84 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. Freezing and Melting Freezing and melting are changes of state from a liquid to a solid, and from a solid to a liquid. Th e most common example of this change of state is water freezing or melting. When water freezes (sometimes called fusion), it goes from a liquid to solid ice. When ice melts, it turns to liquid water. Vaporization and Condensation Th e changes of state most vital to understanding the refrigeration system is the change between liquid and gas. Th e change of state of water from liquid to vapor, or gas, is called vaporizing. Vaporizing is sometimes called boiling. When vapor changes to liquid, the process is called condensing. Sensible and Latent Heat All liquids and gases contain two kinds of heat, sen- sible heat and latent heat. Sensible heat is heat that can be felt, or sensed. A drink from a soda can removed from a refrigerator feels colder than a drink taken from a cabinet. Th is is because the refrigerated soda contains less heat than the other soda. Note: Solids contain only sensible heat since there is no further change of state possible once a substance reaches the solid state. Latent heat is oft en called hidden heat, since it is hidden in the liquid or gas. Th e reason only liquids and gases contain latent heat is because latent heat is absorbed to produce a change to the next higher state. Solids contain no latent heat, but must absorb heat to change to a liquid. To change from a liquid to a gas, more heat must be absorbed. Th is heat does not change the temperature, just the state. For instance, it is possible to have 32°F (0°C) ice and 32°F (0°C) water. However, the water contains more heat the latent heat added to change it from ice into water. Th e types of latent heat are: • Latent heat of freezing or fusion. When a substance gives up enough heat to go from a liquid to a solid. • Latent heat of melting. When a substance absorbs enough heat to go from a solid to a liquid. • Latent heat of vaporization. When a substance absorbs enough heat to go from a liquid to a gas. • Latent heat of condensation. When a substance gives up enough heat to go from a gas to a liquid. A great deal of latent heat is absorbed and released from substances when they change state. See Figure 5-5. Th e potential to remove vast quantities of heat by changing state is what makes latent heat so important to the operation of the refrigeration system. Superheat Many air conditioning manuals refer to superheat. Superheat is any heat added aft er the change to a higher state. For instance, once water is turned to steam at 212°F (100°C), more heat can be added to bring the temperature of the steam even higher. Steam with a temperature of more than 212°F is called superheated steam. Superheat must be removed before the substance can change state. Th erefore, steam at 232°F (111°C), must have enough heat removed to lower its temperature by 20°F (11°C) before removing addi- tional heat will cause it to condense back into water. Figure 5-5. An upward change of state occurs when ice absorbs heat and turns to water, and when water absorbs heat and becomes water vapor. A downward change of state occurs when water vapor gives off heat and becomes water, and when water gives off heat and becomes ice. =