In the study of refrigeration, you must be able to distinguish between sensible heat and latent heat. SENSIBLE HEAT is the heat absorbed or given off by a substance that is NOT in the process of changing its physical state. Sensible heat can be sensed, or measured, with a thermometer, and the addition or removal of sensible heat will always cause a change in the temperature of the substance.

LATENT HEAT is the heat absorbed or given off by a substance while it is changing its physical state. The heat absorbed or given off does NOT cause a temperature change in the substance- the heat is latent or hidden. In other words, sensible heat is the heat that affects the temperature of things; latent heat is the heat that affects the physical state of things. You will find more information on sensible heat and latent heat in chapter 3 of Fireman, NAVEDTRA 10520-H.

Specific Heat

Substances vary with respect to their ability to absorb or lose heat. The ability of a substance to absorb (or lose) heat is known as the SPECIFIC HEAT of the substance. The specific heat of water is 1.0 (1 Btu/lb/F), and the specific heat of every other substance is measured in relation to this standard. Thus, if it takes only 0.5 Btu to raise the temperature of 1 pound of a substance 1F, the specific heat of that substance is 0.5 or one-half the specific heat of water. If you look up the specific heat of ice in a table, you will find it to be approximately 0.5.

Heat Flow

Heat flows only from objects of higher temperature to objects of lower temperature. This was described earlier as the natural flow of heat. When two objects at different temperatures are placed near each other, heat will flow from the warmer object to the cooler one until both objects are at the same temperature. Heat flows at a greater rate when there is a large temperature difference. As the temperature difference approaches zero, the rate of heat flow also approaches zero. Heat transfer will occur between two materials of different temperature. Heat will flow from the high-temperature body to the low-temperature body. However, all materials do not conduct heat at the same rate. Some materials, such as metals, conduct heat readily. Other materials, such as wood and cork, offer considerable resistance to the conduction of heat. The relative capacity of a material to conduct heat is known as its THERMAL CONDUCTIVITY. Heat flow may take place by radiation, conduction, convection, or some combination of these met hods.

RADIATION.-Heat transfer by radiation from one body to another occurs in the form of a wave motion similar to that of a light wave or radio wave. No intervening matter is required for heat transfer to take place by radiation. For example, the sun transmits great amounts of heat energy to the earth by radiation across the vacuum of space. By standing close to a fire, you can readily feel the heat radiated by the fire. Heat transfer by radiation will occur between any two bodies that are visible to one another and that exist at differing temperatures.

CONDUCTION.-Heat transfer by conduc-tion occurs when energy is transferred by direct contact between molecules of a single body or among molecules of two or more bodies in physical contact with each other. Conduction takes place from the area of the higher temperature to that of the lower temperature. For example, if you hold an iron bar in one hand and place the other end of the bar in a bed of hot coals, the heat will pass from the coals to the bar, and then along the bar to your hand. Physical contact is made in each instance; the coals to the bar, and the bar to your hand.

CONVECTION.-Convection is the transfer of heat by the movement of a substance (gas or liquid) through a space. Examples of heat transfer by convection include a current of warm air in a room and warm air rising from hot water.

Refrigeration Ton

The unit of measure for the amount of heat removed is known as the REFRIGERATION TON. The capacity of a refrigeration unit is usually stated in refrigeration tons. The refrigeration ton is based on the cooling effect of 1 ton (2,000 pounds) of ice at 32F melting in 24 hours. The latent heat of fusion of 1 pound of ice (or water) is 144 Btus. Therefore, the number of Btus required to melt 1 ton of ice is 144 x 2,000 = 288,000. The standard refrigeration ton is defined as the transfer of 288,000 Btus in 24 hours. On an hourly basis, a refrigeration ton is equivalent to 12,000 Btus per hour (288,000 is divided by 24). The refrigeration ton is the standard unit of measure that designates the heat flow capacity of a refrigeration unit. It is NOT a measure of the ice-making capacity of a machine, since the amount of ice that can be made depends on the initial temperature of the water and other factors.