How Does a Heat Exchanger Work? Understanding the Mechanism

Heat Exchangers are used to efficiently transfer thermal energy between two fluids, which can be either gas or liquid. The fluids are separated with a solid wall to avoid mixing or direct contact.

Depending on the design, the fluids can flow in various directions within the heat exchanger. Counterflow is the most efficient, however co-current and parallel flow can also be used.

Thermodynamics

According to the Second Law of Thermodynamics, heat always flows from hotter objects to colder ones. In a heat exchanger, this happens through fluid circuits passing in opposite directions. Hot water from your home’s furnace flows into the aluminum finned core of an exchanger. This transfers heat to cold water that is then pushed out through your radiator, baseboard or floor heating system.

There are many different types of heat exchangers. The three most common are concurrent, countercurrent and crossflow. In co-current flow the process and utility fluids flow in parallel directions, which is suitable if the temperature of the two streams is very close to each other.

In countercurrent flow the two fluids are separated by a shell and the tubes inside it are designed to maximize surface area for heat transfer. This arrangement is similar to how our blood vessels widen near the skin to dissipate heat and narrow further back in our bodies, helping regulate body temperature.

Conduction

Heat exchangers transfer heat between mediums by conduction and convection. Conduction occurs when the hot medium touches the cold medium and thermal energy is transferred through the contact surface. For example, when you hold a hot cup of coffee on the table for a few minutes the table will absorb some of the thermal energy and cool down. The heat is transferred through the contact of the cup with the table through conduction.

The Second Law of Thermodynamics states that systems move towards thermal equilibrium. This means that a system with a higher temperature gains heat and increases its entropy, while a system with a lower temperature loses heat and decreases its entropy.

Using this principle, heat exchangers separate the process and utility fluids by a wall that has high thermal conductivity. This separates the two streams of fluids and prevents them from mixing or coming in direct contact. This allows for cooling and heating while maintaining hygienic operations such as pasteurization, sterilization, and clean-in-place processes.

Convection

Heat transfer through convection depends on the difference in densities and viscosities of two different fluids. In industrial exchangers, the hot and cold mediums must have very different properties. For example, one of the mediums must be highly volatile and another must have low boiling and condensation points.

The hot fluid transfers its heat to the tube wall by convection, which is then conducted through the pipe walls to the other side of the heat exchanger. This heat is carried away by the cooling medium that comes in contact with it.

Shell and tube heat exchangers feature a bundle of tubes that sit inside a cylindrical shell. The tubes can be fixed or floating, depending on the design of the heat exchanger. Floating tube bundles are more flexible and can float in the fluid, which allows them to expand and contract. They can also be easily removed for cleaning and inspection. Shell and tube exchangers can be used in high and moderate pressure applications.

Radiation

A heat exchanger allows liquids or gas to transfer heat without direct contact between the two mediums. This is achieved through a transfer process that involves convection and radiation.

When a ray shoots out from an element, it essentially queries the surrounding area for any objects that are specularly reflective. Then, it selects the smallest of those objects that sees a reflection. This small object is then used to subdivide the angular space around that point, generating a set of rays that are launched from that location. The number of rays that are generated is dependent on the Radiation Resolution and Maximum number of adaptations options, with higher values producing more accurate results.

A shell and tube heat exchanger is a type of heat exchanger that consists of a series of tubes located within a sealed shell. These tubes can either be in a single phase (liquid or gas) or in multiple phases (solids, liquids, and gases). These types of heat exchangers are often found on ships and other marine vessels.