The concept and operation of a shell and tube heat exchanger are rather simple and are based on the flow and thermal contact of two liquids. The name of a shell and tube heat exchanger serves as an explanation of the process, which is the exchanging of temperature between two fluids. In a heat exchanger, a heated or hot fluid will flow around a cold fluid and transfer heat in the direction of the flow of the cold fluid.
In any situation where two pieces of material make contact, there will be an exchange or transfer of heat through a conductive surface. The process of a shell and tube heat exchanger provides a place for two fluids to exchange or transfer heat through conductive metals.
In the shell and tube heat exchanger process, one fluid flows through the tubes while the other fluid flows through the shell. In the diagram below, which is of a straight tube shell and tube heat exchanger, the shell inlet for the shell fluid to enter is at the top with the inlet for the tube fluid at the bottom right.
A shell and tube heat exchanger has two compartments or sections: the shell side and the tube side. When working with a shell and tube heat exchanger, it is important to decide on which side the hot fluid will enter and on which the cold fluid will enter; this decision is referred to as fluid allocation.
When there is a difference in pressure between the fluids, the lower pressure fluid enters through the shell inlet since the tubes are designed to handle high pressure.
Shell Side
When determining fluid flow for the shell side, it is important to know that the shell is more expensive to fabricate than the tubes, and it is more difficult to clean. The shell side has baffles that direct the flow of the fluid across the tube bundles.
Viscous fluids and fluids with a high flow rate are processed through the shell side where there is increased turbulence and an increased transfer coefficient that leads to improved heat transfer. Large temperature changes are normally performed on the shell side.
Tube Side
A necessity on the tube side is to create turbulent flow; this is achieved by installing turbulators inside the tubes through the holes in the tube sheet. Like the turbulence in the shell, the turbulence in the tubes increases the heat transfer capacity. A secondary function of the turbulators is to keep the inside of the tubes clean and unfouled. Tubes have lower turbulence and pressure drop and provide a streamlined flow.
Passes
Shell and tube heat exchangers can have one, two, four, six, or eight passes, which are written 1-1, 1-2, 1-4, and so on. The first number is the number of shells. The second number is the number of passes. The number of passes is the number of times the fluid passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes through the shell once. As the number of passes increases, the heat transfer coefficient increases.
An Operating Shell and Tube Heat Exchanger
As can be seen in the graphic, cold fluid enters through the tube or shell inlet and is heated by conductive heating in the tubes; it then exits processed. The image below is a two pass shell and tube heat exchanger.
Turbulent flow in a shell and tube heat exchanger increases the heat transfer rate and avoids fouling in the tubes and shell walls. The constant turbulent flow has a self-cleaning effect that ensures continuous and optimum performance. Baffles that direct the flow in the shell create its turbulence while turbulators installed in the tubes create its turbulation.
The heat exchange process is facilitated by thermal contact between the two fluids contained in the shell and tubes. One of the fluids leaves cooler while the other leaves warmer or hot.