ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment
ashrae.org/ashraeterms

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age of air

time of passage of air from one point to another within an indoor space.

air change effectiveness

Ratio of the room mean age of air for an ideal plug (piston) flow pattern compared to the room mean age of air for the real flow at a test location.

air leakage

1) the flow of air through the building envelope caused by a specified pressure difference, a measure of airtightness, cfm at fixed pressure (m3/s at fixed pressure). 2) undesirable or unwanted leakage of air from within a component within an air-distribution system that could include such items as ducts, air terminal devices, and AHUs.

airflow resistance

deterrent (due to friction, change of direction, etc.) to the passage of air within an air-distribution system and/or equipment.

average age of air

average of local mean ages of air measured throughout an indoor airspace.

float-and-thermostatic trap (F&T)

steam trap that relies on the density of water to raise a float-and-lever mechanism to operate a valve head. It discharges condensate as it forms and enters the trap body. It includes a thermostatic balance pressure or bimetallic air vent to allow free passage of air on start up and discharges incondensable gases reaching the trap during operation.

louver

device consisting of an assembly of parallel sloping vanes, intended to permit the passage of air while providing a measure of protection against environmental influences. Also spelled louvre.

nominal time constant

the indoor air volume of a space or building divided by the rate of outdoor air supply, the nominal time constant also equals the average age of air exiting from the space or building. The reciprocal of the nominal time constant is called the nominal air change rate.

perfect mixing

a theoretical airflow distribution pattern within a ventilated space where the supply air is instantaneously and uniformly mixed with the air in the space such that the concentration of all constituents in the air, and the age of air, are spatially uniform.

smoke barrier

an approved opening protection device designed to resist the passage of air or smoke that meets the requirements of UL Standard 555S-1983, Leakage Rated Dampers for Use in Smoke Control Systems. A combination fire and smoke damper should also meet the requirements of UL Standard 555-1990, Fire Dampers.

steam trap

device for allowing the passage of condensate and preventing the passage of steam, or for allowing the passage of air as well as condensate. Common steam trap types include float and thermostatic (F&T) traps, inverted-bucket traps, and thermodynamic traps.

thermostatic balanced pressure steam trap

trap installed on the discharge side of a heating unit and designed to pass air freely on start-up and condensate at a subcooled temperature, but to prevent steam vapor passing into the return. It can have a bellows or encapsulated metallic diaphragm containing a small quantity of volatile liquid. At the bottom of the diaphragm or bellows is attached a hardened, self centering valve head operating on the pressure side of the valve seat. At ordinary temperatures and atmospheric pressure, the valve is fully open to permit free passage of air and cold condensate. The trap discharges at a fixed temperature below that of steam saturation temperature and closely follows the steam pressure/temperature curve.