Document 4QV4XEQn3DE0pkJqBR0Og659V

American Society of Heating and Ventilating Engineers Guide, 1937 Chapter 36--Insulation of Piping emphasized that the flow rates and periods of time designated apply only for the conditions stated. To estimate for other service conditions the following.method of procedure may.be:used. insulated so that the heat losses from the heating system are not exces sive, and the heating effect is concentrated against the water pipe where is needed. For this form of protection 2 in. of an efficient insulation If Water enters the pipe at 52 F instead of 42 F, the time required to may be applied. cool it to the freezing point will be prolonged to twice that given in the table, or the rate of flow of . water may be reduced so that the quantity required will be one-half that shown in the last column of Table 12. However, if the water enters the pipe at 34 F it will be cooled to 32 F in orie-fifth of the time given in the. table. It will then be necessary to in crease the rate of .flow so that five times'the Specified quantity of water will have to be Supplied' in order to prevent freezing.' ' If: the minimum air temperature is -- 38 F (temperature difference, it 1$ \ 80 F), instead of --18 F, the time required to cool the water to the Table 12. Data for Estimating Requirements to Prevent Freezing of Water in Pipes NoPmipineal (InScohbes) Numbas or Hours .WtoateCrootlo Freezing Point Water Required to Flow , PotuondPsrpeevreLntineFarreeFzoinogt ,of-- Pipe per Hour ..., H l 2 .3 . 4 5 6 8 10 12 ' 1 0.42 0.83 1.40 1.94 3.25 . 4.55 5.92 7.35 10.05 13.00 15.80 2* 0.50 1.02 1.74 2.48 4.27 6.02 7.96 9.88 13.90 18.10 . 22.20 Thickness of Insulation in Inches 3I 0.S7 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 ' 0.40 0.48 0.58 0.64 0.79 0.93 1.06 1.19 1.44 1.70 1.93 freezing point will be 60/80 of the time given in the taffle, or the necessary quantity of water to be supplied will be 80/60 of that given. In making calculations to arrive at the values given in Table 12, the loss of heat stored in the insulation, the effect of a varying temperature dif ference 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 12, the only safe way to insure against freezing is to install a steam dr 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 680 Fig. 2. Thickness of Pipe Insulation to Prevent Sweating3 Solve problems by drawing lines as indicated by dotted line, entering chart at lower left band scale. 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 681