FS 231: Final Exam (5-6-05) Part A (Closed Book): 60 points

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1 Name: Start time: End time: FS 231: Final Exam (5-6-05) Part A (Closed Book): 60 points 1. What are the units of the following quantities? (10 points) a. Enthalpy of a refrigerant b. Dryness fraction of a refrigerant c. Latent heat of vaporization of a refrigerant d. Thermal conductivity of a frozen product e. Apparent specific heat of a frozen product f. Mole fraction g. Specific volume of ambient air h. Convective heat transfer coefficient i. Coefficient of performance of a vapor compression refrigeration system j. Degree of superheating of a refrigerant in the evaporator 2. Are the following statements true or false? (10 points) a. Ammonia refrigeration systems were replaced by Freon refrigeration systems since Freon has a higher latent heat of vaporization than ammonia b. At atmospheric pressure, ammonia is in liquid state c. In a vapor compression refrigeration system, it is desirable to have a refrigerant in as much vapor form as possible at the inlet of the evaporator d. The compression process in a vapor compression refrigeration system is a constant enthalpy process e. Rapid freezing results in extensive structural damage to the food product f. Plank s equation is valid only when freezing is taking place from all three directions when dealing with a brick-shaped product g. High molecular weight solutes affect freezing point more than low molecular weight solutes h. During evaporation, as a solution gets more and more concentrated, its boiling point increases i. Adding a solute to a solvent results in a depression in the boiling point 3. What is meant by one ton of refrigerant? 4. What is dry ice and how does it serve as a refrigerant?

2 5. Which of the following statements are true for a vapor compression refrigeration cycle operating under saturated conditions? EXPLAIN your answer. a. The temperature of the refrigerant at the inlet and exit of the condenser are the same b. The temperature of the refrigerant at the inlet of the condenser is higher than that at the exit c. The temperature of the refrigerant at the inlet of the condenser is lower than that at the exit 6. Draw a typical time-temperature graph for freezing of a potato (initially at 25 C) using blast air (at -30 C) and explain what happens in each section of the graph. 7. List two ways of increasing the rate of freezing. 8. What is the approximate storage temperature for frozen foods? 9. List two purposes of maintaining vacuum in the evaporation chamber.

3 10. Does the rate of heat transfer increase or decrease as the solution gets more and more concentrated? EXPLAIN. 11. What are the techniques that can be used to increase the rate of heat transfer in an evaporator? 12. Explain wet bulb temperature. 13. In the psychrometric chart, constant enthalpy lines are the same as constant lines. 14. How many properties of air are required to locate the conditions of the air in a psychrometric chart? 15. Keeping all other parameters the same, will sub-cooling a refrigerant in the condenser increase or decrease the C.O.P. (= Q e/q w) of a vapor compression refrigeration system? (4 points)

4 16. Will the use of Plank s equation result in over-estimating or under-estimating the freezing time? EXPLAIN your answer. (4 points) 17. You are asked to determine the efficiency of different evaporators. What are the techniques you would use to accomplish this? How would you quantify efficiency? (4 points) 18. From a psychrometric standpoint, explain what happens to the various thermodynamic properties of the air when a humidifier (spray of water) is used in a heated room during winter. (4 points)

5 BONUS QUESTIONS 19. Explain the following terms associated with thermal processing: (6 points) a. D-value b. z value c. F value 20. What is the purpose of having a back pressure valve in an aseptic processing system? 21. What are the advantages of aseptic processing over canning? (3 points) 22. a. What are the gases that are controlled in modified atmosphere packaging? b. What are the effects of these gases on the food product?

6 Name: Start time: End time: FS 231: Final Exam (5-6-05) Part B (Open Book): 40 points 23. The same quantity (m s) of a given solute (of molecular weight (M s) is added to two different solvents (one of molecular weight M 1 and the other of molecular weight M 2) of mass M. If M 2 > M 1, which solution will have a higher boiling point? EXPLAIN how you arrived at the answer without using any numerical values. (4 points) Answer any 3 of the following 4 questions 24. What should be the mass flow rate of Freon (R-12) in a vapor compression refrigeration system (operating between 200 kpa and 800 kpa with 10 C superheating in the evaporator and 5 C sub-cooling in the condenser) if it is desired to cool a 50 kg product of specific heat 2,500 J/kg-K from 80 C to 30 C in 10 minutes? (12 points) 25. A certain cylindrical shaped food product (5 cm diameter, 25 cm height) containing 90% water is being frozen in a blast freezer operating at -40 C. The Biot number in this case is 50 and the initial freezing point of the product is -3 C. If the density and thermal 3 conductivity of the product are 950 kg/m and 0.5 W/m-K respectively, determine the time taken to freeze the product if the product is placed in the freezer when it is at its initial freezing point. What are the assumption(s) involved? (12 points) 26. An evaporator is used to produce 100 kg/hr of a concentrated food product at 70 C using 120 kg/hr of steam of 80% quality at 90 C. If the dilute food product containing 25% solids enters the evaporator at 10 C, determine the solids content of the final product if the specific heat of the feed and final product are 4,000 J/kg-K. (12 points) 27. A stream of air (dew-point temperature = 25 C and relative humidity = 50%) flowing at 5 kg/s is mixed with another stream of air (dry bulb temperature = 60 C and humidity ratio = kg water / kg dry air) flowing at 10 kg/s. This mixture is heated till the enthalpy of the air becomes equal to 140 kj/kg dry air and then used to dry a 500 kg product containing 80% moisture. If the exit air has a dry bulb temperature of 50 C, determine the moisture content of the final product after 20 mins. (12 points)

7 BONUS QUESTIONS 28. What happens to the F value of a process if the reference temperature is increased by an amount equal to the z-value of the target organism? EXPLAIN. (5 points) 29. What is the probability of spoilage (in %) for a batch of cans processed at 268 F for 4.8 s if 5 the initial microbial count is 10 cfu/ml? Assume that D = 1.2 s. (5 points) 268

8 FS 231 Final Exam (Solutions) Part A 1. a. J/kg b. % c. J/kg d. W/m-K e. J/kg-K f. no units 3 g. m /kg h. W/m 2 -K i. No units j. C 2. a. False b. False c. False d. False e. False f. False g. False h. True i. False 3. One ton of refrigerant refers to the power required to melt one ton (2,000 lbs) of ice in 1 day. It is equal to W. 4. Dry ice is the solid form of CO 2. When dry ice is exposed to atmospheric conditions, it sublimates, thereby absorbing the latent heat of sublimation from the product in contact and hence causing a cooling effect. 5. Statement b is true. At the inlet of the condenser, the refrigerant is in a superheated gaseous state due to the compression process undergone in the compressor. In the condenser, the refrigerant loses energy to the surrounding medium (air or water). It begins by losing its degree of superheat (decrease in temperature, but no phase change) and then loses latent heat (phase change occurs, but no temperature change). Thus, in the condenser, the refrigerant is at a higher temperature at the inlet than the outlet.

9 6. A typical time-temperature graph for freezing of potato using blast air is shown below: A to B: Initial cooling B to C: Freezing C to D: Cooling of frozen solid 7. The rate of freezing can be increased by increasing the rate of flow of the freezing medium or decreasing the temperature of the freezing medium. 8. ~ 0 F 9. The purposes of maintaining vacuum in an evaporation chamber are to decrease the temperature at which the product boils (thereby minimizing damage to heat sensitive flavors) and also to increase the rate of heat transfer by increasing the driving force (temperature difference between the product and heating medium). 10. As the product gets concentrated, the rate of heat transfer will decrease as convective heat transfer coefficient is lower for viscous products. In addition, fouling will also increase over time and cause a decrease in the rate of heat transfer. 11. Ways to increase the rate of heat transfer include increasing the flow rate of the heating medium, increasing the temperature difference between steam and the boiling solution or decreasing fouling, viscosity, foaming, and corrosion. 12. When a moist wick is placed over the bulb of a mercury thermometer and unsaturated air at high speed is blown over it, moisture from the wick evaporates since the vapor pressure of the water vapor near the moist wick is higher than that of the bulk of the unsaturated air

10 surrounding it. The evaporation process requires latent heat from the wick and this causes the temperature of the moist bulb to decrease below that of a dry bulb next to it. As the temperature of the wick decreases below the dry bulb temperature of air, sensible heat flows from air to the wick and tends to raise the temperature of the moist bulb. A steady state is attained when the sensible heat flow from the air to the wick is equal to the latent heat required to evaporate moisture from the wick. This steady state temperature of the moist wick is called the wet bulb temperature (or saturation temperature). 13. Wet bulb temperature 14. Two 15. Sub-cooling in the condenser results in a decrease in the value of H 1. C.O.P. = Q e/q w = (H 2 -H 1)/(H 3 - H 2) If we decrease H, C.O.P. increases Plank s equation does not account for the time taken for the product to be cooled from its initial temperature to its freezing temperature. Thus, it underestimates the freezing time. However, Plank s equation assumes that freezing takes place only in one direction, while in reality it could take place in more than direction. This could result in an over-estimation of the freezing time. Thus, the net effect on estimating the freezing time depends on the relative effects of these two factors. 17. The efficiency of energy utilization in an evaporator is related to how much of vapor is produced per amount of steam utilized. Thus, the magnitude of steam economy would be one of the indicators of efficiency. In terms of production, efficiency is related to the amount of concentrated product produced per unit time. This is related to the amount of vapor produced per unit time. Thus, the magnitude of capacity is another measure of the efficiency of the evaporator. In terms of heat transfer, the magnitude of the overall heat transfer coefficient would be an indicator of the efficiency of the evaporator. 18. When you spray cold water into the air, you are adding moisture and cooling it just like in a spray drying operation. Thus, the humidity ratio and relative humidity increase, the dry bulb temperature decreases, and the enthalpy & wet bulb temperature remain constant.

11 BONUS QUESTIONS 19. a. D value is the time required at a constant temperature for an order of magnitude reduction in microbial population. b. Z value is the temperature change required for an order of magnitude change in the D value. c. F value of a process is the time of processing at the reference temperature that results in the same amount of microbial kill as in the given process. 20. A back pressure valve is placed at the end of the cooling unit in an aseptic processing system to ensure that the product does not boil or flash during heating, holding, or cooling of the product. 21. Better product quality (nutrients, flavor, color, texture), less energy consumption, fewer operators, less space requirements, eliminating the need for refrigeration, easy adaptability to automation, use of any size package, use of flexible packages, and cheaper packaging costs are some of the advantages of aseptic processing over the conventional canning process. 22. a. Oxygen, carbon dioxide, ethylene, and nitrogen are some of the gases that are controlled in MAP. b. Oxygen causes oxidative reactions in foods Ethylene causes ripening of climacteric fruits CO 2 has a bactericidal effect on foods Nitrogen has no effect on most foods and is hence used as an inert filler gas

s. Properties for R134a are as follows : Saturated R-134a Superheated R-134a

s. Properties for R134a are as follows : Saturated R-134a Superheated R-134a CHAPTER 9 REFRIGERATION & AIR-CONDITIONING YEAR 2012 ONE MARK Common Data For Q. 1 and Q.2 A refrigerator operates between 120 kpa and 800 kpa in an ideal vapour compression cycle with R-134a as the refrigerant.