What is the effect of altering the flowrate through the system?
As the flowrate through the system changes, the rate at which the water changes temperature is affected.
When the flowrate is small (0.5kg/s) the water cools down at a slower rate. This means that the heater does not have to be switched on as much.
However when the flowrate is large (1.5kg/s) the water cools down quicker due to the increased flow of cold water entering the tank.
Note that with the higher flowrate more power is required to heat the tank (the heater is on for longer) and so this leads to more oscillations in the heat input.
What is the effect of altering the deadtime of the controller?
The deadtime of the controller determines the accuracy of the temperature of the water. For example when the deadtime is 5% the temperature will oscillate between 38 C and 44 C. Likewise for 20% the temperature will oscillate between 32 C and 44 C.
Notice that with on/off control it is impossible to have a deadtime of 0%. There has to be some offset between the setpoint and measured variable.
Finally with 1% the temperature is controlled between 39.5% and 40.5% although there is some overshoot due to the amount of power added.
What are the disadvantages in using on/off control?
The disadvantage in using on/off control is the oscillation around the setpoint. Although on/off control is very simple to implement there will always be this oscillation.
Therefore there has to be a trade off between the accuracy required by the controller (how important is it to be exactly at the setpoint) and the amount of wear and tear on the valve.
Can you think of any situations where on/off control could be used and others where it definately can't?
The main situation where on/off control is used is in a domestic heating system. The boiler is switched on when the water is too cold and off when the water is too hot. This works well since it is not important for the temperature to be exact.
One area where on off control will not work is with controlling flowrate. It is not a good idea to have a flowrate oscillating between fully on and fully off!!
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