Question

Control circuit, ladder diagram, and sequence of operations: 1. The chiller is started by the need for chilled water by one of four area sensors of chilled water temperature in the chilled water distribution loop of four buildings. The sensors call for reduced temperature when the thermo switch registers 50F 2. The chiller runs until the outgoing water temperature is 45 degrees F for four minutes. 3. There is a manual start/ stop switch at the chiller and at the central utility plant SCADA center. 4. There is freeze control which is a failsafe and shuts the chiller off if the chilled water reaches 40 F. 5. There are red/green off/on pilot lights at both the chiller and at the SCADA center.

Use a ladder diagram please!

Answer 1

The output is one command for the Chiller. The lights can be triggered using the same command.

It will be treated as high to run, low to stop, as nothing else is mentioned.

The inputs which can change the state of the Chiller are:

1. Four sensor inputs which trigger the Chiller to turn on when temperature exceeds 50F

2. On/Off switch

3. Outgoing water temperature sensor input: Turn off chiller when temperature falls below 45F for 4 minutes

4. Freeze control to turn off chiller at 40F

We shall look at these points one by one.

1: This is fairly straight forward logic. Since any of the sensors can turn on the Chiller, they should be connected in parallel. Since the logic command Logic would be:

2. The Start commands from the switch can also start the chiller, so they should be connected in parallel with the Sensor commands. Modified logic:

We will look at the connection for the Stop switch a little later.

3. It is stated that when the outgoing water temperature is < 45F for 4 min, the Chiller is turned off. Let us draw the logic for this step alone. We will connect it to the Final run command eventually.

4. When outgoing water temperature is lower than 40F, the Failsafer logic should turn off the chiller. Failsafe logic is:

Let us now talk about the final logic for the Run command. For the case when started by the Sensor logic, temperature begins to fall and when it falls below 50F, the sensor output goes to 0. Hence the output of this logic needs to be latched so that the Run command stays high till temperature falls below 45F.

Hence intermediate logic:

We now need to add the conditions for stopping the Chiller in series with this latched command. The conditions are: Temperature logic (Temp below 45F for 4 min), Failsafe logic and Stop button at Chiller or SCADA. Add the inverted outputs generated above:

It is now seen that all conditions mentioned in the problem are satisfied.

Final ladder diagram showing all rungs together:

Footnote: It was assumed that the start/stop switch has different contacts for Start & Stop. If that is not the case, the switch status can be used to generate start/stop command by this logic:

Running and Not Running here are status of the Chiller.

If this is the case, the "Start" and "Stop" shown here have to be used in the final Run logic in place of the "Start switch" & "Stop switch" from the switch shown in the original logic.