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Chapter 6: Problem 16

Reducing irreversibilities within a system can improve its thermodynamic performance, but steps taken in this direction are usually constrained by other considerations. What are some of these?

Short Answer

Expert verified
Constraints include financial, technological, physical/material, operational/maintenance, and safety/environmental factors.

Step by step solution

01

- Define Irreversibilities

Irreversibilities in a thermodynamic system refer to the inefficiencies that occur due to factors like friction, unrestrained expansion, mixing of different substances, heat transfer across a finite temperature difference, etc. These reduce the system's performance by increasing entropy and energy losses.
02

- Acknowledge Methods to Reduce Irreversibilities

Several methods can be used to reduce irreversibilities, such as optimizing the design of components, improving insulation, reducing friction through lubrication, and increasing heat exchanger effectiveness.
03

- Identify Practical Constraints

While reducing irreversibilities is beneficial, practical constraints often limit the extent of these improvements. These include:
04

Step 3.1 - Financial Constraints

Implementing advanced materials, better designs, and more efficient processes typically requires significant investment. Budget limitations can restrict the extent of these measures.
05

Step 3.2 - Technological Constraints

Current technological limitations might prevent the complete elimination of irreversibilities. For example, perfect insulation or zero-friction surfaces are not yet achievable with present technology.
06

Step 3.3 - Physical and Material Constraints

Physical properties of materials, such as thermal conductivity and strength, can limit the effectiveness of certain techniques to reduce irreversibility. Additionally, space constraints might limit design modifications.
07

Step 3.4 - Operational and Maintenance Constraints

Modifications that reduce irreversibilities might require more complex operations and maintenance. Increased complexity can result in higher operational and maintenance costs, as well as the need for specialized personnel.
08

Step 3.5 - Safety and Environmental Constraints

Changes to reduce irreversibilities must also consider safety and environmental regulations. Some techniques might have trade-offs that could compromise safety or have adverse environmental impacts.

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Key Concepts

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

Entropy Increase
Entropy is a measure of disorder or randomness in a system. In thermodynamics, an increase in entropy indicates a loss of useful energy. Every real process increases the entropy of the universe, making some energy unavailable for work. Processes like heat transfer, friction, and mixing of different substances all contribute to this increase.
To minimize entropy increase, you can:
  • Optimize heat transfer processes to minimize temperature differences.
  • Use advanced materials with better insulating properties.
  • Improve system designs to reduce friction and turbulence.
While reducing entropy increase can lead to more efficient systems, it is constrained by factors like budget, current technology, and physical material properties.
Energy Losses
Energy losses in a thermodynamic system occur due to irreversibilities, resulting in less efficient performance. Common sources of energy losses include friction, uncontrolled expansions, and heat transfer across temperature gradients.
To reduce energy losses, you can:
  • Implement better insulation to reduce heat losses.
  • Optimize fluid flow to minimize frictional losses.
  • Use more efficient components and design improvements.
These improvements, however, are often constrained by factors such as cost, material limits, and maintenance requirements.
Friction Reduction
Friction is a major source of irreversibility in thermodynamic processes, causing both entropy increase and energy losses. Reducing friction can significantly improve system performance.
Techniques to reduce friction include:
  • Using lubrication to reduce surface friction.
  • Implementing smoother surface finishes on machinery components.
  • Adopting aerodynamic and hydrodynamic designs to streamline fluid flow.
While these methods can reduce friction, practical constraints like financial, physical, and technological limitations often impact their implementation. Effective friction management requires balancing these factors to achieve optimal system performance.
Heat Exchanger Effectiveness
Heat exchangers are crucial for transferring heat between fluids without mixing them. Their effectiveness directly influences the overall efficiency of thermodynamic systems. A more effective heat exchanger minimizes temperature differences between fluids, reducing irreversibilities and energy losses.
To improve heat exchanger effectiveness, consider the following:
  • Optimize design to maximize surface area for heat transfer.
  • Use materials with high thermal conductivity.
  • Maintain clean surfaces to prevent fouling, which reduces efficiency.
Practical limitations such as space, cost, and material properties must be taken into account. Achieving a balance between these constraints and heat exchanger effectiveness is essential for improving system performance.

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Most popular questions from this chapter

Air enters an insulated compressor operating at steady state at 1 bar, \(350 \mathrm{~K}\) with a mass flow rate of \(1 \mathrm{~kg} / \mathrm{s}\) and exits at 4 bar. The isentropic compressor efficiency is \(82 \%\). Determine the power input, in \(\mathrm{kW}\), and the rate of entropy production, in \(\mathrm{kW} / \mathrm{K}\), using the ideal gas model with (a) data from Table A-22. (b) \(I T\). (c) a constant specific heat ratio, \(k=1.39\).

Steam is contained in a large vessel at \(100 \mathrm{lbf} / \mathrm{in} .^{2}, 450^{\circ} \mathrm{F}\). Connected to the vessel by a valve is an initially evacuated tank having a volume of \(1 \mathrm{ft}^{3}\). The valve is opened until the tank is filled with steam at pressure \(p\). The filling is adiabatic, kinetic and potential energy effects are negligible, and the state of the large vessel remains constant. (a) If \(p=100 \mathrm{lbf} / \mathrm{in} .^{2}\), determine the final temperature of the steam within the tank, in \({ }^{\circ} \mathrm{F}\), and the amount of entropy produced within the tank, in \(\mathrm{Btu} /{ }^{\circ} \mathrm{R}\). (b) Plot the quantities of part (a) versus presssure \(p\) ranging from 10 to \(100 \mathrm{lbf} / \mathrm{in}\).

The temperature of a 12 -oz \((0.354-\mathrm{L})\) can of soft drink is reduced from 20 to \(5^{\circ} \mathrm{C}\) by a refrigeration cycle. The cycle receives energy by heat transfer from the soft drink and discharges energy by heat transfer at \(20^{\circ} \mathrm{C}\) to the surroundings. There are no other heat transfers. Determine the minimum theoretical work input required by the cycle, in \(\mathrm{kJ}\), assuming the soft drink is an incompressible liquid with the properties of liquid water. Ignore the aluminum can.

Ammonia enters a valve as a saturated liquid at 7 bar with a mass flow rate of \(0.06 \mathrm{~kg} / \mathrm{min}\) and is steadily throttled to a pressure of 1 bar. Determine the rate of entropy production in \(\mathrm{kW} / \mathrm{K}\). If the valve were replaced by a power-recovery turbine operating at steady state, determine the maximum theoretical power that could be developed, in \(\mathrm{kW}\). In each case, ignore heat transfer with the surroundings and changes in kinetic and potential energy. Would you recommend using such a turbine?

Air is compressed in an axial-flow compressor operating at steady state from \(27^{\circ} \mathrm{C}, 1\) bar to a pressure of \(2.1\) bar. The work input required is \(94.6 \mathrm{~kJ}\) per \(\mathrm{kg}\) of air flowing through the compressor. Heat transfer from the compressor occurs at the rate of \(14 \mathrm{~kJ}\) per \(\mathrm{kg}\) at a location on the compressor's surface where the temperature is \(40^{\circ} \mathrm{C}\). Kinetic and potential energy changes can be ignored. Determine (a) the temperature of the air at the exit, in \({ }^{\circ} \mathrm{C}\). (b) the rate at which entropy is produced within the compressor, in \(\mathrm{kJ} / \mathrm{K}\) per \(\mathrm{kg}\) of air flowing.
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