Document 20xY32XDqBD4XGja77Oa1bVR

HEATING VENTILATING AIR CONDITIONING ^GUIDE 1940 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 psychrometric chart. The thickness of insulation required to prevent sweating for rectangular ducts will be slightly greater than the value obtained by solving the equation: (n> where L = thickness of insulation for flat surfaces, inches. Ti = temperature of air in room, degrees Fahrenheit. Tt = temperature of surface of insulation, degrees Fahrenheit. 7j = average temperature of cooler air in duct, degrees Fahrenheit. 4 = conductivity of insulation, Btu per hour per square foot per degree Fahrenheit per inch. ` S ~ heat loss per square foot of outer surface of insulation, Btu per hour. Jo and/j have the same values as given in Formula 4. In any practical case T\ and Tz will be known, and, for any assumed value of relative humidity (7\ -- 7s) can be obtained from a psychro metric chart. Values of q andf0 can be obtained from Tables 1, 2, 3, and 4; /i from Formula 5; and k from conductivity Table 2 of Chapter 5. The approximate thickness of insulation used to prevent condensation 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 Table 16. 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 Pipe Sob (Inches) uk 1 IK 1H 2 2K 3 3K 4 5 6 8 10 12 Ice Water Thickness Brine Thickness Heayt Brine Thickness Thickness of Insulation (Inches) 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 Btu Per Linear . Foot Btu Per Sq Ft ripe Surface 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 Thickness of Insulation (Inches) 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 Btu Per linear Foot Btu Per Sq Ft Pipe Surface 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 Thickness of Insulation (Inches) 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 Btu Per Btu Per Sq Ft linear Pipe Foot . Surface 0.087 0.094 0.104 0.113 0.118 0.134 0.147 0.162 0.176 0.182 0.202 0.228 0.263 0.309 0.364 0.394 0.340 0.294 0.260 0.238 0.214 0.197 0.176 0.167 0:154 0.138 0.130 0.116 0.110 0.108 698 CHAPTER 40. PIPE AND DUCT INSULATION chart. This temperature drop represents the difference between the drybulb temperature and the dew-point temperature for the conditions involved. (See discussion of Condensation in Chapter 5.) 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 Fig. 5. Thickness of Pipe Insulation to Prevent Sweating51 Solve problems by drawing lines as indicated by dotted line, entering chart at lower left hand scale. 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 0.30 Btu per square foot per hour per degree Fahrenheit difference per inch thickness are given in Table 16. Heat gains for insulated ducts are given in the cold air column of Table 15. The heat gains are based on a uniform series of conductivities at 86 F mean temperature and an air temperature of 90 F outside of the duct. The gains may be interpolated for odd material conductivities and 699