Document Bv7QZvknRrZbXzdD80mwmExmX

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 Boiler Horsepower: The equivalent evaporation of 34.5 lb of water per hour from and at 212 F. This is equal to a heat output of 970.3 X 34.5 = 33,475 Btu per hour. British Thermal Unit: The mean British Thermal Unit is 7^= of the heat required to loU raise the temperature of 1 lb of water from 32 to 212 F. It is substantially equal to the quantity of heat required to raise 1 lb of water from 63 to 64 F. One Btu = ,, kwhr. By-pass: A pipe or duct, usually controlled by valve or damper, for short-circuiting fluid flow. Calorie: The mean calorie is of the heat required to raise the temperature of 1 1UU gram of water from Zero C to 100 C. It is substantially equal to the quantity of heat required to raise one gram of water from 14.5 C to 15.5 C. Central Fan System: A mechanical indirect system of heating, ventilating, or air conditioning, in which the air is treated or handled by equipment located outside the rooms served, usually at a central location, and is conveyed to and from the rooms by means of a fan and a system of distributing ducts. (See Chapter 21.) Chimney Effect: The tendency in a duct or other vertical air passage for air to rise when heated, owing to its decrease in density. Coefficient of Transmission: The amount of heat (Btu) transmitted from air to air in one hour per square foot of the wall, floor, roof or ceiling for a difference in temperature of 1 F between the air on the inside and that on the outside of the wall, floor, roof or ceiling. Comfort Line: The effective temperature at which the largest percentage of adults feel comfortable. Comfort Zone (A verage): The range of effective temperatures over which the majority (50 per cent or more) of adults feel comfortable. . Comfort Zone (Extreme): The range of effective temperatures over which one or more adults feel comfortable. (See Chapter 3.) Concealed Radiator: A heating device located within, adjacent to, or exterior to the room being heated but so covered or enclosed or concealed that the heat transfer surface of the device, which may be either a radiator or a convector, does not see the room. Such a device transfers its heat to the room largely by convection air currents. Conductance: The amount of heat (Btu) transmitted from surface to surface in one hour through one square foot of a material or construction, whatever its thickness, when the temperature difference is 1 F between the two surfaces. Conduction: The transmission of heat through and by means of matter unaccom panied by any obvious motion of the matter. Conductivity: The amount of heat (Btu) transmitted in one hour through one square foot of a homogeneous material / in. thick for a difference in temperature of 1 F between the two surfaces of the material. , Conductor (Heat): A material capable of readily conducting heat. The opposite of an insulator or insulation. Constant Relative Humidity Line: Any line on the psychrometric chart representing a series of conditions which may be evaluated by one percentage of relative humidity; there are also constant dry-bulb lines, wet-bulb lines, effective temperature lines, vapor pressure lines, and lines showing other physical properties of air mixed with water vapor. Convection: The transmission of heat by the circulation of a liquid or a gas such as air. Convection may be natural or forced. Convector: A heat transfer surface designed to transfer its heat to surrounding air largely or wholly by convection. Such a surface may or may not be enclosed or concealed. When concealed and enclosed the resulting device is sometimes referred to as a concealed radiator. (See also definition of Radiator.) (See also Chapter 14.) Decibel: A unit commonly used for expressing sound or noise intensities referred to an arbitrary reference level. It is defined by the relation db = 10 logio , where Pi is * o the unknown intensity, and P0 is the reference level which is commonly taken as 10-" watts per square centimeter. Degree-Day: A unit, based upon temperature difference and time, used in specifying the nominal heating load in winter. For any one day there exists as many degree-days 776 CHAPTER 46. TERMINOLOGY as there are degrees Fahrenheit difference in temperature between the average outside air temperature, taken over a 24-hour period, and a temperature of 65 F. Dehumidify: To remove water vapor from the atmosphere; to remove water vapor , or moisture from any material. 4 Uf Density: The weight of a unit volume, expressed in pounds per cubic foot, d = --. Dew-Point Temperature: The temperature corresponding to saturation (100 per cent relative humidity) for a given moisture content. Direct-Indirect Heating Unit: A heating unit located in the room or space to be heated and partially enclosed, the enclosed portion being used to heat air which enters from outside the room. Direct Radiator: Same as Radiator. Direct-Return System (Hot Water): A hot water system in which the water, after it has passed through a heating unit, is returned to the boiler along a direct path so that the total distance traveled by the water is the shortest feasible, and so that there are considerable differences in the lengths of the several circuits composing the system. Down-Feed One-Pipe Riser (Steam): A pipe which carries steam downward to the heating units and into which the condensation from the heating units drain. Down-Feed System (Steam): A steam heating system in which the supply mains are above the level of the heating units which they serve. Draft Head (Side Outlet Enclosure): The height of a gravity convector between the bottom of the heating unit and the bottom of the air outlet opening. (Top Outlet En closure): The height of a gravity convector between the bottom of the heating unit and the top of -the enclosure. Drip: A pipe, or a steam trap and a pipe, considered as a unit, which conducts con densation from the steam side of a piping system to the water or return side of the system. Dry Air: In psychrometric work, dry air is defined as air without water vapor. This state, though not obtained practically, is used as the basis of calculations. Dry-Bulb Temperature: The temperature of the air indicated by any type of ther mometer not affected by the water vapor content or relative humidity of the air. Dry Return: A return pipe in a steam heating system which carries both water of condensation and air. The dry return is above the level of the water line in the boiler in a gravity system. (See Wet Return.) Dust: Solid material in a finely divided state, the particles of which are large and heavy enough to fall with increasing velocity, due to gravity in still air. For instance, particles of fine sand or grit, the average diameter of which is approximately 0.01 centimeter, such as are blown on a windy day, may be called dust. Dynamic Head or Pressure: Same as Total Pressure. Effective Temperature: An arbitrary index which combines into a single value , the effect of temperature, humidity, and movement of air on the degree of warmth or cold felt by the human body. The numerical value is that of the temperature of still, satu rated air which would induce an identical sensation of warmth. Enthalpy: A measure of the amount of heat to be removed at constant pressure from an air-vapor mixture to reduce the temperature of the mixture to a predetermined datum. The enthalpy includes both latent and sensible heat. In Table 6, Chapter 1, separate values for air and water vapor are given with 0 F as the datum for air and 32 F for water vapor. Entropy: A ratio, evaluated for practical purposes by dividing the heat content of a unit weight of a substance by its absolute temperature. Useful in examining changes during a heat cycle. Entropy is constant during a reversible adiabatic change of state. Equivalent Evaporation: The amount of water a boiler would evaporate, in pounds per hour, if it received feed water at 212 F and vaporized it at the same temperature and atmospheric pressure. Estimated Design Load: The sum of the heat emission of the equivalent direct radia tion to be installed plus the allowance for heat loss of the connecting piping plus the heat requirements of any auxiliary apparatus connected with the system. (A.S.H.V.E. Standard Code for Rating Steam Heating Solid Fuel Hand-Fired Boilers--edition of April, 1932.) 777