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Drive Selection
Whether a fan is belt- or direct-driven impacts efficiency.

Belt drives offer the ability to adjust fan speed and balance a system; however, efficiency losses range from a median 15 percent for fractional-horsepower motors to 4 percent for larger motors (Figure 5).4 Additionally, belt-driven fans require regular maintenance. As belts wear, dust accumulates around a fan; in an air handler or makeup-air unit, this causes filters to load faster or, in the absence of filters, compromises air quality.

Because the wheel or impeller is connected directly to the motor shaft, direct-drive fans do not have the losses associated with belt-driven fans. Also, without belts, pulleys, and shaft bearings, they require less maintenance, and, with fewer rotating components, they generally experience less vibration. However, the larger, lower-speed motors associated with larger direct-drive fans generally cost more; thus, return-on-investment calculations should be performed.

Variable-Frequency Drives
Variable-frequency drives (VFDs) are used in both belt-driven and direct-drive systems to maximize energy savings. Reducing input power frequency with a VFD as building load decreases lowers motor speed, airflow, and power. As building occupancy changes, fan speed can be changed and power consumption reduced.

Where Does Lost Energy Go?
Most of the energy lost in a system is converted to heat. For example, mechanical and electrical energy losses in a fan motor raise the surface temperature of the motor. If the motor is in an air stream, the heat will be transferred directly to the air.

In a belt-driven system, losses are the result of belt friction, slippage, and/or flexing. All such losses are converted to heat and, if the belt drive is in an air stream, increase the temperature of air. Fan losses quantified by fan efficiency contribute to air-temperature rise as well. As a fan works on a fluid, the friction attributed to the airflow decreases the fan's efficiency and creates heat.

Figure 6 shows energy transfer in a typical belt-driven fan. The energy flow starts with the power supplied to the motor and ends with the air power generated by the fan. In this example, the motor receives 3,730 W of power, or the equivalent of 5 hp. After losses in the motor, drives, and fan, 2,209 W of air power is delivered. That air power accounts for only 60 percent of the power supplied to the motor. The 1,521 W lost in the process is converted to heat. If the components were located in an air stream, the heat would transfer to and increase the temperature of the air.

As the efficiency of a system decreases, air-temperature rise increases. In a cooling application, this means increased energy consumption on the part of the compressor. Minimizing inefficiencies results in energy savings.

Summary
It is the engineer's responsibility to minimize total system pressure through proper design and layout. This involves balancing economics and efficiencies in specifying a fan.

But not all responsibility resides with the engineer. The contractor must ensure appropriate equipment is installed and not base decisions solely on the lowest bid. The contractor needs to be aware of the consequences of poor installation and minimize system effects.

The losses accrued in a system directly affect the system's power consumption while indirectly increasing the energy consumed by other processes. Energy losses in the form of heat equate to higher operating costs.

Because performance conditions are mandated by codes, proper system design, equipment selection, and installation are the most effective means of minimizing inefficiencies and saving energy.

References

1. ASHRAE. (2007). ASHRAE handbook--HVAC applications. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
2. AMCA International. (2010). ANSI/AMCA standard 99-10, standards handbook. Arlington Heights, IL: Air Movement and Control Association International.
3. AMCA International. (2007). AMCA publication 201-02, fans and systems. Arlington Heights, IL: Air Movement and Control Association International.
4. AMCA International. (2007). AMCA publication 203-90, field performance measurement of fan systems. Arlington Heights, IL: Air Movement and Control Association International.

Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold at scott.arnold@penton.com.

Brian Mleziva is an application engineer for centrifugal, vane-axial, and industrial products for Greenheck Fan Corp. His responsibilities include product development, marketing-support materials, and customer service. He has a degree in mechanical engineering from Michigan Technological University.

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