HS, TS, PV, PH Diagrams in Steam Properties

Definition

In thermodynamics, property diagrams (HS, TS, PV, and PH) are graphical representations used to visualize the state changes of a substance, such as water and steam, during various processes. These diagrams plot two thermodynamic properties against each other to help engineers analyze energy transfers, heat exchange, and work output in steam power cycles.

Main Content

1. PV (Pressure-Volume) Diagram

• The PV diagram plots Pressure (P) on the y-axis and Specific Volume (v) on the x-axis. It is primarily used to calculate the work done during a process, as the area under the curve represents work.
• It helps visualize the expansion or compression of steam, such as in the cylinder of a steam engine.

2. TS (Temperature-Entropy) Diagram

• The TS diagram plots Temperature (T) on the y-axis and Entropy (s) on the x-axis. This is the most crucial diagram for analyzing heat transfer.
• The area under the curve in a TS diagram represents the total heat added or rejected during a reversible process.

3. HS (Enthalpy-Entropy) Diagram (Mollier Chart)

• The HS diagram, often called the Mollier Chart, plots Enthalpy (h) on the y-axis and Entropy (s) on the x-axis. It is highly valued for its simplicity in analyzing adiabatic processes, such as steam expanding through a turbine.
• Vertical lines on this diagram represent isentropic (constant entropy) processes, which are ideal for calculating turbine efficiency.

4. PH (Pressure-Enthalpy) Diagram

• The PH diagram plots Pressure (P) on the y-axis and Enthalpy (h) on the x-axis. It is extensively used in refrigeration and steam plant analysis to track the heat content (enthalpy) changes relative to system pressure.

Basic Phase Dome (Common to most diagrams):
    |       / \
    |      /   \  <-- Superheated Region
    |     /  |  \
 P  |    /   |   \
    |   /    |    \
    |  /     |     \  <-- Wet Steam
    | /______|______\
    +------------------->
             v (Volume)

Working / Process

1. Analyzing Isothermal Processes

• In a TS diagram, an isothermal process appears as a horizontal line because the temperature remains constant.
• This represents a phase change process, such as boiling water into steam at a constant pressure.

2. Analyzing Isentropic Processes

• In an HS diagram, an isentropic process is represented by a perfect vertical line.
• This is the standard method for calculating the ideal work output of a steam turbine, where entropy remains constant.

3. Analyzing Isobaric Processes

• In a PV diagram, an isobaric process is a horizontal line because pressure remains constant.
• This occurs during the constant-pressure heat addition stage in a boiler.

Advantages / Applications

• Turbine Design: HS diagrams allow engineers to quickly determine the quality of steam after expansion in a turbine.
• Heat Engine Analysis: TS diagrams are essential for calculating the thermal efficiency of the Rankine cycle.
• System Safety: PV diagrams help in designing pressure vessels by showing the relationship between volume changes and potential pressure spikes.
• Process Monitoring: PH diagrams are used in power plants to monitor enthalpy changes in the feed-water heaters and boilers.

Summary

• The PV diagram is used to determine work output based on volume changes.
• The TS diagram is used to analyze heat transfer using entropy and temperature.
• The HS diagram (Mollier Chart) simplifies calculations for turbine performance.
• The PH diagram tracks enthalpy changes across pressure variations.
• Key terms: Entropy (measure of disorder), Enthalpy (total heat content), Isentropic (constant entropy), and Isobaric (constant pressure).