Cascade control

The strategy may be simple but I can't figure this out.

Our plant has 8 water cooled chillers and 8 condenser water cooling towers.The condenser water supply temperature setpoint is reset by cascade control based on the average refridgerent diff. pressure of the running chillers.Can anybody please explain how the PLC (AB 5/40E)code may calculate the temperature setpoint based on the chiller refridgerent diff. pressure???


Any suggestions would be really appreciated.

I'm assuming that the chillers are using a freon. If they are you will need to find out what type (r-22,r-422, etc).You also need to know various temperatures and the system pressures.


What it comes down to is at certain temperatures/pressures the freon can remove "x" amount of heat, so if the condensor water temp is adjusted for the actual load and other respective temps it would be easier to determine how many chillers to run. I'm not sure on the math and that might vary depending on the type of refrigerant.

If you need more specifics find out more about the system and post that info.

This link might help explain thingshttp://www.chillers.com/RefrigCycle.htm

Thermodynamic exchanges are governed by rules and formulas including Pressure, Volume, and Temperature (PV = nRT for pure or perfect gas).

That said, even if it's not a gas, in areas where your refridgerent Volume is read as a fixed parameter, a differential pressure between two points should relate to different temperature values. In your case, I suppose that a diff. pressure indicates the system's ability of temperature absorption (targetted delta_T), useful if a temperature value is given (input or output circuit).

Ask your vendor for the coefficients used in their calculations. You could perhaps approximate the result with some linear function.

Tell us if you find what kind of formula has to be used exactly.

Regards

Regardless of the thermodynamics, it sounds to me like what you want to do is create cascaded PID loops. The output of the DP loop is the temperature setpoint for the temperature control loop.

hi,
you say the temperature setpoint is reset...
it may be that a minimum differential pressure (0.5 bar )between coolant and product is not being maintained and this is causing the temperature control to shut down.
If this is the case an alarm should be generated by the diff. press monitoring routine.
I doubt if the diff. pressure is used to contribute to the generation of the temperature control setpoint other than to alarm.