What is Control loop ? Controlling in HVAC System ~ HVACR DICTIONARY

Control loop

The fifth, and final loop of the HVAC system is that the controls loop. Each of the previous four loops contains several components. Each components must be controlled during a particular thanks to ensure proper operation. Typically, each bit of kit (which could also be comprised of 1 or more components of a loop) is provided with a unit-level, automatic controller.

In order to supply intelligent, coordinated control in order that the individual pieces of kit operate together as an efficient system, these individual unit-level controllers are often connected to a central, system-level controller.

Finally, many building operators want to watch the system, receive alarms and diagnostics at a central location, and integrate the HVAC system with other systems within the building. These are some of the functions provided by a building automation system(BAS).

Roof top VAV system

This system uses a packaged rooftop air conditioning to deliver air to many VAV terminal units. This packaged rooftop air conditioner includes a unit-level controller that coordinates the operation of all components packaged inside this piece of equipment, such as the outdoor-air and return-air dampers, the supply and exhaust fans, the compressors, and the condenser fans.

In addition, each VAV terminal unit is provided with a unit-level controller that directs its response to space conditions.

The system-level controller coordinates the operation of the VAV terminal units and the rooftop unit during the various modes of operation, such as occupied, unoccupied, and morning warmup.

Air-cooled Chiller, Fan- Coil System

This system includes fan coil units served by an air cooled chiller and a hot water boiler. A fan coil unit is located in or near each conditioned space, and each one includes its own unit level controller to modulate water flow through the coil in response to the changing load in the space. The unit level controller on the air cooled water chiller ensures the flow of chilled water whenever it's required, and therefore the boiler controller ensures the flow of hot water whenever it is required. Finally, a fanatical outdoor military unit conditions all of the outdoor air brought into the building for ventilation, before delivering it on to the individual spaces.

In this example, a separate, system-level controller coordinates starting and stopping the pumps, the dedicated outdoor military unit , and therefore the stand alone fan . It also determines when to vary over from cooling to heating mode, and coordinates the operation of the chiller and boiler to stop them from operating simultaneously.

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