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HEATING VENTILATING AIR CONDITIONING GUIDE 1943 Fig. 6; Chart for Determining Economical Thickness of Pipe Insolation 788 CHAPTER 43. PIPE AND DUCT HEAT LOSSES UNDERGROUND PIPE INSULATION Underground steam distribution lines are carried in protective struc tures of various types, sizes and shapes. (See Chapter 17.) Detailed data on commonly used forms of tunnels and conduit systems have been published by the National District Heating Association!. Pipes in 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 waterproofing membrane 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 to determine accurately due to the many variables which have to be Table 13. Thickness of Loose Insolation for Use as Fill in Underground Conduit Systems Steam Pressures (Lb Gags) ob Conditions Steam Temperatures Degress Fahrenheit Minimum Thickness op Insulation in Inches Steam Lines Return Line3 Pipes Less Pipes 4 In. Pipes Larger Pipes Less Pipes 4 In. than 4 In. to 10 In. than 12 In. than 4 In. ami Larger Minimum Distance Between AND Return Hot Water, or 0 to 25 212 to 267. m 2 2)4 m m i 25 to 125 267 to 352 2 2)4 3 1)4 1)4 1)4 Above 125, or superheat 352 to 500 2)4 3 3)4 1)4 1)4 considered. As a result of theories3 previously developed, together with other experimental data which have been presented, the usual endeavor is to secure-not less than 90 per cent efficiency for underground piping. Table 13 can be used as a guide in arriving at the minimum thickness of loose insulation fills to use for laying out conduit systems. Other factors such as the number of pipes and their combination of sizes, as well as the standard conduit sizes, are primary controlling factors in the amount and thickness of insulation for use. When sectional insulation is applied to lines in tunnels or conduits, usual practice is to apply the most efficient materials J4 in. less in thick ness than that determined by the use of Fig. 6. The data in Fig. 6 are based on conditions of insulation exposed to the air, whereas normal ground temperature is substituted for air temperature in determining the tem perature difference for use with the chart when applying it for under ground pipe line estimates. Handbook of the National-District Hcalitlg Association, Second Edition, 1932. Theory of Heat Losses from Pipes Buned in the Ground, by J. R. Allen (A.S.H.V.E. Transactions. VoL 26/1920. p. 335).