Working Principle of Finned Tube Drying Heat Exchanger

Working Principle of Finned Tube Drying Heat Exchanger

Finned tube drying heat exchanger is the core component for efficient heat transfer in coating machine, dryer and other equipments. Its working principle is based on the basic laws of heat transfer (conduction, convection, radiation) and the heat transfer process is strengthened by the optimized design of fin structure.

Take the “steam - air heat transfer” commonly used in coating machine drying as an example, the heat transfer path is divided into 4 steps, interlocking:

Heat transfer from steam to the inner wall of the base tube:

High-temperature steam flows inside the base tube, and through the exothermic condensation (steam liquefaction into water releases a large amount of latent heat), the heat is transferred to the inner wall of the base tube (convection heat transfer + phase change heat transfer).

Heat transfer inside the base tube:

Heat is transferred from the inner wall of the base tube to the outer wall of the base tube by heat conduction through the metal material (dependent on the thermal conductivity of the base tube material).

Heat transfer from the outer wall of the base tube to the fins:

Heat from the outer wall of the base tube is transferred to the fins through the close contact formed by the process of welding, expanding or winding (if the contact is not good, “contact thermal resistance” will be generated, which reduces the efficiency of heat transfer, so the process of connecting the fins to the base tube is the focus of optimization).

Heat transfer from the fins to the air:

The fin surface is in contact with the flowing cold air, and the heat is transferred to the air through convection heat transfer to raise the air temperature (which becomes the hot air for drying); at the same time, a small amount of heat is transferred through radiation (a low percentage of the heat transfer, which relies mainly on convection).

Eventually, the heated hot air flows over the surface of the coated material, absorbing the moisture or solvent in the coating and completing the drying process.

Key Factors Affecting Efficiency

Fin Design Parameters:

Shape: Flat fins are simple to process but weakly disturbed; corrugated and serrated fins enhance air turbulence and improve convection heat transfer coefficients (K-values); spiral fins are good for extending air contact paths in small spaces.

Pitch and height: the fin pitch is too small, easy to accumulate dust or condensation (especially coating environment may have dust), too large to reduce the unit length of the heat transfer area; the height of the balance between the area and the resistance of air flow (too high will lead to increased wind resistance, increase fan energy consumption).

Medium flow state:

The flow rate of hot medium (steam / hot water) inside the tube, the flow rate of air outside the tube affects the heat transfer efficiency: the higher the flow rate, the thinner the boundary layer, the stronger the convective heat transfer, but to avoid too high a flow rate leading to vibration of the equipment or a surge in energy consumption.

Material and process:

The material of fins and base tube need to match the thermal conductivity and corrosion resistance (such as coating environment with solvents, the use of stainless steel fins to prevent corrosion); fins and the base tube connection (such as high-frequency welding, the whole rolled) need to reduce the contact thermal resistance, to ensure that the heat transfer is smooth.

Temperature difference control:

The larger the temperature difference (Δt) between the heat medium and air, the stronger the heat transfer power, but need to match the coating process requirements (such as some coatings need to be low-temperature slow baking, to avoid too large a difference in temperature leading to cracking of the coating).