Need help with my Hw please .......... .......................

ChE 350 Problem Set 11 Due Thursday April 14, 2016 Supplementary Reading: Chapter 11 in Bergman 1. A heat exchanger is currently used to heat 2.5 kgs-1 of water from 25C to 65C, using saturated steam at 138C. It is desired to bleed off 0.62 kgs-1 of water at 50C so that the single exchanger will be replaced by a two-exchanger arrangement that will permit this, with the two smaller exchangers having the same 2 heat exchange area. The overall heat transfer coefficient is 1,700 W/m K for the single exchanger and may be taken to be the same for each of the smaller exchangers. Determine the area of the smaller exchangers and the water temperature at the outlet of the two-exchanger arrangement. Assume that the water flows in series through the two exchangers, with the bleed-off taking place between them. 2. The trans-Alaska pipeline, used to transport oil at a rate of 500 kgs-1, is 1.5 m in diameter, covered with a 15 cm-thick layer of insulation (ki = 0.05 Wm-1K-1), and buried in soil (ks = 0.5 Wm-1K-1) at a depth of 3 m. Since the temperature at the ground surface above the pipeline is - 40C, each 100 km-section of the pipeline extends between pumping stations in which the oil is heated to 120C to ensure low viscosity (and hence low pumping power requirements). You may neglect the thermal resistance of the pipe wall, and use the following average physical properties for oil: = 900 kgm-3 ; c = 2,000 Jkg-1K-1 ; k = 0.14 Wm-1K-1 ; = 8.5X10-4 m2s-1 a. Estimate the mean temperature of the oil arriving at a pumping station and the total rate of heat loss from each section of the pipeline between two pumping stations. b. Depending on the soil composition and moisture, the thermal conductivity of the soil can vary in the range of 0.25-1.0 Wm-1K-1, while variations in oil demand may require the oil mass flow rate to be adjusted in the range of 250-500 kgs-1. To keep pumping power requirements manageable, the oil exit temperature at the end of each 100-km segment must be at least 110C. Examine the effects of variations in the soil thermal conductivity and oil demand on the exit oil temperature and the total rate of heat loss between stations (i.e. plot the exit oil temperature and the rate of heat loss as a function of ks for different mass flow rates). Identify the worst case operating condition. c. (Extra credit) Recommend possible strategies to allow pipeline operation in the worst case scenario. Justify your answer. 3. A shell-and-tube heat exchanger is composed of 25 stainless steel tubes (25 mm O.D., 20 mm I.D.) of thermal conductivity 22 Wm-1K-1. Steam flows through the shell surrounding the tubes and condenses on the outside of the tubes at 121C. The heat transfer coefficient between the steam and the external tube surface is 4500 Wm-2K-1. Water flowing at 1.2 kgs-1 on the tube side is to be heated from 38C to 93C. a. Calculate the tube length required to achieve the desired heat transfer performance. b. Having shorter tubes in the heat exchanger is desirable due to space limitations. It is proposed to increase the number of tubes to either 50 or 100 in order to reduce the required tube length. Would you recommend this action? c. (Extra Credit) As an alternative to increasing the number of tubes to reduce the required tube length, it is proposed to install 8 equally-spaced longitudinal fins of rectangular profile (2 mm-thick and 10 mmlong) on the outside of each tube. Would you recommend installing the fins? You may assume the heat transfer coefficient between the steam and the external tube surface (including the fins) to remain the same as before. d. In order to obtain additional heat transfer out of the exchanger with 25 tubes, it is suggested that a portion of the water leaving the exchanger be recycled and mixed with the cold water as it enters the exchanger. If the mass flow rate of recycled water is equal to the cold water feed rate, compute the temperature of the water leaving the heat exchanger