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unit thermal conductance or overall heat transfer coefficient, W/(m2·K) (Btu/(h·ft2·°F).
ultimate strength
the stress at which rupture occurs.
ultraviolet (UV) radiation
range of electromagnetic radiation wavelengths from about 400 nm, just beyond the violet in the visible spectrum, to about 4 nm, on the border of the x-ray region. UV photons have enough energy to initiate some chemical reactions, to degrade most plastics, and to kill microorganisms.
unconditioned space
space within a building that is not conditioned space.
underfloor air-distribution system (UFAD)
an air-distribution system that uses an underfloor plenum (open space between the structural concrete slab and the underside of a raised-floor system) to deliver conditioned air into the space, typically through floor diffusers. Air is returned at a level above the occupied zone (typically at the ceiling level). Under cooling operation, UFAD systems produce a stratified environment, similar in principle to that of displacement systems. The primary difference between these systems is that UFAD outlets deliver air at higher velocity, producing greater mixing in the area near the outlet discharge.
condition where voltage is lower than rated.
unglazed solar collector
a solar collector in which the absorbing surface is directly exposed to the atmosphere.
uniflow compressor
compressor in which the suction gas passes through the piston.
uniform velocity
a smooth, straight line motion of a gas or a liquid across the area of flow.
uninterruptable power supply (UPS)
a system intended to deliver continuous, stable power to the critical load. The majority of modern UPS systems are of two fundamental types: (a) “static,” in which incoming alternating current (AC) power is rectified to direct current (DC) and then inverted back to AC, with batteries in the DC portion that assume the load when incoming power fails or anomalies occur, and (b) “rotary,” in which incoming AC power drives a propulsion unit that turns a generating device, with a heavy flywheel storing kinetic energy that continues to turn the generating portion when incoming power fails or anomalies occur. Either type can be made up of one or more modules running in parallel to add capacity or redundancy or both. DC UPS systems, which eliminate the inverter and deliver DC power to the ITE, are also used.
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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.