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Chapter 4: Problem 126

Consider the freezing of packaged meat in boxes with refrigerated air. How do \((a)\) the temperature of air, (b) the velocity of air, \((c)\) the capacity of the refrigeration system, and \((d)\) the size of the meat boxes affect the freezing time?

Short Answer

Expert verified
Answer: The factors affecting the freezing time of packaged meat include the temperature of air, the velocity of air, the capacity of the refrigeration system, and the size of the meat boxes.

Step by step solution

01

(a) The temperature of air

The temperature of air in the refrigeration system greatly affects the freezing time of the packaged meat. Lower temperatures will cause the meat to freeze faster, while higher temperatures will slow down the freezing process. If the air temperature is maintained at a constant low level, it will lead to a quicker freezing of the meat. Therefore, to minimize the freezing time, one should maintain the lowest possible temperature in the refrigerational air.
02

(b) The velocity of air

The velocity of air plays a crucial role in determining the freezing time of the packaged meat. Faster air circulation helps to minimize the temperature difference between different parts of the meat, ensuring a more uniform freezing process. A higher air velocity will result in shorter freezing times as it allows the cold air to come in contact with the meat more effectively, improving heat transfer. Therefore, increasing the velocity of air in the refrigeration system will result in faster freezing times.
03

(c) The capacity of the refrigeration system

The capacity of the refrigeration system is another essential factor affecting the freezing time of packaged meat. If the system has a higher capacity, it can handle a large quantity of meat and maintain a low temperature, which speeds up the freezing process. In contrast, a lower capacity refrigeration system will struggle to keep up the required low temperatures for large amounts of meat, leading to longer freezing times. Thus, to reduce freezing time, one must increase the capacity of the refrigeration system.
04

(d) The size of the meat boxes

The size of meat boxes is another factor that affects the freezing time of packaged meat. Larger boxes offer a greater surface area for heat transfer, but they may also lead to temperature differences within the container. On the other hand, smaller boxes have less surface area and less room for differences in temperature, potentially resulting in faster freezing times. To optimize the freezing time, one should consider the optimal size of meat boxes based on the refrigeration system's capacity, air temperature, and air velocity.

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Most popular questions from this chapter

A 10-cm-inner diameter, 30-cm-long can filled with water initially at \(25^{\circ} \mathrm{C}\) is put into a household refrigerator at \(3^{\circ} \mathrm{C}\). The heat transfer coefficient on the surface of the can is \(14 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Assuming that the temperature of the water remains uniform during the cooling process, the time it takes for the water temperature to drop to \(5^{\circ} \mathrm{C}\) is (a) \(0.55 \mathrm{~h}\) (b) \(1.17 \mathrm{~h}\) (c) \(2.09 \mathrm{~h}\) (d) \(3.60 \mathrm{~h}\) (e) \(4.97 \mathrm{~h}\)

The walls of a furnace are made of \(1.2\)-ft-thick concrete \(\left(k=0.64 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft} \cdot{ }^{\circ} \mathrm{F}\right.\) and \(\left.\alpha=0.023 \mathrm{ft}^{2} / \mathrm{h}\right)\). Initially, the furnace and the surrounding air are in thermal equilibrium at \(70^{\circ} \mathrm{F}\). The furnace is then fired, and the inner surfaces of the furnace are subjected to hot gases at \(1800^{\circ} \mathrm{F}\) with a very large heat transfer coefficient. Determine how long it will take for the temperature of the outer surface of the furnace walls to rise to \(70.1^{\circ} \mathrm{F}\). Answer: \(116 \mathrm{~min}\)

How does \((a)\) the air motion and (b) the relative humidity of the environment affect the growth of microorganisms in foods?

In an experiment, the temperature of a hot gas stream is to be measured by a thermocouple with a spherical junction. Due to the nature of this experiment, the response time of the thermocouple to register 99 percent of the initial temperature difference must be within \(5 \mathrm{~s}\). The properties of the thermocouple junction are \(k=35 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \rho=8500 \mathrm{~kg} / \mathrm{m}^{3}\), and \(c_{p}=320 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\). If the heat transfer coefficient between the thermocouple junction and the gas is \(250 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the diameter of the junction.

A long cylindrical wood \(\log (k=0.17 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\) and \(\left.\alpha=1.28 \times 10^{-7} \mathrm{~m}^{2} / \mathrm{s}\right)\) is \(10 \mathrm{~cm}\) in diameter and is initially at a uniform temperature of \(15^{\circ} \mathrm{C}\). It is exposed to hot gases at \(550^{\circ} \mathrm{C}\) in a fireplace with a heat transfer coefficient of \(13.6 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) on the surface. If the ignition temperature of the wood is \(420^{\circ} \mathrm{C}\), determine how long it will be before the log ignites. Solve this problem using analytical one-term approximation method (not the Heisler charts).
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