Shell & Tube Evaporators-part 15

In the last article we completed our review of BPHE s. This month we move on to direct expansion water to refrigerant evaporators. These are often referred to as shell & tube evaporators or chillers.

The primary purpose of shell and tube chillers is that of heat exchange, in vessel form, to cool a closed circuit, recirculating fluid flow, using refrigerant as the cooling medium. The thermodynamic process was shown in Part 3 of this series in the P-E diagram.

Shell and tube chillers are extremely efficient. Moreover, they are very compact, requiring only a small footprint and overall height. Maintenance, an important consideration in costs terms, is also fairly straightforward.

The concept of the shell and tube chiller is based on a large number of tubes formed into what is known as a tube bundle. Refrigerant flowing from the expansion device is passed into the tubes and progressively evaporates thereby producing a cooling effect through the latent heat of vaporisation.

The tube bundle is mounted within a steel shell and end caps are fitted to both ends of the shell. Water is passed over the tubes and gives up heat energy to the surface of the tubes at lower temperature. The water therefore leaves the shell and tube chiller several degrees lower than the entering water temperature. This is known as a dry type or direct expansion evaporator. The water enters the side of the shell at one end and leaves the side of the shell at the other. Shell and tube chillers are supplied with screwed or flanged water connections. A drain connection is normally incorporated to allow the water in the shell to be removed.

A typical arrangement of a dry type or direct expansion shell and tube chiller is shown in Fig 1. It is a more common arrangement for the refrigerant inlet and outlet to be at the same end as shown in the cutaway illustration in Fig 2. These are normally used with positive displacement compressors such as reciprocating, rotary or screw machines connection tube.