The following study shows the potential of CFD tools when proposing design alternatives in heat exchangers.
The equipment to be studied is a shell-tube exchanger, in which the shell is fully specified.
The fluids to be used are also fully specified, in this case being water (cold fluid) and oil (hot fluid). The water circulates through the casing and the oil through the interior of the tubes. The oil is at an inlet temperature of 200ºC, while the water enters the system at a temperature of 30ºC. The total inlet flow of each fluid is another of the study’s operating requirements.
The equipment must heat the water up to 55ºC. Since the available oil flow rate is imposed and the casing is fixed, the only way to increase this temperature as much as possible is to increase the heat transfer area between the two fluids. This implies planning the design of the tubes in such a way as to maximize said area, which can be done by changing their morphology, adding accessory elements or increasing the number of tubes themselves. This study focuses on the last possibility, with the main objective of finding out what number of tubes allows the water to be heated to the desired temperature.
Since a priori a direct relationship between the number of tubes and temperature is not known, three alternatives will be evaluated: the use of 18, 32 and 64 tubes. The greater the number of tubes, the smaller the passage area per tube, but the greater the heat transfer area between the tubes and the shell.
The simulation is carried out in OpenFOAM version 8 software, using the “chtMultiRegionFoam” solver. In it, three different regions have been simulated: volume of cold fluid, volume of hot and solid fluid (tubes). The casing is not taken into account in this simulation since the equipment will be assumed to be adiabatic.
The results table is shown below with the inlet and outlet temperatures of each fluid.
As can be seen, it is necessary to use 64 tubes if we want to reach the target temperature (55 ºC).
In this analysis, only the increase in the number of tubes, which already had a certain morphology, has been considered. However, the CFD would also make it possible to study the use of different types of tubes (corrugated, for example), the use of a greater or lesser number of baffles in such a way as to modify the oil flow pattern inside the casing, etc.
If you are interested in knowing how to prepare this type of simulations with heat transfer in OpenFOAM, you can visit our series of tutorials on how to do it.