Document O1dbaX9vkgmYbpEG1vO571zp

Heating Ventilating Air Conditioning Guide 1939 ferred from the boiler or other source of steam to the heating units by means of steam at, above, or below atmospheric pressure. Steam Trap: A device for allowing the passage of condensate andpreventing the passage of steam, or for allowing the passage of air as well as condensate. Superheated Steam: See Steam. Supply Mains (steam): The pipes through which the steam flows from the boiler or source of supply to the run-outs and risers leading to the heating units. Surface Conductance: The amount of heat (Btu) transmitted by radiation, conduction, and convection from a surface to the air or liquid, surrounding it, or vice versa, in one hour per square foot of the surface for a difference in temperature of 1 deg between the surface and the sur rounding air or liquid. Therm: Symbol used in the gas industry representing 100,000 Btu. Thermal Resistance: The reciprocal of conductance. Thermal Resistivity: The reciprocal of conductivity. Thermostat: An instrument which responds to changes in tempera ture and which directly or indirectly controls the source of heat supply. Ton of Refrigeration: The extraction of 12,000 Btu per hour. Ton Day of Refrigeration:' The heat removed by a ton of refriger ation operating for one day; 288,000 Btu. Total Heat: A thermodynamic quantity, variously called heat con tent, thermal potential, enthalpy. It is the heat required per unit mass (Btu per pound) to raise a given substance to a given point from an arbi trary datum point. It is the sum of the heat of the liquid, the latent heat, and any miscellaneous heat which may be present. Total Pressure: The sum of the static or radial pressure and the velocity pressure at the point of measurement. Tube (or Tubular) Radiator: A cast-iron heating unit used as a radiator and having small vertical tubes. 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. Underfeed Distribution System (hot water): A hot water heating system in which the main flow pipe is below the heating units. Underfeed Stoker: A stoker which feeds the coal underneath the fuel bed. Unit: As applied to heating, ventilating and air conditioning equip ment this word means a factory-built and assembled equipment with apparatus for accomplishing some specified function or combination of functions. , 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. 844 Chapter 45. Terminology 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 include free 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 phrases. (See Chapter 23). Up-Feed System (steam): A steam 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 equip ped with the necessary accessory apparatus which will permit operating the system below atmospheric pressure when desired. Vapor: Any substance in the gaseous state. Vapor Heating System: A steam heating system which operates under pressures at or near atmospheric and which returns the condensa tion to the boiler or receiver by gravity. Vapor systems have thermo static traps or other means of resistance on the return ends of the heating units for preventing steam from entering the return mains; they also have a pressure-equalizing and air-eliminating device at the end of the dry return. Direct V.ent Vapor System: A vapor heating system with air valves which do not permit re-entry of air. Vapor Pressure: Thg equilibrium pressure exerted by a vapor in contact with its liquid. Velocity: The time rate of motion of a body in a fixed direction. In the fps system it is expressed in units of one foot per second. V -- -j. Velocity Pressure: The pressure corresponding to the velocity of flow. It is a measure of the lanetic energy of the fluid. . Ventilation: The process of supplying or removing air by natural or mechanical means, to or from any space. Such air may or may not have been conditioned. (See Air Conditioning). Warm Air Heating System: A warm air heating plant consists of a heating unit (fuel-burping furnace) enclosed in a casing, from which the heated air is distributed to the various rooms of the building through ducts. If the motive head producing flow depends on the difference in weight between the heated air leaving the casing and the cooler air entering the bottom of the casing, it is termed a gravity system. A booster fan may, however, be used in conjunction with a gravity-designed system. If a fan is used to produce circulation and the system is designed especially for fan . circulation, it is termed a fan furnace system or a centralfanfurnace system. A fan furnace system may include air washers and filters. Wet-Bulb Temperature: The lowest temperature which a water wetted body will attain when exposed to an air current. This is the temperature of adiabatic saturation. Wet Return: That part of a return main of a steam heating system which is filled with water of condensation. The wet return usually is below the level of the water line in the boiler, although not necessarily so. (See Dry Return). 845