Pressure-drop limitations

Flame arrester pressure drops mnst he taken into consideration when selecting and specifying them, especially when they are installed on vent systems of low pressure storage tanks, snch as API-type tanks. If the total system pressure drop exceeds the design pressure of the tank, the tank will he overpressnred and may he strnctnrally damaged. Refer to Section 6.1 for additional discnssion of this topic. 

In many sitnations several types of flame arresters are snitahle for the application. Often pressure drop dnring normal operation and its possible increase over the flame arrester s service life may he the primary determinant in selection of a snitahle nnit. 

Depending on the design, the pressnre drop throngh a crimped metal ribbon flame arrester may exceed that of a parallel plate type of the same size. If a crimped metal ribbon type arrester is nsed for end-of-line service 

Pressnre taps are often specified on each side of an in-line flame arrester for checking the pressnre differential across the arrester while in service. The taps and instmment installation mnst not provide a flame bypass aronnd the flame arrester. By monitoring pressnre drop, fonling or plngging may be detected early, if there is a snfficient vapor flow rate. 

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, 

Using Bell s method, calculate the shell-side heat transfer coefficient and pressure drop for the exchanger designed in Example 12.1. 

Prandtl number, from Example 12.1 = 5.1 Neglect viscosity correction factor (n/nw). 

Very low, sealing strips needed try one strip for each five vertical rows. 

This for the exchanger in the clean condition. Using the factors given in Table 12.7 to estimate the pressure drop in the fouled condition 

Appreciably lower than that predicted by Kern s method. This shows the unsatisfactory nature of the methods available for predicting the shell-side pressure drop. 

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Available in metal and plastic, used in large and small towers for distillation, absorption, scrubbing, liquid extraction. High efficiency, low HETP, low pressure drop. Limited data available. 

Chattering caused by undersized inlet piping may sometimes be eliminated on pilot operated PR valves if the pilot valve pressure tapping is taken directly from the vessel being protected. However, it is recommended that the above inlet pressure drop limitations still be applied, to avoid the capacity reduction that would result from excessive inlet losses and to ensure freedom from chatter. 

Normal operating pressure Anticipated process operating pressure used to determine pipe diameter requirements and pressure drop limitations for various operating conditions. 

The original optimization problem with five variables was, by choosing the liquid flow rate in section I by pressure-drop limitations and following Equations (35) and (36) to evaluate the switch time interval and the recycling flow rate, reduced to a two-variable optimization problem the choice of liquid flow rates in the two central sections. Table 9-5 summarizes the SMB operating conditions (and equivalent TMB conditions) used in the design of the 7 -711 plot. 

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

Not too suitable for vacuum services due to pressure drop limitations but are used in application. 

In general, tube-side pressure drops less than one psi per pass should not be specified for economical designs. Drops as low as 15 mm Fig. have been specified, and designs obtained which were competitive with cooling tower installations. For viscous materials, pressure drop limitations can markedly influence a design and its economics. 

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. 

The value for is conservatively interpreted as the particle diameter. This is a perfectly feasible size for use in a laboratory reactor. Due to pressure-drop limitations, it is too small for a full-scale packed bed. However, even smaller catalyst particles, dp 50 yum, are used in fluidized-bed reactors. For such small particles we can assume rj=l, even for the 3-nm pore diameters found in some cracking catalysts. 

Since neither design in Figs. 22 or 23 will satisfy the specified pressure drop limitation, the only alternative lies in reducing inlet width without increasing inlet velocity, which requires multiple cyclones operating in parallel as illustrated in the design of Example C. 

Platinum on alumina reforming catalysts are commonly used commercially in the form of cylindrical pellets about X i in. in size, since this is about the smallest size acceptable from the standpoint of avoiding excessive pressure drop. For fundamental studies in small-scale reactors, where pressure drop limitations are less severe, it may be preferable to use the catalyst in the form of small granules to minimize diffusional limitations. 

Two Liquid Phases Two liquid phases often occur in heterogeneous distillation, in steam stripping, and in solvent recovery. Harrison (Chem. Eng. Progrp. 80, November 1990) and Meier et al. [IChemE Symp. Ser. 152, 267 (2006)] saw no reason to expect unusual capacity or pressure drop limitations due to the presence of the second liquid phase, suggesting that standard correlations for these should apply. 

Special pressure drop-limiting changeover valve... 

Where intraparticle diffusion appreciably affects the rate of the reaction, reduction in catalyst particle size would be necessary to increase the effectiveness factor and hence conversion. But this may not be possible due to the pressure drop limitations in conventional packed beds. In such situations, the use of Monoliths would provide the advantage of higher effectiveness factor. 

Considering methane steam reforming [see Eq. (2.12)] as a large-scale process, Xu and Froment found that only the outer 2 mm of the catalyst pellets actually participates in the reaction [41], thus theoretically allowing for a two orders of magnitude reduction in catalyst volume. However, the well-known pressure drop limitations have prevented practical applications in the industrial field so far. 

A very large tube length is required because of the very low inside film coefficient and several passes or indeed a multistage unit would be specified. A better approach would be to increase the tube side velocity by decreasing the number of tubes in each pass, though any pressure drop limitations would have to be taken into account. The use of a smaller tube diameter might also be considered. 

Limitations Solids collect around and at potted ends of fibers Requires air scour to clean Cannot perform true recycle High pressure drop limits fiber length Requires significant filtration for pretreatment... 

In heat-exchanger applications, it is frequently important to match heat-transfer requirements with pressure-drop limitations. Assuming a fixed total heat-transfer requirement and a fixed temperature difference between wall and bulk conditions as well as a fixed pressure drop through the tube, derive expressions for the length and diameter of the tube, assuming turbulent flow of a gas with the Prandtl number near unity. 

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