Control loops: introduction & terminology
Automatic control – when is properly applied – ensures that a correctly designed HVAC system will maintain a comfortable environment and perform economically over a wide range of operating conditions.
The picture above presents a basic HVAC control system. Warm air flows through a finned-tube cooling coil, where heat is transferred from the air passing over the tubes and fins to the water flowing through the tubes. A valve (controlled device) is used to vary the amount of water flowing through the coil and, therefore, the cooling capacity of the coil. In general, this basic control system includes a controlled variable, a sensor, a controller, a controlled device, and a controlled agent. A small analysis of the parameters is given below:
- The controlled variable is the parameter being measured and controlled. In the particular case, the controlled variable is the dry-bulb temperature of the air leaving the cooling coil.
- The sensor measures the condition of the controlled variable and sends an input signal to the controller. Here, the sensor is a dry-bulb temperature sensor located in the airflow.
- The controller is the brain of the system. It compares the measured condition of the controlled variable to the desired condition (setpoint) and transmits a corrective output signal to the controlled device.
- The controlled device is the component that reacts to the output signal from the controller and takes action to vary the controlled agent. Here, the controlled device is the valve.
- The controlled agent is the medium that is manipulated by the controlled device. In the above example, the controlled agent is chilled water. As the valve opens, more chilled water is allowed to flow through the cooling coil, increasing the cooling capacity of the coil.
The coordination of these elements is the basis for automatic control. This systematic operation is frequently referred to as a “control loop”. Typically, there are two types of control loops used in HVAC applications: the open-loop and the closed-loop, which are analyzed below.
The open-loop strategy assumes a fixed relationship between an external condition and the controlled variable. As the above picture demonstrates, the sensor measures the outdoor-air temperature. The controller compares this temperature to a given set of criteria and adjusts the valve to vary the capacity of the coil.
This arrangement assumes a fixed relationship between the outdoor temperature and the required cooling capacity of the system. The disadvantage of the open-loop is that it doesn’t take into account variables that may affect the air temperature downstream of the coil, such as variations in either airflow or water temperature. In this example, the air may be too hot or too cold, resulting in wasted energy or poor comfort control.
This is often the consequence of trying to control the condition of the controlled variable based on an assumed fixed relationship to an external variable. For this reason, open control loops are not often used in HVAC systems.
The closed-loop strategy senses the actual condition of the controlled variable. In this example, the controller compares the temperature of the air leaving the coil to the desired setpoint and adjusts the valve to meet that desired temperature. In other words, closed-loop control is based directly on the condition of the controlled variable, such as the leaving-air temperature in this example.
A closed-loop provides better control than the open-loop strategy, resulting in more efficient use of energy and improved occupant comfort. For this reason, closed-loop control is generally preferred in HVAC applications.
Sometimes, a controller may use a combination of these two loops. In that case, a closed control loop measures the temperature of the air leaving the coil and adjusts the valve to maintain the desired setpoint. A second sensor measures the outdoor temperature. As the temperature of the outdoor air decreases, the controller resets the setpoint to a higher value.
This strategy is called “control reset”. The closed-loop sensor acts as the primary source of information, whereas the open-loop sensor acts as the secondary source. Control reset is often used to minimize energy consumption, while still maintaining acceptable comfort.
The control loops article you just read was based on material found on the following ASHRAE handbooks:
2010 – Fundamentals of HVAC Control Systems
2011 – HVAC Applications
2012 – HVAC Systems And Equipment