4 FM quick reads on fans
1. 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."
2. 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.
4. A VFD Offers Energy Savings, Other Benefits from Part-Load HVAC Operation
Today's tip comes from James Piper, contributing editor for Building Operating Management: Variable frequency drives, or VFDs, offer multiple benefits for HVAC systems.
For more than 20 years, VFDs have successfully been installed on fan and pump motors in wide range of variable load applications. The most significant benefit of the use of a VFD is energy savings. By matching system capacity to the actual load throughout the entire year, major savings in system motor energy use is achieved.
Another benefit of the units is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This inrush current can be three to ten times the full-load operating current for the motor, generating both heat and stress in the motor's windings and other components. For motors that start and stop frequently, the heat and other stresses produced contribute to early motor failures.
In contrast, when a motor connected to a VFD is started, the VFD applies a very low frequency and low voltage to the motor. Both are gradually ramped up at a controlled rate to normal operating conditions. With no significant inrush current, heating and stresses are practically eliminated, extending motor life.
VFDs also provide more precise levels of control of applications. For example, high rise buildings use a booster pump system on the domestic water supply to maintain adequate water pressure at all levels within the building. Conventional pump controls in this type of application can maintain the pressure within a certain range, but a VFD based system can maintain more precise control over a wider range of flow rates, while reducing energy requirements and pump wear.