Another type of flow switch used in the mini chiller units is the differential pressure flow switch.
This flow switch works by measuring the difference in pressure between the inlet and outlet to
detect the flow. The schematic diagram is as follows:
Water out
Water in
BPHE
Flow switch
The flow switch model used is SFS 050.
The main advantages of using this type of flow switch is:
a) It does not have moving parts, i.e. it is more rugged and reliable
b) It does not require any adjustments
c) The internal parts are sealed, i.e. it has good water-proofing.
The flow switch has three wires coming out of it, i.e. common (black), normally open (blue) and
normally closed (brown). The wiring in the chilller has been configured whereby the common and
normally open wires are connected to the PCB.
This means that when there is sufficient water flow, the switch contact will close to give a 240
VAC signal to the PCB.
The operating pressure differential to close the NO contact is 50 mbar, whereas to open it is 27
mbar.
Connection of the switch to the water pipe is done with two flexible hoses, screwed in with
threaded couplings to access valves. This facilitates easy replacement of the switch as it is not
necessary to drain away all the water in the system during servicing.
Section 12 Page 145
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