Pressure Independent Control
Pressure Independent controls in valves an additional input to the control of the VAV terminal unit. The pressure independent control options also monitor and respond to the velocity of the air flow, generally at the inlet of the unit.
The terminal unit damper is positioned from a signal sent from the zone thermostat through a velocity reset controller to the damper actuator. The velocity reset controller then responds to changes in the inlet pressure conditions to maintain the required airflow. Pressure independent controls are frequently used for single duct variable volume control. For a given thermostat setting the controller can position the damper further open if the air flow at the inlet is insufficient to meet the requirement or it can position the damper further closed if the inlet air flow is greater than the requirement.
Another design consideration when using pressure independent controls is the state of the central system. When the central system is shut down or not supplying adequate air to meet the design requirements, the primary flow control damper can drive open looking to satisfy a minimum air flow condition as long as the controls remain active. This feature can be beneficial by providing open dampers at the start of the morning warm-up cycle. However, it may not be desirable in some fan terminal unit applications. Re circulated air could short circuit and flow backup stream.
Pneumatically, the terminal unit damper can be configured to fail in the open or closed position on a loss of main control air pressure irregardless of the thermostat action required. Electrically or electronically the damper must be powered and driven to either of those conditions.
Flow Control Valve
Control valves can also work with hydraulic actuators (also known as hydraulic pilots). These types of valves are also known as Automatic Control Valves. The hydraulic actuators will respond to changes of pressure or flow and will open/close the valve. Automatic Control Valves do not require an external power source, meaning that the fluid pressure is enough to open and close the valve.
Automatic control valves include: pressure reducing valves, flow control valves, back-pressure sustaining valves, altitude valves, and relief valves. An altitude valve controls the level of a tank. The altitude valve will remain open while the tank is not full and it will close when the tanks reaches its maximum level. The opening and closing of the valve requires no external power source (electric, pneumatic, or man power), it is done automatically, hence its name.
To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller will then process this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.