,446 – $2,908 = $538 What codes or standards refer to FEG ratings? As mentioned earlier, FEGs (and FMEGs) were developed to facilitate regulatory measures that seek to increase fan and fan system efﬁciency. AMCA 205 is currently being considered for adoption in an advisory “continuous maintenance proposal” into ANSI/ ASHRAE/IES 90.1-2010. It also is on the path to being adopted into the prescriptive requirements of the IGCC. For more information on these two developments, see related article, “Update on Fan Efﬁciency Grade Option into ASHRAE 90.1 and the International Green Construction Code.” AMCA is reporting progress on AMCA FEG standards and where they are adopted into codes and standards at www.amca.org/feg. If I want to replace my existing fan with a new fan that has an FEG rating, how do I calculate energy savings? Fan replacement is treated in much the same way as a new installation. If required by code, the replacement fan must meet a minimum FEG rating and be selected within 10 points of the peak fan efﬁciency. The economic analysis would then be done using actual energy savings of the new fan over the existing fan. Replacement fans present some challenges in getting the new fan to ﬁt in the same overall space as the old fan and physically moving the new fan into this location. Unfortunately, this is made even more difﬁcult when trying to use a larger, more efﬁcient replacement fan. However, the newer fans and fan systems result in energy (and economic) beneﬁts beyond the fan efﬁciency. Not only can the actual fan efﬁciency be improved, older belt driven fans can be converted to direct driven fans with higher efﬁciency motors. Variable frequency drives can also be added to turn back fan speed during off-peak demand times. Do FEGs relate to sound performance or other fan-performance parameters? Fans, like many other machines, produce the least sound when they are operating most efﬁciently. This relates to FEG ratings in two ways. First, FEG ratings are a measure of peak total
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ASHRAE Journal - September 2011

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