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American Society of .Heating and Ventilating Engineers Guide, 1935 Fig. 3. Chart for Determining Economical Thickness of'Insulation . Chapter 36--Insulation of Piping If the conditions are such that the air may circulate through cracks and crevices in the insulation, the increases may be far greater than those eiven.' Therefore, it is essential that insulation be sealed as tightly as possible. Pipe insulation out-of-doors should be provided with a waterproof jacket, and other outdoor insulation should be thoroughly weatherproofed. ECONOMICAL THICKNESS OF PIPE INSULATION Table 12 shows the thicknesses of insulation which ordinarily are used for various tempierature conditions. Where a thorough analysis of economic thickness is desired, this may be accomplished through 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 temperature difference; thence vertically to the line representing the conductivity of Table 12. Thicknesses of Insulation Ordinarily used Indoors Steam Pressures (Lb Gage) OB CONDITIOKS 0 to 25 25 to 100 100 to 200 Low Superheat Medium Superheat High Superheat Steam Temperatures Degrees Fahrenheit 212 to 267 267 to 338 338 to 388 388 to 500 500 to 600 600 to 700 Thickness op Insulation Pipes Larger Ilian 4 In. 1 in. . 114 in. 2 in. 2M n. 3 in. 3in. Pipes 2 In. to 4 In. 1 in. 1 in. l.H in. 2 in. 2^ in. 3 in. .Pipes Min. to IM In. . 1 in. 1 in. 1 in. 1H in. 2 in. 2 in. All-piping located outdoors or exposed to weather is ordinarily insulated to a thickness M in. greater than shown in this table, and covered with a waterproof jacket. 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. The dotted line on the chart illustrates its use in solving a typical example. Underground Insulation Underground, steam distribution- lines are carried in protective struc tures of various types, sizes and shapes. (See Chapter 37.) Detailed data on commonly used forms of tunnels and conduit systems have been published by the National District Heating Association1. Pipes in tunnels are covered with sectional insulation to provide maximum thermal efficiency and are also finished with good mechanical 'Handbook of the National District;Healing Association, Second Edition. 1932. 635