There are many different arrangements of the shells, tubes and baffles in heat exchangers. Figure 3-6 is a list of TEMA standard classifications for heat exchangers, which helps to describe the various options. Theseare best understood in conjunction with the example configurations given in Figures 3-7 through 3-9.
The first letter designates the front end of the heat exchanger, the second letter designates the shell type, or the middle of the heat exchanger, and the third letter designates the back, or the rear, of the heat exchanger.
An AES classification for a heat exchanger means that the heat exchanger has a channel and removable cover type front end. The cover can be unbolted to perform maintenance and the channel can be unbolted without pulling the tube sheet. The “E” designates a one-pass shell. The shell fluid comes in one end and goes out the other. The rear of the heat exchanger is an internal floating head. The head can move back and forth as the tubesheet expands and contracts.
If there were no removable cover on the front end of the exchanger, it would be designated “BBS.” 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, J, 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.
G is a split flow. The fluid comes in and goes both ways around the longitudinal baffle and then exits. H is very rare; a double split flow. J is a divided flow. K is a kettle type reboiler, which is a special type and is best explained by looking at the example “AKT” in Figure 3-9. Kettle types are common where there is a boiling liquid or where gas is liberated from shell fluid as it is heated. The weir controls the liquid, making sure the tubes are always immersed in liquid. Gas that flashes from the liquid can exit the top nozzle.
Many of the rear end types allow the head to “float” as the tube bundle expands and contracts.
Type S is a floating head type. As the tubes heat up, they expand. As they expand, the floating head moves back and forth, but the pressure seal is not at the sliding joint. The pressure seal is at the fixed shell joint in the outer head, which contains the pressure. The floating head floats free inside the pressure vessel as the tubes move. Types P and W are floating heads where the movement of the head effects the seal between either the shell-side or tube-side fluid and atmosphere.
Type U indicates the tubes are in a U-shaped bundle and no special closure is needed for the rear end, other than the shell itself. The tubes are free to expand or contract. This is by far the least expensive type of floating head.