Pressure drop Fittings

Severe hydraulic disturbances near the distributor inlet may cause excessive (Fig. 2.5e) or insufficient flow. It is best to avoid sharp bends and high-pressure-drop fittings close to the distributor inlet. 

Table 3.4 lists the FTS pressure drop fitting parameters for Dutch Twill, and all other weaves, respectively. Dutch Twills yield smaller FTS pressure drops in both laminar and turbulent regimes, due to more available flow area, higher void fraction, etc. For most accurate results, a LAD designer should fit parameters to each individual screen data from Figures 3.11 and 3.12 and Table 3.4 are meant for preliminary design or where testing is imavailable. 

A )m sc represents miscellaneous pressure drops associated with valves, fittings, etc, in the flow circuit, and these devices are not usually present inside a heat exchanger. 

Viscous Transport. Low velocity viscous laminar dow ia gas pipes is commonplace. Practical gas dow can be based on pressure drops of <50% for low velocity laminar dow ia pipes whose length-to-diameter ratio may be as high as several thousand. Under laminar dow, bends and fittings add to the frictional loss, as do abmpt transitions. 

Vfjp is the friction velocity and =/pVV2 is the wall stress. The friction velocity is of the order of the root mean square velocity fluctuation perpendicular to the wall in the turbulent core. The dimensionless distance from the wall is y+ = yu p/. . The universal velocity profile is vahd in the wall region for any cross-sectional channel shape. For incompressible flow in constant diameter circular pipes, = AP/4L where AP is the pressure drop in length L. In circular pipes, Eq. (6-44) gives a surprisingly good fit to experimental results over the entire cross section of the pipe, even though it is based on assumptions which are vahd only near the pipe wall. 

The viscous or frictional loss term in the mechanical energy balance for most cases is obtained experimentally. For many common fittings found in piping systems, such as expansions, contrac tions, elbows and valves, data are available to estimate the losses. Substitution into the energy balance then allows calculation of pressure drop. A common error is to assume that pressure drop and frictional losses are equivalent. Equation (6-16) shows that in addition to fric tional losses, other factors such as shaft work and velocity or elevation change influence pressure drop. 

Compared with elbow fittings, bends with a centerhne radius of three or five nominal pipe diameters save the cost of joints and reduce pressure drop. Such bends are not suited for instaUation in a bank of pipes of unequal size when the bends are in the same plane as the bank. 

Fittings contribute strongly to the pressure drop on the annulus side. General methods for predicting this are not reliable, and manufacturer s data should be used when available. 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

Figure 3.4.3 shows that the measured results fit the quadratic equation well for pressure generated. It also shows that the pressure generated is independent of flow since three different quantities of catalyst were used. Since the pressure drop remained constant, then flow must have been different over the three quantities of catalysts. The flow adjusted itself to match the constant pressure generated by the blower. 

Orifice plate A metal plate with a hole of diameter smaller than the pipe or duct run in which it is fitted. The pressure drop that takes place across the plate is used to calculate the fluid velocity. 

Straighteners Vanes fitted in ductwork or air handling units before or after a change in section to produce a reduction in pressure drop. 

The total piping system pressure drop for a particular pipe installation is the sum of the friction drop in pipe valves and fittings, plus other pressure losses (drops) through control valves, plus drop through equipment in the system, plus static drop due to elevation or pressure level. For example, see Figure 2-2. 

Pressure Drop in Fittings, Valves, Connections Incompressible Fluid... 

For a system of multiple components of valves, pipe, and fittings. Equation 2-25 can be used to establish a component size to which each separate resistance can be expressed as a common denominator, or common pipe size. Under these conditions, the corrected Rvalues are additive and can be used as one number in Equation 2-27. These types of corrections should be made to improve and more accurately represent the pressure drop calculations. 

Scope, 52 Basis, 52 Compressible Flow Vapors and Gases, 54 Factors of Safety for Design Basis, 56 Pipe, Fittings, and Valves, 56 Pipe, 56 Usual Industry Pipe Sizes and Classes Practice, 59 Total Line Pressure Drop, 64 Background Information, 64 Reynolds Number, R,. (Sometimes used Nr ), 67 Friction Factor, f, 68 Pipe—Relative Roughness, 68 Pressure Drop in Fittings, Valves, Connections Incompressible Fluid, 71 Common Denominator for Use of K Factors in a System of Varying Sizes of Internal Dimensions, 72 Validity of K Values,... 

The method proposed by Bolles fits the average design problem quite satisfactorily. However, for low pressure drop designs as in vacuum towers, it may well require checking by the more detailed method of Dauphine [13]. 

Strigle [139] reports that Kister and Gill s [93] tests indicate that from over 3,000 pressure drop measurements the results fit Figure 9-21-G for 80% (excellent) and another 15% (reasonable) fit. 

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