課程名稱︰單元操作(二) 課程性質︰必修 課程教師︰葛煥彰 開課學院：工學院 開課系所︰化工系 考試日期（年月日）︰2013/1/10 考試時限（分鐘）：120 是否需發放獎勵金：是 （如未明確表示，則不予發放） 試題 : Part I: close book(45%) 1. A cool fluid of temperature 20℃ and specific heat 4000 J/kg‧K enters a concentric tube heat exchanger at flow rate 0.15kg/s to cool a hot fluid of temperature 220℃, specific heat at 3000J/kg‧K, and flow rate 0.10kg/s. The overall heat transfer coefficient of the heat exchanger can be taken as a constant. (a) What is the maxium possible heat transfer rate of the exchange under the operating conditions? (b) What is the heat exchanger effectiveness if the cold fluid leaves at temperature 80℃ (c) What is the ratio of the required areas of the heat exchanger for counter-flow and parallel-flow operations if the cold fluid leaves at temperature 80℃ 2. A spherical drop of liquid A (with density ρA and molecular weight MA) is suspended in a large volume of dry and still air at constant temperature T. The initial radius of the drop is r0, the vaper pressure in the gasphase is a constant p. The diffusion coefficient of vapor of A in the air is DAB and the gas constant is denoted by R. (a) Determine the evaporation rate (molar flux at the surface) of the drop when its radius is r1 (your answer should not contain r0). (Hint: The assumption of a quasi-steady state should be made) (b) Determine the time required for the drop of radius r0 to evaporate completely (your answer should not contain r1). Part II: open book (55%) 3. A continuous single-effect evaporator concentrates 9000 kg/h of a 1.0 wt% salt solution entering at 40℃ to a final concentration of 1.5 wt%. The vapor space is at 1 atm abs. and the steam supplied is saturated at 1.41 atm abs. The overall heat-transfer coefficient is 1700 W/m^2-K. Assume that, since it is dilute, the solution has the same boiling point and specific heat as water Calculate (a) the amount of vapor and liquid products (in kg/h), (b) the amount of steam required (in kg/h), (c) the heat-transfer area required (in m^2) (d) the steam economy 4. Consider a cylindrical furnace with r0=H=1m (radius and height), as shown in the figure below .The top (surface 1) and the base (surface 2) of the furnace have emissivities ε1=0.8 and ε2=0.4, respectively, and are maintained at uniform temperatures T1=700 K and T2=500 K. The side surface (surface 3) closely approximates a black body and is maintained at a temperature of T3=400 K. (a)Determine the view factors F12,F21,F13,F23,F31,and F32. (b)Determine the net rate of radiation heat transfer at each surface at steady state. --

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