Chapter 6 Refrigeration

CHAPTER 6 REFRIGERATION Learning Objective: Describe the stages of heat theory and the principles involved in heat transfer, and recognize various components of refrigeration systems and their application. Recognize the characteristics and procedures required to service and troubleshoot rcfri gerat ion systems. Modern refrigeration has many applications, such as preserving medicine, blood, and the most important application, the preservation of food. Most foods kept at room temperature spoil rapidly. This is due to the rapid growth of bacteria. Refrigeration preserves food by keeping it cold, which greatly slows down the growth of bacteria. In days past, blocks of ice were used in iceboxes to refrigerate food and other items. These iceboxes were small and not very practical. Today, mechanical refrigeration systems make transportation, storage, and use of refrigerated goods easy and practical. The installation, operation, adjustment, and repair of refrigeration equipment are the primary responsibility of the Utilitiesman rating. To perform these duties required of a refrigeration mechanic, you need to understand the principles and theory of refrigeration and recognize system components and understand the way they work within the system. Methods of installing, maintaining, and repairing refrigeration equipment and maintaining, servicing, and repairing domestic refrigerators and freezers are also covered in this chapter. HEAT AND REFRIGERATION PRINCIPLES Learning Objective: Explain the basics of heat theory and the basic principles of refrigeration. REFRIGERATION is the process of removing heat from an area or a substance and is usually done by an artificial means of lowering the temperature, such as the use of ice or mechanical refrigeration. MECHANICAL REFRIGERATION is defined as a mechanical system or apparatus so designed and constructed that, through its function, heat is transferred from one substance to another. Since refrigeration deals entirely with the removal or transfer of heat, some knowledge of the nature and effects of heat is necessary for a clear understanding of the subject. NATURE OF HEAT Heat is a form of energy contained to some extent in every substance on earth. All known elements are made up of very small particles, known as atoms, which, when joined together, form molecules. These molecules are particular to the form they represent. For example, carbon and hydrogen in certain combinations form sugar and in others form alcohol. Molecules are in a constant state of motion. Heat is a form of molecular energy that results from the motion of these molecules. The temperature of the molecules dictates to a degree the molecular activity within a substance. For this reason, substances exist in three different states or forms—solid, liquid, and gas. Water, for example, may exist in any one of these states. As ice, it is a solid; as water, it is a liquid; and as steam, it is a gas (vapor). When heat is added to a substance, the rate of molecular motion increases, causing the substance to change from a solid to a liquid, and then to a gas (vapor). For example, in a cube of ice, molecular motion is slow, but as heat is added, molecular activity increases, changing the solid "ice" to a liquid "water" (fig. 6-1). Further application of heat forces the molecules to greater separation and speeds up their motion so that the water changes to steam. The steam formed no longer has a definite volume, such as a solid or liquid has, but expands and fills whatever space is provided for it. Heat cannot be destroyed or lost. However, it can be transferred from one body or substance to another or to another form of energy. Since heat is not in itself a substance, it can best be considered in relation to its 6-1