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6 CHAPTER 1 1954 Guide sometimes taken as.0 C.andfsometimes as 62 F. One inch .of water at 62 F = 5.197 lb per sq ft. ./; , Insulation {Thermal) ', A materialhaving a relatively high.resistance to heat flow, and used principally, to retard.the. flow of heat. Isobaric :; An adjective used^to indicate a change taking place at constant pressure.: Isothermal : Ad adjective "'used 'to indicate a change taking place at constant temperature. = ; i Load,. Estimated Designin a`heating or cooling system, the .sum. of the useful heat transfer, plus heat transfer from or to the connected; piping, plus heat'transfer occurring in any auxiliary apparatus'cohnected to the system.' The units are Btu per hour or, in heating, equivalent.direct radiation (EDR). ' Load, Estimated Maximum: In a heating or cooling system, the calculated maxi mum heat transfer that the system will be called upon to provide. Manometer; Anjnstrument for .measuring pressures;.essentially a U-tube par tially filled with a liquid, usually water^ mercury, or a light oil, so constructed that the amount of displacement of the liquid indicates the ptessiite being exerted on the instrument. .. .. Mass: A measure of the inertia of a body. It also measures the quantity of matter in a body. Since the only general property of a given portion of matter that cannot be changed is its inertia, it is this property by which quantities of matter are defined. Two bodies which have.equal; inertias are said to have:equa! masses* or to contain equal quantities of matter.(This.definition fails at velocities approaching the veloc-. ity of light.) The mass of a body is numerically, equal to the ratio of the force re quired to give the body a given acceleration, to the acceleration; in = F/a. The common units of mass are the gram and the pound. Mechanical Equivalent of Heat: The quantity of mechanical energy equal to one unit of heat. J = 778.3 ft-lb per Btu = 4.187 X 1,Q7 ergs per gram-calorie. Medium, Heating: A substance such as water, steam, air or furnace gas used to convey.heat from the.boiler, furnace or other source of heat or energy to'the heating unit from which the heat is dissipated. - Micron: A unit of length, the thousandth part of 1mm or the millionth of a meter. Millimeter of Mercury: A. unit of pressure equal to.the pressure exerted by a column of mercury 1 mm high at a temperature of 0 C. One millimeter of mercury atOC = 1.934 X 10"* psi. . Mol: A weightof a substance numerically equal to its molecular weight. If the weight is in pounds the unit, is a Pound Mol, in grams the unit is. a Gram Mol. For perfect gases the volume of 1 mol is constant for all gases at the same temperature and pressure. For real gases this is approximately true at moderate pressures. At 32 F and zero-pressure the value of the product, pressure times specific volume, is 359.045 0.006 atmosphere cubic feet (atm ft*), for 1 mol of any gas. For dry air at 32 F and standard atmospheric pressure, the specific volume is 358.83 cu ft per mol (ft* per mol)'. - One-Pipe Supply Riser--(Steam): A pipe which carries steam vertically to a heat ing unit, and which also carries the condensate from the heating unit. In an up-feed system, steam and condensate flow in opposite directions; in an overhead or downfeed system,'they flow in the same direction. One-Pipe System--(Steam): A steam heating system in which a single main serves the dual purpose of supplying steam to the heating unit and conveying condensate from it. Ordinarily to each beating unit there is but one connection which must serve as both the supply and the return, although separate supply, and return connections may be used. (Hot Water)--A hot water system in which the cooled water from the heating units is returned to the supply main. Consequently, the beating units far thest from the boiler are supplied with cooler water than those near the boiler in the' same circuit. : v; . . Overhead System": Any steam or hot water system in which the supply main is above the heating unit. In a steam system the return must, be below the heating units; in a water system the return may be above or below the heating units. Panel Heating : A heating system in which heat is transmitted by both radiation and convection from panel surfaces to both air and surrounding surfaces. Panel Radiator: A heating unit placed on or flush with a flat wall surface, and in tended to function essentially as a radiator. Perimeter System: See Warm Air Heating System. . Plenum Chamber: An air compartment maintained under pressure, and connected to one or more distributing ducts. Terminology 7 Potentiometer: An instrument for comparing small electromotive forces, or for measuring small electromotive forces by comparison with a known electromotive force. Its principal advantage is that during the measurement, no current flows through the source of electromotive force. Power: The rate of performing work. Common units are horsepower, Btu per hour, and watts. Pressure: Force per unit area. Common units are pounds per square inch, gram per square centimeter, inch of water, millimeter of mercury. Pressure, Absolute: The sum of the gage pressure and the barometric pressure. Pressure, Dynamic: Same as Total Pressure. Pressure,.Gage: Pressure measured from atmospheric pressure as a base. Gage pressure may be indicated by a manometer which has one leg connected to the pres sure source and the other exposed to atmospheric pressure. Pressure, Saturation : The saturation pressure for a pure substance for any given temperature is that pressure at which vapor and liquid, or vapor and solid, can co exist in stable equilibrium. Pressure, Static: The normal force per unit area that would be exerted by a mov ing fluid on a small body immersed in it if the body were carried along with the fluid. Practically, it is the normal force per unit area at a small hole in a wall of the duct through which the fluid flows (piezometer) or on the surface of. a stationary tube at a point where the disturbances, created by inserting the tube, cancel.' It is supposed that, the thermo-dynamic properties of a moving fluid depend on static pressure in exactly the same manner as those of the same fluid at rest depend upon its uniform hydrostatic pressure. Pressure,Total: In the theory of the flow of fluids; the sum of the static pressure and the velocity pressure at the point of measurement. Pressure, Vapor: The pressure exerted by a vapor. If a vapor is kept in confine ment over its liquid so that the vapor can accumulate above the liquid, the tempera ture being held constant , the vapor pressure approaches a fixed limit called the maxi mum, or saturated, vapor pressure, dependent only on the temperature and the liquid. The term vapor pressure is sometimes used as. synonymous with saturated vapor pressure. Pressure, Velocity: In a moving fluid, the pressure capable of causing an equiva lent velocity, if applied to move the same fluid through an orifice such that all pres sure energy expended is converted into kinetic energy. Psychrometer: An instrument for ascertaining the humidity or hygrometric state of the atmosphere. Psychrometric: Pertaining to psychrometry or the state of the atmosphere with reference to moisture. Psychrometric Chart: A graphical representation of the thermodynamic proper ties of moist air. Psychrometry: The branch of physics relating to the measurement or determina tion of atmospheric conditions, particularly regarding the moisture mixed with the air. Pyrometer: An instrument for measuring high temperatures. Radiant Heating: A heating system in which only the heat radiated from panels is effective in providing the heating requirements. The' term1 Radiant Heating is fre: quently used to include both Panel arid Radiant Heating. Radiation: The transmission of energy by means of electromagnetic waves. Radiation, Thermal (Heat) Radiation: The transmission of energy by means of electromagnetic waves of very long wave length. Radiant energy of any wave length may, when absorbed, become thermal energy and result in an increase in the tempera ture of the absorbing body. Radiation, Equivalent Direct (EDR): A unit of heat delivery of 240 Btu per hr. It does not imply 144 sq in. of surface. Radiator: A heating unit exposed to view within the room or space to be heated. A radiator transfers heat by radiation to objects within visible range, and by conduc tion to the surrounding air which in turn is circulated by natural convection; a socalled radiator is also a convector, but the term radiator has been established by long usage. Radiator, Concealed: A heating device located within, adjacent to, or exterior to the room being heated* but so covered or enclosed or concealed that the heat transfersurface of the device, which may be either, a radiator or a convector, is not visible