When the heat loss from a pipe or tube is higher than the capacity of the heat trace cable - the cable must be spiral-wrapped around the pipe or tube.
The "heat loss - cable output" ratio can be calculated as
HR = Qhl / Qt (1)
HR = "heat loss - cable output" ratio
Qhl = heat loss from the insulated pipe or tube (W/m, Btu/h ft)
Qt = heat output from the heat trace (W/m, Btu/h ft)
If the "heat loss - cable output" ratio - HR - is larger than one, the cable must be wrapped. The pitch - the distance between the spirals - can be estimated from the diagram below.
Example - Heat Tracing
The heat loss from a 4" pipe is estimated to be three times (HR = 3) the heat output capacity of the tracing cable. The pitch - the distance between each spiral - can be estimated as indicated in the table below to 5 - 6 inches (125 - 150 mm).
Heat transfer and heat loss from buildings and technical applications - heat transfer coefficients and insulation methods to reduce energy consumption.
Heat loss (W/m) from 1/2 to 6 inches insulated pipes - ranging insulation thickness 10 to 80 mm and temperature differences 20 to 180 degC.
Heat loss (W/ft) diagrams for 1/2 to 6 inches insulated pipes - ranging insulation thickness 0.5 to 4 inches and temperature differences 50 to 350 degF.