Document bBkdG13RKEOJRmk3QKNNkkxRk

,588 CHAPTER 28 1949 Guide cost $0,804 per thousand Mb. Therefore, the monetary value of the saving = 0.804 (dollars) X 148.1 (thousand Mb) = $119.07, or 81.5 per cent of the cost when using ' ' uninsulated pipe. LOW TEMPERATURE PIPE INSULATION Surfaces maintained at temperatures lower than the surrounding air. are insulated to reduce the flow of heat and to prevent condensation. The insulating material should absorb a minimum amount of moisture,because the absorption of moisture substantially increases the con ductivity of the material. This property is particularly important in the insulation of surfaces that are below the dew-point of the surrounding air. In such cases, due to vapor pressure difference, it is necessary to seal the surface of the insulating material against the penetration of water vapor [ which would condense within the material, causing 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 absorp tion might pick up moisture before application and then, when the seal is : in place and the temperature of the insulated surface reduced, release that Pipe: Insulation 589 moisture to the,cold surface. There are a number of methods of pro ducing vapor seals, spine of which have been worked out by . insulation 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 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 Table 8. Heat Gains fob Insulated Cold Pipes ` ' Rates of heat transmission given in Btu per {hour) (Fahrenheit degree temperature difference between fluid in pipe and surrounding still air) Based bn materials having conductivity, k ,= .0M Nominal Pipe Size (Inches) HX 1 IX 1)2 2 2K 3 3X 4 5 6. 8' 10 12 . Ice Water Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface 1.5 1.6 1.6 1.6 1.5 1.5 . 1.5 1.5 1.5. 1.7 r.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 Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface 2.0 2.0 2.0 2.4 2.5 2.5 2.6 2.7 2.9 2.9 3.0 3.0 3.0 3.0 3.0 0.098 0.111 0.124 0.13t 0.134 0.151 0.170 0.186 0.191 0.209 0.241 0.259 0.318 0.383 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 Heavy Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface 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 0.197 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 which a surface resistance of .0.606, corresponding to. a film conductance o 1.65, was used'in calculating the curves, This vAhie`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 ah intermediate thickness is indicated. Heat gains for pipes insulated with a material having. an installed conductivity 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.. .. " . - INSULATION OF PIPES TO PREVENT FREEZING i 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.