Rotating Duty Chiller

Hi all,

I am in the process of re-writing a BMS program to run on an S7 plc. I cannot for the life of me work out how to rotate the sequence of chillers upon restarting the system e.g.

Rotation 1 1234 Outputs Q32.1
Rotation 2 2341 Outputs Q32.2
Rotation 3 3412 Outputs Q32.3
Rotation 2 4123 Outputs Q32.4

Any ideas?

Regards

Paul

Pick a register and put it in four "equal to" functions (equal to 1,equal to 2, etc.) with 0 = to Off, with the outputs to the four chillers on each line. Each time you start a chiller, increment the register. If you need more than one, teach each line to overflow onto the next one. This becomes a drum switch in software.

Best Regards,

Bob A.

Paul,

The usual arguement for doing this is to achieve "even wear" on all equipment. But somebody should ask, "Is that really what is wanted?" Suppose it works and all 4 chillers do wear out at the same time. Will it not be more difficult to repair or replace all 4 at the same time? Do you plan a system shut down while you repair all 4, or do you need to keep running 3 while 1 is repaired?

Instead, if they always run 1-2-3-4, #1 will wear out first, while number 4 is still fairlly unused. #4 can then take #1's place while #1 is being rebuilt, and you are good to go again.

In my case, the reason for even use was so that a maintenance effort could be mounted such that all the machines could be gone over in one mobilization. It lead to greater familization by the technical people and the expendable filters, etc could be obtained in a single order.

Bob A.