Assume that atmospheric pressure air is heated in a long annulus (250-mm ID & 400-mm OD) by steam condensing at 150C on the inner surface. The velocity of the air is 7 m/s, and its bulk temperature is 42C (average). Properties of dry air at atmospheric pressure are given on the next slide. Assume steady state conditions and that the steam temperature is constant and uniform. a) Sketch a diagram showing the inner and outer tubes. b) Find p, k, Cp and u at the bulk temperate of the air. Show all interpolations and organize your results in a table. c) Calculate the Re number based on the hydraulic diameter of the annulus (Raw). What is the type of flow? Show work. d) Calculate the correction factory. e) Calculate the heat coefficient he of the air. Write down the equation(s) used and show work. f) If the total resistance (Butu) between the steam and the air (including the pipe wall and scale on the steam side) is 0.06 m2K/W, calculate the temperature difference between the outer surface of the inner pipe and the air (C). g) What is the percentage of the convective resistance on the air side to the total thermal resistance? h) Sketch the thermal circuit for the heat transfer between the stream and the air. Thermodynamic Properties of Dry Air at atmospheric Pressure MX10 18 Coeficient of Thermal Specific Thermal Thermal Absolute Kinematic Density, Expansion, Heat, Conductivity. Diffusivity, Viscosity, Viscosity, Prandel Temperature, P Bx10 X 10 X 104 Number, (kg/m) (1/K) (J/kg K) (W/m K) (m/s) (N s/m) Pr (1/Km) X 6.243 x 10 x0.5556 X 2.388 x 10-4 x 0.5777 x 3.874 x 10" X 0.6720 x 3,874 x 10 FK C x 1.573 x 10 = (b) = (1/R) = (Btu/lb"F) = (Btu/hr ft F) = (he/hr) = (h/f) = (/hr) (R) 32 273 0 1.252 3.66 1011 0.0237 19.2 17.456 13.9 0.71 68 293 20 1.164 3.41 1012 0.0251 22.0 18.240 15.7 0.71 1.36 104 313 40 1.092 3.19 1014 0.0265 24.8 17.6 19.123 0.71 1.01 140 333 60 1.025 3.00 1017 0.0279 27.6 19.907 19.4 0.71 0.782 176 353 80 0.968 2.83 1019 0.0293 30.6 20.790 21.5 0.71 0.600 212 373 100 0.916 2.68 1022 0.0307 33.6 21.673 23.6 0.71 0.472 392 473 200 0.723 2.11 1035 0.0370 49.7 25.693 35.5 0.71 0.164 572 573 300 0.596 1.75 1047 0.0429 68.9 29.322 49.2 0.71 0.0709 752 673 400 1.49 1059 0.0485 894 32.754 64.6 0.72 0.0350 932 773 500 0.442 1.29 1076 0.0540 113.2 35.794 81.0 0.0193 1832 1273 1000 0.268 0.79 1139 0.0762 240 48.445 181 0.74 0.00236 OSOS 0.72 Source: K. Raznjevi, Handbook of Thermodynamic Tables and Charts, McGraw-Hill, New York, 1976
