According to the US Department of Energy report Energy Savings Potential and RD&D Opportunities for Commercial Building HVAC Systems, (PDF) HVAC accounts for 30% of annual commercial energy consumption. Of that, fans make up about 29% of HVAC energy use. Supply and exhaust fans are the major players, primarily because most fans operate continuously while the building is occupied. There is a wide variation in efficiency between different fan designs (from as low as 40% to as high as 80%). In light of their long operating hours and wide-ranging efficiencies, HVAC fans are often good candidates for energy-efficiency retrofits.
In addition to the efficiency of the fan itself, the low cost and wide availability of variable-frequency drives (VFDs) in all horsepower ranges make this technology an important part of most energy-efficiency upgrade strategies.
What are the options?
Two basic types of fans are used in HVAC applications, classified according to the direction of the airflow through the impeller: axial and centrifugal.
Axial-flow fans. Axial-flow fans are the familiar propeller-type fan, similar in many ways to residential fans that plug into the wall for space cooling. Axial fans are often directly connected to their motors, avoiding losses associated with a drive belt. They also have a central hub that allows the motor to fit neatly behind the fan with little penalty in efficiency. The weight distribution of their blades allows for low starting torque.
Axial fans can be subdivided into three categories (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 motion into longitudinal, straight air motion). Vane-axial fans tend to be the most efficient fans available for HVAC air-handling units—with efficiencies reaching up to 85%—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 pitch in flight,” meaning that the blade angle can be varied as the fan rotates. Fixed-pitch blades are the norm for low-efficiency propeller fans and 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. The mechanism that enables blade pitch to be varied in flight needs constant maintenance to ensure proper operation over time.
Centrifugal fans Centrifugal fans, also known as “squirrel cage” or “utility” fans, have an entirely different design. Instead of passing straight through, air makes a 90-degree-angle turn as it travels from the inlet to the outlet and 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 and have an airfoil shape (similar to a cross section of an airplane wing), 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 shouldn’t 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. The main advantages of fans with forward-curved blades are their low initial cost and compactness. Forward-curved fans are usually used for smaller HVAC units, and more-efficient fan types are usually only available for larger packaged units (15 tons and higher) or from semi-custom manufacturers.
How to make the best choice
Consider size, cost-effectiveness, and variability
Pick a size that’s just right Some fans and motors are larger than needed for their intended use, which is 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 similar to the one 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 figure 3 with data from actual measurements of airflow and pressure to determine the optimal size, rather than just looking for a replacement.
If HVAC fans are oversized, replacing them with ones that are correctly sized (known as “rightsizing”) can be cost-effective. A rightsized system saves energy costs, but there are other advantages, including:
- Lower initial costs. Rightsizing fans reduces the capacity requirement of the fan system, which enables the system to be more accurately tailored to new airflow requirements. Installing smaller, more energy-efficient equipment that meets these requirements also reduces initial costs.
- Comfort. Fans that supply too much air not only waste energy but can compromise comfort as well. Fans that oversupply air can cause disturbing drafts, increased humidity, and noise.
- Equipment life. Consistently operating an oversized motor with a VFD at a very low speed can reduce the useful life of the motor and associated equipment. Properly sized equipment operate better at reduced capacities.
As a first step, the opportunity for rightsizing an air distribution system can usually be determined by building maintenance staff. Once an opportunity has been identified, however, it’s usually necessary to hire an HVAC engineer to verify it, to conduct a more detailed analysis, and to make recommendations for optimizing the system.
If you’ve got a variable air volume HVAC system, the first step in rightsizing the system is to reduce the static pressure setpoint to the minimum setting at which occupant comfort is maintained on hot, humid days. This step in itself can save a lot of energy. Once the lowest acceptable static pressure setpoint has been determined, measure the fan-motor power draw using a true RMS (root-mean-square) power meter under peak load conditions (that is, on a hot, humid day). If the measured power is less than 75% of the motor’s nameplate rating, there’s a good chance that the motor is oversized.
For constant air volume systems, you can determine whether the fan is oversized by comparing the system’s actual static pressure to design pressure. Measure the main supply fan system static pressure on a hot, humid day. Make sure that all fan vanes and dampers are fully open. If the measured static pressure is greater than the design pressure (found in building mechanical drawings), the fan is supplying too much air and is probably a good candidate for rightsizing.
Again, for either type of system, it will usually be necessary to obtain the services of a qualified HVAC engineer to verify whether a fan can be rightsized without compromising indoor air quality. For more information on rightsizing fans, see Chapter 8 of the U.S. Environmental Protection Agency’s Building Upgrade Manual.
Check the cost-effectiveness of high-efficiency options Axial fans are the most efficient, but consider installing backward-curved fans if centrifugal design is necessary. 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 figure 4. Also, consider how VFDs might figure into the equation, especially if there are a significant number of operating hours spent at part load. A VFD provides significant benefit during part-load operation for airfoil and fixed-pitch axial fans, but less benefit when applied to a forward-curved fan. This is because horsepower requirements for forward-curved fans drop off more steeply with reduced airflow than for other fan designs, so the VFD provides less of an efficiency improvement. A VFD shouldn’t be used on a variable- or adjustable-pitch axial fan because those fans are designed to operate at a constant speed, and varying the fan’s speed can cause it to operate at a resonant frequency. This can lead to excessive vibration that can make the fan blades break free from the hub, potentially causing damage to surrounding equipment.
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. Inlet guide vanes, sometimes also called pre-rotation vanes, are used to vary the air delivery of centrifugal fans.
- 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. Doing the opposite—turning the air in the opposite direction from the impeller—is colloquially known as “breaking the back of the velocity profile” and leads to substantial pressure drop.
- If a centrifugal fan with inlet guide vanes needs to be retrofitted with a VFD, remove the inlet vane assembly from the fan inlet and replace it with a smooth bell mouth to improve efficiency.
- For axial fans, use bell mouths, spinner cones, and tapered outlet sections for maximum efficiency.
Consider VFDs for variable flows VFDs—also known as 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, VFDs can reduce energy consumption (a 20% speed reduction can lead to electrical savings up to 45%), 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 capacity-control mechanisms such as outlet dampers or inlet guide vanes. VFDs require a small amount of power to operate, and so fans with a VFD consume more power at full load than single-speed fans—typically 2% to 3% more—but it takes very little time operating at part load to make up the power draw of the VFD. VFDs can be cost-effective in cases with average loads as high as 90%, but operating technicians should analyze the time spent at part-load conditions and the efficiency of the fan with and without the VFD before moving forward. The price of VFDs has continued to decrease, and performance and reliability have increased, resulting in some states’ energy codes now requiring VFDs on almost all fans employed in HVAC systems.
Maintaining fan performance
Visually inspect the fan Make sure nothing is blocking the fan blades and check for obvious signs of damage. Damaged fan blades can reduce performance and ruin a motor.
Check fan motors Verify that the motor amperage is as expected. If it’s not, this could indicate that the motor is failing, or that the fan assembly needs maintenance. Also, verify that the fan motor is running in the correct direction—many HVAC technicians have at least one story of finding a fan that’s running backward. Centrifugal fans still supply some air even when running backward—typically about 50% of rated flow—so the problem may not be readily apparent. The most common cause of reverse fan operation is switched wire leads on the motor; clear labels on the fan housing, pulleys, motor, and wires can help prevent this problem.
Rapid on-off cycling of a condenser fan (three minutes or less) leads to poor control of the refrigeration system and can wear out the fan motor prematurely. If you observe a fan that’s cycling rapidly, call in a qualified HVAC technician to check the settings on the fan controller, as it may need adjustment.
Lubricate bearings Sleeve bearings, which are simple oiled metal-to-metal running surfaces found in older fans, should be lightly oiled two or three times per year with the recommended lubricant. A label near the bearings should indicate the lubrication interval, lubricant type, and perhaps a log of past service. Newer fans are equipped with self-lubricating bearings (sealed-cassette ball-bearing cartridges preloaded with grease). There is no way to regrease these bearings, so when they finally fail—typically after several years of service—the bearing cassette must be replaced. Warning signs of impending failure are excessive noise, vibration, or heat emanating from the bearing.
Conventional greased ball bearings are occasionally found in fans. The most common problem with these bearings is overgreasing—the service technician connects a grease gun to the fill fitting and pumps in the grease until it flows out of the bearing seals. Overgreasing can be as damaging as undergreasing. The proper procedure is to open the drain plug and inject grease through the fill fitting until clean grease comes out of the drain. If it’s possible to safely do so, regrease the bearings while the motor is running to help ensure a complete grease exchange. Take care not to get grease or oil on the pulley wheels or belt, because that will cause slip-stick action that will jar the system.
Clean fan blades If impeller blades are coated with dirt, fan efficiency will suffer. Impeller blades on forward-curved fans are especially prone to filling up with dirt because they’re shaped like scoops. Good filtration helps keep dirt out of the fan, but an annual visual inspection still makes sense. Cleaning the blades on a small fan takes an hour or more because the technician must remove the impeller from the fan housing. Cleaning larger fans, especially those with multiple wheels on a single shaft, can be a major project.
Adjust belts Improperly adjusted belts rob the drivetrain of power, create noise, and require replacement sooner than well-adjusted belts. Loose belts slip on the pulley wheels, causing torque loss and rapid wear. Belts that are too tight put an excessive load on the motor and fan shaft bearings, causing early failure of the bearings or belts. Proper belt tension can be achieved with a deflection strain gauge, but most technicians are familiar enough with the proper tension to adjust it simply by pressing on the belt with a finger. Either method works well if performed consistently. In addition, belts should be aligned with a straightedge to prevent lateral wear.
Some technicians advocate belt changes once or twice a year, whereas others let belts run until they break. Depending on the price of a belt, it may make sense to forestall breakage with periodic replacement. Experts recommend keeping one extra belt (an old one will do if it’s in good shape) inside the cabinet to use as an emergency spare.
Switching from standard to cogged V-belts is an easy upgrade that can improve drivetrain efficiency by 2% to 8% (figure 5).
What’s on the horizon?
Research and development in fan design are 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, improving tolerances, and reducing clearances.
While smart fan technology is mostly being deployed on the residential side, there are also commercial applications, as smart fans show lots of promise in office settings. The US government’s General Services Administration created the infographic Smart Ceiling Fans that outlines energy saving opportunities associated with smart fans.
Who are the manufacturers?
Here is a partial list of industrial and commercial fan manufacturers.
In the US
- Chicago Blower Corp
- Continental Fan Manufacturing
- Moore Fans
- Penn Fan
- Twin City Fan and Blower
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