Material parameters
The lambda value expresses the capacity of a material to limit energy loss. The average lambda value is the parameter used to calculate the actual heat loss in pipe networks that carry gases and fluids from one location to another, and for distribution. The lambda values are calculated on the basis of a pipe section with given dimensions.
They can, however, also be used to calculate heat loss for pipe sections with different dimensions, if these sections are manufactured in the same way.
Straight pipes
The lambda value depends on the temperature. It requires measurements to be taken at at least three different temperatures. The measurements are taken on a section of DN 50/125–140 preinsulated pipe that is at least three metres long. This section is taken from a 12-metre section of pipe from the standard production line. The ambient temperature (ta) is kept constant while the measurements are performed.
The temperature on the outer casing is measured when the temperature of the carrier pipe is constant. The lambda values for the three different temperature conditions can be calculated on the basis of the temperatures of the carrier pipe, outer casing and surroundings.
The lambda value for a preinsulated district heating pipe is always stated as an average temperature measured in the polyurethane foam at a temperature of 50°C. A straight line can be traced from the calculated lambda values, as the lambda value is directly proportional to the temperature. The lambda value at 50°C can then be read off the curve for temperature-related lambda values.