The production process of milk powder consists of multiple stages. This report focuses on the falling film evaporator, which role in the production process is to evaporate the water content from the milk. A falling film evaporator is a large vertically-placed vessel whereby the inside is filled with smaller tubes. Steam enters the vessel and heats up the outside of the smaller tubes. The milk flows in a thin layer only along the inside perimeter of the smaller tubes, so these tubes are not completely filled. The advantage of this flow is that a thin layer of liquid is continuously in contact with the wall so the heating process is equal along the tube. The thin layer is called a falling film because the thickness compared to the length of the flow is very small.

The objective of this research is to determine theoretically the local overall heat transfer coefficient of the falling film and to investigate experimentally the applicability of heat flux sensors by determining the local overall heat transfer coefficient. By investigating the falling film, it is not allowed to disturb the falling film. Once a falling film is disturbed, the falling film will proceed at a different path. Heat flux sensors allow for local non-intrusive measurements. The overall heat transfer coefficient gives information about the thickness of the falling film. Theoretically the overall heat transfer coefficient is calculated by taken the thermal resistances of each component. Experimentally, the overall heat transfer coefficient is measured by the heat flux and the temperature difference between the bulk temperature of the fluid and the sensor at the outside of the tube.

Before the heat flux sensors are used in practice on the falling film evaporator, a setup has been built to experimentally determine the applicability of the heat flux sensor. This setup has been made for a tube filled with water and to create a falling film. The tube filled with water is well-described in theory and used as a reference.

The results of the experimental setup show that the Danfoss 'Koperpasta tube AT' is in good comparison with the theoretical approach. The theoretical overall heat transfer coefficient difference, caused by the mass flow difference of 0.01 kg/s in the falling film evaporator, can be detected by the heat flux sensors taking into account the error margin of the heat flux sensor and temperature sensor.