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Heating Ventilating Air Conditioning Guide 1938 (L. B. McMillan. Proc. National Dili. Healing Ais'n., Vol. 18, p. 138.) Fig. 3. Chart for Determining Economical Thickness of Insulation 736 Chapter 39. Piping and Duct Insulation When water must remain stationary longer than the times designated in Table 17, the only safe way to insure against freezing is to install a steam or hot water line or to place an electric resistance heater along the side of the exposed water line. The heating system and the water line are then insulated so that the heat losses from the heating system are not excessive, and the heating effect is concentrated against the water pipe where it is needed. For this form of protection 2 in. of an efficient insu lation may be applied. ECONOMICAL THICKNESS OF PIPE INSULATION The thicknesses of insulation which ordinarily are used for various temperature conditions are given in Table 18. Where a thorough analysis of economic thickness is desired this may be accomplished through the the use of the chart, Fig. 3. The dotted line on the chart illustrates its use in solving a typical example. In using the chart, start with the scale at the left bottom margin representing the given number of hours of operation per year; then proceed vertically to the line representing the given value of heat; thence horizontally to the right, to the line representing the given tem perature difference; thence vertically to the line representing the con ductivity of the given material; thence horizontally, to the left, to the line representing the given discount on that material; thence vertically to the curve representing the required per cent return on the investment; thence horizontally to the right, to the curve representing the given pipe size; thence vertically to the scale at the top right margin where the economical thickness may be read off directly. UNDERGROUND PIPE INSULATION Underground steam distribution lines are carried in protective struc tures of various types, sizes and shapes. (See Chapter 42). Detailed data on commonly used forms of tunnels and conduit systems have been published by the National District Heating Association2. Pipes ' ih tunnels are covered with sectional insulation to provide maximum thermal efficiency and are also finished with good mechanical protection in the form of metal or waterproofingvmenibrane outer jackets. Conduit systems are in more general use than tunnels. Pipes carried in conduits may be insulated with sectional insulation; however, the more usual practice is to fill the entire section of the conduit around the pipes with high quality, loose insulating material. The insulation must be kept dry at all times', and for this purpose effective waterproofing mem branes enclose the insulation. A drainage system is also provided to divert water which may tend to enter the conduit. The economical thickness of insulation for underground work is difficult of accurate determination due to the many variables which have to be considered. As a result of theories3 previously developed, together 'Handbook of the National District Heating Association, Second Edition, 1932. Theory of Heat Losses from Pipes Buried in the Ground, by J. R. Allen (A. S. H. V. E. Transactions Vol. 26. 1920. p. 335). 737