Pressure-drop procedure

Sample Preparation. Gas phase samples have routinely been prepared in cylindrical Klmax KG-55 ampoules (23) using a pressure-drop procedure to determine mixture compositions (2 -26. J+6). Pressure measurements for H2/C3F6 and CH4/C3Fe samples were carried out to an absolute accuracy of O.O5 Torr by means of a Wallace and Tlernan 0-200 Torr model FA-1 15 mechanical gauge. More recent experiments have utilized a Barocell model U7l(-capacitance manometer provided with a model 5TO-D-1OO0T-1B2-H5 temperature stabilized 0-1000 Torr sensor. Individual component pressures measured with the calibrated Barocell had an absolute accuracy of 0.005 Torr. Unless noted otherwise the total sample pressure corresponding to the irradiation temperature was always 1000 10 Torr. Gravimetric checks (g5. Ut) of the total pressures were routinely carried out. 

On occasion one will find that heat-transfer-rate data are available for a system in which mass-transfer-rate data are not readily available. The Chilton-Colburn analogy provides a procedure for developing estimates of the mass-transfer rates based on heat-transfer data. Extrapolation of experimental or Jh data obtained with gases to predict hquid systems (and vice versa) should be approached with caution, however. When pressure-drop or friction-factor data are available, one may be able to place an upper bound on the rates of heat and mass transfer, according to Eq. (5-308). 

An equation for use with venturi meters was given by Chisholm [Br Chem. Eng., 12, 454—457 (1967)]. A procedure for determining steam quahty via pressure-drop measurement with upflow through either venturi meters or sharp-edged orifice plates was given By Colhus and Gacesa [J. Basic Eng., 93, 11-21 (1971)]. 

The values of h, and AP, calculated by this procedure are for clean exchangers and are intended to be as accurate as possible, not conservative. A fouled exchanger will generally give lower heat-transfer rates, as reflected by the dirt resistances incorporated into Eq. (11-2), and higher pressure drops. Some estimate of fouling effects on pres-... 

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a hquid-level-controUer. The subcoohng heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33 ), (5-33Z )], with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

The same procedure may be applied in principle to design of forced-recirculation reboilers with shell-side vapor generation. Little is known about two-phase flow on the shell side, out a reasonable estimate of the fric tion pressure drop can be made from the data of Diehl and Unruh [Pet Refiner, 36(10), 147 (1957) 37(10), 124 (1958)]. No void-fraction data are available to permit accurate estimation of the hydrostatic or acceleration terms. Tnese may be roughly estimated by assuming homogeneous flow. 

The values for pressure ratio in a practical case must include allowance for pressure drop in the interstage piping. In the sizing procedure used by manufacturers, certain adjustments must be made to the ideal for incremental cylinder sizes and allowable rod loading. Efficiency is represented by r. yi. 

There are conflicting procedures for predicting pressure drop across fibrous filters. The three chief methods of predicting pressure drop are based on the hydraulic... 

A slit die is designed on the assumption that the material is Newtonian, using apparent viscous properties derived from capillary rheometer measurements, at a particular wall shear stress, to calculate the volumetric flow rate through the slit for the same wall shear stress. Using the correction factors already derived, obtain an expression for the error involved in this procedure due to the melt being non-Newtonian. Also obtain an expression for the error in pressure drop calculated on the same basis. What is the magnitude of the error in each case for a typical power law index n = 0.377... 

The design equations for pressure drops can also be used for evaluation procedures (Section 10..5. ... 

Volume 1, Chapter 9 explains the criteria for choosing a diameter and wall thickness of pipe. This procedure can be applied to choosing a coil diameter in an indirect fired heater. Erosional flow criteria will almost always govern in choosing the diameter. Sometimes it is necessary to check for pressure drop in the coil. Typically, pressure drop will not be important since the whole purpose of the line heater is to allow a large pressure drop that must be taken. The allowable erosional velocity is ffiven bv ... 

An alternate procedure is to calculate (1) the pressure drop series-wise one section of the line at a time, or (2) capacity for a fixed inlet pressure, series-wise. 

Example 8-41 Procedure for Calculating Valve Tray Pressure Drop (after Klein [201])... 

As a comparison or alternate procedure, the pressure drop at the flooding point as indicated by the upper break in the pressure drop curve can be estimated from Table 9-33 and Figure 9-34D for rings and saddles [81]. The values in the table multiplied by the correction ratio gives the pressure drop for the liquid in question, expressed as inches of water. 

To complete the design, the tower should be checked for loading and flooding conditions, and the pressure drop established. However, this procedure will not be repeated as it can be found elsewhere in this text. 

Total shell side Ap (refer to the pressure drop section of this chapter) can be neglected for pressure units unless an unusual condition or design exists. To check, follow procedure for unbaffled shell pressure drop. 

Follow steps 7 (Gilmour method), etc., of the procedure for vaporization only. If baffles are added for sensible heat (not assumed in free convection), then pressure drop will be affected accordingly. Gr is the Grashof number using properties at average fluid temperature, = Dj pgP At/p. ... 

The vertical thermosiphon reboiler is a popular unit for heating distillation column bottoms. However, it is indeed surprising how so many units have been installed with so little data available. This indicates that a lot of guessing, usually on the very conservative side, has created many uneconomical units. No well-defined understanding of the performance of these units exists. Kern s recommended procedure has been found to be quite conservative on plant scale units yet it has undoubtedly been the basis for more designs than any other single approach. For some systems at and below atmospheric pressure operation, Kern s procedure gives inconsistent results. The problem is in the evaluation of the two-phase gas-liquid pressure drop under these conditions. 

Nakayama suggests essentially the same procedure except recommending that specific manufacturers data for various units be assembled and correlated for use in the detailed design of film coefficients and pressure drop. 

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