Ventilaation System Sizing

By Author: Susan Terlitski
Total Articles: 57
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HVAC design engineers face many choices throughout the planning process, 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

where:
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, 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, 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, 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, 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 ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, 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 restrictions/obstructions, 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, 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 ANSI/AMCA Standard 210-07/ANSI/ASHRAE 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, 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, 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.

Most of the energy lost in a system is converted to heat. For example, mechanical and electrical energy losses in a fan motor raise the surface temperature of the motor. If the motor is in an air stream, the heat will be transferred directly to the air.

In a belt-driven system, losses are the result of belt friction, slippage, and / or flexing. All such losses are converted to heat and, if the belt drive is in an air stream, increase the teperature of air. Fan losses quantified by fan efficiency contribute to air-temperature rise as well. As a fan works on a fluid, the friction attributed to the airflow decreases the fan's efficiency and creates heat. As the efficiency of a system decreases, air-temperature rise increases. In a cooling application, this means increased energy consumption on the part of the compressor. Minimizing inefficiencies results in energy savings. It is the engineer's responsibility to minimize total system pressure through proper design and layout. This involves balancing economics and efficiencies in specifying a fan. But not all responsibility resides with the engineer. The contractor must ensure appropriate equipment is installed and not base decisions solely on the lowest bid. The contractor needs to be aware of the consequences of poor installation and minimize system effects.

The losses accrued in a system directly affect the system's power consumption while indirectly increasing the energy consumed by other processes. Energy losses in the form of heat equate to higher operating costs.

Because performance conditions are mandated by codes, proper system design, equipment selection, and installation are the most effective means of minimizing inefficiencies and saving energy.

Oleg Tchetchel
Building HVAC Specialist
Canada Blower Co.
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