Allowable Pressure Drop

Packed fiber bed mist eliminators can be designed to operate at almost any desired particle collection efficiencies, depending on the allowable pressure drop and cost. A good discussion of sulfuric acid mist generation, control, and mist eliminator design is available (109,110). 

The problem presented to the designer of a gas-absorption unit usually specifies the following quantities (1) gas flow rate (2) gas composition, at least with respect to the component or components to be sorbed (3) operating pressure and allowable pressure drop across the absorber (4) minimum degree of recoverv of one or more solutes and, possibly, (5) the solvent to be employed. Items 3, 4, and 5 may be subject to economic considerations and therefore are sometimes left up to the designer. For determining the number of variables that must be specified in order to fix a unique solution for the design of an absorber one can use the same phase-rule approach described in Sec. 13 for distillation systems. 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

The allowable pressure drop influences the choice of the particle size and helps determine the column length. Equations for estimating the pressure drop in packed beds are given in Section 6. 

Process Gas flow rate and velocity Pollutant concentration Variability of gas and pollutant flow rates, temperature, etc. Allowable pressure drop... 

These techniques have been described previously in the pressure measurement section. Usually, one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressure drop, flow type, accuracy required, and cost. 

Temperature and quantity of cooling water available for the intercondensers. Also cooling water allowable pressure drop for the intercondensers. 

API mandates that the pressure drop across a clean filter element be no more than 15% of the allowable pressure drop when dirty. An upper limit of 5 psi drop is set for clean filters. This is a reasonable criterion. If a little arithmetic is performed, the head rise for a centrifugal pump may be calculated. The specifications on both of these items must be coordinated and made compatible. 

Size a Nutsch filter for processing a suspension containing 15 % solids. The desired cake wetness is 45 % and the filter capacity for the filtrate should be 5 mVhr. The allowable pressure drop is 500 mm Hg. The cake specific resistance is r = 9 X 1011 m and the filter plate resistance is Rf = 2 x 10 ... 

Step 4 Reading vertically up from this intersection, it can be seen that a 1-in. line will produce more than the allowed pressure drop, so a i)f<-in. size is chosen. 

In order to ensure good efficiencies and yet a definite seal consistent with allowable pressure drops, suggested values for hds are modified from the references and shown in Table 8-18. 

To preclude thermal degradation of the amine solution, the reboiler pressure must be limited to 10 mm Hg absolute pressure. Total allowable pressure drop is 3 mm Hg. 

Values include dirt factors of 0.003 and allowable pressure drops at 5—10 psi on the controlling steam. DOWTHERM fluid would be included as light organic. (Kern. Process Heat Transfer, 1 Ed., p. 840 1950.)... 

When using this method, it is necessary to assume an allowable pressure drop though the pulsation drum. This pressure drop does not need to be large (V4 -1 lb). However, the higher the allowable pressure drop, the smaller the resulting bottle, because the diameter and the length of the drum vary as (pressure drop) or the volume of the drum varies as (pressure drop) . ... 

P = pressure in drum, psia Cq = orifice or short pipe coefficient = 0.62 for sharp-edged orifice = 0.81 for short pipe h = allowable pressure drop, psi V = specific volume, fP/lb, at 14.7 psia and 60°F f = frequency, cycles/sec rpm... 

These devices are generally designed to meet specific pressure-drop limitations. For ordinary installations operating at approximately atmospheric pressure, fan limitations dictate a maximum allowable pressure drop corresponding to a cyclone inlet velocity in the range of 20 to 70 feet per second. Consequently, cyclones are usually designed for an inlet velocity of 50 feet per second. 

Typical pipe velocities and allowable pressure drops, which can be used to estimate pipe sizes, are given below ... 

For vacuum distillations the maximum allowable pressure drop will be determined by the process requirements, but for satisfactory liquid distribution the pressure drop should not be less than 8 mm water per m. If very low bottom pressures are required special low pressure-drop gauze packings should be considered such as Hyperfil, Multifil or Dixon rings see Volume 2, Chapter 4. 

Pressure drop. For the same pressure drop, higher heat-transfer coefficients will be obtained on the tube-side than the shell-side, and fluid with the lowest allowable pressure drop should be allocated to the tube-side. 

Too low to make effective use of the allowable pressure drop, try 4 passes. 

Though Bell s method will give a better estimate of the shell-side pressure drop than Kern s, it is not sufficiently accurate for the design of exchangers where the allowable pressure drop is the overriding consideration. For such designs, a divided-flow model based on Tinker s work should be used. If a proprietary computer program is not available,... 

If other actual I.D. pipe sizes are used, or if local superheat exists on 175 psig or 30 psig systems, the allowable pressure drop shall be the governing design criterion. 

For viscous fluids, the velocity may be constrained by the allowable pressure drop or shear degradation of the fluid (e.g. large molecules breaking down into smaller molecules from high shear rates). Typical values are given in Table 13.6. 

Pressure drop. Rather than specify a fluid velocity, it is often preferred to specify the pressure drop across the heat exchanger. In retrofit situations, where a new heat exchanger is to be installed in an existing plant, the allowable pressure drop is often highly constrained. This is because in a retrofit situation, it is often desirable to avoid... 

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