Understanding Chiller Flow Switches: Preventing Evaporator Freezing

The Ultimate Safety Interlock for Water Chillers

In a chilled water plant, the evaporator barrel acts as a massive heat exchanger, using liquid refrigerant (at temperatures as low as 1°C) to chill the factory's water loop down to 7°C. If the water stops moving while the compressor is running, that water will freeze solid into ice in seconds, rupturing the steel barrel and destroying the machine. The flow switch prevents this catastrophic event.

1. How a Paddle Flow Switch Works

The flow switch is installed directly into the chilled water piping leaving the evaporator. It features a physical metal paddle or target that dangles into the water stream.

• When the chilled water pumps are running correctly, the velocity of the water pushes the paddle forward, mechanically closing a microswitch inside the electrical housing.
• This closed circuit sends a 24V "Proof of Flow" signal to the main chiller microprocessor, granting it permission to engage the massive compressors.

2. Diagnosing False Trips and Failures

Flow switches operate in a harsh, wet environment and are prone to mechanical failure.

• Scale and Debris: Rust flakes or calcium scale can jam the paddle hinge, locking it in the "Open" position, preventing the chiller from ever starting.
• The Danger of Bypassing: Incompetent technicians will often use a jumper wire to bypass a broken flow switch to force the chiller to run. If a pump actually fails later that day, the chiller will not know, and it will freeze and shatter the internal tubes, leading to multi-lakh repair bills.

3. Calibration and Thermal Dispersion Switches

Modern industrial plants are upgrading from mechanical paddle switches to electronic thermal dispersion switches. These have no moving parts; instead, they use heat sensors to accurately measure fluid velocity, eliminating mechanical jamming risks.

Never bypass safety interlocks. Book a certified chiller safety audit with Prime Cool.