Refrigerant is flowing while a ¾ inch O.D. copper tube under conditions such that its mixed temperature is 15 °F. Condensation of moisture from atmosphere on the outside of the tube is considered a nuisance, so it is decided to protect the tube with glass wool [k = 0.025 Btu/ (h) (ft) (°F)]. Discover minimum thickness of glass wool necessary to prevent condensation on the outer surface of insulation. Air in the room is at 70 °F but it has a dew-point temperature of 45 °F. The natural convection heat-transfer coefficient between insulation surface and the air in the room is represented by h = 0.5 (Δt)1/4, in which Δt is temperature drop in °F between the insulation surface and the air in the room, and h is in Btu/(h)(ft2)(°F). The convective coefficient between inner wall of the tube and refrigerant is sufficiently large that resistance to heat transfer in the refrigerant is negligible. The resistance of the copper tube to heat transfer is also insignificant.