Document qaeM4M97Zggdd8D8ra09Ex2kq

540 CHAPTER 28 1948 Guide CL. B. McMillan. Proc. National District Heatini Association, VoL 18. p. 138). Fig. 6. Chart for Determining Economical Thickness of Pipe Insulation Pipe Insulatien 541 In making calculations to arrive at the values given in Table 9, the loss of heat stored in the insulation, the effect of a varying temperature difference due to the cooling of pipe and water, and the resistance of the outer surface of the insulation to the transfer of heat to the air have all been neglected. When these factors enter into the computations it is necessary to enlarge the factor of safety. Also as stated, the time shown in the table is that required to lower the water to the freezing point. A longer period would be required to freeze the water but the danger point is reached when freezing starts. The flow, of water will stop and the entire line will be in danger as soon as the water freezes across the section of the pipe at any point. When water must remain stationary longer than the times designated in Table 9, 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. TAble 10. Thickness of Pipe Insulation Ordinarily Used Indoors Steam Pressure Psic or Condition 0 to 25 25 to 100 100 to 200 Low Superheat Medium Superheat High Superheat Steam Temperature Fahrenheit Degrees 212 to 267 267 to 338 338 to 388 388 to 600 600 to 600 600 to 700 Thickness of Insulation Pipes Larger Than 4 In; 1 in. IX in- 2 in. 2} in. 3 in. 3)4 in. Pipes 2 In. to 4 In. 1 in. 1 in. IX in- 2 in. 2H in. 3 in. Pipes M In. to 1H In* 1 in. 1 in. 1 in. IX in- 2 in. 2 in. aAll 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. ECONOMICAL THICKNESS OF PIPE INSULATION The thicknesses of insulation which ordinarily are used for various temperature conditions are given in Table 10. Where a.thorough analysis of economic thickness is desired this may be accomplished through the use of the chart, Fig. 6. 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 29). Detailed data