Trinity Persful 2016-08-03 10:40:20
Fans consume 14% of the electricity purchased by industrial and commercial buildings (Nadal et al. 2002). Yet, fan efficiency has escaped the attention of those who design efficiency into the air systems they specify. That is about to change. The US Department of Energy (DOE) will publish their proposed rule this year covering fans from 1 to 200 horsepower. This new regulation will determine compliance at the design point of the fan. If you follow ASHRAE 90.1 or model energy codes, you may have discovered requirements for fan efficiency based on the metric Fan Efficiency Grade (FEG), which is tied to the fan’s peak efficiency. Unfortunately, FEG has been proven inadequate to the task of lowering fan energy use. That is because the fan’s performance at its peak efficiency is poorly correlated with the fan efficiency and power use at its design operating point. The operating efficiency of every fan varies dramatically from single digits to peak levels that are greater than 90%. Unfortunately, a fan with a 90% peak efficiency will generally operate at lower efficiency—how low depends on where the fan is selected. The DOE will now embrace a new fan efficiency metric called the “Fan Efficiency Index” (FEI). The proposed DOE standard will establish a maximum power input that will vary by a formula tied to the design point flow and pressure. The DOE maximum input power requirement: the DOE calls this the Fan Electrical Power Standard, or FEPstd (Equation1). It has been proposed that the fans with an unducted discharge use static pressure rise and a target static efficiency equal to 62%. For fan categories that are normally ducted, the formula will use total pressure rise and a total efficiency of 68%. Since the FEPstd varies with flow and pressure at the fan design point, the regulation applies at an infinite set of conditions, which defines the particular design and selection point flows and pressure rises that are offered for sale. FEI is the ratio of the DOE standard’s maximum allowable watts into the fan motor, over the actual fan electrical power at the design point (Equation 2). The proposed FEI metric compares the DOE-determined maximum fan input power-FEPstd (numerator) to the actual fan power at the system design point i-FEPi (denominator). An FEI of 1.0 or greater meets the proposed DOE regulation. An FEI of 1.1 will use 10% less energy than the DOE standard. That makes FEI especially helpful. It informs the public immediately of the percentage savings relative to a fixed benchmark of DOE regulation—at design conditions. FEI is deterministically linked to the fan’s relative operating efficiency at design conditions. If one were to plot the compliant operating range (Figure 1) of a fan, where FEI is greater than 1.0, several interesting observations could be made that impact design practice. Every fan has a compliant range. The proposed DOE regulation will not arbitrarily force any fan off the market. Instead, the proposed regulation will constrain fan applications to a compliant range. Every fan has a non-compliant range. That means that manufacturers are rewarded for improving the efficiency of every fan they sell to expand the compliant range. A non-compliant selection is usually resolved with a larger-diameter fan, or one which is more aerodynamically designed. Which is a better fit with job site and air system requirements? Which is less expensive? Answers to these questions will change the competitive dynamic in the fan industry. Since the proposed DOE standard will be “wire-to-air,” improvements in motor, transmission, and variable speed drive efficiency expand the fan’s compliance envelope, making it possible to satisfy design requirements with a smaller diameter fan. Owners are more likely to compare actual design conditions (and actual FEI) recorded during commissioning to those specified. Clients will have a clearer expectation of the power use of their fan, thanks to DOE regulation and the FEI metric. Getting the air system design right will become equally important. Fan power varies with the cube of any change in airflow. So, for a given air system and fan, if the flow is reduced by half, the energy draw will drop to one-eighth. Most fan systems are either on or off, but the loads they serve are continuously variable. In the case of the fan, varying the speed to match the load pays great dividends. (Essentially, the fan is oversized when running at partial speed–oversized fans are generally more efficient). So, variable speed is an attractive option to improve system efficiency. All fans offered for sale at operating conditions that require one break horsepower to nominally 200 break horsepower will be covered by the rule, unless they are on an exclusion list. The DOE is reviewing a long list of exclusions, either because fan energy is already part of another DOE regulation, the fan application requires a design that compromises efficiency, or because the fan type is so rarely used that its aggregate energy use is trivial. There are many impacts of the new DOE rule to system designers. A closer look reveals a shift in the marketplace to direct-driven fans, partial-width fans, and an increase in variable speed drives. Under the proposed DOE regulation, the fan is always considered to have a motor and a belt drive. All fan default values will include a motor to drive the equipment (turbine-driven fans, et al, are excluded from the regulation) as well as a belt drive. It becomes obvious that if a fan system is not belt-driven, it will not be penalized with a belt loss, and will automatically benefit. That is to say an arrangement eight fan will be more efficient than the corresponding arrangement nine fan. In regulatory terms, a direct-driven fan and motor combination will comply with the DOE regulation better than a belt, fan, and motor combination. In the DOE Rule, the belt loss varies per the size of the motor. This can also be seen in the Air Movement and Control Association (AMCA) Standard 203. The calculated belt loss in the DOE rule can range from 4 to 10% of the fan system. Simply put, a direct-driven fan will have a 4 to 10% advantage over the same belt-driven fan. As quickly as the benefit for direct-driven systems becomes apparent, so does the benefit for partial-width fan impellers. Although belt driven fans have an efficiency loss, they are extremely flexible in being able to match a fan rpm with a flow requirement. In order to accommodate the inflexibility of direct-drive fan synchronous motor speeds and design points, fan manufacturers must now seek a solution with partial width fans or variable speed drives (VFD.) Partial-width fan engineering is accomplished by determining the fan speed at the design point and adjusting the impeller width accordingly. Many fan companies lack the ability to manufacture partial-width fans. It may be a lack of engineering protocols and standards or a lack in manufacturability. In either case, those fan companies will be at a distinct disadvantage with respect to the fan rule. Direct-driven fans will also find flexibility in speed control. Fans may be slightly oversized to account for unknown system resistance, and be direct-driven to avoid the belt loss, but will add a VFD in order to reduce the energy cost in a variable volume system. It will eventually become necessary to shift fan specifications to account for these new realities. The Notice of Proposed Rule regarding the Commercial and Industrial Fan and Blower regulation should be published in the second quarter of 2016. There will be a comment period of 60 days, with a final rule published in late 2016. There is much to understand in the proposed DOE rulemaking. It will have many implications with regard to engineers’ design practices. Further details can be accessed at http://1.usa.gov/28L24bs. BE Trinity Persful was a member of the DOE rulemaking negotiations for the fan industry. He is also the chairman of the Air Movement and Controls (AMCA) System Efficiency Task Force and serves as the Marketing Director and Energy Initiatives Director at Twin City Fan Companies Ltd.
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