Secondary factors in electrical heat tracing design

Electrical heat tracing has become the preferred method of heat tracing in many applications. The standard IEEE 515 heat transfer equation defines the heat losses for an infinite pipe with homogeneous insulation. In practical applications pipes freeze at valves, pipe supports, open ended vents and other anomalies. Heat tracing design must provide design rules for these anomalies and estimate the heat losses. This paper extends the design criteria for what we call secondary factors. These secondary factors become critical when temperature at these items is not maintained. In the past, vendors have provided a singular heat loss value for valves, pipe supports and open ended vents that were linearly dependent on pipe diameter. Analysis shows these features of a piping system have heat losses that are not linear with pipe diameter. Valve heat loss is a function of the area of the valve and not its pipe diameter. Extensive finite element analysis has been performed on these features, and the results are reported in this paper with estimates of the factors that may be used for estimating heat loss. These factors are explained for manual calculations and for computer aided design. It has been estimated that up to 30% of total heat loss is due to valves and supports. Proper engineering, including better thermal insulation, can significantly reduce both installed and operational costs.