1) Problem 4.3-1 (Page 345): Insulation Needed for Food Cold Storage Room. 4.3-1 Insulation Needed for Food Cold Storage Room. A food cold storage room is to be constructed of an inner layer of 19.1mm of pine wood, a middle layer of cork board, and an outer layer of 50.8mm of concrete. The inside wall surface temperature is 17.8C and the outside surface temperature is 29.4C at the outer concrete surface. The mean conductivities are for pine, 0.151; cork, 0.0433; and concrete, 0.762W/m. K. The total inside surface area of the room to use in the calculation is approximately 39m2 (neglecting corner and end effects). What thickness of cork board is needed to keep the heat loss to 586 W? A8:Ans, 0.128m thickness 2) Problem 4.3-6 (Page 346): Heat Loss by Convection and Conduction. 4.36 Heat Loss by Convection a nd Conduction. A glass window with an aren of 0.557m2 is installed in the wooden outside wall of a room. The wall dimensions are 2.443.05m. The wood has a k of 0.1505W/m. K and is 25.4mm thick. The glass is 3.18mm thick and has a k of 0.692. The inside room temperature is 299.9K(26.7C) and the outside air temperature is 266.5 K. The convection coefficient hy on the inside wall of the glass and the wood is estimated as 8.5W/m2.K; the out side h0 is also estimated as 8.5 for both surfaces. Calculate the heat loss through the wooden wall, through the glass, and the total. A13:Ans. 569.2W (wood) (1942btu/h),77.6W (glass) (265btu/h),646.8W (total) (2207btu/h) Heat Tra nsfer in Stea m Heater. Water at an average of 70F is flowing in a 2-in. steel pipe, schedule 40 . Steam at 220F is condensing on the outside of the pipe. The convective coefficient for the water inside the pipe is h=500btu/hft2F and the condensing steam coefficient on the out side is h=1500. a. Calculate the heat loss per unit length of 1ft of pipe using resistances. b. Repeat, using the overall U1 based on the inside area A1. c. Repeat, using U0. A15:Ans.(a)q=26710btu/h(7.828kW),(b)Ui=329.1btu/hft2F(1869W/mK),(c)Uo=286.4btu/hftF(1626W/m2K) 4) Problem 4.3-10 (Page 346): Effect of Convective Coefficients on Heat Loss in Double Window. 4.310 Effect of Convective Coefficients on Heat Loss in Double Window. Repeat Problem 4.33 for heat loss in the double window. However, include a convective coefficient of h=11.35W/m2 - K on one outside surface of one side of the window and an h of 11.35 on the other outside surface. Also calculate the overall U. A17:Ans.q=106.7W,U=2.29W/m2K 5) Problem 4.3-13 (Page 347): Temperature Rise in Heating Wire. 4.3-13. Temperature Rise in Heating Wire. A current of 250A is passing through a stainless-steel wire having a diameter of 5.08mm. The wire is 2.44m long and has a resistance of 0.0843. The outer surface is held constant at 427.6K. The thermal conductivity is k=22.5W/mK. Calculate the center-line temperature at steady state. Problem 4.3-14 (Page 347): Critical Radius for Insulation. 4.314 Critical Radius for Insulation. A metal steam pipe having an outside diameter of 30mm has a surface temperature of 400K and is to be insulated with an insulation having a thickness of 20mm and a k of 0.08W/mK. The pipe is exposed to air at 300K and a convection coefficient of 30W/m2K. a. Calculate the critical radius and the heat loss per m of length for the bare pipe. b. Calculate the heat loss for the insulated pipe assuming that the surface temperature of the pipe remains constant. A21:Ans. (b) q=54.4W
