Thermal Flow Switches
Thermal flow switches detect and monitor the flow of a fluid (liquid or gas) in a pipeline or system by measuring changes in temperature. It operates based on the principle of heat transfer, when a fluid flows over a heated sensor, it cools the sensor due to the thermal conductivity of the fluid. The rate of cooling provides an indication of the fluid's flow rate. When the flow falls below or exceeds a predefined setpoint, the thermal flow switch triggers an electrical circuit to take specific actions, such as activating an alarm, shutting down equipment, or controlling pumps and valves.
Thermal flow switches contain a platinum RTD (Resistance Temperature Detector). This sensor is heated either by an external electrical supply. A second temperature sensor is used as a reference to measure the ambient or baseline temperature of the fluid, this reference sensor helps in determining the rate of cooling of the heated sensor. An electronic controller processes the signals from the sensors, comparing the heated sensor’s temperature to the reference temperature. The controller determines whether the flow is above or below the setpoint and activates an electrical circuit accordingly. The switch includes output relays or contacts that open or close when the flow rate reaches a specific threshold. These relays can be connected to alarms, pumps, valves, or other control systems.
A thermal flow switch works based on the principle of convective heat transfer, the heated sensor is placed in the flow path of the fluid. An electrical current heats the sensor, raising its temperature above the ambient temperature of the fluid. When the fluid is flowing, it cools the heated sensor by carrying away the heat. The amount of cooling depends on the fluid’s flow rate: higher flow rates cause more rapid cooling, while lower flow rates result in less cooling. The controller continuously monitors the temperature of the heated sensor and compares it to the reference sensor or baseline. If the flow rate drops below or rises above a preset level, the thermal flow switch detects this change in temperature difference. When the flow rate reaches the set threshold, the switch triggers its output relay or contacts to take action—such as turning off a pump, closing a valve, sounding an alarm, or sending a signal to a control system.
Thermal flow switches are used to protect pumps from running dry or operating under low flow conditions. If the flow rate drops below the setpoint, the switch can shut down the pump to prevent damage. In chemical, petrochemical, and pharmaceutical industries, thermal flow switches are used to monitor and control the flow of various process fluids, providing safety interlocks and ensuring consistent production quality. In cooling and heating applications, such as HVAC systems or industrial process cooling, thermal flow switches monitor the flow of coolant or heating fluids to ensure adequate circulation and prevent overheating or freezing. In water treatment, filtration, or chemical dosing systems, ther help ensure that the required flow rate is maintained for optimal treatment or mixing. In air and gas flow monitoring systems, and pneumatic systems, compressed air lines, and gas delivery systems, thermal flow switches detect low or no-flow conditions to prevent issues such as air supply loss or gas leakage.
Thermal flow switches are suitable for both liquids, gases, and slurries. Thermal flow switches have no moving parts, making them less prone to mechanical failure, reducing maintenance requirements, and enhancing durability in harsh environments. Available in insertion or inline configurations, thermal flow switches can be installed in various setups and orientations, providing flexibility for different applications. Thermal switches are capable of providing accurate flow detection even under varying temperature and pressure conditions, ensuring reliable performance.
