Commercial and Industrial Fans: An Approach for Pending Efficiency Standards to Address the Importance of Fan Selection
The actual efficiency of a fan in the field depends enormously on how it is applied.
Fans are used in a wide variety of commercial and industrial applications such as in commercial building HVAC systems, commercial kitchen exhaust systems, agricultural ventilation, and material handling. According to the Energy Information Administration (EIA), ventilation in commercial buildings accounts for about 200 billion kilowatt-hours (kWh) of electricity use per year, or about 15% of total commercial building electricity consumption. 1
Over the past 30 years, national efficiency standards have been established for about 55 types of appliances, equipment, and lighting products. There are currently no efficiency standards for commercial and industrial fans, but the U.S. Department of Energy (DOE) is in the middle of a rulemaking that would establish the first standards for these fans.
Fans are available with a wide range of efficiencies, and some models on the market have very efficient designs. However, the actual efficiency of a fan in the field depends enormously on how it is applied. A very efficient fan can perform very inefficiently if not properly applied. Therefore, improved fan selection represents a huge opportunity for energy savings for the nation and energy bill savings for businesses.
The performance of every fan can be described by a series of curves. The fan curve describes the air flow rate a given fan can deliver as a function of pressure resistance to air flow. A fan can operate anywhere along the fan curve, but its actual operating point in a given application is at the intersection of the fan curve and the system curve. The system curve describes the pressure loss in the system as a function of air flow rate. In a system such as a duct system, as air flow rate increases, system resistance (or pressure loss) increases with the square of the flow rate as shown in figure 1.
Every fan also has an efficiency curve, which describes the efficiency of the fan at each potential operating point along the fan curve. A fan’s peak efficiency occurs at a single point, and efficiency can drop off significantly at operating points away from the peak efficiency point. In the example shown in figure 1, the fan’s actual operating point (where the fan curve and system curve intersect) is very close to the peak efficiency point. However, if a given system curve instead intersects the fan curve at a point far from the peak efficiency point, the fan will operate at an efficiency significantly lower than its peak efficiency.
In today’s market, fan purchases are often driven by first cost. Customers who make purchasing decisions based on first cost tend to purchase undersized fans which operate at high speeds to meet the customer’s flow and pressure requirements. In these situations, the fan will often operate far from its peak efficiency point and waste a significant amount of energy compared to a larger fan operating at a lower speed and closer to its peak efficiency point.
Table 1 shows an example of three exhaust fans from a manufacturer’s selection software that can all deliver an air flow of 20,000 cubic feet per minute (CFM) at 0.15 inches of static pressure. These three fans are part of the same family of models but have different diameters—36”, 42”, and 48”. The peak efficiency of each of the three fans is almost identical—about 55%. However, at the required flow and pressure (20,000 CFM at 0.15 in.), the actual efficiency of the 48” fan is 34% compared to just 14% for the 36” fan, and the power consumption of the 48” fan (1.45 hp) is less than half that of the 36” fan (3.49 hp).
In the example in table 1, the 36” fan is not inherently inefficient, and it can deliver certain flow and pressure combinations efficiently. However, for this particular application, the larger 48” fan would operate much more efficiently and yield energy savings of more than 50% relative to the 36” fan.
In recognition of the importance of appropriate fan selection in addition to efficient fan design, fan manufacturers and efficiency advocates developed an approach for efficiency standards to drive improved fan selection that is now being considered in the current DOE rulemaking for commercial and industrial fans. Under this approach, instead of certifying a single value for the efficiency of a given fan, manufacturers would certify all the operating points (flow and pressure) at which the fan meets the standard. (The standard would be a function of flow and pressure to account for the inherent efficiency differences of providing different air flow rates and pressures.) The rated maximum speed (RPM) of a fan would be the highest speed at which the fan meets the standard. In addition, under this approach, manufacturers would be able to market their fans for only those operating points that meet the standard. For example, a manufacturer’s fan selection software could return only those fan selections that meet the standard at the operating point specified by the user.
In addition to driving improved fan selection, this approach for fan efficiency standards would also encourage manufacturers to develop more efficient fans. Manufacturers would have a market incentive to improve overall fan efficiency through better design in order to be able to market their fans for a wider range of operating points and to be able to offer lower-cost solutions for individual applications. Figure 2 shows examples of two different fans with the shaded regions showing the operating points that meet the standard. The fan on the left could be marketed for a larger range of operating points and a higher rated maximum RPM compared to the fan on the right.
In 2015, a working group consisting of representatives from fan, motor, and HVAC manufacturers, efficiency advocate organizations, utilities, and DOE was formed to negotiate test procedures and efficiency standards for commercial and industrial fans. The working group completed discussions in September 2015 with agreement on many items related to a test procedure and efficiency metric consistent with the approach described above. DOE is now working on developing proposed rules for test procedures and standards based on the working group recommendations.
Pending efficiency standards for commercial and industrial fans have the potential to achieve very large energy and economic savings. An approach for standards addressing fan selection will both drive improved fan design as well as help ensure that fans are appropriately applied in order to reduce power consumption.