Dairy Engineering Questions and Answers – Heat Exchangers – 1

This set of Dairy Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Heat Exchangers – 1”.

1. Pasteurization aid in which of the following?
a) killing tubercle bacillus
b) killing spores
c) increasing fat content
d) lowering temperature
View Answer

Answer: a
Explanation: The pasteurization of milk is a special type of heat treatment which can be defined as “any heat treatment of milk which secures the certain destruction of tubercle bacillus (T.B.) without markedly affecting the physical and chemical properties”.

2. Which of the following is the time-temperature combination for HTST pasteurization?
a) 72°C to 74°C for 15 to 20 seconds
b) 135°C to 140°C for 2 to 4 seconds
c) 63°C for 30 minutes
d) 57°C to 68°C for 15 min
View Answer

Answer: a
Explanation: High temperature short time (HTST) pasteurization. Heat the milk to between 72°C to 74°C for 15 to 20 seconds. Targets resistant pathogenic bacterial spores (Clostridium botulinum spores).

3. Which of the following is the time-temperature combination for LTST pasteurization?
a) 72°C to 74°C for 15 to 20 seconds
b) 135°C to 140°C for 2 to 4 seconds
c) 63°C for 30 minutes
d) 57°C to 68°C for 15 min
View Answer

Answer: c
Explanation: Low temperature long time (LTLT) pasteurization involves heating the milk to 63°C for 30 minutes. The extended holding time causes alteration in the milk protein structure and taste.
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4. Which of the following is the time-temperature combination for UHT pasteurization?
a) 72°C to 74°C for 15 to 20 seconds
b) 135°C to 140°C for 2 to 4 seconds
c) 63°C for 30 minutes
d) 57°C to 68°C for 15 min
View Answer

Answer: b
Explanation: UHT Pasteurization involves heating the milk to between 135°C to 140°C for 2 to 4 seconds. The extreme heat targets Coxiella burnetii, which causes Q-fever. The heat kills all the vegetative forms of bacteria and the milk can survive for 9 months.

5. Which of the following is the time-temperature combination for Thermization?
a) 72°C to 74°C for 15 to 20 seconds
b) 135°C to 140°C for 2 to 4 seconds
c) 63°C for 30 minutes
d) 57°C to 68°C for 15 min
View Answer

Answer: d
Explanation: Thermization involves heating the milk to between 57°C to 68°C and hold for 15 minutes. Thermization targets pathogenic bacteria while leaving the good bacteria in the product. The low temperatures do not alter the structure and taste of the milk.
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6. Which of the following is the time-temperature combination for Sterilization?
a) 72°C to 74°C for 15 to 20 seconds
b) 135°C to 140°C for 2 to 4 seconds
c) 63°C for 30 minutes
d) 115 – 120°C for some 20 – 30 minutes
View Answer

Answer: d
Explanation: Sterilization is a wet treatment of canned milk products in an autoclave/specialized treatment chambers. Heat to between 115°C to 121°C for 10 to 20 minutes.

7. “Heat transferred from the bowl of the spoon to the handle” is an example of which of the following way of heat transfer?
a) Conduction
b) Convection
c) Radiation
d) Filtration
View Answer

Answer: a
Explanation: Conduction means a transfer of thermal energy through solid bodies and through layers of liquid at rest (without physical flow or mixing in the direction of heat transfer). The question shows an example of heat conduction to a teaspoon in a cup of hot coffee. Heat is transferred by conduction to the handle, which becomes warmer.
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8. “The spoon is rinsed in running cold water. Heat gets absorbed by the water and the spoon gets cooler, until the spoon and the water are at the same temperature” is an example of which of the following way of heat transfer?
a) Conduction
b) Convection
c) Radiation
d) Filtration
View Answer

Answer: b
Explanation: Convection consequently involves mixing. When the teaspoon is rinsed with running cold water, heat gets transferred from the spoon to the water, which is heated in the process. The heated water when replaced by cold water, which in turn absorbs heat from the spoon. Heat transfer by convection continues until the spoon and the running water have the same temperature.

9. “A roof accumulates solar heat during the day and radiates the heat at night” is an example of which of the following way of heat transfer?
a) Conduction
b) Convection
c) Radiation
d) Filtration
View Answer

Answer: c
Explanation: Radiation is the emission of heat from a body which has accumulated thermal energy. In the given example the thermal energy is converted into radiant energy, emitted from the body and absorbed by other bodies which it strikes. Almost all substances emit radiant energy.
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10. Which of the following is the application of direct heating in the dairy industry?
a) Heating curd in the manufacture of cheese
b) Solar heating
c) Heating with gamma rays
d) Oxidation
View Answer

Answer: a
Explanation: Direct heating means that the heating medium is mixed with the product. This technique is used in heating the curd in the manufacture of cheese in dairy industry.

11. Heat exchanger works on which of the following principle?
a) Direct heating
b) Indirect heating
c) Slow heating
d) Fast heating
View Answer

Answer: b
Explanation: A heat exchanger is used to transfer heat by the indirect method. Hot water flows through one channel and milk through the other. Heat is transferred through the partition.

12. If the product flow rate in a plant is to be increased from 10 000 l/h to 20 000 l/h, the heat exchanger must be extended to_______ the original size, provided the flow rates of the service media are also doubled, other factors being constant.
a) Double
b) Triple
c) Same
d) Quadruple
View Answer

Answer: a
Explanation: In the question the size needs to be doubled as The flow rate, V, is determined by the planned capacity of the dairy. The higher the flow rate, the larger the heat exchanger needed.

13. Calculate temperature change: 20 000 l/h cheese milk (V1) is to be heated from 4°C to 34°C by 30 000 l/h hot water (V2) at 50°C. Density (r) and specific heat (cp) for milk are about 1020 kg/m3 and 3.95 kJ/kg, K and for water 990 (at 50°C) and 4.18.
a) 20.5℃
b) 21.5℃
c) 18.8℃
d) 19.5℃
View Answer

Answer: d
Explanation: The temperature change for the hot water can then be calculated:
20 000 x 1 020 x 3.95 x (34 – 4) = 30 000 x 990 x 4.18 x Dt2
Dt2 = 19.5°C. The hot water temperature will drop by 19.5 from 50 to 30.5°C.

14. The temperature difference between the two liquids is best utilized if they flow in opposite directions through the heat exchanger. This type of flow is called ________
a) Countercurrent
b) Concurrent
c) Rapid flow
d) Slow flow
View Answer

Answer: a
Explanation: The cold product then meets the cold heating medium at the inlet and a progressively warmer medium as it passes through the heat exchanger. During the passage, the product is gradually heated so that the temperature is always only a few degrees below that of the heating medium at the corresponding point. This type of arrangement is called a countercurrent flow.

15. ______________ tells how much heat passes through 1 m2 of the partition per 1°C of differential temperature.
a) Friction coefficient
b) Temperature coefficient
c) Overall heat transfer coefficient
d) Time coefficient
View Answer

Answer: c
Explanation: The overall heat transfer coefficient refers to how well heat is conducted over a series of mediums. This factor, k, is a measure of how efficient the heat transfer is.

Sanfoundry Global Education & Learning Series – Dairy Engineering.

To practice all areas of Dairy Engineering, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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