Document k612NGJXO969dK9BGEyqrDKkD

684 CHAPTER 31 l- . i 19S2 Guide the^'inevitable'- 4<5ss due to tutbiilence. The principle of static pressure regain is, however,of importance in. the economical design' of duct.systems. Fig. 9 shows the application of static pressure regain to a simple fan and discharge duct. The fan in the upper part of the figure has a free inlet and discharges air through' a straight duct, the diameter of which is equal to,the;fan outlet.. ..The total pressure.which pnist be provided by the fan. is therefore the sum of the pressure that is necessary t,o.overcome the fric tion in the duct (no dynamic pressure loss), plus the velocity pressure Which, in this case, is the same at any location along the length of the duct. In arrangement B in theTower part of Fig. 9, a diverging section, with after'section, has been added to the straight duct. The velocity in the diverging section is therefore decreased, and velocity pressure converted Arrangement A \-t/Oecrease in'TotatPressure I * Due to Static Pressure Regain Velocity Pressure ofthe MrLeaving StaticPressure Re-.. gain in Expanding Section. Fig. 9. Application of Static Pressure Regain to a Simple. Fan 'and Discharge Duct into static pressure before the air is released'to the atmosphere. ' It can be seen that in case B,. the total pressure at the fan outlet is less than in case A, and thus a saving in .horsepower .can be effected. The regain iri static pressure h, in an abruptly expanded section is the difference in the velocity pressures of the'small and the large duct, minus the dynamic pressure loss (Equation 6): (13) or simplified (14) where At = regain: in static pressure, feet of fluid, flowing. Air Duct Design 685 Vi and vt = mean velocities in inlet and outlet duct sections, respectively, feet per second. '. The static, pressure regain in a gradually expanding transition, followed by an after section, may be expressed, as *"[2-3 -[^] or (16) 2g where Ci = .an experimentally determined regain constant depending on nature of con struction.: Curves have been developed showing the static pressure regain and the theoretical efficiency of conversion in abrupt expansion, and in diverging * sections in smooth circular ducts.16'18 DUCT DESIGN The discussion of duct design in this chapter refers to ducts in fan sys tems for central heating, ventilating and air conditioning. Additional data for heating ducts used in residences are to be found in Chapter 18 (Gravity Warm Air! Systems) and Chapter 19 (Forced Warm Air Systems). The design of ducts in industrial exhaust systems is discussed in Chapter 45. The following general rules should be followed in design: 1. The air should be conveyed as directly as possible at the permissible velocities to obtain the desired results with greatest economy of power, material, and space. 2. Sharp elbows and bends should be avoided. Splitters and turning vanes should be used to reduce the elbow or outlet pressure loss. 3. Diverging transformation pieces should be made as.long as practicable. As shown in the section on area changes, losses in sudden enlargements are high,' and abrupt enlargements should be avoided. The included angle of divergence for enlargements should not exceed 20 deg. Losses in contractions are low, but, the inT eluded angle .of convergence should not be larger than 60 deg. ,~A 4. Special care should be taken to avoid restriction'of flow'in elbows or trans formation pieces. 5. Where the greatest air carrying capacity per square foot1 of sheet metal is de sired, rectangular ducts should be made as nearly square as possible. Aspect ratios greater than 10 to 1 should be avoided. 6. Ducts should, be constructed of smooth material, such as steel or aluminum sheet metal. For ducts made from other materials, for example masonry, proper allowance for the surface friction coefficient should be made. Procedure for Duct Design The general procedure for design is outlined as follows: 1. Study the plan of the building and draw in roughly the most convenient system of ducts, taking cognizance of the building construction, avoiding, all obstructions in steel work and equipment, and at the same time maintaining a simple design. 2. Arrange the positions of duct outlets to insure the proper distribution of air. 3. Divide the building into zones and proportion the volume of air necessary for each zone. 4. Determine the size of each outlet, based on the volume as obtained in the pre ceding paragraph; for the proper outlet velocity and throw. In case of some ceiling diffusers, determine size of outlet for proper throat velocity, and radius of diffusion.