Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSTOTAL PRESSURESTATIC PRESSUREVELOCITY PRESSUREFANAIRFLOWA. PRESSURE IN THIS DUCT ABOVE ATMOSPHERIC PRESSURETOTAL PRESSURE STATIC PRESSUREVELOCITY PRESSUREFANAIRFLOWB. PRESSURE IN THIS DUCT BELOW ATMOSPHERIC PRESSUREFig. 8. Relationships of Total, Static, and Velocity Pressures for Positive and Negative Duct Static Pressures.C2643TOTALPRESSURESTATICPRESSURETOTAL PRESSUREVELOCITYPRESSURETOTALPRESSURESTATICPRESSUREFRICTION LOSSTOTAL PRESSUREVELOCITYPRESSUREATMOSPHERICPRESSUREVELOCITYPRESSUREATMOSPHERICPRESSUREVELOCITYPRESSUREDIRECTIONDIRECTIONOF AIRFLOWOF AIRFLOWAIR DUCTFig. 9. Theoretical Changes in Pressurewith Changes in Duct Area.OPEN ENDOF DUCTC2644AIR DUCTFig. 10. Actual Changes in Pressurewith Changes in Duct Area.OPEN ENDOF DUCTC2645In a theoretical duct system without friction losses, the totalpressure is constant along the entire duct (Fig. 9). The static andvelocity pressures, however, change with every change in the ductcross-sectional area. Since the velocity decreases in larger ductsections, the velocity pressure also decreases, but the static pressureincreases. When theoretical ducts change size, static pressure istransformed into velocity pressure and vice versa.An actual duct system (Fig. 10) encounters a phenomenon calledpressure loss or friction loss. Pressure loss is caused by frictionbetween the air and the duct wall. Dynamic losses also occur dueto air turbulence caused by duct transitions, elbows, tees, and otherfittings. At the open end of the duct in Figure 10, the static pressurebecomes zero while the velocity pressure depends solely on theduct size. The pressure loss due to frictionappears to be a static pressure loss. However, in reality the totalpressure decreases because the pressure loss due to friction alsoindirectly affects the air velocity in the duct. When the duct inletand outlet sizes are identical, the velocity pressures at both placesare equal and the difference in static pressure readings actuallyrepresents the pressure loss due to friction.In most applications, the duct outlet is larger than the duct inlet(velocity is lower at the outlet than at the inlet). When the ductsize increases, a small part of the initial velocity pressure isconverted into static pressure and lost as friction loss (Fig. 11).This concept is called static regain. Similar to water flow througha pipe, a larger airflow through a given duct size causes a largerpressure loss due to friction. This pressure drop or friction losscannot be regained or changed to static or velocity pressure.ENGINEERING MANUAL OF AUTOMATIC CONTROL275