When any medium which has been heated to a temperature above that of the external environment is transported through pipes, there will inevitably be a loss of heat from the medium being transported, through the pipe wall and into the external environment. This heat loss is expressed in the amount of heat that will be lost in an applicable unit (W) times the length of pipe run (m). For the purposes of the calculation, the medium in the pipe is considered to be stationary. The result is a heat loss factor expressed in terms of energy lost per unit length of pipe (W/m).
The amount of heat lost to the outside environment is directly linked to the thermal characteristics of the material from which the pipe is manufactured. In general, metals exhibit high rates of thermal conductivity, whilst most plastics from which pipes are manufactured have relatively low rates of thermal conductivity. Because of this, plastic piping systems will transfer much smaller amounts of energy from the transported medium to the external environment, meaning that in plumbing terms, heated water will remain hotter when transported in plastic pipes than in metal pipes, and cold water is less likely to freeze in plastic pipes than in metal pipes when exposed to very low temperatures.
The thermal conductivity of Polyethylene is 0.4 W/m per ˚C.
Thermal expansion is the tendency for any material to either expand or contract due to changes in temperature.
All materials will exhibit this property, but some may be affected to a greater extent than others. The change in size can be calculated for a given temperature rise and expressed as a unit measure of length per degree rise in temperature.
In general, plastic materials exhibit greater thermal expansion than metals, so it is to be expected that plastic piping systems will expand more as they increase in temperature than similar metal systems. Thermal expansion should thus always be taken into account when desiging and installing plastic plumbing systems by making allowances to accommodate for the movement of pipes due to temperature changes.
The thermal expansion co-efficient of Polyethylene is 0.26 mm/˚C.