Document RaQ9xJpMDJ7BbKKERb7J8MV3a

524 ' CHAPTER 28 1946 Guide . users. Unless time proven methods are known, specifications of insulation manufacturers should be obtained and followed carefully. The thickness of insulation required to prevent condensation on the' outer surface is that thickness which will raise the temperature of the outer surface of the insulation to a point slightly higher than the dew point of the surrounding vapor. The dew-point for various humidities. can be readily ascertained from a psychrometric chart. The: approximate required' thickness of insulation to prevent conden sation on pipes and flat metallic surfaces may be obtained from Fig. 5 in which a surface resistance of 0.606 corresponding to a film conductance of 1.65, was used in calculating the curves. This value provides a slight factor of safety and its use is known to give satisfactory field results. In using the chart it is advisable to specify the next thicker, rather than Table 9. Data for Estimating Requirements to Prevent Freezing of Water in Pipes with Surrounding Air at --18 F Nominal Pipe Sub (Inches) F` Number op Hours to Cool 42 Water , . to Freezing Point FWater Flow Required at 42 to Pretent Freezing, Pounds per Linear Foot or Pipe per Hour Vi i m 2 3 4 5 6 8 10 12 2 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 (Conductivity, k ** 0.30) 3 4 2 `3 0.50 1.02 1.74 2.48 4.27 6.02 7.96 9.88 13.90 18.10 22120 ' 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 0.45 0.55 0.68 0.75 - 0.94 1.11 1.29 1.46 1.78 2.12 . 2.45 4 0.40 0.48 0.58 0.64 0.79 0.93 1.06 1.19 1.43 1.70 1.93 . . the next thinner, commercial insulation in cases where an intermediate thickness is indicated. Heat gains for pipes insulated with a material having an installed conductivity of ,0.30 Btu per (square foot) (hour) (Fahrenheit degree per inch) are given in Table 8. This table may be used for any of the com mercial insulations offered for this purpose since they have conductivities very near the 0.3 value used. INSULATION OF PIPES TO PREVENT FREEZING ' If the surrounding air temperature remains sufficiently low for an ample period of time, insulation cannot prevent the freezing of still water, or of water flowing at such a velocity that the quantity of heat carried in the. water is not sufficient to take care of the heat losses which will result and cause the temperature of the water to be lowered to the freezing point. Insulation can materially prolong the time required for the water to give up its heat, and if the velocity of the water flowing in the pipe is main-, tained at a sufficiently high rate, freezing may be prevented. Table 9 may be used for making estimates of the thickness of insu lation necessary to take care of still1 water in pipes at various water and surrounding air temperature conditions. Because of the damage- and Pipe Insulation 525 service interruptions which may result from frozen water in pipes, it' is essential that an efficient insulation be utilized. ' This table is based on the use of a material having a conductivity of 0.30. The initial water tem perature is assumed to be 10 F above, and the surrounding air temperature 50 F below the freezing point of water (temperature difference, 60 F). The last column of Table 9 gives the minimum quantity of water at initial temperature of 42 F which should be supplied every hour for each linear foot of pipe, in order to prevent the temperature of the water from being lowered to the freezing point. The weights given in this column should be multiplied by the total length of the exposed pipe line expressed in feet. As an additional factor of safety, and in order to provide against temporary reductions in flow occasioned by reduced pressure, it is ad visable to double the rates of flow listed in the table. It must be empha-. sized 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. If water enters the pipe at 52 F instead of 42 F, the time required to 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 9. 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 increase 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 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. 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.