Document YrLyo8w6zqjybpeOmn6vjKd00

American Society of Heating and -Ventilating Engineers Guide, 1932 (L. B. McMillan, Proc. National Dist. Heating Ass'n., Vol. 18, p. 131.) Fig. 4. Chart for Determining Economical Thickness of Insolation 202 Chapter 12--Pipe Insulation In the case of well-insulated surfaces the increases in losses due to air velocity^ are very small as compared with increases shown for bare surfaces, because of the fact that air flowing over the surface of the insulation can. increase, only the rate of heat transfer from surface- to air, and cannot change the internal resistance to heat flow inherent in the insulation itself. The maximum increase in loss due to air velocity ranges from about 30 per'cent in the-case of 1 in. thick insulation,, to about 10 per cent ini the case of 3 in. thick insulation, provided that the insula-^ tion is thoroughly-sealed so that airman flow only over the surface. 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 given. Therefore, it is essential that insulation be sealed as tightly Table 11. Thicknesses of Insulation Ordinarily used Indoors3 Steam Pressures (Lb Gage) ob Condition 0 to 25 25 to 100 100 to 200 Low Superheat Medium Superheat High Superheat Steaju Temperatures Decrees Fahrenheit 212 to 267 267 to 338 338 to 388 388 to 500 500 to 600 600 to 700 Thickness op Insulation Pipes Larger Than 4 In. 1 in. 1}4 in. 2 in. 2'A in. 3 in. 3H in. Pipes 2 In. to 4 In. 1 in. 1 in. 1^ in. 2 in. 2^ in. 3 in. Pipes MI*. to 1M In. 1 in. 1 in. 1 in. 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. as possible. Pipe insulation out-of-doors should be provided' with a weather-proof jacket, and other outdoor insulation should be thoroughly weather-proofed. ECONOMIC THICKNESS OF PIPE INSULATION Table 11 shows the thicknesses of insulation which ordinarily are used for various temperature conditions. Where a thorough analysis of economic thickness is desired, this may be accomplished through the use of the chart, Fig. 4. 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 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. 203