4 FM quick reads on fans
1. Life-cycle Costs Justify Investment In Energy-Efficient Fans
As any seasoned facility manager knows, weighing lowest first cost against lowest lifetime cost can be a difficult conversation to have with management, especially in government organizations, where one might be bound by low-bid stipulations. Nevertheless, life-cycle cost considerations are crucial for justifying investments in energy-efficient fans.
"One design trend to bear in mind is that fan sizing/selection software often gives a range of fan sizes to meet a given project requirement," says Michael Ivanovich, director of strategic energy initiatives for the Air Movement Control Association (AMCA) International.
Obviously, all of the fans in such a selection will meet airflow and pressure requirements of a system, but, Ivanovich notes, the smaller fans will have to work a lot harder to do so.
"Efficient products that are selected during the design and specification process may be 'value engineered' to smaller, less costly, lower efficient products during the bidding phase," says Tim Mathson, principal engineer for Greenheck and member of AMCA's air control task force. "For a typical fan performance requirement, there are almost always multiple fan sizes that can do the same duty. The smaller sizes almost always come with a lower price tag, but they also almost always perform at a lower efficiency."
And with the harder-working nature of smaller fans also comes the potential for decreased fan longevity and increased system noise, which, apart from the system efficiency, potentially affects productivity.
To minimize energy use, facility managers also have to be mindful of "system effects," says Mathson. System effects happen when ductwork and fittings slow the movement of air. That causes fans to work harder and be less efficient, and also often increases system noise and vibration. One way of preventing this inefficiency is to minimize static pressure in the system by providing proper inlet and outlet conditions at air handlers and other equipment where fans are installed, says Ivanovich.
He also cites two other practices that lead to less energy use by fans. First, regularly check the fan schedules in controllers and automation systems. Fans that are off when they should be use the least amount of energy. Secondly, regularly check VFD settings and belt drives to ensure they are properly set and maintained.
Mathson warns that building owners can't solely focus on fan efficiency; rather, they need to keep a focus on how much energy is being consumed by the fans in the system, he says.
"It's safe to say that the most important factor in fan energy consumption is how much pressure the fan must overcome," Mathson says. "Look for ways to reduce the system pressure by removing or opening up dampers, sealing leaks in ducts, or improving airflow into and out of the fan."
He also notes that owners could consider replacing constant volume systems with variable volume. Then, once the system requirements are minimized, compare fan the various fan options based on energy consumed.
"Look at the total cost of ownership for a period of time, say five to 10 years," Mathson says. "More efficient fans can often pay for themselves in reduced energy costs just one to two years."
2. Fans Offer Opportunities For Significant HVAC Energy Savings
Fans offer opportunities for significant HVAC energy savings. According to Michael Ivanovich, director of strategic energy initiatives for the Air Movement Control Association (AMCA) International, fans account for 80 percent of the so-called "parasitic" load — that is, HVAC loads other than prime movers like chillers and boilers. Starting in 2007, AMCA began developing a method of rating the efficiency of fans. This work led to fan efficiency gradients (FEGs) — a measurement of peak fan efficiency independent of the motor and drive.
Understanding the basic nature of these FEGs is important for savvy facility managers, principally because these efficiency grades, essentially an index of inherent aerodynamic quality, are referenced in last year's International Green Construction Code (IGCC), and the 2013 update of ASHRAE 90.1.
More adoption of the AMCA fan standards is likely, says Ivanovich, as AMCA members begin to work with 2015 IECC code language and members of the ASHRAE 189.1 committee.
The gist of AMCA 205 is that:
- It applies only to fan types and sizes covered by the referenced test standards (some products, therefore, such as air curtains, are not covered).
- It defines a new industry metric, Fan Efficiency Grade (FEG), that will help engineers, owners/managers, contractors, and code officials have an indication of how efficient a fan is out of the box.
- It nudges system designers and contractors and owners who make purchasing decisions to select fans that will operate in the higher-efficient regions of the fan curve.
According to Tim Mathson, principal engineer for Greenheck and member of AMCA's air control task force, the last point is invaluable for fan selection among facility managers. He notes that ASHRAE 90.1 specifies a minimum FEG rating of 67.
"Since the FEG values are based on the peak total efficiency, and the actual fan efficiency varies significantly along a fan curve, there is also a requirement to select the fans within 15 percentage points of their peak total efficiency," Mathson says. "This selection requirement is a key point because it's the real mechanism that will save energy."
3. Pay Attention To Part Load Efficiency Of Heating And Cooling Distribution System
Today's tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. The creation of energy-efficient HVAC systems can be difficult. Many different and often conflicting factors must be optimized to achieve the best system. The prevailing climate and the function of the conditioned space are the main determinants of the most effective system. The challenge is to recognize the opportunities inherent in the climate and application so as to select the best heating and cooling sources and distribution system. One key step in the process of creating an energy-efficient HVAC system is to improve the efficiency of the heating and cooling distribution system for the building. This improvement should be thought of not only as improvement of peak-load efficiency but also of part-load efficiency, because most HVAC systems spend the preponderance of their operating life at part load.
Good part load efficiency for distribution system components often involves the use of variable speed drives along with components that allow those drives to operate at lower frequencies as often as possible. For fans and pumps, facilitating variable flow operation is a must.
For variable flow systems to be effective, capacity reduction should be accompanied by flow reduction. Two-way control valves should almost always be specified for hydronic distribution systems. Some systems require a minimum flow rate, so the use of a controlled minimum flow bypass may be required.
The bypass is preferred to the option of utilizing a limited number of three-way valves, because the three-way valves will increase flow through the system when the actual required flow is above the minimum, resulting in increased pumping energy.
Appropriate selection of the prime movers is also important for energy efficient distribution systems. Pump and fan curves can be compared to find the best selection for each application. In general, larger diameter, slow rotation speed selections are more efficient, up to a point, although the designer should avoid selections for which a slight miscalculation of the system pressure drop might result in an undesirable operating point.