Heat Loss from a Horizontal Tube. A horizontal tube carrying hot water has a surface temperature of 355.4 K and an outside diameter of 25.4 mm. The tube is exposed to room air at 294.3 K. What is the natural convection heat loss for a 1-m length of pipe? Natural Convection Cooling of an Orange. An orange 102 mm in diameter having a surface temperature of 21.1C is placed on an open shelf in a refrigerator held at 4.4C. Calculate the heat loss by natural convection, neglecting radiation. As an approximation, the simplified equation for vertical planes can be used with L replaced by the radius of the sphere (M1). For a more accurate correlation, see (S2). Natural Convection in an Enclosed Horizontal Space. Repeat Example 15.5-3, but for the case where the two plates are horizontal and the bottom plate is hotter than the upper plate. Compare the results.