HISTORICAL DEVELOPMENT
Natural ice was shipped from the New England states throughout the western world from 1806 until the early 1900s. Although ice machines were patented in the early 1800s, they could not compete with the natural ice industry. Artificial ice was first commercially manufactured in the southern United States in the 1880s.
Domestic refrigerators were not commercially available until about 1920. See Fig. 2-1. During the 1920s, the air-conditioning industry also got its start with a few commercial and home installations. The refrigeration industry has now expanded to touch most of our lives. There is refrigeration in our homes, and air conditioning in our place of work, and even in our auto- mobiles. Refrigeration is used in many industries, from the manufacture of instant coffee to the latest hospital surgical techniques.
Fig. 2-1 One of the first commercial home refrigerators. (General Electric)
STRUCTURE OF MATTER
To be fully acquainted with the principles of refrigeration, it is necessary to know something about the structure of matter. Matter is anything that takes up space and has weight. Thus, matter includes everything but a perfect vacuum.
There are three familiar physical states of matter: solid, liquid, and gas or vapor. A solid occupies a definite amount of space. It has a definite shape. The solid does not change in size or shape under normal conditions.
A liquid takes up a definite amount of space, but does not have any definite shape. The shape of a liquid is the same as the shape of its container.
A gas does not occupy a definite amount of space and has no definite shape. A gas that fills a small container will expand to fill a large container.
Matter can be described in terms of our five senses. We use our senses of touch, taste, smell, sound, and sight to tell us what a substance is. Scientists have accurate methods of detecting matter.
ELEMENTS
Scientists have discovered 105 building blocks for all matter. These building blocks are referred to as elements. Elements are the most basic materials in the universe. Ninety-four elements, such as iron, copper, and nitrogen, have been found in nature. Scientists have made 11 others in laboratories. Every known sub- stance, solid, liquid, or gas, is composed of elements. It is very rare for an element to exist in a pure state. Elements are nearly always found in combinations called compounds. Compounds contain more than one element. Even such a common substance as water is a compound, rather than an element. See Fig. 2-2.
Fig. 2-2 Two or more atoms linked are called a molecule. Here two hydrogen atoms and one oxygen atom form a molecule of the compound water H2O.
ATOM
An atom is the smallest particle of an element that retains all the properties of that atom, that is, all hydrogen atoms are alike. They are different from the atoms of all other elements. However, all atoms have certain things in common: They all have an inner part—the
nucleus. This is composed of tiny particles called pro- tons and neutrons. An atom also has an outer part. It consists of other tiny particles, called electrons, which orbit around the nucleus. See Figs. 2-3 and 2-4.
Neutrons have no electrical charge, but protons have a positive charge. Electrons are particles of energy and have a negative charge. Because of these charges, protons and electrons are particles of energy. That is, these charges form an electric field of force within the atom. Stated very simply, these charges are always pulling and pushing each other. This makes energy in the form of movement.
The atoms of each element have a definite number of electrons, and they have the same number of protons. A hydrogen atom has one electron and one proton. An aluminum atom has 13 of each. The opposite charges—negative electrons and positive protons— attract each other and tend to hold electrons in orbit. As long as this arrangement is not changed, an atom is electrically balanced. When chemical engineers know the properties of atoms and elements they can then engineer a substance with the properties needed for a specific job. Refrigerants are manufactured in this way.
PROPERTIES OF MATTER
It is important for a refrigeration technician to under- stand the structure of matter. With this knowledge, the person can understand those factors that affect this structure. These factors can be called the properties of matter. These properties are chemical, electrical, mechanical, or thermal (related to heat). Some of these proper- ties are force, weight, mass, density, specific gravity, and pressure.
Force is described as a push or a pull on anything. Force is applied to a given area. Weight is the force of gravity pulling all matter toward the center of earth. The unit of weight in the English system is the pound. The unit of mass in the metric system is the gram.
Fig. 2-3 Atoms contain protons, neutrons, and electrons.
Fig. 2-4 Molecular structure.
Mass is the amount of matter present in a quantity of any substance. Mass is not dependent on location. A body has the same mass whether here on earth, on the moon, or anywhere else. The weight does change at other locations. In the metric system, the kilogram (symbol kg) is the unit of mass. In the English system, the slug is the unit of mass.
Density is the mass per unit of volume. Densities are comparative figures, that is, the density of water is used as a base and is set at 1.00. All other substances are either more or less dense than water.
The densities of gases are determined by a comparison of volumes. The volume of 1 lb of air is com- pared to the volume of 1 lb of another gas. Both gases are under standard conditions of temperature and pressure.
The specific gravity of a substance is its density compared to the density of water. Specific gravity has many uses. It can be used as an indicator of the amount of water in a refrigeration system. Testing methods are discussed in later chapters.
