Document MJXypqvME7Gvo7w4w5o7V4Qdy

American Society of Heating and Ventilating Engineers Guide, 1934 (iNCHEB)Ji i 1H 2 3 4 5 6 8 10 12 Table 11. Data for Estimating Requirements to Prevent Freezing of Water in Pipes to Cool Water to Freezing Point Water Required to Flow to Prevent Freezing Pounds per Linear Foot op Pipe per Hour 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 31 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 0.40 0.48 0.58 0.64 0.79 0.93 1.06 1.19 1.44 1.70 1.93 required will be one-half that shown in the last column of Table 11. However, if the water enters the pipe at 34 F it will be cooled to 32 F in one-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, 80 F), instead of --18 F, the time required to cool the water to the freezing point will be 60/80 of the time given in the table, 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. 11, 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 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 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 exces sive, and the heating effect is concentrated against the 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 514 Chapter 35--Heat Losses from Bare and Insulated Pipes 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 Fig. 2. Thickness of Pipe Insulation to Prevent Sweating3 Solve problems by drawing lines as indicated by dotted line, entering chart at lower left hand scale. given may be followed with no alteration on account of the 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). 515