by mickiboi » Thu Feb 27, 2014 2:52 pm
If anyone is interested I built this for a friend years ago.
Theory of operation
1. Close power switch. Power Light ON. Power applied to LLC1 (Liquid Level Control 1).
2. If boiler is full R2 energised, if boiler is empty R3 energised. Only one of these can be energised at one time.
3. Boiler is full, R3 energised, boiler Full Lamp ON. Power is now available at START push button. C1 energised via Low Level Relay R3 being de-energised. Power available at SSR. Heater enabled lamp ON.
4. To start the system Push START button, power applied to R1. R1 is held energised permanently via self-holding contact in series with itself and the closed R3 contact. Power applied to TC1 and TC2. Temperature control now running. Heater lamp ON. SSR should be on 100%. TC2 valve should be fully closed.
5. Boiler will now heat up. As the boiler temperature increases above setpoint (SV) on TC1, the output of the PWM on the SSR will decrease to maintain the set value. This is reverse acting temperature control (RA). The temperature in the column now starts to rise. Depending on what the setpoint is on TC2 will determine what the 2 way valve position will be. TC2 is Direct Acting (DA), as the temperature goes above the setpoint the output will increase opening the valve allowing cooling water to flow through the condenser keeping the temperature constant. The more the temperature goes up the more cooling water will flow through the condenser.
6. You can change the boiler temperature or the condenser temperature independently of each other.
7. Should the boiler level drop too low R2 will de-energise and R3 will energise, Boiler Full Lamp will go off and Boiler Empty Lamp will be ON. C1 will de-energise and R1 will de-energise as R3 breaks this self-holding circuit. All temperature controls lose power shutting the system down.
8. To stop push the STOP button. This breaks the circuit to R1. Note that power is still available to the SSR as the boiler Low Level has not energised but the power to the temperature controllers is removed.
Remember these are PID controllers so the output will never be proprtional to the proportional band set in the controller. You may at different times have different output levels for the same setpoints.
As usual if you have no electrical or controls experience then GET SOMEONE THAT DOES HAVE build it for you. Shouldn't cost anymore than about 200 in parts.
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