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nadir
(1) object point that is directly beneath an aircraft flying an airborne imaging survey. (2) point of the celestial sphere that is directly opposite the zenith and vertically downward from the observer.
NAICS
North American Industry Classification System, as published by the Office of Management and Budget, 1997.
nameplate rating
full-load, continuous rating of a generator, prime mover, or other equipment under specified conditions, as designated by the manufacturer and usually indicated on an attached plate. Compare to capacity.
naphthenic oil
a petroleum-lubricating oil fraction in which naphthenic ring-type hydrocarbon structures predominate.
nationally recognized laboratory
see laboratory.
natural air circulation
(1) air circulation caused by air movement or pressure differences. (2) air circulation caused by differences of density induced by differences of temperature and/or humidity.
natural convection
heat transmission by movement of a fluid, caused by density differences. Usually created by a thermal heat source or heat sink.
natural convection cooling
(1) cooling a fluid by natural convection over the cooling surface. (2) cooling a space by means of air cooled by natural convection.
natural draft
airflow resulting from the difference between atmospheric density and some lower density; chimney effect.
natural gas
naturally occurring mixture of hydrocarbon and nonhydrocarbon gases found in porous geologic formations beneath the earth’s surface, often in association with petroleum. The principal constituent is methane.
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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.