The design of a heat exchanger relies on the understanding of heat transfer processes, such as:
- Forced convection: heat transfer between a surface and a moving fluid, driven by mechanical devices such as pumps or fans.
- Conduction: heat transfer through the solid materials of the exchanger, without movement of matter, based on thermal agitation.
- Thermal radiation: generally considered minor in heat exchangers.
Note: the information given here is simplified for a better understanding. In practice, the calculation of a heat exchanger may involve more complex parameters and advanced formulas.
Key steps for sizing a heat exchanger
1. Technology selection
Choosing the right technology for a heat exchanger means considering:
- The thermal goals (expected temperatures, exchanger efficiency…).
- The nature of the fluids involved.
- The specific requirements of the customer.
- Space and maintenance constraints.
These factors determine the type of exchanger (plate, shell and tube, Platular®, etc.) and the materials to be used.
The Heat Exchanger Advice Tool makes this first selection for you, based on 10 criteria.
2. Thermal sizing
Once the technology has been chosen, the thermal sizing is carried out:
2.a. Validation of the thermal programme
The thermal specifications are validated using formulas defining the duty, the area and the geometry of the exchanger.
2.b. Heat exchange area calculation
The required area is estimated from the log mean temperature difference (LMTD) and the total exchanged duty, with the formula:
Total exchanged duty P = K × S × ΔTLM
Where K is the heat transfer coefficient and S is the area in m².
2.c. Heat transfer coefficient (K)
The coefficient K takes fouling into account and is calculated from the fluid properties and the operating conditions.
3. Pressure drop calculation
The energy losses due to friction and obstacles along the fluid path are assessed to ensure proper circulation through the exchanger.
The thermal sizing and pressure drop calculation steps are interdependent and iterative.
Heat exchanger yield and efficiency
It is essential to distinguish the yield of an exchanger from its efficiency. The efficiency is ideally 1, meaning no heat loss. The yield is determined by the application requirements and is often specified by the customer. The goal is to reach the best compromise between temperature needs and energy consumption.