Use of Steam Tables and Properties of Steam

Definition

Steam tables are comprehensive reference data sets that provide the thermodynamic properties of water and steam—such as pressure, temperature, specific volume, enthalpy, and entropy—at various states. These tables are essential tools for engineers to determine the energy state of steam without needing complex experimental measurements for every calculation.

Main Content

1. Thermodynamic States of Water

Compressed Liquid: Water exists in a liquid state below its saturation temperature for a given pressure.
Saturated Liquid/Vapor: The point at which water is at the exact temperature where it is about to boil (liquid) or has just finished condensing (vapor).
Superheated Steam: Steam heated to a temperature higher than its saturation temperature, containing no liquid droplets.

2. Properties Recorded in Tables

Specific Volume ($v$): The volume occupied by a unit mass of the substance ($m^3/kg$).
Enthalpy ($h$): The total heat content of the system, defined as $u + Pv$ (Internal energy + Flow work).
Entropy ($s$): A measure of the thermal energy per unit temperature that is unavailable for doing useful work.

3. The Quality Factor ($x$)

Definition: The dryness fraction, representing the mass ratio of vapor to the total mass in a mixture of liquid and steam.
Formula: $h_{total} = h_f + x(h_g - h_f)$, where $h_f$ is saturated liquid enthalpy and $h_g$ is saturated vapor enthalpy.

Phase Change Diagram (T-v)
Temperature (T)
      |         / (Superheated)
      |        / 
      |  (Sat)|  (Liquid)
      |  Vapor|      
      |  /    |      
      | /     |      
      |/______|______ Volume (v)
       Critical Point

Working / Process

1. Identifying the State

Determine the known parameters (usually Pressure and Temperature).
Check if the steam is Saturated (Pressure = Saturation Pressure) or Superheated (Temperature > Saturation Temperature).

2. Selecting the Correct Table

Use Saturated Steam Tables if the state is on the saturation curve (Pressure-based or Temperature-based tables).
Use Superheated Steam Tables if the temperature provided is higher than the saturation temperature at the given pressure.

3. Interpolation for Intermediate Values

When given values fall between two rows in the table, use linear interpolation.
Example: If the pressure is 1.5 bar and the table lists 1.0 and 2.0 bar, find the average to approximate the specific property.

Advantages / Applications

Power Plant Efficiency: Steam tables are used to calculate the performance of steam turbines and boilers in electricity generation.
System Design: Engineers use these to size piping and heat exchangers to ensure the steam carries the required amount of energy to the process.
Safety Analysis: They help in predicting pressure relief requirements by determining how steam volume changes under specific thermal conditions.

Summary

Steam tables serve as the standardized lookup resource for analyzing the thermodynamic properties of water and steam across different phases. By utilizing these tables, engineers can accurately determine internal energy, enthalpy, and entropy, which are crucial for the efficient design and operation of thermal systems.

Important terms to remember: Saturation Temperature, Enthalpy of Vaporization, Quality (Dryness Fraction), and Specific Volume.