Baffle spacing, heat exchanger

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. 

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-... 

Gentry, G. G. and W. M. Small, RODbaffle Exchanger Thermal-Hydraulic Predictive Models Over Expanded Baffle-Spacing and Reynolds No. Ranges, National Heat Transfer Gonference, Boulder, GO., Aug. 5-8, (1985). 

TWo tubular heat exchangers are available each with a 0.44 m i.d. shell fitted with 166 tubes, 19.0 mm o.d. and 15.0 mm i.d., each 5.0 m long. The tubes are arranged in two passes on 25 mm square pitch with a baffle spacing of 150 mm. There are two passes on the shell side and operation is to be countercurrent. With benzene passing through the tubes, the anticipated film coefficient on the tube side is 1000 W/m2K. 

At the first level of detail, it is not necessary to know the internal parameters for all the units, since what is desired is just the overall performance. For example, in a heat exchanger design, it suffices to know the heat duty, the total area, and the temperatures of the output streams the details such as the percentage baffle cut, tube layout, or baffle spacing can be specified later when the details of the proposed plant are better defined. It is important to realize the level of detail modeled by a commercial computer program. For example, a chemical reactor could be modeled as an equilibrium reactor, in which the input stream is brought to a new temperature and pressure and the... 

In the case where the fluid flow is parallel to the tubes, as in a shell-and-tube heat exchanger without transverse baffles, the equivalent diameter d of the shell side space is calculated as mentioned above, and h at the outside surface of tubes can be estimated by Equation 5.8a with the use of d. ... 

In the case where fluid flows in the shell side space of a shell-and-tube-type heat exchanger, with transverse baffles, in directions that are transverse, diagonal, and partly parallel to the tubes, very approximate values of the heat transfer coefficients at the tube outside surfaces can be estimated using Equation 5.12a, if is calculated as the transverse velocity across the plane, including the shell axis [1]. 

Problem Below is a cross-sectional view of a heat exchanger consisting of a multi-pass configuration with baffles and five pipes running down the middle. The pipe diameter is small compared to the combined width of the baffles ( of dimension 1 each) and the distance between them. The baffles are spaced at a distance 4d apart with the five pipes positioned as shown in the diagram. The central cooling pipe is at distance 0.51 from the heat exchanger wall. The other four pipes are positioned at a distance... 

The potential for fouling on the shell side of shell and tube heat exchangers is well known. The problem arises largely because the fluid in the shell does not flow uniformly across the tubes. As a result, it is possible that deposits will form where the velocity is low. By the use of helical baffles, it is possible to redirect the fluid flow so that dead spaces are eliminated. ... 

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