Shell types passes

The divided flow and split-flow arrangements (G and J shells) are used to reduce the shell-side pressure drop where pressure drop, rather than heat transfer, is the controlling factor in the design. 

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Figure 12.12. Shell types (pass arrangements), (a) One-pass shell (E shell) (b) Split flow (G shell) (c) Divided flow (J shell) (d) Two-pass shell with longitudinal baffle (F shell) (e) Double split flow (H shell)...

If there were no removable cover on the front end of the exchanger, it would be designated BES." The second nozzle and pass partition in the front end are discretionary depending upon the shell type. Types A and B bolt onto the shell. In type C, the head cannot be unbolted for maintenance. 

The shell types are E, F, G, H. i, and K. E is a one-pass shell. The fluid comes in on one side and goes out the other side, F is a two-pass shell with a longitudinal plate in it. The fluid in the shell makes two passes. 

An economic exchanger design cannot normally be achieved if the correction factor Ft falls below about 0.75. In these circumstances an alternative type of exchanger should be considered which gives a closer approach to true counter-current flow. The use of two or more shells in series, or multiple shell-side passes, will give a closer approach to true counter-current flow, and should be considered where a temperature cross is likely to occur. 

The required heat-transfer area of 19.5 m2 is based on an overall heat-transfer coefficient of 102 W/(m2 K). The best exchanger geometry for this application includes six internal baffles, one shell-side pass and two tube-side passes. The shell is fabricated from standard carbon steel piping of nominal pipe size 30, schedule number 80. The 112 tubes required are each 1.83 m long and 38.1 mm (1.5 in.) o.d. (BWG 12). The tubes must be fabricated from stainless steel type 250 for reasons of temperature tolerance. 

Other combinations of shell-side passes and tube-side passes are used, but the 1-2 and the 2-4 types are the most common. 

In Fig. 4.9-2b a 1-2 parallel-counterflow exchanger is shown. The liquid on the tube side flows in two passes as shown and the shell-side liquid flows in one pass. In the first pass of the tube side the cold fluid is flowing counterflow to the hot shell-side fluid, and in the second pass of the tube side the cold fluid flows in parallel (cocurrent) with the hot fluid. Another type of exchanger has 2 shell-side passes and 4 tube passes. Other combinations of number of passes are also used sometimes, with the 1-2 and 2-4 types being the most common. 

Egg shell type catalysts are used in the catalytic test measurements. Thin layers of the active compounds have been fixed on nonporous steatite pellets (2-3 mm diameter) using a colloidal silica solution (DuPont Ludox AS-40) as a binder. The tests are performed in an integral flow reactor using a standard gas mixture containing 4% O2,1000 ppm NO and 1200 ppm NHj in nitrogen. The gas stream passes over 5 g catalyst with 3 wt% of the active compound. The space velocity, calculated with respect to the total catalyst volume including the inert carrier, is maintained at 29000 h. ... 

The shell-and-tube heat exchanger is probably the most common type of exchanger used in the chemical and process industries. The simplest type of such device is the 1-1 design (1 shell pass, 1 tube pass), as illustrated in Fig. 7.7a. Of all shell-and-tube types, this comes closest to pure countercurrent flow and is designed using the basic coimtercurrent equation ... 

The Stainicaibon process is described in Figures 3—7. The synthesis section of the plant consists of the reactor, stripper, high pressure carbamate condenser, and a high pressure reactor off-gas scmbber. In order to obtain a maximum urea yield pet pass through the reactor, a pressure of 14 MPa (140 bar) and a 2.95/1 NH —CO2 molar ratio is maintained. The reactor effluent is distributed over the stripper tubes (falling-film type shell and tube exchanger) and contacted by the CO2, countercurrendy. This causes the partial NH pressure to decrease and the carbamate to decompose. 

Typical Examples (A) Split-ring floating-heat exchanger with removahle channel and cover, single-pass shell, 591-mm (23V4-in) inside diameter with tubes 4.9 m (16 ft) long. SIZE 23-192 TYPE AES. 

Tube-Side Passes Most exchangers have an even number of tube-side passes. The fixed-tube-sheet exchanger (which has no shell cover) usually has a return cover without any flow nozzles as shown in Fig. 11-35M Types L and N are also used. All removable-bundle designs (except for the U tube) have a floating-head cover directing the flow of tube-side fluid at the floating tube eet. 

Moving-Bed Type This concept uses a single-pass tube bundle in a vertical shell with the dividea solids flowing by gravity in the tubes. It is little used for sohds. A major difficulty in divided-sohds apphcations is the problem of charging and discharging with uniformity. A second is poor heat-transfer rates. Because of these hmita-tions, this tube-bundle type is not the workhorse for solids that it is for liquid and gas-phase heat exchange. 

Double-Pipe Scrapea-Surface Crystallizer This type of equipment consists of a double-pipe heat exchanger with an internal agitator fitted with spring-loaded scrapers that wipe the wall of the inner pipe. The cooling hquid passes between the pipes, this annulus being dimensioned to permit reasonable shell-side velocities. The scrapers prevent the buildup of solids and maintain a good film coefficient of heat transfer. The equipment can be operated in a continuous or in a recirculating batch manner. 

The following are several examples TEMA K-type shells, whieh allow for proper liquid disengagement for reboilers TEMA J-type shells, whieh aecommodate high vapor flows by allowing for divided flow in the shellside Two-pass TEMA F-type shells, whieh ean be used for applieations when a temperature eross exists (below) TEMA D-type front head designs, whieh are often the answer for high-pressure tubeside applieations. 

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