ASHRAE Terminology

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

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entering wet-bulb (EWB) temperature

the temperature that the wet-bulb portion of a psychrometer would measure if exposed to air entering the evaporator coil. For a draw-through fan with no heat gains or losses in the ductwork and no outdoor air mixed with return air, this would also be the zone air wet-bulb temperature. For a similar configuration, but when outdoor air is mixed with return air, EWB equals the mixed-air wet-bulb temperature. For mixtures of water vapor and dry air at atmospheric temperatures and pressures, the wet-bulb temperature is approximately equal to the adiabatic saturation temperature (temperature of the air after undergoing a theoretical adiabatic saturation process). The wet-bulb temperature given in psychrometric charts is really the adiabatic saturation temperature.

entrainment flow

(also known as conventional mixing). Systems with ceiling-based supply air diffusers and return air grilles are common examples of air-distribution systems that produce entrainment flow.

exhaust terminal unit

terminal unit for regulating exhaust or return airflow.

extract air

airflow leaving a treated space. Compare with return air.

filter mixing box

in air-handling units, the section containing the filters and the outside air/return air dampers and mixing plenum.

recirculated air

air taken from a space and returned to that space, usually after being passed through a conditioning system. The part of the return air that is reused. Air removed from a space and reused as supply air.

relief air

(1) all return air that is discharged directly to the outside or exhausted by separate exhaust fans. (2) building return air discharged by the air-handling unit (AHU) equipment to control building pressure when an HVAC system is operating in the economizer cycle.

return air

air removed from a space to be recirculated or exhausted. Air extracted from a space and totally or partially returned to an air conditioner, furnace, or other heating, cooling, or ventilating system.

unit ventilator

fan coil unit package devised for applications in which the use of outdoor and return air mixing is intended to satisfy tempering requirements and ventilation needs.