Document VJMxx6841wggkJ6YNor76Ow6w

820 CHAPTER 41 1949 Guide discharge duct.1 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 must be provided, by the fan is therefore the sum of the pressure that is necessary to overcome the fric tion in the duct (no shock 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 the lower part of Fig, 6 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 f Velocity Pressure \ ofthe Air Leaving ; p the System. fAtmospheric Pressure Arrangement B jr After Section Expanding Section 3* /Decrease in Tota/Pressure r Due to Static Pressure Regain yAtmospheric' Velocity Pressure ofthe AirLeaving the System-^ > & StaticPressure Re gain in Expanding Section. Fig. 6. Application of Static PhessuebRegain 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 in static pressure Kt in an abruptly expanded section is-the difference in the velocity pressures of the small and the large duct, minus the shock pressure loss (Equation 6): or simplified r_ 4] [ft.r,)'1 V * L*J (12) where cfti -- fl) 9 (13) h, =' regain in static pressure, feet of fluid flowing, ri and c = mean Velocities in smaller and larger "duct sections, respectively, ' -{ feet per second. . ; Air Duct Design 821 The static pressure regain in. a gradually expanding transition followed by an after section may be expressed as: ~ * [ J_ cH _ [eft. - ),~| A, = 2g\ (14) or Cj -- c) --' eft. -- yj* K 2g (16) 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.1*'14 DUCT DESIGN The discussion of duct design in this chapter refers to ducts in fan sys tems for central heating, ventilating and air conditioning systems. Addi tional data for heating ducts used in residences are to be found in Chapter 21 (Gravity Warm Air Systems) and Chapter 22 (Mechanical Warm Air Systems). The design of ducts in industrial exhaust systems is discussed in Chapter 46. The following general rules should be followed in the design of a duct system: 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. Carefully designed splitters and turning vanes should be used to reduce the elbow or outlet pressure loss. ` (See sec tion Pressure Loss in Elbows). 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 in cluded angle of divergence should not be larger than 60 deg. 4. Special care should be taken to avoid restriction of flow in elbows or trans formation pieces. 5. Rectangular ducts should be made as nearly square as possible. . Good practice limits the ratio between the long side and the short side 3 to 1. In no case should this . ratio exceed 10 to 1. 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 designing a duct system is outlined in the several items listed herewith: 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. 6. Calculate the sizes of all main and branch ducts by one of the three methods of sizing air simply systems in common use, the velocity reduction method, the equal .-friction method or.thestatic regain method.