Document rxvXkeX8yD5LeMRLY184vpRy7

American Society o/.Heating and Ventilating Engineers Guide, 1937 F .ig 3. Chart for Determining Economical Thickness of Insulation 682 Chapter 36--Insulation of Piping ' However, it has been found that the resistance for asphaltic and roofing surfaces is practically the same as. for canvas surfaces, so that the curves given may be followed with no alteration for surfaces commonly used. Moisture will be deposited on a surface whenever its temperature falls to that of the dew point. The maximum permissible temperature drop is indicated on Fig. 2 at the point where the guide line passes through the horizontal scale at the left center of the chart. This temperature drop represents the difference between the dry-bulb temperature and the dew point temperature for the conditions involved. . (See discussion of con densation in Chapter 7.) The rate of heat loss from a surface maintained at constant temperature is greatly increased by air circulation over the. surface.. 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 in the case of 3-in. thick insulation, provided that the insulation is thoroughly sealed so that air can 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 as possible. Pipe insulation out-of-doors should be provided with a waterproof jacket, and other outdoor insulation should be thoroughly weatherproofed. ECONOMICAL PIPE THICKNESS Table 13 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. 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 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 Insulation Underground steam distribution lines are carried in protective struc tures of various types, sizes and shapes. (See Chapter 37.) -Detailed