Automatic Control Systems
HVAC systems are dynamic:
Sized for extreme conditions
Most operation is part load / off-design
Deviation from design => imbalance since Capacity > Load
Without control system, HVAC would overheat or overcool spaces.
“A system that reacts to a change or imbalance in the variable it controls by adjusting other variables to restore the desired balance.”
Modem computer-based systems manage system resources (“supervisory”):
• Reduce energy use
• Identify maintenance problems
Essential components of a control system:
Controlled variable is a characteristic of system to be regulated.
“set point” is desired value “control point” is actual value
“set point”- “control point” = “error” or “offset”
Sensor measures actual value of controlled variable.
Controller modifies action of controlled device in response to error.
Controlled device acts to modify controlled variable as directed by controller.
Example: Water tank level control
Simple water-level controller. The set point is the full-water level; the error is the difference between the full level and the actual level.
Example: Steam heating coil.
Simple heating coit control system showing the process (coil and short duct length), controller, controlled device (valve and its actuator), and sensor. The set point entered externally is the desired coil outlet temperature.