TV Diagram (Temperature-Specific Volume Diagram)

Definition

The TV diagram (Temperature-Specific Volume diagram) is a thermodynamic graphical representation used to visualize the phase changes of a pure substance, such as water, as it transitions between liquid, wet vapor, and superheated steam states when heated at a constant pressure.

Main Content

1. The Saturation Dome

• The TV diagram is characterized by a "dome" shape that separates the liquid, liquid-vapor mixture, and vapor phases.
• The left side of the dome represents the Saturated Liquid Line (where the substance is entirely liquid at boiling point), while the right side represents the Saturated Vapor Line (where the substance is entirely dry steam).

2. Phase Regions

• Compressed Liquid Region: Located to the left of the saturation dome, where the substance exists as a subcooled liquid.
• Wet Vapor (Mixture) Region: The area inside the dome where liquid and vapor coexist in equilibrium.
• Superheated Vapor Region: Located to the right of the saturation dome, where the steam temperature exceeds the saturation temperature for a given pressure.

3. Constant Pressure Lines (Isobars)

• On a TV diagram, constant pressure lines move upward and to the right.
• As heat is added at constant pressure, the specific volume increases; once the substance enters the two-phase region, it undergoes a phase change while the temperature remains constant.

Temperature (T)
^
|      / \  (Critical Point)
|     /   \
|    /     \
|   /       \
|--/---------\---- Constant Pressure Line
| /  (Dome)   \
|/_____________\__________> Specific Volume (v)
   Liquid | Mixture | Vapor

Working / Process

1. Heating of Compressed Liquid

• The process begins with liquid water being heated. As energy is added, the temperature increases, but the specific volume remains relatively constant.
• This continues until the liquid reaches its saturation temperature for that specific pressure.

2. Phase Transition (Evaporation)

• Once the saturation temperature is reached, adding more heat causes the liquid to turn into vapor without an increase in temperature.
• During this stage, the substance moves horizontally across the inside of the dome from left (saturated liquid) to right (saturated vapor).

3. Superheating of Vapor

• After all the liquid has evaporated into dry saturated steam, additional heat input raises the temperature of the vapor.
• The substance now occupies the superheated region, and both its temperature and specific volume increase significantly.

Advantages / Applications

• It is essential for engineers to determine the state of steam in power plants and boilers.
• It helps in calculating the "Quality of Steam" (dryness fraction) within the mixture region.
• It provides a visual tool for analyzing thermodynamic cycles, such as the Rankine cycle, which powers steam turbines.

Summary

The TV diagram is a vital thermodynamic tool that maps the relationship between temperature and specific volume to track how water transforms into steam. By identifying positions relative to the saturation dome, engineers can determine whether a substance is a liquid, a mixture, or superheated steam.

• Critical Point: The peak of the dome where liquid and vapor phases become indistinguishable.
• Dryness Fraction (x): A measure of the mass of vapor compared to the total mass in the mixture region.
• Sensible Heat: Heat required to raise the temperature to boiling.
• Latent Heat: Heat required to convert liquid to steam at a constant temperature.