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GLOSSARY

Air-conditioning. The process by which the heat and humidity of the air in an enclosed space are maintained within certain limits, the air may also be filtered and purified in the process.

Air, latent heat in. The latent heat of vaporization of the water vapor in air.

Air, saturated. Air that contains all the water vapor it can hold at a given temperature.

Air, sensible heat of. The heat of the air only, without regard to the heat of any water vapor that may be mixed with the air.

Air, total heat of. The sum of both the latent heat and the sensible heat in any sample of air.

British thermal unit. A measure of heat quantity, defined as the quantity of heat required to raise the temperature of one pound of pure water one degree Fahrenheit at 39.10 degrees F. the amount of heat required varies slightly at other temperatures, but for practical engineering purposes, is usually considered as constant. (Abbreviation, Btu.)

Condensation. The change of state from a vapor to a liquid.

Dewpoint. The temperature at which water vapor in any sample of air begins to condense.

Ebullition. Vaporization or change of state from a liquid to a vapor in a rapid, active, and visible process; also called boiling.

Evaporation. Vaporization or change of state from a liquid to a vapor in a slow, inactive, and invisible process.

Freezing. The change of state from a liquid to a solid.

Heat. A vibratory form of energy, perceptible to human beings as the sensations to which the names cold, cool, warm, hot, and similar terms are applied.

Heat, intensity of. That characteristic of heat, the variations in which are indicated by such terms as cold and hot, or measured by numbers on the scale of a thermometer.

Heat, latent. The heat that changes the physical state of a substance; see Latent heat of vaporization and Latent heat of fusion.

  Heat load. The amount of heat to be removed from a space to be air-conditioned or refrigerated to satisfy the requirements of the given installation. The total heat load is the sum of various separate heat loads coming from such sources as: sensible heat, latent heat, crew, electric lights, electrical equipment, engines and stoves, sunlight, and foodstuffs.

Heat, quantity of. The quantity of heat possessed by a substance depends on the size, nature, and temperature of that substance; it is measured in British thermal units.

Heat, sensible. The heat that raises the temperature of a substance but does not change its physical state. It is measured by the thermometer.

Heat, specific. The heat required to raise the temperature of a unit mass of a substance one degree. Vapors and gases have two specific heats, one when the volume is held constant, and one when the pressure is held constant.

Heat, total. In air-conditioning, the term means the total heat, including both sensible and latent heat, to be eliminated from an air conditioned space. The term heat content is frequently used as a synonym for it. In the purely scientific field, the term total heat means the total heat energy present in a substance on the absolute scale.

Heat transfer. The flow or transmission of heat from a region of higher temperature to a region of lower temperatures.

Heat transfer by conduction. The transmission of heat from one part to another of a single body or substance; or from one body or substance to another in contact with it.

Heat transfer by convection. The carrying of heat from one location to another by the molecules of a substance in motion.

Heat transfer by radiation. The transmission of heat energy through space by the propagation of wave forms in a medium.

Humidity. Water in the physical state of vapor mixed in the air. Humidity is invisible; humidity does not include any water present in liquid form, however finely divided, such as

 
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mist or fog. The space that drops of liquid water occupy does not contain air.

Humidity, absolute. The weight of water vapor, usually stated in grains per cubic foot of air.

Humidity, relative. The ratio of the weight of water vapor in a sample of air to the weight of water vapor that same sample of air contains when saturated; usually stated as a percentage.

Humidity, specific. The weight of water vapor, usually stated in grains per pound of dry air.

Insulator. Any material that transmits heat at a very slow rate.

Latent heat of fusion. The heat that changes the physical state of a substance from a liquid to a solid, or from a solid to a liquid; no temperature change is shown by a thermometer during the conversion process.

Latent heat of vaporization. The heat that changes the physical state of a substance from a liquid to a vapor, or from a vapor to a liquid; no temperature change is shown by a thermometer during the conversion process.

Melting. The change of state from a solid to a liquid.

Pressure. The result of the action of an external force on an enclosed fluid, which may be a liquid, vapor, or gas; usually expressed in pounds per square inch.

Pressure, absolute. The true total pressure inside an enclosed vapor system, that is, the gage reading plus standard atmospheric pressure of 14.7 psi.

Pressure, atmospheric. The pressure exerted by the earth's atmosphere. For engineering purposes, usually taken as that which normally exists at sea level, 14.7 pounds per square inch.

Pressure, back. In refrigeration, back pressure is the difference between the vapor pressure in the suction line (including the evaporator) and the head pressure of the compressor.

Pressure, gage. The pressure indicated by a pressure gage; the zero mark on a pressure gage in reality means an interior pressure of 14.7 psi, corresponding to normal atmospheric pressure.

  Pressure, head. The pressure produced by the pistons of a compressor.

Pressure, negative. In engineering practice, a pressure below normal atmospheric or 14.7 psi, is called a negative pressure, or partial vacuum; usually stated in inches of vacuum.

Psychrometer. An instrument for deter mining wet-bulb temperature. The form most generally used, called the sling psychrometer, has a wet-bulb and a dry-bulb thermometer attached to a small support. By means of a handle or chain, the device can be swung around rapidly. The strong current of air thus caused evaporates the moisture in the sleeve of the wet-bulb thermometer, whereby the wet-bulb temperature is indicated.

Refrigerant. A substance capable of carrying heat, which it picks up at a low temperature level, and is then compressed to a higher pressure and temperature, where the heat can be removed by the condensing medium, air or water.

Refrigeration ton. A unit used in measuring the elimination of heat; one refrigeration ton is the removal of the heat that would be required to melt one ton of ice at 32 degrees F in 24 hours.

Saturation temperature. The temperature at which a liquid substance boils under a given pressure.

Specific volume. The number of cubic feet occupied by one pound of a substance at a given pressure and temperature.

State of a substance. A substance may exist in three different physical states: solid, liquid, and gaseous. For example, water (H2O) can exist in all three states, depending upon temperature and pressure: solid (ice); liquid; and gaseous (steam, or water vapor). A substance may change its state on variation of temperature or pressure, or both.

Superheat. The heat above saturation temperature in a vapor.

Temperature, dewpoint. The temperature at which the water vapor present in the air begins to condense, depending upon the amount of humidity of the air.

Temperature, dry-bulb. The temperature of the sensible heat of the air, as measured by an ordinary, or dry-bulb, thermometer.

 
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Temperature, saturation. The temperature at which a liquid substance boils under a given pressure.

Temperature, wet-bulb. The temperature indicated by a thermometer around the bulb of which is placed a wet cloth sleeve; a strong current of air, causing evaporation at the wet bulb, lowers the thermometer reading by a definite amount, called the wet-bulb depression, which depends upon the amount of moisture present in the air.

Thermal capacity. The capacity, or ability, of a substance to receive and store heat; equals the specific heat of a substance times its mass.

Thermal conductance. The heat conduction power of a substance; substances with a very low thermal conductance are called insulators.

Thermometer, centigrade. An instrument for measuring temperature in which the scale is numbered zero at the freezing point of pure

  water and 100 degrees at the boiling point, at standard sea-level atmospheric pressure,

Thermometer, Fahrenheit. An instrument for measuring temperature in which the scale is numbered 32 degrees at the freezing point of pure water and 212 degrees at the boiling point, at stand and sea-level atmospheric pressure.

Vapor, dry. A saturated vapor containing no suspended mist or liquid.

Vapor, saturated. A vapor that is at the temperature corresponding to the boiling point of a substance at a given pressure.

Vapor, superheated. A vapor the temperature of which is above the boiling point of the substance for a given pressure.

Vapor, wet. A saturated vapor that contains some suspended liquid in the form of mist.

Vaporization. The change of state from a liquid to a vapor.

 
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