National Oilheat Research Alliance 5 Total Pressure Total pressure is the sum of the static and the velocity pressures in the duct system. It is the pressure produced by the fan or blower. Due to the low pressures inside a duct system, an incline manometer (Figure 4a), a digital manometer (Figure 4b) or a magnehelic type (Figure 4c) are used to measure duct static, velocity and total pressures in inches of water column (in. w.c.). Figure 4a Figure 4b Figure 4c every one pound per square inch of pressure, a column of water will rise to a height of 27.68″ or about 2.3′. In special low-pressure applications, such as blower door testing, pascals (pa) are used as the units of measurement. Normal pressure at sea level is 101.3 kilopascals (kPa), usually rounded to 100 kPa. To convert psi to kPa, multiply psi by 6.895. Exactly one inch of water column equals 249 pascals. As a point of reference, the typical over-the-fire draft of 0.2 inches of water column equals about 5 pascals. Fans and Blowers The fan or blower provides the pressure difference necessary to force the air into the supply duct, through the grilles and registers and into the conditioned space. The blower must overcome the resistance of any component in the airflow, such as: air filters, evaporator coils, heat exchangers and dampers, as well as the pressure loss involved in the return of air through the return ductwork. The words fan and blower are often used interchangeably; however, those specific terms usually describe their function. For example, the word blower describes applications where the device must work against the resistance of a duct system, such as a forced air system. The word fan describes applications where high quantities of air are needed with little resistance to airflow, as with a condenser fan. Two types of blowers are commonly used in heating and cooling systems: belt drive and direct drive. Belt drive blowers (Figure 5a on following page) have the blower motor mounted outside the blower housing and are connected to the blower wheel by pulleys and a belt. The blower speed is adjusted manually by a change or adjustment of the pulleys. Increasing the size of the motor pulley will increase the rpm of the blower. Increasing the size of the blower pulley will decrease the rpm Because the readings are very low, usually less than 0.025 pounds per square inch, we measure pressure in inches of water column. Inches of water column is the height in inches the pressure will lift a column of water; 27.7 inches of water column equals one pound per square inch. Since atmospheric pressure is 14.7 psi (14.696 psi) at sea level with 70o F dry air, this atmospheric pressure will support a column of water 33.9′ or 406.9″ high. For
NORA Advanced Air Flow
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