Steam: Generation, Types and Volume | Energy Management

In this article we will discuss about:- 1. Introduction to Steam 2. Generation of Steam 3. Types 4. Specific Volumes.

Introduction to Steam:

Steam comes under the category of pure substance. A pure substance is one whose chemical composition does not change with the change of phase during thermodynamic processes. It is a homogeneous substance.

A pure substance can exist in three phases:

(a) Solid phase as ice

(b) Liquid phase as water

(c) Gaseous phase as steam

When ice melts with the addition of heat, it enters liquid phase to form water. Similarly, when water is heated, it evaporates and liquid phase is converted to steam, which may be called as vapor. During this transformation, known as vaporization, it remains as a two-phase mixture of water and steam. After the completion of vaporization, it remains in gaseous phase as steam.

The important properties of steam are pressure, temperature, specific volume, enthalpy, internal energy, and entropy.

Generation of Steam:

Steam is considered as the best working substance for steam prime mover. The generation of steam takes place in a boiler/steam generator. The water is supplied to the boiler at atmospheric pressure and temperature, and water is transformed into steam with the application of heat produced after the combustion of fuel in the boiler furnace. As the generation of steam takes place continuously, its pressure increases and it is supplied to the steam prime mover at constant pressure.

To know about the different properties of steam at a particular pressure, water is considered at 0°C at a constant pressure p. Since the generation of steam takes place at constant pressure, the amount of heat supplied to convert water into steam is equivalent to its enthalpy. Enthalpy h as per definition,

h = u + pv

From the first law of thermodynamics,

dq = du + p dv

= du + d(pv)

= d(u + pv)

= dh

At constant pressure, amount of heat added to convert water into steam becomes equal to the change in enthalpy. The generation of steam can be understood as shown in Fig. 2.26.

There are six stages, as shown in Fig 2.26, which can be explained as follows:

Consider 1 kg of water at 0°C is kept inside a cylinder fitted with a frictionless piston with a weight placed over it so that the total weight of piston and weight exerts a pressure p.

The initial position of water is indicated by “a” on temperature- enthalpy plot as shown in Fig. 2.27. When 1 kg of water at saturation temperature t_s converts into 1 kg of dry steam at the same saturation temperature corresponding to a given constant pressure, it is called latent heat of evaporation or enthalpy of evaporation denoted by h_fg. Any state in between “bc” is called as wet steam, i.e., the state will be partly steam and partly water.

On heating of water beyond point “a,” when the water is heated at constant pressure, its temperature rises till it reaches a saturation point (boiling point) corresponding to the pressure indicated by “b.” There will be slight increase in volume. The saturation temperature (t_s) at which boiling occurs depends on the generation pressure.

The saturation temperature is defined as the temperature at which the water starts boiling corresponding to the pressure at which generation of steam takes place. The heating of water from 0°C to t_s at constant pressure is represented by “ab” on graph. The saturation temperature of water increases with the increase of pressure (t_s ∝ p) The amount of heat required to raise the temperature of 1 kg of water from 0°C to t_s (at pressure p) is known as sensible heat of water or enthalpy of liquid or liquid heat.

On further heating, the evaporation of water starts while temperature remains constant as t_s. The evaporation process is continued at constant temperature t_s until the whole water is completely converted to steam as shown in Fig. 2.26(e) and the point “c” on graph (Fig. 2.27).

Constant temperature, constant pressure, and heat addition are represented by “bc” on the graph (Fig. 2.27). The amount of heat required to convert saturated water to dry and saturated steam corresponding to same pressure and saturation temperature t_s is known as heat of evaporation or latent heat denoted by h_fg.

The amount of heat is further required to evaporate at the same pressure, the temperature of steam increases above the saturation temperature t_s. The temperature of steam above the saturation temperature at a given pressure is called superheated temperature. During this process of heating, the dry steam will be heated from its dry state and this process of heating is called superheating of steam.

In this state, steam is called superheated steam and the process of superheating is shown by “cd” (Fig. 2.27). The amount of heat required to increase the temperature of dry steam from its saturation temperature to any desired higher temperature at a constant pressure is called enthalpy of superheat.

The difference between the superheated temperature and the saturation temperature is known as degree of superheat. If t_sup is the temperature of superheated steam, then

Degree of superheat = t_sup – t_s

Types of Steam:

There are three types of steam:

(a) Wet steam,

(b) Dry and saturated steam, and

(c) Superheated steam.

(a) Wet Steam:

When water is heated beyond saturation temperature at constant pressure, it starts evaporating. Yet the whole of its latent heat has not been utilized. During this process, the homogeneous mixture of steam and water is formed and this is called wet steam. Thus, wet steam is defined as a two-phase homogeneous mixture of water and steam in equilibrium at saturation temperature corresponding to a given pressure for unit mass.

h = hf + xh_fg

where h is the enthalpy of wet steam, h_f the liquid heat, and h_fg the latent heat of evaporation.

Dryness Fraction:

A wet steam may be of different dryness fraction or qualities depending on the proportion of dry steam with respect to the amount of mixture. The quality of wet steam is known as dryness fraction and denoted by x. Thus, the dryness fraction of steam is defined as the ratio of the mass of dry steam present in a mixture to the total mass of wet steam (mass of dry steam + mass of water).

If m_s is the mass of dry steam present in wet steam and m_f is the mass of water present, then

x = m_s/m_s + m_f

(b) Dry and Saturated Steam:

A saturated steam at saturation temperature corresponding to a given pressure will have no water particle and is known as dry and saturated steam. Its dryness fraction is unity. The whole of latent heat of evaporation has been fully utilized.

The enthalpy of dry and saturated steam is defined as the amount of heat supplied at a given pressure to convert 1 kg of water into 1 kg dry and saturated steam at saturation temperature.

h_g = h_f + h_fg

where h_f = sensible heat

h_fg = latent heat of vaporization

h_g = enthalpy of dry saturated steam

(c) Superheated Steam:

For superheating the steam, heat is to be added at constant pressure beyond saturation to increase its temperature. The enthalpy of superheated steam is defined as the amount of heat supplied at a given constant pressure to convert 1 kg of water at 0°C into 1 kg of superheated steam and is denoted by h_sup. Thus,

h_sup = h_f + h_fg + c_ps (t_sup – t_s) (kJ/kg)

where c_ps is the specific heat of superheated steam depending on temperature. However, c_ps may be taken for all calculation = 2.2 kJ/kg K. t_sup is the temperature of superheated steam and t_s is the temperature of dry saturated steam.

The difference between the superheated temperature and the saturation temperature is known as degree of superheat, i.e., t_sup – t_s = degree of superheat.

Advantages of Superheated Steam:

(a) For a given pressure, the superheated steam contains more heat energy than saturated steam or wet steam; hence more work can be developed by the expansion of steam in a prime mover.

(b) When superheating is done in a superheated tube by the hot combustion flue gases in a boiler, there is a saving of heat energy which improves the thermal efficiency of boiler.

(c) When expansion of steam takes place in a steam turbine, it reduces the condensation and in extreme cases it prevents the condensation.

Specific Volumes of Steam:

In thermodynamics, the specific volume of a substance is the ratio of the substance’s volume to its mass. It is the reciprocal of density and is an intensive property of substance. The specific volume of saturated water is denoted by v_f. The specific volume of dry saturated steam is denoted by v_g.

Specific Volume of Wet Steam (v):

v – xv_g + (1 -x)v_f

Since (1 – x)v_f is very small, it can be neglected.

Hence, v = xv_g

where x is the dryness fraction.

Specific Volume of Superheated Steam (v_sup):

A superheated steam behaves like a perfect gas; hence v_sup can be calculated with the application of Charles’ law: