Introduction to Steam

  1. Basic Knowledge of Steam

    1. Water phases


    Most substances exist in one of three kinds of phase: solid phase, liquid phase, or gas phase. Substances in these phases are called solids, liquids, and gases respectively. For water, ice, water and steam correspond respectively to solid, liquid and gas.A good way to learn about the characteristics of steam is to understand the outline of the molecular structure of substances and apply them to ice, water and steam. Molecular structures are easy to understand if you imagine molecules to be electrically bonded together.


    A molecule is the smallest unit which has all the chemical characteristics that the substance have.

    A compound consisting of two hydrogen atoms and one oxygen atom is expressed as H2O. The chemical formula to describe it is. The reason why a lot of water exists on earth is that both hydrogen and oxygen are the most common molecule in the universe.

    1. Ice (solid phase)


      The molecules of ice bind rigidly together because they are close to each other and are arranged in an orderly grid structure. The activity (vibration) of each molecule is limited to a small range in this crystal structure. As it is progressively heated, the vibrational strengthening of the molecules finally causes the binding force between some parts of the molecules to be destroyed. It begins to melt and change from solid to liquid. The temperature at which melting starts is called the melting point.


      The melting point is 0°C under atmospheric pressure. The higher the pressure is, and the higher the melting temperature will be. The temperature of a (liquid) water and ice mixture stays at the melting temperature while the water molecule grid of the ice is dissolved (also known as melting). Thus, all the applied heat is spent on melting. This heat quantity is called heat of fusion or (more accurately) enthalpy of fusion. The phase changing from solid to liquid is invertible, and liquid solidifies while emitting the same heat. This amount of heat (equal to the fusion heat) is called the heat of solidification.


      The density of most substances is reduced when the substance changes from solid to liquid. Thus, the space between liquid molecules becomes larger than that between solid molecules. However, the case of H2O is an exception, as its density increases by melting. That is why ice floats on water.

    2. Water (liquid phase)


      In the liquid phase, molecules move freely and often collide with each other. However, the space between molecules is still narrow since the forces attracting the water molecules to each other remain strong. Movement and collisions of the molecules are increased and temperature is raised by applying heat, until the boiling point is reached.

    3. Steam (gas phase)


As the temperature of water rises, its density decreases. Provided the heat source is not on top, this causes the warmer molecules to rise up and the colder ones to sink down in a process called convection. Some water molecules on the surface may become gaseous despite being below saturated temperature if there is a gas phase of different substances (e.g. air) above a process called evaporation. The amount of evaporated water molecules increases until the water temperature reaches saturated temperature, at which point the phase change from liquid to gas becomes boiling instead of evaporation. The boiling occurs at the heat source, not the surface (unlike evaporation), and can thus be identified by the steam bubbles rising from the heat source to the surface.



If the pressure remains constant, the temperature does not rise even if further heat is added to the water until all the water becomes steam. All that heat is used by the process of evaporation and thus called latent heat or heat of evaporation. The temperature of the boiling water (saturated water) and saturated steam is the same, but the heat energy per mass unit of steam is much larger than that of water. The process of evaporation is reversible, like the phase-change from ice to water. The process of change from (gaseous) steam to (liquid) water is called condensation and the liquid resulting from that process may be called condensate. During condensation, heat is emitted by condensing the steam in the same quantity that was absorbed during evaporation (assuming an identical pressure).


This heat is called heat of condensation.