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2 CHAPTER 1 1946 Guide For unit heaters, the distance an air stream travels from a heater without a perceptible rise due to temperature difference and loss of velocity. Boiler Heating Surface: That portion of the surface of the heat-transfer apparatus in contact with the fluid being heated on one side and the gas or refractory being cooled on the other, in which the fluid being heated forms part of the circulating system; this surface shall be measured on the side receiving heat. This includes the boiler, water , walls, water screens, and water floor. (A.S.M.E. Power Test Codes, Series 1929.) 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: Classically the Btu is defined as the quantity of heat required to raise the temperature of 1 lb of water 1 Fahrenheit degree. By this definition , the exact value depends upon the initial temperature of the water. Several values of the Btu are in more or less common use, each differing from the others by a slight amount. One of the more common of these is the mean Btu which is defined as 1/180 of the heat .required to raise the temperature of 1 lb of water from 32 F to 212 F at a constant atmospheric pressure of 14.696 lb per square inch absolute. For most accurate work the International Table (I.T.) Btu is usually used. This is defined by the relation: 1 (I.T.) Btu per (pound) (Fahrenheit degree) = 1 (I.T.)calorie per (gram) (Centigrade degree). This value corresponds to the amount of heat required to raise the temperature of 1 lb of water 1 Fahrenheit degree at 58 F and also at 149 F. The mean Btu corresponds to 1.0008 (I.T.) Btu. By-Pass: A pipe or duct, usually controlled by valve dr damper, for conveying a fluid around an element of a system. Calorie (Gram Calorie): Classically the calorie is defined as the quantity of heat required to raise the temperature of 1 gram of water 1 Centigrade degree. By this definition the exact value depends upon the initial temperature of the water. Several values of the calorie are in more or less common use, each differing from the others by a slight amount. Among these are the 15 C calorie and the C calorie. The mean calorie, i.e. 1/100 the quantity of heat required to raise the temperature of' 1 gram of water from 0 C to 100 C, is also extensively used. For the most accurate work the International Table (I.T.) calorie, defined in terms of the international electrical units, is usually used: 1 (I.T.) calorie = 1/860 inter national watt-hour = 3,600/860 international watt-seconds or international joules. The kilocalorie =1,000 cal. 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 43.) Chimney Effect: The tendency of air or gas in a duct or other vertical passage to rise when heated due to its lower density compared with that of the surrounding air or gas. In buildings, the tendency toward displacement (caused by the difference in tem perature) of internal heated air by unheated outside air due to the difference in density of outside and inside air. Comfort Air Conditioning: The process by which simultaneously the temperature, moisture content, movement and quality of the air in enclosed spaces intended for human occupancy may be maintained within required limits. (See Air Conditioning.) Comfort Line: The effective temperature at which the largest percentage of adults feels comfortable. Comfort Zone (Average): The range of effective temperatures over which, the majority (50 per cent or more) of adults feel comfortable. (See Chapter 12.) Conductance, Surface (Unit): The amount of heat transferred by radiation, con duction, and convection from unit area of a surface to the air or other fluid in contact with it, or vice versa, in unit time for a unit difference in temperature .between the surface and the fluid. The common unit is: Btu per (hour) (square foot) (Fahrenheit degree). Symbol/. The temperature of the fluid should be taken in a plane sufficiently far from the surface that it will not be affected by the temperature of the surface. Conductance, Thermal: The time rate of heat flow through unit area of a body,' of given size and shape, per unit temperature difference. Common unit is: Btu per (hour) (square foot) (Fahrenheit degree). Symbol C. Conduction, Thermal: The process of heat transfer through a material medium in which kinetic energy is transmitted by the particles of the material from particle toparticle without gross displacement of the particles. Conductivity, Thermal: The time rate of heat flow through unit area of a homo geneous substance under the influence of a unit temperature gradient. Common units are: Btu per (hour) (square foot) (Fahrenheit degree per inch). Symbol k. > .Tiprniinology 3 Conductor, Thermal: A material which readily transmits heat by means of . conduction. Convection: The motion resulting in a fluid from the differences in density and the action of gravity. In heat transmission this meaning has been extended to include both forced and natural motion or circulation. Convective Heat Transfer: The transmission of heat by either natural or forced motion of a fluid (liquid or gas). Convector: An agency of convection. In heat transfer, a surface designed to transfer its heat to a surrounding fluid largely or wholly by convection. The heated fluid may be removed mechanically or by gravity (Gravity Convector). 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 25.) Decibel: A unit used to express the relation between two amounts of power. By definition the difference in decibels between two powers Pi and Pt, Pt being the larger, is db difference = 10 log Pt_ Pi In acoustics the threshold of hearing at 1,000 cycles per second has been standardized at 10"16 watts per square centimeter. If Pt is the power in watts per square centimeter of a measured sound, then 10 logio is the db difference above the threshold and is known as the intensity level. This is a definite recognized way of describing the intensity of a sound. Degree-Day: A unit, based upon temperature difference and time, used in estimating fuel consumption and specifying nominal heating load of a building in winter. For any one day, when the mean temperature is less than 65 F, there exists as many degree-days as there are Fahrenheit degrees difference in temperature between the mean temperature for the day and 65 F. Dehutnidify: To reduce, by any process, the quantity of water vapor within a given space. Dehydrate: To remove water in all forms from matter. Liquid water, hygroscopic water, and water of crystallization.or water of hydration are included. Density: The ratio of the mass of a specimen of a substance to the volume of the specimen. The mass of a unit volume of a substance. When weight can be used without confusion, as synonymous with mass, density is the weight of a unit volume of a sub- ' stance. Dew Point: See Temperature, Dew Point. . 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-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 drains. 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: A current of air. Usually refers to the pressure difference which causes a current of air or gases to flow through a flue, chimney, heater or space. 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 tie 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 io the water or return side of the system; -Dry: To separate or remove a liquid or vapor from another substance. The liquid . may be water but the term is also used for the removal of liquid or vapor forms of other substances. : Dust: An air suspension (aerosol) of solid particles of any material. (See-also Chapter 10, p. 184.) Enthalpy: A term applied to the Gibbs function, h -- u -f pv and superseding total heat, heat content, and heat offormation. The increase of enthalpy of a system is equal y'