Document M4zM94EZbeLJogBBR8vZ33eva

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 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; usually expressed in units of horsepower, Btu per hour, or watts. Prime Surface: See Heating Surface. Psyehrometer: An instrument for ascertaining the humidity or hygrometric state of the atmosphere. Psychrometric: Pertaining to psychrometry or the state of the atmo sphere as to moisture. Psychrometry: The branch of physics that treats of the measure ment 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. Relative Humidity: See Humidity; also discussion relative humidity, Chapter 1. Return Mains: The pipes which return the heating medium from the heating units to the source of heat supply. Reversed-Retum 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 sub-division of the system are practically of equal length. Roof Ventilator: A device placed on the roof of a building to facilitate egress of air. Saturated Air: Air in a space 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 the water at the existing temperature. 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. 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 thesmoke produced. Specific Gravity: The ratio of the weight of a body to the weight of an equal volume of water at some standard temperature, usually 39.2 F. Specific Heat: The ratio of thermal capacity of a substance to that of water at 63 F: Since thermal capacity is measured by unit mass raised one degree in temperature, this ratio is numerically equal to the Btu required to raise one pound of the substance through one degree Fahrenheit. Specific Volume: The volume, expressed in cubic feet, of one pound of a substance. s = lH-d = V -i- W. Split System: A system in which the heating arid ventilating are accomplished by means of radiators or convectors supplemented by mechanical circulation of air (heated or unheated) from a central point. Square Foot of Heating Surface (Equivalent): Equivalent Direct Radiation (EDR). By definition, that amount of heating surface which will give off 240 Btu per hour. The equivalent square feet of heating surface may have no direct relation to the actual surface area. Stack Height: The height of a gravity convector between the bottom of the heating unit and the top of the outlet opening. Standard Air: As defined by A.S.H.V.E. Codes, standard air is air weighing 0.07488 lb per cubic foot, which is air at 68 F dry-bulb and 50 per cent relative humidity with a barometric pressure of 29.92 in. of mercury. (Most engineering tables and formulae involving the weight of air are based on air weighing 0.07492 lb per cubic foot, which is 780 CHAPTER 46. TERMINOLOGY dry air at 70 F dry-bulb with a barometric pressure of 29.921 in. of mercury. The error involved in disregarding the difference between the above two weights is very slight and in most instances may be neglected.) Static Pressure: The compressive pressure existing in a fluid. It is a measure of the potential energy of the fluid. Steam: Steam is water vapor which exists in the vaporous condition because sufficient heat has been added to the water to supply the latent heat of evaporation and change the liquid into vapor. Steam in contact with the water from which it has been generated may be dry saturated steam or wet saturated steam. The latter contains more or less actual water in the form of mist. If steam is heated, and the pressure maintained the same as when it was vaporized, its temperature will increase and it will become super heated. Steam Heating System: A heating system in which heat is transferred 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 and preventing 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, conduc tion, 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 surrounding 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 temperature and which directly or indirectly controls the source of heat supply. Ton of Refrigeration: A unit of power equal to the extraction of 12,000 Btu per hour. Ton Day of Refrigeration: The heat removed by a ton of refrigeration operating for one day; 288,000 Btu. Total Heat: See Enthalpy. Total Pressure: In the theory of the flow of fluids; 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 unit. Underfeed Stoker: A stoker which feeds the coal underneath the fuel bed. Unit: As applied to heating, ventilating and air conditioning equipment this word means a factory-built and assembled equipment with apparatus for accomplishing some specified function or combination of functions. (See Chapters 22 and 23.) 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 phrases. (See Chapter 23.) 781