A dual chiller changeover panel, drawn once and built again and again for a hire fleet

The requirement

Two chillers on a hire site share one cooling load. The changeover panel decides which one runs, brings the second in when the first cannot keep up, swaps duty so both wear evenly and stops calling a chiller that has tripped. The hire company already had a panel design that did this, but it was being built as a one-off each time. They wanted it standardised: the same drawing, the same parts and the same wiring every time, in a version for indoors and one for outdoors, so their depots could hold panels in stock and pick one off the shelf when a job came in at short notice.

The constraints

• Their design, their product code: we were improving an existing panel, not starting again, and every change had to be agreed with their technical manager.
• The panel only enables the chillers and reads their faults; the chillers are powered separately, so the signals going out to them had to be safe to handle on site.
• A failed or disconnected fault signal had to read as a fault, not as healthy.
• Indoor and outdoor versions that an engineer could wire, test and fault-find the same way.
• Repeat orders from more than one depot and buyer, so every panel had to match the drawing exactly.

What we built

We redrew the panel with circuit breakers in place of fuses and changed the control from 110V AC to 24V DC, so the volt-free circuits out to the chillers are extra-low voltage. The logic runs on a Siemens LOGO! with display: the start signal enables the duty chiller, an over-temperature stat brings in the assist chiller, duty rotates once a week at a set day and time, a faulted chiller is never called and the healthy one becomes fixed duty, and a front switch can force chiller 1 or chiller 2. Fault inputs are fail-safe, with short delays so a flicker does not cause a changeover, and there are separate volt-free outputs for a common fault and a chiller alarm. Cable colours follow the customer's own standard. The indoor version is a steel wall box; the outdoor version is a polyester enclosure with a clear outer door. The same approach went into a triple chiller changeover panel (two running, one standby) and a defrost panel for low-temperature air cooler coils, built to the customer's drawing.

Delivery

The first standard panel set the drawing, which the customer approved before the repeat orders started. Since then the dual changeover panels have typically gone out one to three weeks from order, one at a time or in batches of two or three, with the drawing and test certificate in each.

The outcome

Twenty dual chiller changeover panels so far, nine indoor and eleven outdoor, plus triple changeover and defrost panels, all built to one set of drawings. The hire company holds them in its depots, so an urgent job can go out with a panel that is already on the shelf; one buyer ordering three more noted they already had one in stock for an urgent job.