1. Define dryness fraction and degree of superheat and show their applications in a steam power plant.
2. Explain the use of steam table and Mollier diagram
The Correct Answer and Explanation is :
1. Dryness Fraction and Degree of Superheat
Dryness Fraction:
The dryness fraction (or quality) of steam is the ratio of the mass of dry steam to the total mass of the steam-water mixture. It is a measure of the amount of moisture present in the steam. It is denoted by (x) and is expressed as:
[
x = \frac{m_{\text{dry steam}}}{m_{\text{total steam-water mixture}}}
]
- If (x = 1), the steam is considered to be dry, meaning there is no moisture.
- If (x = 0), the steam is completely wet, consisting entirely of water.
Degree of Superheat:
The degree of superheat is the amount by which the temperature of steam exceeds its saturation temperature at a given pressure. It is expressed as:
[
\text{Degree of Superheat} = T_{\text{actual}} – T_{\text{saturation}}
]
Where:
- (T_{\text{actual}}) is the actual temperature of the steam.
- (T_{\text{saturation}}) is the temperature at which steam would exist in equilibrium with liquid water at the same pressure.
Applications in a Steam Power Plant:
- Dryness Fraction: In a steam power plant, it is crucial to monitor the dryness fraction because wet steam (low dryness fraction) can cause damage to turbines and reduce efficiency. Keeping steam dry ensures the proper functioning of turbines and improves efficiency.
- Degree of Superheat: Superheated steam is used to drive turbines more effectively. Higher superheating allows for greater energy transfer and improves turbine efficiency. Superheating also prevents the formation of condensation in the turbine blades, which could damage the blades or reduce performance.
2. Use of Steam Table and Mollier Diagram
Steam Table:
A steam table is a reference table that provides the thermodynamic properties of water and steam at various pressures and temperatures. It contains information about:
- Saturation temperature
- Saturation pressure
- Enthalpy of water and steam
- Entropy of water and steam
The steam table is useful for determining the phase and properties of steam at different conditions. For instance, when the pressure and temperature of steam are known, the steam table allows engineers to find out the specific enthalpy or entropy values, which are essential for calculating the work output in steam engines and turbines.
Mollier Diagram (h-s Diagram):
The Mollier diagram is a graphical representation of the relationship between enthalpy (h) and entropy (s) for steam. It is a powerful tool for analyzing steam cycles, especially for calculating the work and efficiency of steam engines and turbines.
In a Mollier diagram:
- The horizontal axis represents entropy (s).
- The vertical axis represents enthalpy (h).
- The curve represents the saturated liquid and vapor states.
- Areas within the diagram can show superheated steam conditions, allowing engineers to trace steam properties over various processes, such as expansion in a turbine or heat addition in a boiler.
Applications in a Steam Power Plant:
- Steam Table: Used to calculate the steam’s energy content and to track the performance of the boiler, condenser, and turbines. It helps engineers to evaluate efficiency, determine the dryness fraction, and monitor the health of the system.
- Mollier Diagram: Used to visualize and optimize the Rankine cycle by analyzing enthalpy and entropy changes through the turbine, pump, and condenser. The diagram is key in determining ideal turbine work, pump power, and overall system efficiency.
Both tools are essential for designing, optimizing, and maintaining steam power plants, as they provide accurate thermodynamic information needed for performance analysis.