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656 CHAPTER 28 1955 Guide manufacturers to suit their products, and others by applicators and users. Unless time-proven methods are known, specifications of insulation man ufacturers should be obtained and followed carefully. Piping must be carefully protected against corrosion caused by condensa tion of water vapor. All metal!IG surfaces should be coated with a vapor- Fig. 5. Thickness of Pipe Insolation to Prevent Condensation on Outer Surface* impervious barrier (some types , of which have an asphaltic or tar base) without any breaks or openings in order to prevent corrosion. 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 psychrometrie chart. The external vapor barrier must be made as nearly perfect as possible in order to prevent leakage of vapor into the insulation. The approximate required thickness of insulation to prevent condensa pipe and Industrial Insulation 657 tion 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 the next thinner, commercial insulation in cases where an intermediate thickness is indicated. Heat gains for pipes insulated with a material having an installed con ductivity of 0.30 Btu per (sq ft) (hr) (F deg per in.) are given in Table 8. 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. Table 8. Heat Gains for Insulated Cold Pipes Rales of heat transmission given in Btu per (hour) (Fahrenheit degree temperature difference between fluid in pipe and surrounding still air) Based on materials having conductivity, k - 0.S0 Nominal Pips Size (Inches) Ice Water Thickness Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface Thickness of . Insulation (Inches) Btu Per Linear Foot Btu Per So Ft Pipe Surface Heavy Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface X 1 1J< 1M 2 2J5. 3 SH 4 6 6 8 10 12 1.5 1.6 1.6 1.6 1.5 1.5 - -1.5 - 1.5 1.5 1.7 1.7 1.7 1.9 1.9 1.9 0.110 0.119 0.139 0.155 0.174 0.200 -0.228 0.269 0.295 0.294 0.349 0.404 0.455 0.559 0.648 0.502 0.431 0.403 0.357 0.351 0.322 - 0.303 0.293 0.282 0.248 0.239 0.233 0.201 0.198 0.194 2.0 0.098 2.0 0.111 2.0 0.124 2.4 0.131 2.5 0.134 2.5 0.151 ------2.6------- 0.170 2.7 0.186 2.9 0.191 2.9 0.209 3.0 s 0.241 3.0 0.259 3.0 0.318 3.0 0.383 3.0 0.438 0.446 - 0.405 0.352 0.300 0.270 0.244 0.226 0.202 0.183 0.176 0.165 0.150 0.140 0.135 0.131 2.8 2.9 3.0 3.1 3.2 3.3 3.3 " 3.4 3.5 3.7 3.9 4.0 4.0 4.0 4.0 0.087. 0.394 0.094 0.340 0.104 0.294 0.113 0.260 0.118 0.238 0.134 0.214 0.147" ' 0;i97 0.162 0.176 0.176 0.167 0.182 0.154 0.202 0.138 0.228 0.130 0.263 0.116 0.309 0.110 0.364 0.108 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 insula1Qn necessary to take care of still water in pipes at various water and surrounding air temperature conditions. . Because of the damage and rvice interruptions which may result from frozen water in pipes, it is sential that an efficient insulation be utilized. This table is based on e Use I a material having a conductivity of 0.30. The initial water tern-