What Are the Options?

Axial fans can be subdivided into three categories (see Figure 1): propeller fans (used to move high air volume against low or no static pressure), tube-axial fans (fans that encase the propeller in a duct section), and vane-axial fans (fans that use straightening fins to convert circular, twisting air to get the fan moving). Vane-axial fans tend to be the most efficient fans available for HVAC air-handling units -- with efficiencies in the high 80s -- largely because the direction of the airflow is little changed as it passes through the fan.

The pitch of axial fan blades can be fixed, adjustable, or variable (adjustable during use). Fixed pitch is the norm for low-efficiency propeller fans and for constant-volume fans. Adjustable-pitch fans allow the user to manually adjust blade pitch to tune the flow -- a useful feature for commissioning or for building in a safety factor without penalizing efficiency. Variable-pitch blades can be adjusted "in flight" by pneumatic or electric actuators; they provide efficient volume control without changing the speed of the fan.

Centrifugal fans, also known as "squirrel cage" fans, have an entirely different design. The air, instead of passing straight through, makes a 90-degree-angle turn as it travels from the inlet to the outlet, and it is "thrown" from the blade tips. Centrifugal fans have more mass farther from the axle, which requires more starting torque, but they're generally quieter than axial fans.

There are several arrangements of fan blades for centrifugal impellers. The highest efficiency centrifugal fans use airfoil or backward-curved impeller blades (Figure 2). Airfoil blades are curved backward but have an airfoil shape, while backward-curved blades are of a single thickness of metal. Straight radial fan blades are used mostly in industrial applications. The main advantage of radial blades is that they permit the passage of foreign objects in the airstream such as sawdust, metal filings, and other debris. They have no advantages for HVAC use, however, and should not be used for handling ventilation air in buildings. Forward-curved fan blades have low efficiency and are typically used to move high volume against low pressure in applications such as window air conditioners and hotel unitary packages. Low purchase cost and compactness are the principal advantages of fans with forward-curved blades; they are still being built and installed in great quantities.

Despite their lower efficiencies, centrifugal fans greatly outnumber axial fans -- anecdotal estimates indicate that centrifugals make up 80 to 90 percent of the HVAC supply fans currently used in the U.S. and nearly 100 percent of fans in packaged air handlers.

Top of Page

Pick a size that's just right. There is broad anecdotal evidence that many fans and motors are larger than necessary for their intended use. One investigator concluded after making field measurements on about 1,000 motors that about half operated at less than 60 percent of their rated load and a third operated at less than half their rated load. Probably the fans whose motors were attached were similarly oversized. That's bad news for those who pay the energy bills, because fans operate at their highest efficiency within a relatively small range. Outside of that range, efficiency drops off dramatically.

To pick the appropriate size, use a fan chart such as that shown in Figure 3. For new construction, carefully calculate the airflow and pressure drop and then add a safety factor. In a retrofit case, use the chart with data from actual measurements of flow and pressure to determine the optimum size, rather than looking for a like replacement.

Check the cost-effectiveness of high-efficiency options. Axial fans are the most efficient, but consider backward-curved fans where centrifugal design must be used. To evaluate the cost-effectiveness of high-efficiency fans, estimate the time spent in full- and part-load operation and calculate the potential savings as shown in Table 1.

	Table 1: Sample savings calculation for high-efficiency fans This table illustrates the calculations required to evaluate the cost-effectiveness of a high-efficiency fan. The calculations assume a full load of 10 kilowatts and a part load of 5 kilowatts, operating time of 3,000 hours per year at full load and 1,000 hours per year at part load, and an electricity cost of $0.08 per kilowatt-hour. Use a fan curve (see Figure 3) to find the efficiency at the desired operating conditions. Note that the heat generated by the fan adds to the cooling load -- the energy required to remove that heat is calculated assuming a cooling coefficient of performance of 3.4.
	Characteristic	Centrifugal fan, forward- curved	Axial fan with vanes
	Fan efficiency, full load (percent)	63	78
	Power requirement, full load (kilowatts)	15.9	12.8
	Fan efficiency, part load (percent)	47	63
	Power requirement, part load (kilowatts)	10.6	7.9
	Annual fan energy use (kilowatt-hours)	58,300	46,300
	Annual cooling energy required (kilowatt-hours)	17,147	13,618
	Annual energy use (kilowatt-hours)	75,447	59,918
	Annual energy cost ($)	6,036	4,793
	Annual energy savings ($)	NA	1,242
	Incremental fan cost ($)	NA	600
	Simple payback period (years)	NA	0.5
	NA = not applicable	Source: E SOURCE

Pay attention to entrance and exit conditions. The conditions at the entrance and exit to a fan greatly influence fan system efficiency. Following these guidelines can help you get the most out of your fan system:

• Use long, straight duct runs upstream and downstream of the fan.

• Use gradual slopes when ducts expand or contract. A slope of 1:7 usually works well.

• For single-inlet centrifugal fans, place the drive system opposite the inlet to keep the inlet clear of obstructions.

• Avoid spinning the air into the impeller of centrifugal fans. Bringing the air in axially produces the best efficiency unless the impeller is specifically designed for either pre-rotation or counter-rotation.

• If duct elbows must be used near a fan inlet or outlet, install turning vanes. If an elbow is installed near the outlet of a centrifugal fan, have it turn in the same direction as the fan impeller.

• For axial fans, use bell mouths, spinner cones, and tapered outlet sections for maximum efficiency.

Consider ASDs for variable flows. Adjustable-speed drives (ASDs) -- also known as variable-speed drives, variable-frequency drives, or variable-frequency inverters -- use electrical waveform modification to vary the voltage and frequency of the alternating current that drives the motors. By controlling motor speed so that it closely corresponds to varying load requirements, ASD installations can reduce energy consumption (in some cases, energy savings can exceed 50 percent), improve power factor, and provide other performance benefits such as soft-starting and overspeed capability. They also can eliminate the need for expensive and energy-wasting throttling mechanisms such as outlet dampers. ASDs require a small amount of power to operate, and so fans with an ASD consume more power at full load than single-speed fans. However, it takes very little time operating at part load to make up the difference. ASDs can be cost-effective in cases with average loadings as high as 90 percent, but an analysis should be performed for each individual case based on the time spent at part load conditions and the efficiency with and without the ASD. (For more information about ASDs, click here.)

Top of Page

What's on the Horizon?

Research and development in fan design is generally focused not on efficiency improvements, but on increasing the mechanical strength of fan blades or extending airfoil performance into uncharted regions of speed, pressure, and temperature.

However, opportunities do exist for improving the aerodynamics of HVAC-duty fans. These include adding airfoils to support struts, tapering inlet cones to centrifugal fans, cleaning up the aerodynamics of small details such as axial fan blade roots and centrifugal fan blade-wheel connections, and improving tolerances and reducing clearances. One NASA engineer believes that the best opportunities for improved HVAC fan performance lie in reduced tip clearance, better blade shaping, and eliminating the use of non-airfoil blades.

Top of Page

Who Are the Manufacturers?

Here is a partial list of industrial and commercial fan manufacturers. (Neither this list, nor any mention of a specific vendor or product in this guide, constitutes an endorsement or recommendation of any vendor or product by E Source, Inc., nor does this guide constitute an endorsement or recommendation, explicit or otherwise, of your service provider's various technology-related programs.)

Aerovent: www.aerovent.com

Chicago Blower Corp.: www.chiblo.com

Continental Fan Manufacturing: www.continental-fan.com

Cook: www.lorencook.com

Howden: www.howden.com

Leader Fan Industries Ltd.: www.leaderfan.com

Moore Fans: www.moorefans.com

Penn Fan: www.pennfan.com

Twin City Fan and Blower: www.tcf.com/TCFBlower/index.asp

In Canada:

Aeroflo Inc.: www.aeroflo.com

Co-Vent Fans Inc.: www.coventfans.ca

Northern Blower: www.northernblower.com

Top of Page

Index
Copyright 2002 - Platts, a unit of The McGraw-Hill Companies, Inc.
All Rights Reserved