Engineering Physics (Food and Beverage)

Session 2, 2015 ENG210 Page 2 of 13 Part A: Multi-choice Questions Answer all the following multi-choice questions. Each question is worth one mark. Select the answer that you think is the best or closest to the most correct value and enter that answer on the General Purpose Answer Sheet. 1. What is the absolute viscosity of water at 25 °C? (a) 880.637 Pas (b) 880.637 x 10-6 Pas (c) 0.884 x 10-6 Pas (d) 0.884 Pas 2. The hydrostatic pressure at the base of a tank of liquid filled to a height of 3 m is measured at 4.27 psig. Does this imply that: (a) The fluid is more viscous than water. (b) The fluid has a lower density than water. (c) The fluid has a very similar density to water. (d) The fluid has a much higher density than water. 3. The mass flowrate of milk ( = 1025 kg/m3) in a consumer milk plant is 650 kg/min. This is equivalent to a volumetric flowrate of: (a) 39,000 L/h (b) 38.0 kg/s (c) 11.10 kg/s (d) 0.634 m3/min 4. Which of the following best describes the likelihood of slip in a rotary lobe pump: (a) Slip increases with increased pressure and viscosity, and decreases with increased clearance. (b) Slip decreases with a decrease in all of pressure, viscosity and clearance. (c) Slip decreases with a decrease in pressure and clearance, and an increase in viscosity. (d) Slip increases with an increase in clearance and viscosity, and a decrease in pressure. 5. When performing heat exchanger design calculations using the equation , what is the temperature difference you should use for a counter-current heat exchanger where the coolant enters at -5°C and exits at 1 °C, and the product enters at 36 °C and exits at 5 °C. (a) 22.5 °C (b) 15.2 °C (c) 20.0 °C (d) 33.0 °C Session 2, 2015 ENG210 Page 3 of 13 6. If the cooling duty in a refrigeration unit is 300 kW, the true power input to the compressor motor is 80 kW, and the compressor operates with an efficiency of 90%, the expected coefficient of performance (CoP) for the system is closest to: (a) 3.75 (b) 4.17 (c) 3.37 (d) 2.75 7. Which of the following best explains the major difference in how a primary refrigerant and a secondary refrigerant (e.g. brine) produce cooling. (a) Primary refrigerants change in temperature, secondary refrigerants undergo phase change (b) Primary refrigerants have a higher volumetric flow rate than secondary refrigerants (c) Secondary refrigerants change in temperature, primary refrigerants undergo phase change (d) The temperature of secondary refrigerants is lower than the temperature of primary refrigerants 8. An input (driving) gear (60 teeth) is connected to an output (driven) gear (45 teeth). Calculate the reduction ratio. (a) 0.75 (b) 1 (c) 1.33 (d) 15 9. If a single phase conveyor motor operating off standard domestic voltage draws 6 amp and the true power is measured as 1250 W, the power factor would be expected to be: (a) 0.52 (b) 1.152 (c) 0.95 (d) 0.87 10. In a processing plant there are many electric motors. The plant is operating at 240 V and 50 Hz. The total current drawn is 54 A. If the power factor is currently 0.84, calculate the kVA of the factory. (a) 4.4 kVA (b) 10.9 kVA (c) 13.0 kVA (d) 12960 kVA END OF MULTI-CHOICE QUESTIONS Session 2, 2015 ENG210 Page 4 of 13 Part B: Short Answer Questions Answer any FOUR of the following questions. All are worth a total of 10 marks each. Start each question on a new page of your answer booklet. Question 11 (a) Beer is pumped at 24,000 L/h in 63 mm pipe. (i) Estimate the velocity of the fluid in the pipe. (ii) Calculate the mass flowrate if the density is 1002 kg/m3. (iii) At one point the beer is pumped through a short section of 75 mm pipe. Calculate the fluid velocity and the change in pressure expected once the flow pattern has stabilised in the 75 mm pipe. Clearly state all assumptions you have made. [5 marks] (b) Briefly describe the major features and operating principles of a volute centrifugal pump. Explain how these characteristics contribute to these pumps often being labelled . [2 marks] (c) Explain the importance of providing both sufficient depth of insulation and ensuring the integrity of sealing of that insulation on pipes carrying glycol or similar refrigerants. [3 marks] Question 12 (a) Briefly describe the major features of a plate heat exchanger and explain in what situations this type of heat exchanger may be preferred over a tubular heat exchanger. [3 marks] (b) Water is heated in a heat exchanger using condensing dry steam at 1 atm absolute pressure. Assume no sensible cooling of the condensate occurs. (i) If the water enters at 20 C and exits at 70 C with a flowrate of 45 US gal/min, what flowrate of steam is required? The specific heat capacity of the water is 4180 J/kg. C and steam latent heat is 2,257 J/kg. (ii) What pipe size would you recommend for that water flowrate? (iii) If the water piping comprised 30 m of straight pipe and fittings equivalent to another 8 m of straight pipe, what pressure drop would be expected across the pipe? [4 marks] (c) Describe the key features of a dissolved gas flotation tank used in solid liquid separations. Use one or more diagrams to illustrate your answer. [3 marks] Session 2, 2015 ENG210 Page 5 of 13 Question 13 (a) Explain, with appropriate illustrations, the methods by which the flowrate of fluid pumped through a piping system can be controlled and the consequences of these different control strategies for energy use and flexibility of operation within a beverage manufacturing plant. [4 marks] (b) Must is cooled from 25 C to 15 C in tubular heat exchanger. The must has a density of 1080 kg/m3 and is pumped through 75 m tubing at 10,000 L/h. The must specific heat capacity is 3750 J/kg. C. The must is cooled using a circulating glycol solution with inlet temperature of 0 C, density of 985 kg/m3 and specific heat capacity is 3.6 kJ/kg. C. The overall heat transfer coefficient is 3000 W/m2 C. Calculate: (i) The mass flowrate of must pumped to the heat exchanger. (ii) The heat flow transferred from the must. (iii) The overall temperature difference and the outlet temperature of the glycol. (iv) The required mass flowrate of the glycol. [6 marks] Question 14 (a) Explain the four main stages of a refrigeration cycle, illustrating each stage on a sketch of the pressure-enthalpy diagram. Be sure to note the physical processes occurring at each point and the state of the refrigerant at the exit of each process. [3 marks] (b) The evaporation and condensation temperatures of a refrigeration plant using Freon 22 (R22) are measured at -15 C and 30 C, respectively. If the outlet temperature from the compressor is 80 C, calculate: (i) The changes in enthalpy occurring in the evaporator and condenser. (ii) The mass flowrate of R22 if the refrigeration capacity is 50 kW. (iii) The estimated coefficient of performance. [4 marks] (c) Estimate the time for homogenised juice pulp particles to settle in juice concentrate through a distance of 1.5 m. The pulp particles can be assumed spherical with a mean diameter of 0.5 mm and density of 1400 kg/3. The juice concentrate has a density of 1300 kg/m3 and viscosity of 50 cp. [3 marks] Session 2, 2015 ENG210 Page 6 of 13 Question 15 (a) Discuss how the fluid delivery conditions into pipes and tanks can influence the effectiveness of cleaning during CIP (clean-in-place) operations. [4 marks] (b) For cleaning in place of a 50 mm nominal diameter pipe: (i) What minimum volumetric flowrate would you recommend? (ii) What Reynolds number does this correspond to? [3 marks] (c) Briefly outline the causes of a low power factor, why this is important in food and beverage manufacturing operations, and how this problem of a low power factor can be addressed in a manufacturing plant. [3 marks] END OF EXAMINATION Session 2, 2015 ENG210 Page 7 of 13 Pressure-enthalpy diagram for Refrigerant R22 Session 2, 2015 ENG210 Page 8 of 13 ENG210 Useful equations and data and = = = = = = ho Ao 1 2 L k ln ro/ri hi Ai 1 UT AT 1 ho ..... 1 k2 x2 k1 x1 hi 1 U 1 x A (T1 T2) q h A (T1 T2) q UA (T1 T2) q k q m Cp (T2 T1) q m (H2 H1) q A (Th4 Tc4) 1 2 LM 2 1 ln Session 2, 2015 ENG210 Page 9 of 13 9549 P n P .... P N r r r .... r r N D r D T r T overall 1 2 n input output overall 1 2 n Output Input Input Output Input Output Session 2, 2015 ENG210 Page 10 of 13 Session 2, 2015 ENG210 Page 11 of 13 Session 2, 2015 ENG210 Page 12 of 13 Table: Dimensions of Schedule 40 sanitary stainless piping Nominal diameter Internal diameter Outside diameter mm inches (ID, mm) (OD, mm) 25 38 51 63 76 102 1 1.5 2 2.5 3 4 22.9 35.6 47.5 60.2 72.9 97.4 25.4 38.1 50.8 63.5 76.2 101.6 Session 2, 2015 ENG210 Page 13 of 13