What is a Terminal Unit?
The control of air distribution in modern buildings is typically accomplished by the use of air terminal units. A terminal unit is a device that is used to regulate the volume of conditioned primary air from the central air handler to the occupied space. When establishing the required air volume for each space, one should consider both the thermal conditions, as well as fresh air requirements based on occupancy.
A terminal unit controls the primary air volume to a zone through the use of a primary air valve that contains an inlet, damper, and controller. This control of primary air to the occupied space, in communication with the thermostat measuring real-time temperature in the occupied space, allows the desired temperature set-point to be maintained.
Terminal unit selection and application is therefore a critical component of the overall HVAC building system, and will have a direct impact on both the occupant comfort, as well as perceived sound levelswithin the built and furnished occupied space.
Constant Air Volume
Constant volume operation is generally desirable if the load in the intended zone does not fluctuate. This type of air volume strategy is often employed with single speed air handlers without variable frequency drive (VFD) capability.
A single duct terminal with constant volume operation may or may not have an actuator, flow sensor, and controls. If the unit does not have an actuator, it is typically supplied with a manually locking quadrant that allows the damper blade to be locked into a single position.
Variable Air Volume
Variable volume operation is desirable if the occupancy and/or load in the intended zone is expected to fluctuate. To handle the fluctuating system air volumes, central air handlers must either have VFD capability, or there needs to be a bypass relief to the return of the air handler. Variable volume operation requires the DDC package to communicate with the actuator on the damper shaft to modulate the amount of airflow leaving the terminal unit.
With the increased focus within the industry on energy efficiency, variable volume control strategies with pressure independent capability are the basis of design for many consulting Engineers.
Electric Reheat Coils for Terminal Units or Duct Heaters
Electric coils are available as an accessory for single duct and fan powered terminal units, or as a stand-alone duct heater.
Electric coils may be used as sources of heat on most types of terminal units, and are typically located at the discharge of the terminal. They operate based on a line voltage passing through a resistive load (ie. the element rack comprised of coiled wire), which thereby creates heat. Our industry generally assumes electric coils to be 100% efficient. Because these devices require line voltage to operate, they are UL certified for the intended application.
For Venturi Valve (Air Valve pressure independent)
Venturi Valves are mechanically pressure independent airflow control valves designed specifically for room pressure and fume hood control applications.