Controlling
Shell and Tube Exchangers

All above Shell and Tube Exchangers simply explained by Walter Diredger


Introduction
Shell and tube heat exchangers are among the more confusing pieces of equipment for the process control engineer. The principle of operation is simple enough: Two fluids of different temperatures are brought into close contact but are prevented from mixing by a physical barrier. The temperature of the two fluids will tend to equalize. By arranging counter-current flow it is possible for the temperature at the outlet of each fluid to approach the temperature at the inlet of the other. The heat contents are simply exchanged from one fluid to the other and vice versa. No energy is added or removed. Since the heat demands of the process are not constant, and the heat content of the two fluids is not constant either, the heat exchanger must be designed for the worst case and must be controlled to make it operate at the particular rate required by the process at every moment in time. The heat exchanger itself is not constant. Its characteristic changes with time. The most common change is a reduction in the heat transfer rate due to fouling of the surfaces. Exchangers are initially oversized to allow for the fouling which gradually builds up during use until the exchanger is no longer capable of performing its duty. Once it has been cleaned, it is again oversized.

Where Do We Measure?
At the fundamental level, there is only one variable that can be controlled - the amount of heat being exchanged. In practical situations it is not possible to measure heat flux. It is always the temperature of one fluid or the other which is being measured and controlled.
It is not possible to control both since the heat added from one is taken from the other. Therefore, the first consideration is to specify the place at which the temperature is to be kept constant. This is usually within a piece of equipment somewhere downstream of the outlet of one of the fluids. Assuming there is not much temperature change along the piping, the measurement may be anywhere between the outlet itself and the point of interest, perhaps at the base of a distillation tower. In cases where the measurement is being made downstream of a bypass valve, the further downstream, the better the mixing will be, and the more representative the measurement. On the other hand, too far down-stream may result in process dead time that can make control difficult. In cases where the “other” fluid is the one being manipulated, it is often quite sufficient to make the measurement directly downstream of the outlet nozzle of the exchanger.

Which Stream Do We Manipulate?
The second consideration is which stream to manipulate. The complications arise from the fact that exchangers have four ports and involve two different fluids, either of which may change phase. The former feature alone allows eight different valve arrangements.
Figure 1 allows the reader to figure them all out. The diagram assumes that it is the fluid on the shell side whose temperature is being controlled. As likely as not, it is the one on the tube side. It does not really make any difference to the control strategy. The real issue is which fluid is to be manipulated by the valves. For the sake of discussion we will term the two streams the “process” side and the“heat exchange medium” side. A complete tabulation of all the possibilities is:
Process side, outlet throttling
Process side, inlet throttling
Process side, bypass with outlet restriction
Process side, bypass with inlet restriction
Medium side, outlet throttling
Medium side, inlet throttling
Medium side, bypass with outlet restriction
Medium side, bypass with inlet restriction

cont....

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