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|HVAC design engineers face many choices throughout the planning process Kentavius Street 49ers Jersey , perhaps few as crucial as that of fan equipment, a principal consumer of energy. This article will discuss options for improving energy efficiency when designing an HVAC system and selecting a fan.
Fan power consumption is directly proportional to system flow rate and static pressure. For theoretically perfect efficiency, the minimum power required to move air against resistance is defined as:
AHP = (Q × P) ÷ 6,356
AHP = air horsepower
Q = volumetric flow rate (cubic feet per minute)
P = pressure (inches of water gauge) or resistance
Flow rates are predetermined based on space type and occupancy. Although local codes determine minimum requirements for HVAC systems, ASHRAE Handbook--HVAC Applications1 provides general design criteria for various commercial and public buildings. These criteria include air movement, room circulation Tarvarius Moore 49ers Jersey , noise, and filter efficiency. Although all are important, room circulation, in air changes per hour, typically determines airflow requirements. Because flow rate is driven by design criteria, a design engineer's primary means of reducing energy use is to minimize the static pressure needed to move air through a system.
Air Movement and Control Association (AMCA) International defines system pressure loss as "the sum of the static-pressure losses due to friction Fred Warner 49ers Jersey , shock, dissipation of velocity pressure at the system discharge, and the static-pressure differences between the entry and discharge openings of an air system. The static pressure a fan must overcome is dependent on many variables, only some of which the design engineer can control. The location of equipment often is determined by the architect and, therefore, limits the engineer's options.
Duct configuration and fittings used to connect components are large contributors to static pressure. Other sources of system pressure loss are balancing and control dampers Eli Harold 49ers Jersey , variable-air-volume (VAV) boxes, diffusers, louvers, coils, filters, and other components in an air stream. Given that velocity pressure is proportional to the square of velocity Jaquiski Tartt 49ers Jersey , pressure loss in most of these components is proportional to velocity squared. This makes size an important factor, as cross-sectional area dictates fluid velocity. For example, reducing air velocity throughout a system by 10 percent would result in a 20-percent reduction in system static pressure. With air power proportional to pressure, this would equate to a 20-percent reduction in energy consumption.
In addition to accounting for all static-pressure loss in a system and achieving required room airflow, a design engineer must adhere to Section 6.5.3 of ANSIASHRAEIESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings Arik Armstead 49ers Jersey , which limits the power (horsepower) a fan can consume per cubic foot per minute of airflow the fan generates. With flow requirements defined, a design engineer must limit system static pressure to meet this power limitation.
A major contributor to energy consumption that often is ignored is system effect. AMCA International defines system effect as "a decrease in fan performance capability, observed as a pressure loss, which results from the effect of fan-inlet restrictionsobstructions, fan-outlet restrictions, or other conditions influencing the performance of the fan when it is installed in a system."2 System effect is a reduction in a fan's ability to generate pressure and can be looked at as an additional system pressure loss. It can lead to underperformance DeForest Buckner 49ers Jersey , excessive noise, and vibration in a fan and system.
System effect can be described by its impact on a fan curve. Fan curves are produced from laboratory testing, with fans configured for ideal installations. Testing is performed in accordance with ANSIAMCA Standard 210-07ANSIASHRAE Standard 51-2007, Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating. A fan curve displays performance for a constant speed (revolutions per minute) in terms of static pressure vs. volumetric flow rate. In Figure 1, the intersection of the system-resistance curve and the pressure curve is the operating point of the fan. To move along the system curve to alter fan performance, one must increase or decrease fan speed accordingly. Operating power is where the power curve intersects with a vertical line running through the operating point.
The ideal inlet and outlet conditions under which fans are tested rarely are seen in the field. As a result Joe Williams 49ers Jersey , a fan will overperform or underperform in terms of flow rate, static pressure, or both. In Figure 2, a suitable fan was selected, but system effect was ignored. The blue lines represent how the fan would perform under AMCA International test conditions. Actual measured performance is indicated by the red dot. The red line shows how the fan would perform in an AMCA International air test when operating at design speed, but takes into account system effect. The only way to achieve the desired performance is to speed the fan to the rate designated by the green line. The consequence is a higher operating speed C.J. Beathard 49ers Jersey , higher brake horsepower, and higher sound levels.
Another key to reducing power consumption is fan selection. Generally, propeller or tube-axial fans are more efficient for relatively low static pressures, while centrifugal-type fans are used for relatively high static pressures. Too often, fan selection is based solely on first cost. The consequence is that a relatively small-diameter, high-speed fan is used. A small fan operating at a high speed generally requires more operating power and produces more noise than a large fan operating at a low speed.
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