293 kgs discussion note that the rate of condensation

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Unformatted text preview: is still 6 m2. Then the rate of condensation heat transfer in the tilted plate case becomes · Q hAs(Tsat Ts) (5641 W/m2 · °C)(6 m2)(100 80)°C 6.77 105 W (b) The rate of condensation of steam is again determined from · m condensation · Q h* fg 6.77 2314 105 J/s 103 J/kg 0.293 kg/s Discussion Note that the rate of condensation decreased by about 3.6 percent when the plate is tilted. EXAMPLE 10–6 Condensation of Steam on Horizontal Tubes The condenser of a steam power plant operates at a pressure of 7.38 kPa. Steam at this pressure condenses on the outer surfaces of horizontal pipes through which cooling water circulates. The outer diameter of the pipes is 3 cm, and the outer surfaces of the pipes are maintained at 30°C (Fig. 10–32). Determine (a) the rate of heat transfer to the cooling water circulating in the pipes and (b) the rate of condensation of steam per unit length of a horizontal pipe. SOLUTION Saturated steam at a pressure of 7.38 kPa (Table A-9) condenses on a horizontal tube at 30°C. The rates of heat transfer and condensation are to be determined. Assumptions 1 Steady operating conditions exist. 2 The tube is isothermal. Properties The properties of water at the saturation temperature of 40°C corresponding to 7.38 kPa are hfg 2407 103 J/kg and 0.05 kg/m3. The properties of liquid water at the film temperature of Tf (Tsat Ts)/2 (40 30)/2 35°C are (Table A-9) l l 994 kg/m3 0.720 10 3 kg/m · s Cpl kl 4178 J/kg · °C 0.623 W/m · °C Analysis (a) The modified latent heat of vaporization is h* fg hfg 0.68Cpl (Tsat Ts) 2407 103 J/kg 0.68 2435 103 J/kg (4178 J/kg · °C)(40 30)°C Noting that 994), the heat transfer coefficient for conl (since 0.05 densation on a single horizontal tube is determined from Eq. 10–31 to be Steam, 40°C 30°C Cooling water FIGURE 10–32 Schematic for Example 10–6. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 544 544 HEAT TRANSFER h 1/4 g l( l ) h* k3 1/4 g 2 h* k3 fg l l fg l 0.729 (Tsat Ts) D Ts) D 1 (Tsat (9.81 m/s2)(994 kg/m3)2 (2435 103 J/kg)(0.623 W/m ·...
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