Classification based on construction

Fixed tubesheet.

A fixed-tubesheetheat exchanger (Figure 2) has straight tubes that are secured at both ends to tubesheets welded to the shell. The construction may have removable channel covers (e.g., AEL), bonnet-type channel covers (e.g., BEM), or integral tubesheets(e.g., NEN).

U-tube.

As the name implies, the tubes of a U-tube heat exchanger (Figure ) are bent in the shape of a U. There is only one tube sheet in a U-tube heat exchanger. However, the lower cost for the single tubesheet is offset by the additional costs incurred for the bending of the tubes and the somewhat larger shell diameter (due to the minimum U-bend radius), making the cost of a U-tube heat exchanger comparable to that of a fixed tube sheet exchanger. The advantage of a U-tube heat exchanger is that because one end is free, the bundle can expand or contract in response to stress differentials. In addition, the outsides of the tubes can be cleaned, as the tube bundle can be removed.The disadvantage of the U-tube construction is that the insides of the tubes cannot be cleaned effectively, since the U-bends would require flexible-end drill shafts for cleaning. Thus, U-tube heat exchangers should not be used for services with a dirty fluid inside tubes.

 

Floating head:

The floating-head heat exchanger is the most versatile type of STHE, and also the costliest. In this design, one tubesheet is fixed relative to the shell, and the other is free to “float” within the shell.  This permits free expansion of the tube bundle, as well as cleaning of both the insides and outsides of the tubes. Thus, floating-head SHTEs can be used for services where both the shellside and the tubeside fluids are dirty — making this the standard construction type used in dirty services, such as in petroleum refineries. There are various types of floating-head construction. The two most common are the pull-through with backing device (TEMA S) and pull through (TEMA T) designs.

End Channels and Bonnets:

End channels or bonnets are typically fabricated or cast and control the flow of the tubeside fluid in the tube circuit. They are attached to the tube sheets by bolting with a gasket between the two metal surfaces. In some cases, effective sealing can be obtained by installing an O-ring in a machined groove in the tube sheet. The head may have pass ribs that dictate if the tube fluid makes one or more passes through the tube bundle sections. Front and rear head pass ribs and gaskets are matched to provide effective fluid velocities by forcing the flow through various numbers of tubes at a time. Generally, passes are designed to provide roughly equal tube-number access and to assure even fluid velocity and pressure drop throughout the bundle. Even fluid velocities also affect the film coefficients and heat transfer rate so that accurate prediction of performance can be readily made. Designs for up to six tube passes are common. Pass ribs for cast heads are integrally cast and then machined flat. Pass ribs for fabricated heads are welded into place. The tube sheets and tube layout in multipass heat exchangers must have provision for the pass ribs. This requires either removing tubes to allow a low cost straight pass rib, or machining the pass rib with curves around the tubes, which is more costly to manufacture. Where a full bundle tube count is required to satisfy the thermal requirements, this machined pass rib approach may prevent having to consider the next larger shell diameter.