Exercise X
153
Take a second length of tube with a quick-release fitting at one end and an open end
at the other. Connect the quick release fitting to the other side of the coil, and direct
the opposite end of the tube to a suitable drain.
Check that the drain valve on the end of the plinth is open and that it is connected to
a suitable drain.
Procedure
SAFETY NOTE: During this exercise the apparatus may reach temperatures hot
enough to cause burns or scalds. Do not touch the process vessel during the
exercise, or remove the coil or sensors from within the vessel. Allow the apparatus to
cool before draining the water or disconnecting any of the tubes from the process
vessel.
Run the PCT40 software.
Select ‘Configure...’ from the ‘Sample’ menu, and set the software to Automatic
sampling with a sample interval of 10 seconds and Continuous duration.
Setting the On/Off Controller
In the PCT40 software, select ‘Control’ to open the PID controller window.
Set the Proportional Band (P) to 0%, the Integral Time (I) to 0s and the Derivative
Time (D) to 0s. The controller is now set as a simple On/Off controller with fixed
hysteresis of 0.5% of full scale. Select ‘Apply’ and close the control window.
Set the PSV to 20% to give a low flow rate through the coil in the small process
vessel.
Fixed (manually set) ratio
In the PCT40 software, select ‘Control’ to open the PID controller window.
In Manual Control set the percentage time during which the heater will be on. 20% is
suggested as a starting value
At the bottom left, set the Cycle Time to 10s- this is the time over which the control
percentage will be applied (i.e. for a percentage of 20% and a cycle time of 10s, the
heater will be switched on for 2s and switched off for 8s).
Select the Manual Control radio button and click on ‘Apply’. The heater should begin
to operate intermittently as the time proportional controller sends signals to control it.
Check that there is a new data sheet for data logging.
Select the
icon to begin data logging, and observe the temperature of the fluid
leaving the coil, T3. When the temperature has changed by more than 10°C, select
the
icon to finish data logging.
Adjust the control percentage according to the change in temperature. If the change
was rapid, reduce the setting by 5 to 10%. If it was gradual, reduce the value by 2 or
3%.
Continue to monitor the temperature and adjust the percentage value until the
temperature maintains a constant level.
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