Book cover for Thermodynamics: An Engineering Approach

Thermodynamics: An Engineering Approach

Yunus A. Cengel, Michael A. Boles

ISBN #9781259822674

9th Edition

2,694 Questions

Group icon
59,300 Students Helped

Homework Questions

Right arrow
Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter explores the principles of gas–vapor mixtures and their critical role in air-conditioning and human comfort applications. Key properties such as specific and relative humidity are defined and analyzed with tools like psychrometric charts. The chapter covers a variety of processes including heating, cooling, humidification, dehumidification, and evaporative cooling, and explains the importance of energy and mass balance equations combined with ideal gas assumptions. Students learn to apply these concepts to practical engineering problems involving dew-point temperature, wet-bulb temperature, adiabatic saturation, and mixing of airstreams.

Learning Objectives

1

Describe the fundamental properties of gas–vapor mixtures including specific humidity and relative humidity.

2

Utilize psychrometric charts to analyze and visualize air-conditioning processes such as heating, cooling, humidification, and dehumidification.

3

Apply energy and mass balance equations along with ideal gas assumptions to solve practical engineering problems in air-conditioning.

4

Explain the concepts of dew-point temperature, wet-bulb temperature, and adiabatic saturation in gas–vapor mixtures.

5

Analyze real-world processes including evaporative cooling, adiabatic mixing of airstreams, and the operation of wet cooling towers.

Key Concepts

CONCEPT

DEFINITION

Gas–Vapor Mixtures

A combination of gases where one component is a vapor, commonly encountered in atmospheric air used in air-conditioning applications.

Specific Humidity

The mass of water vapor per unit mass of dry air, a key parameter in assessing moisture content in air.

Relative Humidity

The ratio of the actual mass of water vapor in air to the maximum possible mass of water vapor at the same temperature, expressed as a percentage.

Dew-Point Temperature

The temperature at which air becomes saturated and water vapor begins to condense into liquid.

Wet-Bulb Temperature

The lowest temperature that can be reached solely by evaporative cooling of a water-soaked surface, indicated on psychrometric charts.

Adiabatic Saturation

A process in which air is brought into contact with water and allowed to reach saturation without the transfer of heat to or from the surroundings.

Psychrometric Chart

A graphical representation that shows the thermodynamic properties of moist air and enables tracking changes such as heating, cooling, humidification, and dehumidification.

Energy and Mass Balance Equations

Fundamental equations based on the conservation of energy and mass which are applied in analyzing air-conditioning processes under ideal gas assumptions.

Example Problems

Example 1

What is the difference between dry air and atmospheric air?

Example 2

What is vapor pressure?

Example 3

What is the difference between the specific humidity and the relative humidity?

Example 4

Can the water vapor in air be treated as an ideal gas? Explain.

Example 5

Explain how vapor pressure of the ambient air is determined when the temperature, total pressure, and the relative humidity of air are given.
Scroll left
Scroll right

Step-by-Step Explanations

QUESTION

Given an air sample with a specific humidity of 0.010 kg water/kg dry air at a temperature where the saturation specific humidity is 0.015 kg water/kg dry air, determine the relative humidity.

STEP-BY-STEP ANSWER:

Step 1: Identify the actual specific humidity (0.010 kg water/kg dry air).
Step 2: Identify the saturation specific humidity at the given temperature (0.015 kg water/kg dry air).
Step 3: Calculate the ratio by dividing the actual specific humidity by the saturation specific humidity.
Step 4: Multiply the ratio by 100 to convert to a percentage.
Final Answer: (0.010 / 0.015) x 100 = 66.67% relative humidity.

Determining Relative Humidity

QUESTION

How can a psychrometric chart be used to determine the dew-point temperature for given air conditions?

STEP-BY-STEP ANSWER:

Step 1: Plot the point on the psychrometric chart corresponding to the measured dry-bulb temperature and relative humidity.
Step 2: Follow the constant moisture content line horizontally leftwards until it intersects the saturation curve.
Step 3: The temperature at this intersection is the dew-point temperature.
Final Answer: The dew-point temperature is identified at the saturation curve intersection along the constant specific humidity line.

Utilizing a Psychrometric Chart

QUESTION

What steps are involved in analyzing a process where air is heated while maintaining its humidity level constant?

STEP-BY-STEP ANSWER:

Step 1: Use the initial state from the psychrometric chart to determine the starting specific humidity and dry-bulb temperature.
Step 2: Identify the heating process on the chart, which moves the state point vertically upward (increase in dry-bulb temperature) at constant specific humidity.
Step 3: Determine how the relative humidity changes as the dry temperature increases while the moisture content stays the same.
Step 4: Conclude the final state by reading off the new dry temperature and corresponding relative humidity from the chart.
Final Answer: The final state is determined by the vertical movement on the psychrometric chart, preserving specific humidity while increasing temperature, resulting in lower relative humidity.

Analyzing Heating with Humidification

Scroll left
Scroll right

Common Mistakes

  • Confusing specific humidity with relative humidity; specific humidity is a mass ratio, whereas relative humidity is a percentage of saturation.
  • Assuming that changes in dry-bulb temperature do not affect relative humidity, despite specific humidity remaining constant during certain processes.
  • Over-reliance on simplified ideal gas assumptions without considering potential measurement errors or non-ideal behavior in real-world applications.
  • Misinterpreting psychrometric chart lines, such as constant moisture content lines, which leads to inaccurate determinations of properties like dew-point and wet-bulb temperatures.
  • Neglecting the impact of energy and mass balance in more complex processes such as adiabatic mixing or evaporative cooling.