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# 42540_22 - CHAPTER 22 PIPE SIZING Pressure Drop Equations...

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22.1 CHAPTER 22 PIPE SIZING Pressure Drop Equations ......................................................... 22.1 WATER PIPING ....................................................................... 22.5 Flow Rate Limitations .............................................................. 22.5 Hydronic System Piping ........................................................... 22.6 Service Water Piping ................................................................ 22.8 STEAM PIPING ..................................................................... 22.12 Low-Pressure Steam Piping ................................................... 22.13 High-Pressure Steam Piping .................................................. 22.13 Steam Condensate Systems .................................................... 22.13 GAS PIPING .......................................................................... 22.20 FUEL OIL PIPING ................................................................ 22.21 HIS CHAPTER includes tables and charts to size piping for T various fluid flow systems. Further details on specific piping systems can be found in appropriate chapters of the ASHRAE Handbook. Two related but distinct concerns emerge when designing a fluid flow system: sizing the pipe and determining the flow-pressure rela- tionship. The two are often confused because they can use the same equations and design tools. Nevertheless, they should be determined separately. The emphasis in this chapter is on the problem of sizing the pipe, and to this end design charts and tables for specific fluids are pre- sented in addition to the equations that describe the flow of fluids in pipes. Once a system has been sized, it should be analyzed with more detailed methods of calculation to determine the pump head required to achieve the desired flow. Computerized methods are well suited to handling the details of calculating losses around an extensive system. PRESSURE DROP EQUATIONS Darcy-Weisbach Equation Pressure drop caused by fluid friction in fully developed flows of all “well-behaved” (Newtonian) fluids is described by the Darcy- Weisbach equation: (1) where ' p = pressure drop, lb f /ft 2 f = friction factor, dimensionless (from Moody chart, Figure 13 in Chapter 3) L = length of pipe, ft D = internal diameter of pipe, ft U = fluid density at mean temperature, lb m /ft 3 V = average velocity, fps g c = units conversion factor, 32.2 ft·lb m /lb f ·s 2 This equation is often presented in head or specific energy form as (2) where ' h = head loss, ft g = acceleration of gravity, ft/s 2 In this form, the density of the fluid does not appear explicitly (although it is in the Reynolds number, which influences f ). The friction factor f is a function of pipe roughness H , inside diameter D , and parameter Re, the Reynolds number: (3) where Re = Reynolds number, dimensionless H = absolute roughness of pipe wall, ft P = dynamic viscosity of fluid, lb m /ft·s The friction factor is frequently presented on a Moody chart (Figure 13 in Chapter 3) giving f as a function of Re with H / D as a parameter. A useful fit of smooth and rough pipe data for the usual turbulent flow regime is the Colebrook equation : (4) Another form of Equation (4) appears in Chapter 3, but the two are equivalent. Equation (4) is more useful in showing behavior at limiting cases—as H / D approaches 0 (smooth limit), the 18.7/Re term dominates; at high H / D and Re (fully rough limit), the 2 H / D term dominates. Equation (4) is implicit in f ; that is, f appears on both sides, so a value for f is usually obtained iteratively.

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42540_22 - CHAPTER 22 PIPE SIZING Pressure Drop Equations...

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