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American Society of Heating and Ventilating Enginee Guide, 1935 Nominal! Pipe Size (Inches) A l iA 2 3 4 5 6 8 10 12 Table. 11. Data for Estimating Requirements to Prevent .....Freezing of Water in Pipes * Ncmbeb op Hours Water Required to "flow to Cool Water to Freezing Point to Prevent Freezing Pounds per Linear Foot or Pipe per Hour "" ~ 0.42 0.83 1.40 1.94 3.25 4.55 5.92 7.35 10.05 13.00 15.80 Thickness of Insulation in Inches 2 3 1| 0.50 1.02 . 1.74 2.48 4.27 6.02 7.96 9.88 13.90 18.10 22.20 * 0.57 1.16 2.02------2.90 5.08 7.20 9.69 12.20 17.25 22.70 28.10 0.54 0.68 0:84 0.95 1.24 1.47 1.73 1.98 2.46 2.96 3.43 2 0.45 0.55. . 0.68 0.75 0.94 1.11 1.29 1.46 1.78 2.12 2.46 3~ 0 0 A4 0 79 0 93 1 06 1.19 1.44 1.70 1.93 been neglected. When these factors enter into the computations it is necessary to enlarge the factor of safety. Also as stated, the tftne 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 11, 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 tbe exposed water line. The heating system and the water line are then insulated so that the heat losses from the heating system are not exces sive,, and the heating effect is concentrated againstthe water pipe where it is needed. For this form of protection 2 in. of an efficient insulation may be applied. Pipe Sweating In some cases the prevention of condensation rather than the con servation of heat is the governing factor in determining the thickness of insulation required. Fig. 2 may be used for determining the thickness of any material of known conductivity which should be used to prevent con densation on pipes and flat metallic surfaces. The surface resistances used for calculating the family of curves in Fig. 2 are based on the results of tests made on canvas-covered pipe insulation surfaces at Mellon Institute. 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 632 Chapter 36--Insulation of Piping 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 Fig. 2. Thickness of Pipe Insulation to Prevent Sweating Solve problems by drawing lines as indicated by dotted line, entering chart at lower left hand scale. 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. 633