from the operating static pressure. Most equipment is designed to operate at peak airflow and efficiency at .5 to .6 static pressure. Limiting Air Noise The velocity of the air as it travels through the ducts must be considered as high velocities may cause objectionable noise levels. It is possible to select a duct size that will maintain a workable static pressure with a velocity that creates acceptable levels of noise. To minimize noise, most residential systems are sized in the 600-900fpm range as more than 900fpm almost always causes excessive noise. Air Distribution System Measurement Airflow measurements measure the velocity and volume of air as it travels through the system. Feet per minute (FPM) is a measurement of how fast the air is traveling. Cubic feet per minute (CFM) is a measurement of the volume of airflow. Having the correct air volume CFM is critical to any forced air system. Three common instruments are used to measure the static pressure, total pressure and air velocity in an air distribution system: • Manometers • Differential pressure gauges • Pitot tubes, and static pressure tips Manometers Manometers are used to measure the lowlevel static, velocity and total air pressures found in an air distribution system. Manometers used for air distribution systems are calibrated in inches of water column (in. w.c.). Manometers can use water or oil which has a specific gravity of 0.826 as the measuring fluid. The manufacturer specifies the type of oil to be used. Manometers come in many types, including U-tube, inclined and combined U-inclined. Electronic manometers are also widely used. Pitot tubes or static pressure tips are always used with manometer. 16 National Oilheat Research Alliance Individual U-tube and inclined manometers are available in many pressure ranges. Inclined manometers are usually calibrated in the lower pressure ranges and are more sensitive than U-tube manometers. U-inclined manometers combine both the sensitivity of the inclined manometer with the high range capability of the U-tube manometer in one instrument. Inclined vertical manometers combine an inclined section for high accuracy and a vertical manometer section for extended range. They also have an additional section that indicates air velocity in feet per minute (fpm). Electronic manometers typically measure differential pressures in inches of water column (in w.c.). Differential pressure gauge These provide a direct reading of pressure. These gauges are typically used to measure fan and blower pressures, filter resistance, and air velocity and furnace draft. Some are capable of measuring just pressure or both pressure and air velocity. Pitot tube and the static pressure tips These are probes used with manometers and pressure gauges for taking measurements inside duct work. Pitot tubes come in various lengths ranging from 6" to 60". Static pressure tips, like pitot tubes, are used with manometers and differential pressure gauges to measure static pressure in a duct system. They are typically L-shaped with four radial drilled 0.04" sensing holes. Inclined manometers and pitot tubes measure the pressure in a duct. They cannot be used to measure airflow at a register or grille. Instruments are available that read FPM, CFM or both directly, without having to read the pressure in the duct. Some direct reading airflow instruments used are the rotating vane anemometer, the thermal anemometer and the flow hood.
NORA Advanced Air Flow
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