Definition of Heat Exchanger

Candela Rocío Barbisan
Chemical engineer

Container under pressure in which an exchange of energy between two fluids occurs, that is, a transfer of heat from a zone of high temperature to a zone of low temperature. The process can be direct, if the two media are in contact with each other, or indirect if there is another fluid that stands between them.

Fundamentals of heat transfer

For the transfer of heat from one medium to another to be effective, there must be a driving force, in this case, we call the temperature difference between the two fluids the driving force. In this sense, energy is transferred from the hotter area to the colder area.

Although it is expected that all the heat given up by the hottest phase is heat absorbed or gained by the coldest phase, the balance is not direct, there are heat losses.

The phenomenon of heat transfer is based on the Zero Principle of Thermodynamics, which explains the way in which fluids come into contact they reach their thermal equilibrium, equalizing their temperatures if the time and the area of ​​contact are adequate enough to reach the point of balance.

Typologies

When we talk about typology, we refer to the largest division that exists based on the type of contact between phases. As we well said, if the contact between the media is direct, that is, there is no other flow of heat transfer between the fluids that it is desired to exchange energy, then it responds in such a way straight. Cooling towers are a typical case of this type of exchange, since, for example, cooling water or tower water enters through the upper part and, by the lower part, against the current, an air current is induced so that there is the transfer of heat and mass expected.

On the other hand, when we refer to indirect exchangers, we are talking about equipment that has an intermediate heat transfer surface, such as a solid or even another fluent. In this case, the process occurs with convection or conduction. In most exchangers, the mass transfer mechanisms are by: convection, when the hot fluid transfers its heat to the wall internal of a tube and then this transmits it from the external wall to the other fluid in contact and, on the other hand, internal conduction in the tube wall.

They can also be classified based on the type of flow: parallel flow, if the currents travel in the same direction, countercurrent, if they exchange heat when traveling in opposite directions or, cross flow when the fluids travel through the equipment perpendicular to each other.

Likewise, there are single-pass or multi-pass exchangers, depending on how many times the fluid travels the length of the exchanger and comes into contact with the other phase.

Finally, the classification can be given based on its geometry: roughly we find: tube and shell heat exchangers and plate heat exchangers. In the first of them, it consists (as its name indicates) of a casing or casing and heads that hold the bundle of tubes inside. Through the tubes circulates a certain fluid that exchanges heat with the fluid that circulates through the casing, flooding it. The fluid can go through a single step or it can go through more than one. In this case, its construction cost is high; however, they can operate at high pressures and are designed based on the necessary contact area for heat transfer, so its size can vary according to the needs of the process. Maintenance is simple, although the tubes may be somewhat difficult to clean, this can be done chemically or mechanically and it is possible to remove the tube holder plate for clogging inspection of some of they.

In the case of plate heat exchangers, they are made up of several plates that, interspersed, circulate through each of them the different exchange fluids. Between plates there is air. As expected, the area of ​​the plates is much larger than the area of ​​a tube, so the exchange capacity is greatly increased. While tube bundles work at high pressures, plate heat exchangers are limited to pressures no greater than 25 bar. They are generally used in the food industry, where extreme hygiene care is required, since due to their structure they are easier to clean and prevent sedimentation.