Flow switch

A flow switch proves that water is actually moving through a chiller or a fan coil before the compressor or the control sequence is allowed to run. It is a safety device first and a control device second: without it, a blocked strainer, a failed pump or a closed valve leads to a frozen evaporator, and that is a repair measured in thousands rather than in the price of a switch.

The range covers three connection sizes - 1/2, 3/4 and 1 inch - in the HFS, F61KB, FQS and LKB-01D families. Paddle sets are supplied with each switch (one to five paddles depending on model) so the same body can be set for different pipe diameters: the paddle has to reach the flow stream without touching the far wall.

Fluid temperature is rated to +120 C, maximum working pressure is 1.0 to 1.7 MPa depending on model, and the standard enclosure is IP54, with the F61KB-11C using a HEMA1 housing. The switches are adjustable, so the trip point can be set to the actual flow rather than to a factory value.

Selection is by pipe size and by the minimum flow that must be proved - not by the nominal pipe diameter alone, because flow switches trip on velocity and the same switch behaves differently at the bottom of a wide pipe.

Two rules from the field. First, these switches must not be used where the liquid can drop below freezing: ice will damage the paddle and give a false "flow" reading. Second, mount the switch in a straight run with the arrow following flow, at least five diameters downstream of any elbow or valve, or the paddle will sit in turbulence and chatter.