![]() ![]() Little or no air comes out the other end. ![]() Now try to blow through a hose 50 feet long. You can feel a good stream of air coming out of the tube. Think of blowing through a piece of garden hose 6 inches long. The flow of air creates friction as it rubs against the side of the duct, and the friction creates resistance to the airflow. The higher the pressure difference, the greater the air velocity and the greater the quantity of air that will flow from the duct.įriction is a resistance which slows down airflow. ![]() The amount of air flowing through a duct is regulated by the amount of pressure difference and by the system resistance. A positive pressure (+) is above atmospheric pressure. Pressure in a duct refers to the pressure that is higher or lower than the atmospheric pressure of 14.7 psi. The system shown in Figure 1 does not provide for heating or cooling the air.Īll air in an open system is under atmospheric pressure, which is normally 14.7 psi at sea level. As the OA damper opens, the RA damper closes so that the same amount of air remains in the system.Īfter the air is mixed in the proper proportions, the mixed air is drawn through a filter before it enters the fan and returns to the conditioned spaces. The OA and the EA dampers open together and close together to balance the air entering and leaving the building. An automatic control system operates the damper motors to maintain the proper mix of air. The exhaust air (EA), outside air (OA), and return air (RA) ducts all operate together. When outside air enters the building, the same amount of air must be removed from the building through the exhaust air (EA) outlet or other exhaust air systems. In Figure 1 the outside air inlet is in the lower left. To avoid this, fresh air is brought into the system through the outside air (OA) intake. The air would become stale if the same air were used over and over again. Only a percentage of the return air can be reused. (Click on the illustration for an enlarged view.) Hot water system added to air handling system.
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