Document O1ZEMV9r90nbqMzYpDLx25Vzp

718 CHAPTER 27 1958 Guide X third inches would be averaged and the pipe size for list price would be that nearest the average pipe size of the second and third inch. A rapid method for determining the economical thickness of insulation by use of tables has been published.* * Solve problems as indicated by dotted line, entering chart at lower left hand scale Fig. 7. Thickness op Pipe Insulation to Prevent Condensation on Outer Surface* LOW TEMPERATURE PIPE INSULATION Surfaces maintained at temperatures lower than the surrounding sh 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 conduc tivity 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 vapor Pipe and. Industrial Insulation 719 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 pro ducing vapor seals, some 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 manu facturers should be obtained and followed carefully. Equipment must be carefully protected against corrosion caused by con densation of water vapor. All metallic surfaces should be coated with a vapor-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 Table 8. Heat Gains fob Insulated Cold Pipes Rates of heat transmission given in Bin -per (hour) (Fahrenheit degree temperature difference between fluid in pipe and surrounding still air) Based on materials having conductivity, k = 0.S0 Nominal Pipe Size (Inches) u w 1 iH in 2 3 3X 4 5 6 8 10 12 Ice Water Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Bta Per Sq Ft Pipe Surface 1.5 0.110 0.502 1.6 0.119 0.431 1.6 0.139 0.403 1.6 0.155 0.357 1.5 0.174 0.351 1.5 0.200 0.322 1.5 0.228 0.303 1.5 0.269 0.293 1.5 0.295 0.282 1.7 0.294 0.248 1.7 0.349 0.239 1.7 0.404 0.233 1.9 0.455 0.201 1.9 0.559 0.198 1.9 0.648 0.194 Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface 2.0 0.098 0.446 2.0 0.111 0.405 2.0 0.124 0.352 2.4 0.131 0.300 2.5 0.134 0.270 2.5 0.151 0.244 2.6 0.170 0.226 2.7 0.186 0.202 2.9 0.191 0.183 2.9 0.209 0.176 3.0 0.241 0.165 3.0 0.259 0.150 3.0 0.318 0.140 3.0 0.383 0.135 3.0 0.438 0.131 Heavy Brine Thickness Thickness of Insulation (Inches) Btu Per Linear Foot Btu Per Sq Ft Pipe Surface 2.8 0.087. 0.394 2.9 0.094 0.340 3.0 0.104 0.294 3.1 0.113 0.260 3.2 0.118 0.238 3.3 0.134 0.214 3.3 0.147 0.197 3.4 0.162 0.176 3.5 0.176 0.167 3.7 0.182 0.154 3.9 0.202 0.138 4.0 0.228 0.130 4.0 0.263 0.116 4.0 0.309 0.110 4.0 0.364 0.108 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 external vapor barrier must be made as nearly perfect as possible in order to prevent migration of vapor into the insulation. The approximate required thickness of insulation to prevent condensa- "?? i?n P^P68 and flat metallic surfaces may be obtained from Fig. 7 in lfi'; a sur^ace resistance of 0.606, corresponding to a film conductance of fact' W- use<* calculating the curves. This value provides a slight actor of safety and its use is known to give satisfactory field results, th USln ^e chart it is advisable to specify the next thicker, rather than e next thinner, commercial insulation in cases where an intermediate thickness is indicated. Hn?f-aVa*ns ^0r P'Pes insulated with a material having an installed con- This thT Btu per (sq ft) (hr) (F deg per in.) are given in Table 8. this 6 may be used for any of the commercial insulations offered for Purpose since they have conductivities very near the 0.3 value used.