2. Properties of Steam Notes

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Subject - THERMAL ENGINEERING - II ME 4003
Branch - Mechanical Engineering
Semester - 4th Semester

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PROPERTIES OF STEAM

Steam plays a crucial role in power generation, industrial heating, and mechanical applications. It is produced by heating water in a steam boiler and is classified into different types based on pressure, dryness fraction, and energy content.

This section covers the types of steam boilers, steam properties, steam tables, Mollier charts, and steam calorimeters used to analyze steam conditions in practical applications.

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2.1 Different Types of Steam Boilers and Their Classification

A steam boiler is a closed vessel in which water is heated to produce steam. Boilers are classified based on different parameters such as working pressure, circulation method, fuel type, and design.

Classification of Steam Boilers

1. Based on Water and Hot Gas Flow

   • Fire-Tube Boiler – Hot gases pass through tubes surrounded by water (e.g., Cochran Boiler, Lancashire Boiler).

   • Water-Tube Boiler – Water flows inside tubes, and hot gases surround them (e.g., Babcock & Wilcox Boiler).

2. Based on Pressure

   • Low-Pressure Boiler – Operates below 15 bar (e.g., Cochran Boiler).

   • High-Pressure Boiler – Operates above 15 bar (e.g., Benson Boiler, Lamont Boiler).

3. Based on Circulation of Water

   • Natural Circulation Boiler – Water circulates naturally due to density difference (e.g., Lancashire Boiler).

   • Forced Circulation Boiler – Uses a pump to circulate water (e.g., Velox Boiler, LaMont Boiler).

4. Based on Fuel Type

   • Coal-Fired Boilers – Use coal as fuel.

   • Oil-Fired Boilers – Use petroleum-based fuels.

   • Gas-Fired Boilers – Use natural gas.

   • Electric Boilers – Use electricity to generate steam.

5. Based on Mobility

   • Stationary Boiler – Used in fixed locations (e.g., Power Plants).

   • Mobile Boiler – Used in locomotives, ships, etc.

6. Based on Application

   • Utility Boilers – Used for electricity generation in thermal power plants.

   • Industrial Boilers – Used in chemical plants, refineries, and food processing industries.

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2.2 Definitions of Properties of Steam

Steam has several important properties that determine its energy content and behavior in applications like turbines and heat exchangers.

1. Saturation Temperature ($T_s$)

• The temperature at which water starts boiling at a given pressure.

• Example: At atmospheric pressure (1 bar), water boils at 100°C.

2. Saturation Pressure ($P_s$)

• The pressure at which water and steam coexist at a given temperature.

• Example: At 100°C, the saturation pressure is 1.013 bar.

3. Dryness Fraction ($x$)

• The ratio of dry steam mass to total mass (steam + water).

• Values:

  • $x = 0$ → Saturated Water (Liquid Phase)

  • $0 < x < 1$ → Wet Steam

  • $x = 1$ → Dry and Saturated Steam

4. Enthalpy of Steam ($h$)

• The total heat content of steam, measured in kJ/kg.

• Types of enthalpy:

  • Sensible Heat ($h_f$) – Heat required to raise water temperature to boiling.

  • Latent Heat ($h_{fg}$) – Heat required to convert water to steam.

  • Total Enthalpy ($h_g$) – Sum of sensible and latent heat:

    $$h_g = h_f + h_{fg}$$

5. Specific Volume ($v$)

• Volume occupied by 1 kg of steam.

• Wet steam, dry steam, and superheated steam have different specific volumes.

6. Internal Energy ($u$)

• The total energy stored in the steam due to heat.

7. Superheated Steam

• Steam heated above its saturation temperature at a given pressure.

• Example: If steam at 1 bar is heated beyond 100°C, it becomes superheated.

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2.4 Determination of Properties of Steam Using Steam Tables and Mollier Chart

1. Steam Tables

Steam tables provide values for enthalpy, entropy, specific volume, and temperature at different pressures. These tables help engineers determine the properties of steam in different states.

Example from a Steam Table (1 bar pressure):

• Saturation Temperature = 100°C

• Enthalpy of water ($h_f$) = 419 kJ/kg

• Latent Heat ($h_{fg}$) = 2257 kJ/kg

• Enthalpy of steam ($h_g$) = 2676 kJ/kg

2. Mollier Chart (h-s Diagram)

• A graphical representation of enthalpy (h) vs. entropy (s).

• Used to find steam properties when one parameter (pressure, temperature, enthalpy) is known.

• Commonly used in turbine and refrigeration calculations.

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2.5 Simple Problems Using Steam Tables and Mollier Chart

Example 1: Find the Enthalpy of Wet Steam

Given:

• Pressure = 10 bar

• Dryness fraction = 0.8

• From steam tables:

  • $h_f=\mathrm{762\; kJ/kg}$

  • $h_{fg} = 2015$ kJ/kg

Solution:

$$h = h_f + x \cdot h_{fg}$$ $$h = 762 + (0.8 \times 2015) = 2374 \text{ kJ/kg}$$

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2.6 Steam Calorimeters

A steam calorimeter is a device used to measure the dryness fraction (x) of steam. It helps determine the quality of steam used in turbines and boilers.

Types of Steam Calorimeters:

1. Throttle Calorimeter

   • Steam is throttled (expanded) to a lower pressure, and its temperature is measured.

   • The dryness fraction is calculated using energy balance equations.

2. Separating Calorimeter

   • Separates water droplets from wet steam to determine the dryness fraction.

3. Combined Calorimeter

   • Uses both throttling and separating methods for accurate results.

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Conclusion

Steam is an important working fluid in thermal power plants, industries, and mechanical systems. Understanding steam properties and using steam tables, Mollier charts, and calorimeters helps engineers optimize efficiency and performance.