Shell-side cross-flow

Streamline flow shell-side cross-flow modified Donohue ... 

The baffle used in the preceding equation has 20% segmental cuts. Shell-side cross-flow velocity ... 

Divide by 144 to obtain the shell-side cross-flow area in square feet. 

The function of seal strips is to interfere with, and hence reduce, the fluid flow through the bypass area. Often, one pair of seal strips is used for every 18 in of shell ID (inner diameter). These seal strips encourage good shell-side cross-flow velocity and also help reduce localized fouling, caused by low velocity. 

The shell-side cross-flow velocity may be altered in much smaller increments, by changing the tube support baffle spacing. This is one advantage of placing the fluid with the poorer heat-transfer properties on the shell side. But there is another, far more critical, advantage in placing the fluid with the poorer heat transfer properties on the shell side. 

Shell-Side Calculations. In the shell, kerosene flows between the baffles over the tubes. The area for cross-flow between the tubes is given by the length of tube between baffles, Z.T/(/VB + 1), multiplied by the shell internal diameter, DiS, adjusted by the fractional available area for flow between the tubes, (Ft — DoT)/PT. Thus, the shell-side cross-flow area is... 

Ti, 11 supply ternperamre of hot and cold streams, F 72, 2 target temperature of hot and cold streams, °F AT i hot end temperature approach, °F AT 2 cold end temperature approach, °F AT ijn logarithmic mean temperature difference (LMTD), °F u shell side cross-flow velocity or tube velocity, ft/h U overall heat transfer coefficient, Btu/(ft °F h)... 

I ve read about the importance of shell-side cross-flow in heat-exchanger design books. Higher cross-flow velocities reduce film resistance and promote increased heat-transfer rates. Recently, I had a dramatic personal experience to support this engineering principle. 

When fluid flows at a 90° angle across a tube, it creates turbulence, which is called vortex shedding. The swirls created when a river flows across a tree stump is a common example. The swirls destroy the film resistance to heat transfer created by laminar layers of fluid surrounding the tube. My experience indicates that a shell-side cross-flow velocity of 3 to 5 ft/s is sufficient to obtain this benefit. 

In the common types of baffled shell-and-tube exchangers, the shell-side fluid flows across the tubes. The equations for predicting heat-transfer coefficients under these conditions are not the same as those for flow of fluids inside pipes and tubes. An approximate value for shell-side coefficients in a cross-flow exchanger with segmental baffles and reasonable clearance between baffles, between tubes, and between baffles and shell can be obtained by using the following correlation ... 

Re = d Vs/v d = outside diameter of fiber Vg is based on the shell-side cross-sectional area available for flow. 

X shell is a cross flow shell. It does not have baffles. Shell side fluid flows across tube bundle to the other tdde of the shell. Pressure drop is low. For long tubes, more than one set of inlet/outlet nozzles will be used. Sometinies, perforated plates are used to help distribute the shell side flow across the tube bundle. It is mainly used for low pressure gas condensing service. [2]... 

The shell side mass velocity normal to tubes at centerline of exchanger is calculated by dividing the shell side mass flow rate to the cross-sectional area of the shell area between baffle and shell axis. The cross-sectional area between baffles and shell axis, A, ... 

APm.AP,., Pressure drop for ideal-tube-bank cross-flow and ideal window respectively AP for shell side of baffled exchanger kPa Itf ft ... 

Baffles in a horizontal in-shell condenser are oriented with the cuts vertical to facilitate drainage and eliminate the possibility of flooding in the upward cross-flow sections. Pressure drop on the vapor side can be estimated by the data and method of Diehl and Unruh [Pet. Refiner, 36(10), 147 (1957) 37(10), 124 (1958)]. 

Acoustic Coupling When the shell-side fluid is a low-density gas, acoustic resonance or coupling develops when the standing waves in the shell are in phase with vortex shedding from the tubes. The standing waves are perpendicular to the axis of the tubes and to the direction of cross-flow. Damage to the tubes is rare. However, the noise can be extremely painful. 

Maximum shell-side heat-transfer rates in forced convection are apparently obtained by cross-flow of the flmd at right angles to the tubes. In order to maximize this type of flow some heat exchangers are built with segmental-cut baffles and with no tubes in the window (or the baffle cutout). Maximum baffle spacing may thus equal maximum unsupported-tube span, while conventional baffle spacing is hmited to one-h f of this span. 

Figure 10-8. Single-pass shell and tube Teflon tube heat exchanger, countercurrent flow. Tube bundles are flexible tube Teflon joined in integral honeycomb tubesheets. Shell-side baffles are provided for cross-flow. Standard shell construction is carbon steel shell plain or Teflon (LT) lined. Heads are lined with Teflon . Tube diameters range from 0.125-0.375 in. O.D. the temperature range is 80-400°F pressures range from 40-150 psig. (Used by permission AMETEK, Inc., Chemical Products Div., Product Bulletin Heat Exchangers of Teflon . )...

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-... 

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