U-value: all over heat transfer coefficient
The heat transfer coefficient serves as a measure to quantify the heat transfer in three stages: from a gas or liquid to a wall, through the wall, and emerging on the other side of the wall into a liquid or gaseous substance. In the field of construction, this property is referred to as the U-value, crucial for assessing heat loss, while in process engineering, it is known as the k value, essential for calculating heat exchanger efficiency.
Heat transfer is influenced not only by the temperature difference between the media on either side of the wall but also significantly by the thermal conductivity of the wall, the material properties of the media, and the flow conditions within the media.
Forced convection dominates in process engineering, occurring in piping and equipment that convey materials using pumps and compressors. In contrast, free convection is prevalent in construction, driven by temperature-induced pressure differences in the medium.
Heat transfer coefficients provide a consolidated representation of these processes, indicating how much heat flows per square meter of wall surface with a one-Kelvin temperature difference between the media on both sides. This value is expressed in the unit [W / (m² * K)].
In construction, the aim is to minimize heat losses by seeking the lowest possible U-value. Given similar flow conditions in different structures and the consistent use of air as the flowing medium, the assessment of heat transfer primarily centers on the thermal conductivity of building materials. The Heat Flow Meter (HFM) is a specialized measuring device developed for characterizing insulating materials with low heat conduction.