Document 377zLmjVk2G5KeYLo3y5pweY3

*;;CHAPTER I ___________________ 1946 Guide . Return Mains: Pipes or conduits which return the heating or cooling medium from the heat transfer .unit to the source of heat or refrigeration. Reversed-Return System: A system in which the heating or cooling medium from several heat transfer units is returned along paths arranged so that all circuits com posing the system or composing a major sub-division of the system are of practically equal length. Saturation: The condition for co-existence in stable equilibrium of a vapor and liquid or a vapor and solid phase of the same substance. Example: Steam over the water from which it is being generated. Saturation, Degree of, or Per Cent: The ratio of the weight of a given volume of. water vapor to the weight of an equal volume of saturated water vapor at the same temperature. Smoke: An air suspension (aerosol) of particles, usually but not necessarily solid, often originating in a solid nucleus, formed from combustion or sublimation. Also defined as 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. Smokeless Arch: An inverted baffle placed in an up-draft furnace toward the rear to aid in mixing the gases of combustion and thereby to reduce the smoke produced. Sorbent: Material which has the property of dehumidifying gases without the help of refrigeration. The term generally refers to materials which have a large capacity for moisture compared to their bulk or weight. Sorption: Adsorption or absorption. Split System: A system in which the heating is accomplished by means of radiators or- convectors supplemented by mechanical circulation of air (heated or unheated) from a central point. Ventilation may be provided by the same system.' Square Foot of Heating Surface {Equivalent): This term is synonymous with Equivalent Direct Radiation (EDR). Stack Height: The height of a gravity convector between the bottom of the heating unit and the top of the outlet opening. Steam: Water in the vapor phase. Dry Saturated Steam is steam at the saturation temperature corresponding to the pressure, and containing no water in suspension. Wet Saturated Steam is steam at the saturation temperature corresponding to the pressure, and containing water particles in suspension. Superheated Steam is steam at a tem perature higher than the saturation temperature corresponding to the pressure. Steam Heating System: A heating system in which heat is transferred from the boiler or other source of heat to the heating units by means of steam at, above, or below atmospheric pressure. Steam Trap: A device for allowing the passage of condensate, or of air and con densate and preventing the passage of steam. Supply Mains: The pipes through which the heating medium flows'from the boiler < or source of supply to the run-outs and risers leading to the heating units. Surface, Heating: The exterior surface of a heating unit. Extended heating surface {or extended surface):, Heating surface consisting of fins, pins or ribs which receive heat by conduction from the prime surface. Prime Surface: Heating surface having.the heating medium on one side and air (or extended surface) on the other. (See also Boiler Heating Surface.) *, Temperature: The thermal state of matter with reference to its tendency to com- municate heat to matter in contact with it. If no heat flows upon contact, there is?no difference in temperature. Temperature, Absolute: Temperature expressed in degrees above absolute zero. Temperature, Dry-Bulb: The temperature of a gas or mixture of gases indicated > by an accurate thermometer after correction for radiation. Temperature, Dew-Point: The temperature at which the condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature of the vapor is reduced. The temperature corresponding to saturation (100 per cent- relative humidity) for a given absolute humidity at constant pressure. Temperature, Effective: An arbitrary index which combines into a single value the effect of temperature, humidity, and air movement on the sensation of warmth or cold felt by the human body. The numerical value is that of the temperature of still, saturated air which would induce an identical sensation. Temperature, Wet-Bulb: Thermodynamic wet-bulb temperature is the tem perature at which liquid or solid water, by evaporating into air, can bring the.air to saturation adiabatically at the same temperature. Wet-bulb'temperature (without qualification) is the temperature indicated by a wet-bulb psychrometer constructed and Terminology 9 used according to specifications. {A.S.M.E. Power Test Codes, Series 1932, Instru ments and Apparatus, Part 18.) `' Thermodynamics, Laws of: Two laws upon, which rest the classical theory of thermodynamics. These laws have been stated in many different,, but equivalent ways. The First Law: (1) When work is expended in generating.heat, the quantity of heat nroduced is proportional to the work expended and, conversely; when heat is employed in the performance of work, the quantity of heat which disappears is proportional to the work done. (Joule)b (G.P.); (2) If a system is caused to change from an initial state to a final state by adiabatic means only, the work done is the same for all adiabatic paths connecting the two states. (Zemansky); (3) In any power cycle or refrigeration-cycle the net heat absorbed bv the working substance is exactly equal to the net work done. The Second Law: (1) It is impossible for a self acting machine, unaided by anv external atrency to convey heat from a body of lower to one of higher temperature. (Clausius) (G P V (2) It is impossible to derive mechanical work from heat taken from a body unless there is available a body of lower temperature into which the residue not so used- mav be discharged (Kelvin) (G.P.); (3) It is impossible to construct an engine that, operating in a cycle, wilf produce no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work (Zemansky). Thermostat: An instrument which responds to changes in temperature and which directly or indirectly controls temperature. Transmittance, Thermal: The time rate of heat flow, from the fluid on the warm side to the fluid on the cold side, per (square foot) (degree temperature difference be tween the two fluids). Sometimes called Over-all Coefficient of Heat Transfer. Common unit is Btu per (hour) (square foot) (Fahrenheit degree). Symbol U. Two-Pipe System {Steam or Water):. A heating system in which one pipe is used for the supply of the heating medium to the heating unit and another for the return of the heating medium to the source of heat supply. The essential feature of a two-pipe system is that each heating unit receives a direct supply of the heating medium which medium cannot have.served a preceding heating unit. Unit: As applied to heating, ventilating and air conditioning equipment this word means factory-built and assembled equipment with apparatus for accomplishing some specified function or combination of functions. (See Chapters 26 and 36.) It is loosely applied to a great variety of equipment. Usually the function is included in the name, and hence come terms like Unit Heater, Unit Ventilator, Humidifying Unit; and Air Conditioning Unit. ' Units are said to be direct or room, when intended for location, or located.in, the treated space; indirect or remote, when outside or adjacent to the treated space. They are ceiling units when suspended from above, and floor when supported from below. Other descriptive words includefree delivery when the unit is not intended to be attached to ducts or similar resistance-producing devices, and pressure when for use with such ducts. Complete description requires the use of several of these qualifying words or phases. (See Chapters 26 and 36.) . Up-Feed System: A heating system in which the supply mains are below the level of the heating units which they serve. Vacuum Heating System: A two-pipe,steam heating system equipped with the necessary accessory apparatus which will permit operating the system below atmospheric pressure when desired. Vane Ratio: In air distributing devices the ratio of depth of vane to shortest opening width between two adjacent grille barsT Vapor: The gaseous form of substances which are normally in the solid or liquid state and which can be changed to these states either by increasing the pressure or decreasing the temperature. Vapors diffuse. (-4.5./!. definition.) Vapor Heating System: A steam'heating system which operates under pressures at or near atmospheric and which returns the condensation to the boiler or receiver by gravity. Vapor systems have thermostatic traps or other means of resistance on the return ends of the heating units for preventing steamfrom entering the return mains; they also have a pressure-equalizing and air-eliminating device at the end of the dry return. Velocity: A vector quantity which denotes at once the time rate and the direction of a linear motion. V = at. Fo- r uniform linear motion V = --t~ . Common units are: feet per second. bNames of authors who first stated law are given in parentheses).