Pipe insulation thermal conductivity under dry and wet condensing conditions with moisture ingress: A critical review

Condensate that appears on mechanical pipe insulation systems might deteriorate the insulation thermal performance and lead to failure of the pipelines. An optimized solution that accounts for cost and system energy efficiency must consider the rate of moisture absorption at various operating conditions, and how the pipe insulation thermal conductivity varies with moisture content. This article reviews the most up-to-date work available in the public domain and observes that a controversy may exist about the similarities and differences of thermal conductivity of pipe insulation systems and flat slab configurations. Since the dissimilar behavior can be associated with the testing methodology from which the thermal conductivity values are originally derived, this article first discusses the methodologies for measuring thermal conductivity of pipe insulation systems with the intention of providing some clarification about such controversy. Steady-state and transient methods are discussed, and the measurements from these two methods are critically compared. The thermal conductivities of several pipe insulation systems are also summarized under dry operating conditions. For wet insulation, four main methods for preparing the wet samples during laboratory measurements have been identified, and it was observed that they yielded very different results. The advantages and shortcomings of each moisturizing strategy discussed at length, and the thermal conductivities of a few available pipe insulation systems in wet conditions are compared. To date, challenges still exist with the measurement of actual thermal conductivity of pipe insulation systems with moisture ingress, and future research needs in this area are discussed.