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Heating Ventilating Air Conditioning Guide 1938 Isothermal: An adjective used to indicate a change taking place at constant temperature. Latent Heat: See Heat. Laws of Thermodynamics: The first law states that the total energy of an isolated system remains constant and cannot be increased or dimin ished by any physical process whatever. The second law states that no change in a system of bodies that takes place of itself can increase the available energy of a system. Manometer: An instrument for measuring pressures; essentially a U-tube partially filled with a liquid, usually water, mercury, or a light oil, so the amount of displacement of the liquid indicates the pressure being exerted on the instrument. Mass: The quantity of matter, in pounds, to which the unit of force (one pound) will give an acceleration of one foot per second per second. m = --W . . g ' Mb, Mbh1: Symbols which represent, respectively, 1000 Btu and 1000 Btu per hour. Mechanical Equivalent of Heat: The mechanical energy necessary to produce 1 Btu-of heat energy. J = 777.5 ft-lb. Micron: A unit of length, the thousandth part of one millimeter or the millionth of a meter. Mol: The unit of weight for gases. It is defined as m lb where m denotes the molecular weight of a gas. For any gas the volume of 1 mol at 32 F and standard atmospheric pressure is 358.65 cu ft and the weight of a cubic foot is 0.002788 m lb. Neutral Zone: The level within a room or building at which the pressure is exactly equal to the outside barometric pressure. One-Pipe Supply Riser (steam): A pipe which carries steam upward to a heating unit and which also carries the condensation from the heating unit in a direction opposite to the steam flow. One-Pipe System (hot water): A hot water system in which the water flows through more than one heating unit before it returns to the boiler; consequently, the heating units farthest from the boiler are supplied with cooler water than those near the boilet in the same circuit. One-Pipe System (steam): A steam heating system consisting of a main circuit in which the steam and condensate flow in the same pipe, usually in opposite directions. Ordinarily to each heating unit there is but one connection which must serve as both the supply and the return, although separate supply and return connections may be used. Overhead System: Any steam or hot water system in which the supply main is above the heating units. With a steam system the return must be below the heating units; with a water system, the return may be above the heating units. Panel Radiator: A heating unit placed on or flush with a flat wall surface and intended to function essentially as a radiator. 1These symbols were approved by the A.S.H.V.B., June, 1933. ;826 Chapter 45. Terminology Panel Warming: A method of heating involving the installation of the heating units (pipe coils) within the wall, floor or ceiling of the room, so that the heating process takes place mainly by radiation from the wall, floor or ceiling surfaces to the objects in the room. Plenum Chamber: An air compartment maintained under pressure and connected to one or more distributing ducts. Potentiometer: An instrument for measuring or comparing small electromotive forces. Power: The rate of performing work, expressed in units of horse power, one of which is equal to 550 ft-lb of work per second, or 33,000 ft-lb per minute. Prime Surface: See Heating Surface. Psychrometer: An instrument for ascertaining the humidity or hygrometric state of the atmosphere. Psychrometric: Pertaining to psychrometry or the state of the atmosphere as to moisture. Psychrometry: The branch of physics that treats of the measurement of degree of moisture, especially the moisture mixed with the air. Pyrometer: An instrument for measuring high temperatures. Radiation: The transmission of heat through space by wave motion. Radiator: A heating unit exposed to view within the room or space to be heated. A radiator transfers heat by radiation to objects "it can see" and by conduction to the surrounding air which in turn is circulated by natural convection; a so-called radiator is also a convector but the single term radiator has been established by long usage. Recessed Radiator: A heating unit set back into a wall recess but not enclosed; Refrigerant: A substance which produces a refrigerating effect by its absorption of heat while expanding or vaporizing. Register: A grille with a built-in multiblade damper or shutter. Relative Humidity: See Humidity: see also discussion of relative humidity in Chapter 1. Return Mains: The pipes which return the heating medium from the heating units to the source of heat supply. Reversed-Return System (hot water): A hot water heating system' in which the water from several heating units is returned, along paths arranged so that all circuits composing the system or composing a major subdivision of the system are practically of equal length. Roof Ventilator: A device placed on the roof of a building to permit egress of air. Saturated Air: Air containing as much water vapor as it can hold without any condensing out; in saturated air, the partial pressure of the water vapor is equal to the vapor pressure of water at the existing tem perature. Sensible Heat: See Heat. Smoke: Carbon or soot particles less than 0.1 micron-in size which result from the incomplete combustion of carbonaceous materials such as coal, oil, tar, and tobacco. . .. 827