Shell-Side Construction

Impingement plates are another shell-side construction feature that may be required to prevent tube vibration or erosion. The impingement plate is placed under the shell-side inlet nozzle and should be larger than the nozzle opening, with an escape area off the edges of the plate that is twice the flow area of the nozzle. The impingement plate is attached to the shell or the tie rods. TEMA standards specify the conditions under which an impingement plate is required. 

When alloy construction for one of the two fJiiids is required, a carbon steel shell combined with alloy tube-side parts is less expensive than alloy in contact with the shell-side fluid combined with carbon steel headers. 

The price of a shell and tube exchanger depends on the type of exchanger, i.e., fixed tube, U-tube, double tube sheets, and removable bundles. The tube side pressure, shell side pressure, and materials of construction also affect the price. If prices cannot be obtained from endors, correlating in-house data by plotting /fr vs. number of ft with correction factors for the variables that affect price will allow estimating with fair accuracy. If not enough in-house data is available to establish good correlations. it will be necessary to use the literature, such as References 16. 17. and 18. 

Some general considerations to bear in mind are (1) In all start-up and shutdown operations, fluid flows should be regulated so as to avoid thermal shocking the unit, regardless of whether the unit is of either a removable or non-removable type of construction (2) For fixed tubesheet (i.e., non-removable bundle) type units, where the tube side fluid cannot be shut down, it is recommended that both a bypass arrangement be incorporated in the system, and the tube side fluid be bypassed before the shell side fluid is shut down (3) Extreme caution should be taken on insulated units where fluid flows are terminated and then restarted. Since the metal parts eould remain at high temperatures for extended periods of time, severe thermal shock could occur. 

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 . )...

Figure 10-22A. Construction details of two-pass expanding shell-side baffle. (Used by permission Struthers-Wells Corp., Bui. A-22.)...

Eor one shell and multipass on the tube side, it is obvious that the fluids are not in true counter-current flow (nor co-current). Most exchangers have the shell side flowing through the unit as in Eigure 10-29C (although some designs have no more than two shell-side passes as in Eig-ures 10-IJ and 10-22, and the tube side fluid may make two or more passes as in Eigure 10-IJ) however, more than two passes complicates the mechanical construction. 

Design a partial condenser to cool a mixture of hydrogen chloride-water vapor from 178°F to 90°F using 60 gal per min of chilled water at 70°F The unit is to have the acid mixture in the tubes, because this will allow for a cheaper construction than if this material were in the shell. The tube-side material is to be impervious graphite, and the shell and shell-side baffles are to be steel. The acid vapor is essentially at its dew point. 

Pressure losses through the shell side of exchangers are subject to much more uncertainty in evaluation than for tube side. In many instances, they should be considered as approximations or orders of magnitude. This is especially true for units operating under vacuum less than 7 psia. Very little data has been published to test the above-atmospheric pressure correlations at below-atmospheric pressures. The losses due to differences in construction, baffle clearances, tube clearances, etc., create indeterminate values for exact correlation. Also see the short-cut method of reference 279. 

In these designs there is one pass for the fluid on the shell-side and a number of passes on the tube-side. It is often an advantage to have two or more shell-side passes, although this considerably increases the difficulty of construction and, very often therefore, several smaller exchangers are connected together to obtain the same effect. 

Another factor that may determine whether a material should flow inside or outside the tube is the material of construction. If expensive alloys are required for only one of the fluids, it is cheaper to plate the inside of the tubes than the shell side. 

Steam Superheater This unit superheats saturated steam from 250°C (and 4000kPa) to 380°C. The product steam is of medium pressure and suitable quality for in-house application and also for export. The superheater cools the reaction gases from the reactor exit temperature of 645°C to 595°C. Design pressure on the shell side is approximately 5000 kPa. The steam superheater is constructed from mild steel. 

Water flows inside the tubes, and vapors condense on the shell side. Cooling water is normally chilled, as in a cooling tower, and reused. Air-cooled surface condensers and some water-cooled units condense inside the tubes. Air-cooled condensers are usually constructed with extended surface fins. 

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