Programming Advice

We are seeking programming advice on a new cooling water application. Our objective is to maintain a Temp. Setpoint of the water leaving the cooling towers entering the process. There are four new cooling towers, each having eight fans. Water leaving the cooling towers goes to a common line with an Analog temperature transmitter which will be our only input to control the fans. We want to start and stop fans in pairs of two with on/off control to maintain our setpoint. We are trying to accomplish this using a Allen-Bradley SLC 5/05 ethernet processor.


We are seeking a wide variety of programming schemes to accomplish our objective.

One possibility would be to use comparison instructions. This would
involve taking the analog input from the temp. transmitter and
scaling it to your desired engineering units. The input could
then be compared to a pre-determined number by which certain
sets of fans would be turned on. Logic may need to be written
so fans would remain on for a certain amount of time to prevent
rapid on/off control.