Exercise W
149
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.
Switch on mains power to the console and run the PCT40 software.
Check that the temperature readings T1, T2 and T3 are giving sensible values
(usually between 5 and 30 degrees Celsius depending on mains water temperature).
Select ‘Configure...’ from the Sample menu and set data logging to Automatic 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.
In the software PID controls, set the Set Point temperature for fluid leaving the coil in
the process vessel. 50°C is suggested as a suitable value. 30°C is suggested, unless
the local water supply is close to this temperature when a higher value may be
selected. Values of greater than 50°C will be difficult or impossible to achieve using
indirect heating.
Apply the settings and close the controller window.
Select the
icon to begin data logging.
Note the behaviour of the heating element as the temperature approaches the set
point. It is possible to observe heated water rising from the heating element when the
power is on. The heater also responds audibly when power is switched on or off.
Additionally, the switch for the heater is displayed on the mimic diagram screen
within the software, allowing observation of the heater action. An indicator light on the
software screen also indicates whether the thermostat is on (sending a signal to the
heater) or off (sending no signal to the heater).
The change in temperature may be most easily observed using the graph screen in
the software. If the graph does not appear automatically when the graph screen is
selected, select ‘T3’ for the Y axis and ‘Time’ for the x-axis, then plot the graph. New
points will be added as they are logged.
Continue logging for 10 minutes. Select the
icon to finish data logging.
Select the
icon to create a new results sheet.
Set the gear pump to 50% to circulate water through the process vessel, and then
repeat the procedure above.
Return the gear pump setting to 0%.
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