A heat exchanger is a device whose function is to transfer heat between two fluids without them mixing. These fluids can be in contact or separated by a solid barrier. For a good exchange to take place, the temperature of one of the fluids will have to be cold and the other hot.
Heat exchangers are a key element in industrial refrigeration, heating, air conditioning, energy production and chemical processing systems.
Uses of the heat exchanger
Heat exchangers are used for different uses :
- To raise the temperature of a fluid using another with a higher temperature.
- To cool a fluid using another with a lower temperature.
- To condense gases using cold fluids.
- To bring fluid to a boil through another with a higher temperature.
- To bring fluid to a boil while condensing another with a hotter one.
Types of heat exchangers according to the degree of fluid contact
Direct contact exchanger
These are exchangers where the process is done by physical mixing of the fluids. This type of exchanger is not the most widely used because it involves contamination of one or both fluids.
Indirect contact exchanger
In this case, there is no contact between the fluids and therefore they do not mix. The fluids are separated by a partition or by space and heat transfer occurs by conduction and convection through the dividing wall.
Within this type of exchangers are alternative exchangers and surface exchangers.
In this type of exchanger, the two fluids run through the same space but alternately. A hot fluid will pass through a solid surface and, later, that heat will be transmitted to the colder fluid when it passes through that same surface. In this case, the fluids can come into contact. However, it is not significant as contamination does not occur.
Surface heat exchanger
The heat transmission is related to the solid that separates both fluids, which means that there is no minimum contact between them. It is the most widely used type of exchanger.
Depending on the type of surface it can be classified into:
- Plate exchangers: The fluid is separated through a flat wall. The plate heat exchanger consists of a series of very thin corrugated metal sheets with a high transfer coefficient. One of the fluids circulates on one side of the sheet and the other will circulate on the adjacent sheet so that they never mix. This design allows for good maintenance since disassembly for cleaning is easy.
- Tube exchangers: It is the type of heat exchanger most used in chemical plants and refineries. The fluid separation will be the wall of a cylindrical tube. One of the fluids will circulate inside the tube, which will be called “ internal flow ” and the other will circulate outside, which will be called “ external flow “. If the fluids present a large pressure difference, the higher pressure will be the one that will circulate inside the tube. These devices are easy to build and easy to clean. According to the direction of the fluids, another classification could be made:
- Cross flows: The fluids circulate forming an angle of 90°, that is, in a perpendicular way. This type occurs more when the exchange occurs between a liquid and a gas.
- Parallel flows: Both fluids enter the same side of the exchanger and run in the same direction in a parallel way. There can be two cases: they flow in the same direction and the same direction, or the same direction but in the opposite direction. </ Li >