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.
Figure 3: Sample fan curve
Fan curves show the relation between the quantity of air that a fan will deliver and the pressure against which it can discharge the air. The curves also indicate the horsepower required from the drive motor for the corresponding airflow, and the fan efficiency. For a given application, pick a fan that operates most of the time at the highest part of the efficiency curve. As shown, choosing a fan too far to the right will ensure plenty of airflow, but at a penalty in efficiency.
If HVAC fans are oversized, replacing them with ones that are correctly sized—"rightsizing" them—can be cost-effective. A rightsized system saves energy costs, but there are other advantages as well:
- Lower first costs. Because the capacity required from the fan system is reduced, the system can be more accurately tailored to the new airflow requirements. By installing smaller, more energy-efficient equipment that meets these requirements, first costs are also reduced.
- Comfort. If the fan system supplies too much air to occupants, energy is wasted and comfort can be compromised. Too much air can result in disturbing drafts, increased humidity, and noise.
- Equipment life. Prolonged operation at very low speed of an oversized motor with a variable speed drive can reduce the useful life of the motor and associated equipment. Properly sized equipment will be more suited to operation 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 is 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 percent 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 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. In addition, consider how variable-frequency drives (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 (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 should not be employed on a variable- or adjustable-pitch axial fan. Such fans are designed to operate at a constant speed, and varying fan speed can cause it to operate at a resonant frequency, causing excessive vibration that can actually make the fan blades break free from the hub (potentially causing substantial damage to surrounding equipment).
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.
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. For example, 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 is to be retrofitted with a VFD, remove the inlet vane assembly from the fan inlet and replace it with a smooth bell mouth in order to improve efficiency.
- For axial fans, use bell mouths, spinner cones, and tapered outlet sections for maximum efficiency.
Consider VFDs for variable flows. Variable-frequency drives—also known as variable-speed 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, VFDs 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 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 percent 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 percent, but an analysis should be performed for each individual case based on the time spent at part-load conditions and the efficiency of the fan with and without the VFD. The price of VFDs has continued to decrease, while performance and reliability have increased. As a result, energy codes in some states now require VFDs on almost all fans employed in HVAC systems. (For more information about VFDs, see "Motors: Adjustable Speed Drives.")