The liquid film flow rate has an important influence on the heat transfer coefficient. The liquid film flow velocity is too low will lead to low heat transfer coefficient, beyond a certain range will cause unstable turbulent flow, making the heat transfer coefficient change uncertainly.
In general, the flow rate range of the liquid film in a shell tube falling film evaporator should be between 0.2 and 2.0 m/s. This range achieves a high heat transfer coefficient, usually between 1000-5000 W/(m²-K), and the specific heat transfer coefficient is influenced by other factors such as tube material, pressure, liquid properties and temperature.
The relationship between the flow rate of the liquid film and the heat transfer coefficient can be expressed by the Nusselt number (Nu), a dimensionless parameter used to describe the heat transfer strength in fluid heat transfer, which represents the ratio of the heat transfer rate to the heat transfer capacity inside the fluid. For a shell tube falling film evaporator, the relationship between the Nusselt number of the liquid film and the Reynolds number (Re) can be described by the Chen equation:
Nu = 0.023 Re⁰·⁸³ Pr⁰·³⁷⁸
where Pr is the Prandtl number of the fluid. the Reynolds number, on the other hand, is the ratio of the inertial to viscous forces within the fluid and can be expressed as
Re = ρvd/μ
Where, ρ is the fluid density, v is the fluid velocity, d is the thickness of the liquid dense film layer, and μ is the fluid viscosity. The relationship between the Nusselt number and Reynolds number can be obtained by the above two equations, and thus the relationship between the liquid film flow rate and the heat transfer coefficient can be deduced.
In general, the flow rate of liquid film is a very important parameter in the tube falling film evaporator, which has a direct influence on the operation efficiency and heat transfer effect of the equipment.