Document rkBnr3vReekBzgoDYBXpY0qG

332 __________________ __________________________Chapter 17.______________________ 1945 Guide water vapor which would corid'erise^ witKin tKe"material7causing a serious increase in heat flow, possible breakdown of the material, and corrosion of metal surfaces. An insulating material with a high degree of moisture absorption might pick up moisture before application and then, when the seal is in place and the temperature of the insulated surface reduced, release that moisture to..the cold surface. There are a number of methods of producing vapor seals, some of which have been worked out by insula tion manufacturers to suit their products and others by applicators and 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 bn 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- Table 9. Pipe Covering Factors Types of Insulating Materials Corrugated Asbestos--Type Diatomaceous Silica--Type.............. Brown Asbestos Fiber--Type... ;.... Temperature Difference, Pipe to Air, Deg F 100 200 300 400 . 500 1.30 1.19 1.15 0.96 0.98 1.37 0.86 1.36 . 1,23 1.19 0.98 1.00 1;36 0.88 1.42 1.27 1.23 1.00 1.02 1.35 0.91 1.02 1.05 1.35 0.93 1.04 . 1.07 1.34 0.96 sation on pipes and flat metallic surfaces may be obtained from Fig. 4 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 the next thinne,r, 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 (sq ft) (hr) (deg F per inch) are given in Table 10. This table may be used for any of the commercial 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 11 may be used for making estimates of the thickness of insu Pipe Insulation 333 lation necessary to take care of still water in pipes at various water and surrounding air temperature conditions. Because of the damage and seryice 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). Fig. 4. Thickness of Pipe Insulation to Prevent Condensation on Outer Surface3 . .T*1? last column of Table 11 gives the minimum quantity of water at initial temperature of 42 F which should be supplied every hour for each inear foot of pipe, in order to prevent the temperature of the water from l ere^ to the freezing point. The weights given in thus column s ould be multiplied by the total length of the exposed pipe line expressed in eet. As an additional factor of safety, and in order to provide against mporary reductions in flow occasioned by reduced pressure, it is ad*^e rates of flow listed in the table. It must be empha- ,. . a* ^ow rates and periods of time designated apply only for the " stated. To estimate for other service conditions the following method of procedure may be used.