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radial-bladed impeller (paddle-bladed impeller)
one in which the blades are flat and disposed radially from the fan hub to the outer edge.
radial-energy distribution system
system of radial feeders extending outward from a centrally located district energy plant (heating or cooling). Each feeder normally is composed of one supply pipe and one return pipe.
radiance
total radiant flux emitted from a surface through unit projected area by unit solid angle. It includes the self-emitted radiation plus reflections from sources other than the object evaluated as interpreted from the direction of measurement. The term intensity of radiation is sometimes used as a synonym for radiance. See also radiosity.
radiance at a point on a surface
the quotient of the flux incident on an element of a surface containing the point, by the area of that element, measured in watts per square meter, W/m2 (Btu/ft2).
radiant
radiating rays of light; emitting or reflecting beams of light; vividly shining; glowing; brilliant.
radiant asymmetry temperature
difference between the plane radiant temperature of the two opposite sides of a plane element. See plane radiant temperature.
radiant barrier
a surface of low emissivity (less than 0.1) placed inside an attic or roof space above (but not touching) the distribution system to reduce radiant heat transfer.
radiant energy
energy passing through space in the form of electromagnetic radiation (such as light or ultraviolet or infrared radiation) or as a stream of particles (e.g., electrons or protons).
radiant flux
the time rate of flow of radiant energy (watts).
radiant flux density
measure of radiant power per unit area flowing across or onto a surface. (Also called irradiance).
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About ASHRAE Terminology

ASHRAE Terminology, a free resource, is a comprehensive online glossary of more than 3700 terms and definitions related to the built environment, with a focus on heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, and measurement terms.

This searchable glossary was designed for use by engineering professionals but is also useful for architects, building owners and operators, educators, public officials, and homeowners.

The terms in ASHRAE Terminology have been assembled and defined and are updated on an ongoing basis by ASHRAE Technical Committee (TC) 1.6, Terminology. TC 1.6 is concerned with terminology, nomenclature, definitions, abbreviations, symbols, and the International System (SI) and Inch-Pound (I-P) units used in the fields of heating, ventilating, air conditioning, water heating, and refrigeration.

To request a free electronic file of the complete list of terms and definitions, email ashraetermseditor@ashrae.org.