/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 93 A 50 -mm-thick air gap separates... [FREE SOLUTION] | 91Ó°ÊÓ

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A 50 -mm-thick air gap separates two horizontal metal plates that form the top surface of an industrial furnace. The bottom plate is at \(T_{h}=200^{\circ} \mathrm{C}\) and the top plate is at \(T_{c}=50^{\circ} \mathrm{C}\). The plant operator wishes to provide insulation between the plates to minimize heat loss. The relatively hot temperatures preclude use of foamed or felt insulation materials. Evacuated insulation materials cannot be used due to the harsh industrial environment and their expense. A young engineer suggests that equally spaced, thin horizontal sheets of aluminum foil may be inserted in the gap to eliminate natural convection and minimize heat loss through the air gap.

Short Answer

Expert verified
In summary, inserting aluminum foil sheets in the 50-mm-thick air gap between the two metal plates of the industrial furnace can potentially minimize heat loss due to conduction and convection. This could lead to greater efficiency in the industrial furnace. However, radiation heat transfer would not be significantly impacted. Given the constraints of using foamed, felt, or evacuated insulation materials, the aluminum foil solution may be a viable option, but further calculations and analysis are needed to validate its effectiveness compared to other insulation options.

Step by step solution

01

Understand the heat loss mechanisms

There are three mechanisms of heat loss from the bottom plate to the top plate: 1. Conduction through the air gap 2. Convection within the air gap 3. Radiation between the two plates Adding horizontal sheets of aluminum foil within the air gap can potentially minimize conduction and convection heat transfer.
02

Identify the effect of aluminum foil on conduction and convection heat transfer

By inserting aluminum foil sheets into the air gap, we effectively divide the original gap into smaller parts. This can reduce heat loss due to conduction and convection because these heat transfer mechanisms depend on the gap thickness. The conduction heat transfer will be reduced because the air gaps between the sheets are smaller, and the aluminum foil will also act as a barrier, reducing natural convection within the air gap. Therefore, placing aluminum foil sheets can theoretically minimize heat loss through conduction and convection.
03

Discuss the role of radiation heat transfer

Radiation heat transfer is another mechanism contributing to the overall heat loss. However, inserting aluminum foils in the air gap will not significantly impact this heat transfer mechanism since the radiation heat transfer mainly depends on the temperatures of the surfaces and their emissivities.
04

Analyze the feasibility of the aluminum foil solution

Considering that the use of foamed, felt, and evacuated insulation materials has been ruled out for this industrial application, the idea of using aluminum foil sheets may serve as a viable option for reducing heat loss through the air gap. By minimizing conduction and convection heat transfer, heat loss through the air gap between the two metal plates would be reduced, leading to greater efficiency in the industrial furnace. However, further calculations and analysis of the specific heat flux would be needed to validate the effectiveness of the aluminum foil in minimizing heat loss and comparing it with other available insulation options.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Conduction Heat Transfer
Conduction is a heat transfer mechanism that occurs within materials and between substances in direct contact. As the particles become excited due to high temperature, they transfer kinetic energy to neighboring particles, creating a chain effect. The rate of conduction through a material depends on its thermal conductivity, which is a measure of how well a substance conducts heat.

In the industrial furnace scenario, the conduction heat loss through the air gap is significant because air itself isn't a great insulator. This is why the young engineer's suggestion to insert aluminum foil into the air gap could reduce heat conduction. As the aluminum sheets divide the air gap and make it thinner, it's harder for heat to be passed from one side to the other, thus reducing the amount of heat transferred by conduction.
Convection Heat Transfer
Convection is the process of heat transfer through fluids, such as liquids or gases, by the movement of the fluid itself. It occurs due to the natural tendency of hot fluids to rise and cool fluids to sink, creating a convection current. In an industrial setting, preventing unwanted heat loss through convection can significantly improve energy efficiency.

For the air gap in the industrial furnace, natural convection would move hot air from the hotter bottom plate to the cooler top plate, contributing to heat loss. By placing thin sheets of aluminum foil in the air gap, convection currents are disrupted, thus reducing the convection heat transfer and improving insulation.
Radiation Heat Transfer
Radiation heat transfer involves the transfer of energy by electromagnetic waves. All objects emit thermal radiation, which does not require a medium to travel and can occur in a vacuum. The rate of radiation heat transfer depends mainly on the temperature of the emitting surface, its emissivity, and the temperature of the surroundings.

In the context of the industrial furnace, while the insertion of aluminum foil sheets into the air gap has minimal direct impact on reducing radiation heat transfer, the foil's polished surface can potentially reflect some of the radiated heat back to the source, indirectly lowering the total heat loss by radiation.
Industrial Furnace Insulation
Industrial furnaces require effective insulation to maintain high temperatures while minimizing energy costs. Traditional insulation materials, like foamed or felt insulation, are not suitable in the given scenario due to high temperatures and industrial environment challenges. Evacuated insulation also gets ruled out because of its cost.

Aluminum foil emerges as a viable alternative thanks to its reflective properties and the ability to block conduction paths. It is a practical solution within the constraints given, highlighting the necessity to adapt insulation strategies to both the operating conditions and the financial parameters specific to the industrial process.
Heat Transfer Minimization Strategies
To minimize heat loss in industrial systems, it's essential to understand all mechanisms of heat transfer and employ the appropriate strategies that address each mechanism. Layering materials with different heat transfer properties, controlling fluid movement, and reflecting thermal radiation are key strategies.

In our industrial furnace example, using aluminum foil sheets tackles both conduction and convection mechanisms. It creates multiple layers with air pockets that inhibit the flow of hot air and, to some extent, reflect thermal radiation. This method showcases an innovative approach to heat transfer minimization by utilizing material properties strategically within specified constraints.

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Most popular questions from this chapter

The space between the panes of a double-glazed window can be filled with either air or carbon dioxide at atmospheric pressure. The window is \(1.5 \mathrm{~m}\) high and the spacing between the panes can be varied. Develop an analysis to predict the convection heat transfer rate across the window as a function of pane spacing and determine, under otherwise identical conditions, whether air or carbon dioxide will yield the smaller rate. Illustrate the results of your analysis for two surface-temperature conditions: winter \(\left(-10^{\circ} \mathrm{C}, 20^{\circ} \mathrm{C}\right)\) and summer \(\left(35^{\circ} \mathrm{C}, 25^{\circ} \mathrm{C}\right)\).

An aluminum alloy (2024) plate, heated to a uniform temperature of \(227^{\circ} \mathrm{C}\), is allowed to cool while vertically suspended in a room where the ambient air and surroundings are at \(27^{\circ} \mathrm{C}\). The plate is \(0.3 \mathrm{~m}\) square with a thickness of \(15 \mathrm{~mm}\) and an emissivity of \(0.25\). (a) Develop an expression for the time rate of change of the plate temperature, assuming the temperature to be uniform at any time. (b) Determine the initial rate of cooling (K/s) when the plate temperature is \(227^{\circ} \mathrm{C}\). (c) Justify the uniform plate temperature assumption. (d) Compute and plot the temperature history of the plate from \(t=0\) to the time required to reach a temperature of \(30^{\circ} \mathrm{C}\). Compute and plot the corresponding variations in the convection and radiation heat transfer rates.

Liquid nitrogen is stored in a thin-walled spherical vessel of diameter \(D_{i}=1 \mathrm{~m}\). The vessel is positioned concentrically within a larger, thin-walled spherical container of diameter \(D_{o}=1.10 \mathrm{~m}\), and the intervening cavity is filled with atmospheric helium. Under normal operating conditions, the inner and outer surface temperatures are \(T_{i}=77 \mathrm{~K}\) and \(T_{o}=283 \mathrm{~K}\). If the latent heat of vaporization of nitrogen is \(2 \times 10^{5} \mathrm{~J} / \mathrm{kg}\), what is the mass rate \(m(\mathrm{~kg} / \mathrm{s})\) at which gaseous nitrogen is vented from the system?

The heat transfer rate due to free convection from a vertical surface, \(1 \mathrm{~m}\) high and \(0.6 \mathrm{~m}\) wide, to quiescent air that is \(20 \mathrm{~K}\) colder than the surface is known. What is the ratio of the heat transfer rate for that situation to the rate corresponding to a vertical surface, \(0.6 \mathrm{~m}\) high and \(1 \mathrm{~m}\) wide, when the quiescent air is \(20 \mathrm{~K}\) warmer than the surface? Neglect heat transfer by radiation and any influence of temperature on the relevant thermophysical properties of air.

A solar collector design consists of an inner tube enclosed concentrically in an outer tube that is transparent to solar radiation. The tubes are thin walled with inner and outer diameters of \(0.10\) and \(0.15 \mathrm{~m}\), respectively. The annular space between the tubes is completely enclosed and filled with air at atmospheric pressure. Under operating conditions for which the inner and outer tube surface temperatures are 70 and \(30^{\circ} \mathrm{C}\), respectively, what is the convective heat loss per meter of tube length across the air space?

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