Steam properties and use of steam tables

Definition

Steam properties are the thermodynamic characteristics that describe the state and behavior of steam, such as pressure, temperature, specific volume, enthalpy, entropy, and quality. Steam tables are tabulated reference data that list these properties of water and steam at different pressures and temperatures, allowing engineers and students to find unknown values without performing complicated calculations.

Main Content

1. Thermodynamic properties of steam

Pressure, temperature, and specific volume

• are the primary state variables used to describe steam. Pressure is the force exerted by steam per unit area, temperature indicates the level of thermal energy, and specific volume is the volume occupied by unit mass of steam. These properties together determine the state of steam in a system.

Enthalpy, internal energy, and entropy

• are especially important in engineering calculations. Enthalpy represents the total heat content of steam, internal energy refers to the energy stored within the steam due to molecular activity, and entropy measures the degree of disorder or energy unavailable for work. For example, in turbines and boilers, enthalpy change helps calculate work and heat transfer.

2. States of water and steam

Compressed (subcooled) liquid, saturated liquid, wet steam, dry saturated steam, and superheated steam

• are the major states of water and steam. A compressed liquid exists when water is below saturation temperature at a given pressure. Saturated liquid is about to vaporize, and dry saturated steam is just vapor with no liquid droplets.

Wet steam and superheated steam

• are very important in practical systems. Wet steam is a mixture of liquid water and vapor, described by dryness fraction or quality. Superheated steam is steam heated beyond the saturation temperature at the same pressure. For example, steam entering a turbine is often superheated to reduce moisture formation during expansion.

3. Steam tables and their use

Saturation tables, superheated steam tables, and compressed liquid tables

• are the main forms of steam tables. Saturation tables provide properties at boiling conditions for different pressures and temperatures, while superheated tables give properties when steam is heated above saturation. Compressed liquid tables are used for liquid water under pressure, though approximations are often used because liquid properties change very little with pressure.

Steam tables help determine unknown properties

• when only two independent properties of a state are known. For instance, if pressure and temperature are given, the corresponding enthalpy and entropy can be read from the table. If pressure and dryness fraction are known, the exact values of specific volume, enthalpy, and entropy of wet steam can be calculated using mixture relations.

Working / Process

1. Identify the state of steam

Determine whether the fluid is compressed liquid, saturated liquid, wet steam, or superheated steam using the given pressure, temperature, or quality. This is the first and most important step because the correct table depends on the state.

2. Select the appropriate steam table

Use saturation tables when the steam is at boiling conditions, superheated tables when temperature is above saturation temperature, and compressed liquid values when the water is pressurized below the saturation state. Matching the state correctly avoids major errors in calculation.

3. Read or calculate the required properties

Once the proper table is selected, use the given data to locate the row or column and read the needed values such as enthalpy, entropy, or specific volume. For wet steam, apply mixture formulas:

• $v = v_f + x(v_g - v_f)$
• $h = h_f + x(h_g - h_f)$
• $s = s_f + x(s_g - s_f)$ where $x$ is the dryness fraction. Example: if steam is wet with $x = 0.8$, the enthalpy is found by combining the saturated liquid and saturated vapor values in the table.

Advantages / Applications

Accurate thermodynamic calculations

• for boilers, turbines, condensers, and nozzles, making steam tables essential in power plant analysis and design.

Simple and reliable property lookup

• without requiring complex equations of state, especially useful for students and engineers during examinations and practical work.

Wide industrial use

• in thermal power stations, steam engines, process heating, sterilization, textile industries, refineries, and food processing, where steam is used as a source of heat and work.

Summary

• Steam properties describe the thermodynamic state of water and steam.
• Steam tables provide reliable reference values for engineering calculations.
• Correct identification of steam state is necessary to use the proper table.
• Steam tables are fundamental in power generation and thermal system analysis.
• Important terms to remember: pressure, temperature, specific volume, enthalpy, entropy, quality, saturated steam, superheated steam.