National Oilheat Research Alliance 9 Fan Performance Tables Manufacturers publish performance tables for each specific fan. Performance tables show the fan performance at specific operating conditions and the fan CFM compared to the static pressure. Fan performance tables also include: outlet velocity, fan rpm and motor horsepower requirements for the selected condition (Figure 6). These fan performance tables can also be used to find relationships that exist for a set of system conditions involving s.p., blower/fan rpm and cfm. If you know the values for any two of the characteristics shown on the chart, you can easily find the third. Fan Performance Curves The fan curve is a graphical representation of the fan performance (Figure 7). Airflow is typically plotted along the x axis and power, pressure and efficiency are plotted along the y axis. Fan curves are used in system design to select a fan capable of moving the correct amount of air at the desired operating conditions. Fan curves can also be used to predict the effect any change in operating conditions will have on fan performance. Fan curves generally show volume of air moved in CFM compared to the power delivered to the fan shaft in brake horsepower. Brake horsepower (bhp) is the measure of a motor’s horsepower before the loss in power caused by the gearbox, drive train and the resistance in inches of water column. Air Distribution Systems Duct systems are used to distribute conditioned air throughout a building. Proper duct system design is critical to the energy efficiency of a structure. Ductwork design must include a load calculation to properly size the heating/cooling equipment and duct system. Technicians need to understand duct systems and airflow to be able to troubleshoot and maintain an HVAC system. Things that adversely affect system airflow will also have a negative effect on system efficiency and reliability. Forced air systems distribute conditioned air in buildings. Air is heated or cooled and distributed through a duct system throughout the building. The basic components of a forced air system are the blower, the return air duct, which carries air to the blower and the supply ductwork, which carries the conditioned air to the building. The blower provides a pressure difference in order to move air through the ductwork. The amount of air the blower can move and the energy required to move it are controlled by the resistance to airflow from the ductwork and all the components in the airstream. Ducts create resistance to airflow, causing a pressure drop. Additionally, every component that air has to travel through creates more drops in pressure. These include filters, humidifiers, heat exchangers, evaporator coils, and the registers and grilles. The amount of pressure available to move air through the ductwork is the difference between the pressure the blower can produce and the total amount of pressure drop from the combined system components. The four most common duct configurations are radial, reducing radial, extended plenum, and reducing extended plenum. A less common type is the perimeter loop system. Radial Duct Systems A radial duct system (may be referred to as an octopus system) is designed so that all the duct runs originate at the central plenum. Radial systems are frequently installed in attics but can be installed in crawl spaces, basements or slabs. Radial systems are also commonly used in small homes of 1200 square feet or less.
NORA Advanced Air Flow
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