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TRG-TRC014-EN 87
changeover/bypass VAV system A system that allows variable supply air to
the spaces but uses a constant-volume central supply fan. This is accomplished
by using a large damper to bypass the excess air. This is common in smaller
buildings requiring individual space comfort control.
Coanda effect Concept behind the operation of a linear slot diffuser. Air is
discharged at a relatively high velocity along the surface of the ceiling, creating
an area of low pressure that causes the supply air to hug the ceiling. As it
travels along the ceiling, air from the space is drawn into, and mixed with, the
supply airstream. When the air settles to the occupied levels of the space, it has
reached an average temperature.
constant-volume system Type of air-conditioning system that varies the
temperature of a constant volume of air supplied to meet the changing load
conditions of the space.
cooling-only terminal unit Simplest type of single-duct VAV terminal unit. It
has the capability of varying the airflow, but has no method for space heating.
diffuser Device connected to the end of the supply duct system, used to
distribute the supply air effectively to the conditioned space.
direct digital control (DDC) Method of terminal-unit control using an electric
motor to operate the airflow modulation-device activator. It uses a
microprocessor that enables digital communication between the unit controller
and a central building automation system.
discharge dampers Method of supply fan modulation that matches the static
pressure supplied by the fan with the static pressure required by the system.
This is accomplished by using a modulating damper a little bit downstream of
the fan to create a static-pressure drop in the system.
dual-duct terminal unit VAV terminal unit consisting of two primary airflow
modulation devices, one for cool primary air and one for warm primary air.
These units can be controlled to provide either a constant volume or a variable
volume of supply air to the space.
electronic control Method of terminal unit control using an electric motor
to operate the airflow modulation-device actuator. The motor can drive the
modulation device open or closed.
equal friction duct design method Method of designing an air duct system
that results in an equal static-pressure drop per foot of duct. Equal-friction duct
systems can be easily designed by hand.
fan modulation curve Curve that illustrates the static pressure requirement
of the VAV system fan over the range of airflows.
fan outlet static-pressure control Method of VAV system static-pressure
control that mounts the static-pressure sensor near the outlet of the main
supply fan and maintains a constant static pressure at the sensor.
fan performance curve Plot of the airflow capacity of a specific fan at a given
speed (rpm) versus the static pressure it generates.
Glossary