7.4 Thermodynamic charts and their applications

Thermodynamic charts are visual tools that help us understand how fluids behave under different conditions. They show relationships between properties like pressure, temperature, and entropy, making it easier to analyze and solve complex problems.

These charts are crucial for engineers and scientists working with fluids. By using them, we can quickly see how changes in one property affect others, helping us design more efficient systems and make better decisions in real-world applications.

Thermodynamic Diagrams

Mollier Diagram and Temperature-Entropy (T-s) Diagram

• Mollier diagram plots enthalpy (h) on the vertical axis and entropy (s) on the horizontal axis
• Used to analyze thermodynamic processes and cycles involving steam or other working fluids
• Temperature-entropy (T-s) diagram plots temperature on the vertical axis and entropy on the horizontal axis
  • Useful for analyzing ideal gas processes and cycles
  • Constant pressure lines are typically shown as near-vertical lines
  • Constant volume lines are typically shown as near-horizontal lines
• Both diagrams allow for easy visualization of thermodynamic processes such as isothermal (constant temperature), isobaric (constant pressure), and isentropic (constant entropy) processes

Pressure-Enthalpy (P-h) Diagram and Psychrometric Chart

• Pressure-enthalpy (P-h) diagram plots pressure on the vertical axis and enthalpy on the horizontal axis
  • Commonly used for analyzing refrigeration cycles and heat pump systems
  • Saturated liquid and saturated vapor lines form a dome-shaped region on the diagram
  • Constant temperature lines are typically shown as near-horizontal lines within the saturated region
• Psychrometric chart is a graphical representation of the thermodynamic properties of moist air
  • Plots various properties such as dry-bulb temperature, wet-bulb temperature, relative humidity, and specific humidity
  • Used in the analysis and design of air-conditioning systems and other processes involving moist air
  • Processes such as heating, cooling, humidification, and dehumidification can be easily visualized on the chart

Compressibility Factor Charts

Compressibility Factor and Its Applications

• Compressibility factor (Z) is a measure of the deviation of a real gas from ideal gas behavior
  • Defined as the ratio of the actual volume of a gas to the volume it would occupy if it were an ideal gas at the same temperature and pressure
  • Z = 1 for an ideal gas, and Z ≠ 1 for a real gas
• Compressibility factor charts, also known as Z-charts, plot the compressibility factor as a function of reduced pressure and reduced temperature
  • Reduced pressure (P_r) is the ratio of the actual pressure to the critical pressure of the gas
  • Reduced temperature (T_r) is the ratio of the actual temperature to the critical temperature of the gas
• Z-charts are used to determine the compressibility factor of a real gas at given conditions, which is essential for accurate calculations involving real gas properties and processes

Generalized Enthalpy and Entropy Departure Charts

• Generalized enthalpy departure charts plot the difference between the actual enthalpy of a real gas and the enthalpy of an ideal gas (enthalpy departure) as a function of reduced pressure and reduced temperature
  • Used to determine the actual enthalpy of a real gas at given conditions
  • Enthalpy departure is typically expressed as (h - h_ideal) / RT_c, where R is the gas constant and T_c is the critical temperature
• Generalized entropy departure charts plot the difference between the actual entropy of a real gas and the entropy of an ideal gas (entropy departure) as a function of reduced pressure and reduced temperature
  • Used to determine the actual entropy of a real gas at given conditions
  • Entropy departure is typically expressed as (s - s_ideal) / R, where R is the gas constant
• Both enthalpy and entropy departure charts are essential tools for analyzing thermodynamic processes and cycles involving real gases, as they account for the non-ideal behavior of gases at high pressures and low temperatures