Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSleveled 10:1 inclined manometer calibrated against a hookgauge can read to approximately ±1.25 Pa. A standard pitottube with an inclined manometer can be used with the followingdegree of accuracy:areas are taken. Readings are taken along at least two diametersperpendicular to each other. In rectangular ducts, readings at thecenters of equal rectangular areas are taken. The velocities arethen mathematically totaled and averaged.Table 2. Pitot Tube Accuracy.Velocity (m/s) Percent Error (±)20 1.0415 1.6010 2.944 6.63 18.6It can be seen that the use of the pitot tube in practicalapplications is limited at velocities lower than 3 to 4 m/s."An analysis of the accuracy of the pitot tube at 4 m/s follows:EQUALCONCENTRICAREASCENTERSOF AREA OFTHE EQUALCONCENTRICAREAS0.316 R0.548 R0.707 R0.837 R0.949 RROUND DUCTCENTERSOF AREAS16-64 EQUALRECTANGULAR AREASRECTANGULAR DUCTVelocity Pressure:VP = (4 ÷ 1.3) 2= 9.467 PaAccuracy:High9.467 Pa+1.250 Pa10.717 Paor 4.26 m/sLowRange9.467 Pa–1.250 Pa8.217 Paor 3.73 m/s4.26 m/s–3.73 m/s0.53 m/sor 100.4 ÷ 2= ±0.265 m/sPercent Error:(±0.265 ÷ 4) x 100 = ±6.6%In practical situations, the velocity of the air stream is notuniform across the cross-section of a duct. Friction slows theair moving close to the walls so the velocity is greater awayfrom the wall.To determine the average velocity, a series of velocity pressurereadings at points of equal area is found. It is recommended touse a formal pattern of sensing points across the duct cross-section.These readings across the duct cross-section are known as traversereadings. Figure 17 shows recommended pitot tube locations fortraversing round and rectangular ducts. In round ducts, velocitypressure readings at the centers of the areas of equal concentricPITOT TUBE STATIONS INDICATED BYFig. 17. Pitot Tube Locations for TraversingRound and Rectangular Ducts.AIRFLOWC2658Fig. 18. Tcheb Tube Sensors and Manifold.C2657The principle behind the manual pitot tube transverse is simpleand straightforward. However, care must be taken to obtainaccurate readings without inadvertent violation of the formaltraverse pattern. With the manual pitot tube traverse, individualvelocity readings must be calculated mathematically, totaled, anddivided by the number of readings to obtain average velocity.TOTAL AND STATIC PRESSURE SENSORSOther arrangements are available to instantaneously averagesensed pressures and manifold these values to the exterior ofthe duct. The Tchebycheff (Tcheb) tube method (Fig. 18) isone such arrangement. This method separately manifolds thetotal and static pressure sensors. Inside each manifold is a tubewith a single slot which receives an average pressure signalfrom the manifold. The averaged signals from the total andstatic pressure tubes may be used for indication and controlfunctions. This method assures an accurate reading of flowconditions and a steady signal.278ENGINEERING MANUAL OF AUTOMATIC CONTROL