Document bBewQenBVb3K4jnJVoLdgrgRy

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 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 absorption might pick up moisture before application and then, when Fig. 5. Thickness of Pipe Insulation to Prevent Sweating. Solve problems by drawing lines as indicated by dotted line, entering chart at lower left hand scale: the seal is in place and the temperature of the insulated surface reduced, release that moisture to the cold surface. The thickness of insulation required to prevent sweating is that thickness which will raise the temperature of the outer surface of the insulation to a point slightly higher than the dew-point for the corre sponding air temperature and relative humidity. The difference in tem perature between the air and the dew-point for various humidities can be readily ascertained from a psychrometnc chart. The approximate required thickness of insulation to prevent conderi- 780 CHAPTER 43. PIPE AND DUCT HEAT LOSSES sation on pipes and flat metallic surfaces may be obtained from Fig. 5. The maximum permissible temperature drop is indicated at the point where the guide line passes through the horizontal scale at the left center of the chart. This temperature drop represents the difference between the dry-bulb temperature and the dew-point temperature for the conditions involved. (See discussion of Condensation in Chapter 4.) The surface resistances used for calculating the family of curves in Fig. 5 are based on tests made on canvas covered pipe insulation surfaces at Mellon Institute. However, it has been found that the resistance for asphaltic and roofing surfaces is practically the same as for canvas surfaces, so that the curves may be followed with no alteration for surfaces commonly used. Heat gains for pipes insulated with a material having a conductivity of Table 10. Heat. Gains for Insulated Cold Pipes Rates of heat transmission given in Btu per hour per degree Fahrenheit temperature difference between fluid in pipe and surrounding still air Based on materials having conductivity, k = 0.S0 Nominal Pips SlEB (Inches) K 1 Ye m IK 2 2K 3 3K 4 5 6 8 10 12 Ice Watch Thickness Thickness of Insulation (Inches) Btu Per linear Foot Btu 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 Bants 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 0.131 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 Heavt Bhinb Thickness Thickness of Insulation (Inches) Btu Per linear Foot Btu Per Sq Ft Pipe 8urtace 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 0.30 Btu per square foot per hour per degree Fahrenheit difference per inch thickness are given in Table 10. 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 lation necessary to take care of still water in pipes at various water and 781