Accessory List

Glossary
384
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Auxiliary Device (Pneumatic): A control module which is
generally placed between the controller and actuator that
modifies the controller signal in some manner before the
signal reaches the actuator. Example: Relays and switches.
Averaging Element: A sensing device that can extend
across the entire duct and sense the average temperature.
Axial Fan: A fan with multiple impeller blades that move air
along the length of the fan’s axle. Its flanged, heavy-gauge
cylindrical enclosure mounts directly into round ductwork.
Boiler: A closed vessel in which fuel is burned to generate
steam or to heat water.
Brake Horsepower: The total horsepower applied to the
drive shaft of any piece of rotating equipment. The actual
power required to drive a fan or pump.
Branch Lines (Pneumatic): The tubing in a pneumatic
control system which carries the output signal from controller
to auxiliary devices or actuators.
Bridge (TAC System 8000): A device that receives the
resistance change from the sensor and produces a low
voltage change that is amplified to a higher level which
becomes the controller output. Usually a part of the controller
or can be a separate module.
Btu (British Thermal Unit): The energy or heat required to
raise the temperature of one pound of water 1°F under
standard pressure.
Butterfly Valve: A cylindrical flanged-end body with an
internal, rotatable disc serving as a fluid flow regulating device.
Bypass Control Damper: A damper mounted inside a duct
that provides an alternate path for air to flow.
Calibration Point: The output voltage of a controller when
the setpoint and control point are equal.
TAC System 8000: Normally this would be 7.5 Vdc (midpoint
of controller output (6 to 9 Vdc)). It may also be midpoint of
actuator ranges.
Pneumatic: Normally this would be 8 psig (midpoint of
controller output (3 to 13 psig)). It may also be midpoint of
actuator ranges: 3-8=5.5, 5-10=7.5, 8-13=10.5, etc.
Cavitation: The phenomenon occurring in a flowing liquid
when the pressure falls below the vapor pressure of the liquid,
causing the liquid to vaporize and form bubbles. The bubbles
in the flowing liquid are carried through the pump or valve inlet
to a zone of higher pressure where they suddenly collapse or
implode with terrific force.
Centrifugal Fan: A fan housed in a scroll-type housing.
Those used in HVAC systems usually have impellers of the
airfoil, backward-inclined or backward-curved, or forward-
curved type, for different applications. May have a single or
double inlet. May or may not have inlet vane damper(s).
CFM (Cubic Feet Per Minute): A rate of air volume delivery.
Standard measure for HVAC ducted systems.
Chiller: A machine, usually centrifugal or reciprocating, that
chills the water used to cool a building. Heat removed from the
water is rejected to a remote air-cooled condensing unit, or to
a water-cooled condenser that is usually an integral part of the
chiller.
Close-Off: The maximum allowable pressure drop to which
a valve may be subjected while fully closed.
Cold Deck: The portion of the duct containing the chilled
water coil or DX coil. Generally parallel with a bypass deck or
hot deck.
Comfort Zone: The range of temperatures and humidities
over which the majority of people feel comfortable. Generally,
between 68°F and 79°F and 20% to 60% relative humidity.
Conduction: Heat energy transmitted by direct contact.
Constant Volume Control: Constant volume control
systems change the supply air temperature in response to the
space load, while maintaining constant air flow.
Control Point: The actual value of the controlled variable
which the controller operates to maintain (under any fixed set
of conditions).
Controlled Device (C/D): An apparatus that receives the
signal from a controller and positions the damper or valve to
match the capacity to the load. Example: Motorized damper or
valve.
Controller: A device that monitors a controlled variable and
changes the position of final control devices (such as valves,
dampers, or contacts) to maintain the value of the controlled
variable at or near the controller’s setpoint.
Convection: Heat that moves from on place to another by
means of currents that are set up within some fluid medium,
vapor or liquid.
Corrosion Test Coupon: A weighed strip of metal that is
suspended in a flowing water stream for a specified time. The
corrosion rate is calculated from weight loss during the period
of exposure, using procedure ASTM D2688.
Cubic Feet/Minute: A rate of air volume delivery. Standard
measure for HVAC ducted systems.
Cv (Flow Coefficient): The flow of water in gallons per
minute (at 60°F) that causes a pressure drop of 1 psi across a
fully open valve.
Damper: A valve used to regulate the flow of air or some
other gas.
DDC (Direct Digital Control): Microprocessor-based control
systems that provide direct control of the individual
components of an HVAC system without the use of
conventional control devices such as thermostats.
Deadband: An area of no change. For example, a point’s
value must go above or below the deadband to trigger an
alarm or control action.