Heat Exchangers are used for transferring heat between two or more fluids and depending on the type of machine, the heat transfer could be between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact.
This process involves convection in each fluid and conduction through the separating wall, which prevents the fluids from mixing or coming into direct contact. Heat can flow only from hotter to cooler fluids, as per the second law of thermodynamics. Essentially this is what facilitates the heat exchange.
This article focuses on heat exchange design characteristics and the common applications for different types of heat exchangers available.
Heat Exchanger Design Characteristics
While all heat exchangers have the same basic fundamental principles, they are categorized differently based on three types of design features and they are:
- Flow configuration or arrangement
- Construction method
- Heat transfer mechanism
- Cocurrent flow is a type of heat exchanger arrangement designed in a way that fluids flow in the same parallel direction. The concurrent configuration is considered not as effective as their counterpart, particularly the counter flow configuration heat exchangers, but it ensures uniform thermal heat transfer across the walls.
- Countercurrent flow configuration, allows the fluids to move in parallel but opposite direction to each other inside the device. It’s a widely used type the flow configuration because of it’s high efficiency in transferring a huge amount of heat between fluids, causing a considerable temperature change.
- Crossflow configuration heat exchangers are designed such that the fluids fo flow in a perpendicular motion to one another. It’s relatively efficient than cocurrent heat exchangers but not as good as countercurrent flow ones.
- Hybrid flow heat exchangers features the design characteristics of the the above mentioned flow arrangements. A hybrid flow device may have multiple flow configurations such as counter flow and crossflow and are primarily designed to overcome shortcomings commonly experienced in conventional flow configurations.
- Recuperative heat exchangers, where fluids move simultaneously through their own channels to and Regenerative heat exchangers that allows both warm and cool fluid to flow through the same channel within the device.
- Direct and indirect contact transfer are the two types of processes employed to transfer heat between fluids. In direct contact transfer process, the heat is transferred from one fluid to another within a heat exchanger. In indirect contact transfer process, on the other hand, there are thermal conductive components that separate the fluids.
- Static heat exchangers have fixed components that don’t move during the heat transfer process while dynamic heat exchangers feature movable components as the fluid passes within the device.
- Types of materials used in construction
When it comes to heat exchangers, there are quite a number of materials and components employed in their construction. Depending on the type of heat exchanger and its requirements, the construction materials used could be different and some examples of these components and materials include tubes, plates, fins, shells, coils, composites, ceramics, plastics and more.
Flow Configuration or Arrangement
The flow configuration of a heat exchanger is basically the direction in which the fluids flow within the device in relation to each other.
The essentials flow arrangements commonly used in heat exchangers are:
Another common design feature that categorizes heat exchangers is in their construction method. With this design characteristic, heat exchangers are classified according to:
Heat Transfer Mechanism
Single-phase and two-phase are the two types of heat transfers employed in heat exchangers. The single-phase devices allow the fluids — warm and cooler fluids, to flow within the exchanger in a single phase without changing state of the matter.
The other type, two-phase heat exchangers, allows the fluids to undergo a phase change from liquid to gas or the inverse. This type of heat exchangers are more complex in design than single-phase devices. Examples of two-phase heat exchangers include steam condensers and waste heat boilers.
We’ve seen how heat exchangers are classified — based on flow configuration, construction materials, and heat transfer mechanism, and these features are essential to help you with ratings and sizing calculations.