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A unit for estimating the energy needed for cooling a building; one unit is given for each degree Fahrenheit that the daily mean temperature exceeds 75 F (24 C).

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(Btu) 1. A unit of heat energy equal to the heat needed to raise the temperature of 1 pound of air-free water from 60 to 61 F at a constant pressure of 1 standard atmosphere; it is found experimentally to be equal to 1054.5 joules. Also known as sixty degrees Fahrenheit British thermal unit (Btu60/61). 2. A unit of heat energy that is equal to 1/180 of the heat needed to raise 1 pound of air-free water from 32 F (0 C) to 212 F (100 C) at a constant pressure of 1 standard atmosphere; it is found experimentally to be equal to 1055.79 joules. Also known as mean British thermal unit (Btumean). 3. A unit of heat energy whose magnitude is such that 1 British thermal unit per pound equals 2326 joules per kilogram; it is equal to exactly 1055.05585262 joules. Also known as international table British thermal unit (BtuIT).


A measure of the departure of the mean daily temperature from a given standard; one degree-day is recorded for each degree of departure above (or below) the standard during a single day; used to estimate energy requirements for building heating and, to a lesser extent, for cooling.


A standard measure of energy in the British unit system. 1 Btu is the amount of heat required to raise the temperature of a liquid by 1 degree.


The total amount of heat energy that must be removed from a system by a cooling mechanism in a unit time, equal to the rate at which heat is generated by people, machinery, and processes, plus the net flow of heat into the system not associated with the cooling machinery.


The amount of energy required to raise the temperature of 1 gram of water by 1 degree C. The kilocalorie (kcal) is a typical unit of measure in the process industry, 1 kcal = 1000 calories.


The volume of fluid flowing through the cross- section of a conduit in unit time at the pressure and temperature prevailing in that section.


The time required for a fixed amount of an oil to flow through a capillary tube under the force of gravity. The unit of kinematic viscosity is the stoke or centistoke (1/100 of a stoke). Kinematic viscosity may be defined as the quotient of the absolute viscosity in centipoises divided by the specific gravity of a fluid, both at the same temperature Centipoises / Specific Gravity = Centistokes .


The quantity of heat required to raise a system one degree in temperature in a specified way, usually at constant pressure or constant volume. Also known as thermal capacity.


The mass per unit volume of a substance at specified conditions of temperature and pressure.


The lowest temperature to which air can be cooled at any given time by evaporating water into it at constant pressure, when the heat required for evaporation is supplied by the cooling of the air. This temperature is indicated by a well-ventilated wet-bulb thermometer. See also FREE-AIR TEMPERATURE.

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