Divided shell heat exchanger

Figure E.l. If the real (noninteger) number of shells is calculated, the heat exchange profiles Ccm be divided in any way and the sum is always the same, (From Ahmad, Linnhoff, and Smith, Trans. ASME, J. Heat Transfer, 110 304, 1988 reproduced by permission of the American Society of Mechanical Engineers.)...

The large number of matches assumed in Eq. (E.2) is not a complication in establishing the target. This is so because the additive property shows that the total fractional number of shells is independent of how many vertical sections are used to divide a given heat exchange profile. 

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. 

The layout of the heat exchanger tubesheet determines the number of tubes of a selected size and pitch that will fit into a given diameter of shell. The number of tubes that will fit the shell varies depending upon the number of tube-side passes and even upon whether there is a shell-side pass baffle that divides the shell itself into two or more parts. 

For surface condensers, the tubes, tubesheets, and shell should be consistent with experiences in heat exchanger construction. In sea or brackish water, one of the cupronickels or aluminum brass may be a good choice for tubes. The water boxes may be vertically divided to allow half of the unit to operate while the other half is being opened for repair or inspection. 

If the heat exchange involves desuperheating as well as condensation, then the exchanger can be divided into zones with linear temperature-enthalpy profiles in each zone. Figure 15.12a illustrates desuperheating and condensation on the shell-side of a horizontal condenser. The total heat transfer area is the sum of the values for each zone ... 

Pressure drop during condensation results essentially from the vapor flow. As condensation proceeds, the vapor flowrate decreases. The equations described previously for pressure drop in shell-and-tube heat exchangers are only applicable under constant flow conditions. Again the exchanger can be divided into zones. However, in preliminary design, a reasonable estimate of the pressure drop can usually be obtained by basing the calculation on the mean of the inlet and outlet vapor flowrates. 

FIGURE 3 Sectional view of a typical fixed tubesheet shell-and-tube heat exchanger (A) tubes (B) tubesheets (C) shell (D) tube-side channel and nozzles (E) channel cover (F) pass divider plate (G) stationary rear head (bonnet type) (H) tube support plates (or baffles). 

FIGURE 6.25 Diagram of a typical fixed tubesheet heat exchanger, TEMA AEL, with two tube-side passes. (A tubes, B tubesheets, C shell, D tube-side chaimels and nozzles, E channel covers, F pass dividers, G baffles)... 

Spang et al. [22] and Xuan et al. [23] have analyzed 1-N TEMA G (split flow) and 1-N TEMA J (divided flow) shell-and-tube exchangers, respectively, with an arbitrary number of passes N, arbitrary surface area (NTU,) in each pass, and arbitrary locations of inlet and outlet shellside nozzles in the exchangers. Baclic et al. [24] have analyzed two-pass crosscounterflow heat exchanger effectiveness deterioration caused by unequal distribution of NTU between passes. 

The vessel geometries can be broadly divided into plate- and shell-type configurations. The plate-type construction used in flat covers (closures for pressure vessels and heat exchangers) resists pressure in bending, while the shell-type s membrane action operates in a fashion analogous to what happens in balloons under pressure. Generally speaking the shell-type construction is the preferred form because it requires less thickness (as can be demonstrated analytically) and therefore less material is required for its manufacture. Shell-type pressure components such as pressure vessel and... 

Y. Xuan, B. Spang, and W. Roetzel, Thermal Analysis of Shell and Tube Exchangers with Divided-Flow Pattern, Int. J. Heat Mass Trans., Vol. 34, pp. 853-861,1991. 

Example. Given Shell exit temperature, Tout = 100 F tube fluid temperature, t, at reversal of tubes = 71° F overall heat transfer coefficient, C/, multiplied by the surface area of the exchanger per baffle section, Ag (sq.ft.) or UAg = 2,000 shell flow rate, W, (lb./hr.) multiplied by the specific heat of the sheU fluid, C, (Btu/lb. °F) or WC = 10,000. Ratio of stream heat capacities, JR, equals the tube fluid flow rate, w (Ibs./hr.) times the specific heat of the tube fluid, c, (Btu/lb. °F) divided by WC equals one, or i = wcjWC = 1 magnitude of the by-pass stream expressed as a fraction of total flow, I = 0.6. 

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