Chemglass CG-1972-65 - Manual (Page 11)

11
3.3 Setting the Controller for Safe Operation. Two completely separate controllers are contained in the
J-KEM Model HCC and it’s important to understand what each controller does in order to take full
advantage of the Model HCC’s safety benefits.
8 0. 0
o C
*
Setpoint
Controller
The meter on the far left, labeled Setpoint Controller is the main process controller and
is responsible for all aspects of temperature control including, heating, cooling,
maintaining the temperature, tuning, etc. The desired solution or process temperature
is entered into this controller.
8 0. 0
o
C
*
Limit
Controller
The meter on the right, labeled Limit Controller has no influence on the heating
process, which is under the complete control of the meter on the left. The purpose of
this unit is to monitor the temperature of the reaction or process and take control if a
heating error occurs thus acting as a watchdog or backup unit to the setpoint
controller. In normal use, a temperature is entered into this controller, which is several
degrees (≈ 5
o C) above the temperature entered into the setpoint controller. If the
process heats to the temperature entered into the limit controller, power is physically
disconnected from outlets 8 and remains disconnected until the controller is manually
reset.
Setting up an experiment with the Model HCC is quite simple. Place the dual element
thermocouple in the process to be heated and turn the controller on. Enter the desired temperature into
the setpoint controller (i.e., the meter on the left) (for example 80
o C) then a slightly higher temperature
into the limit controller (for example 85 o C). Press the reset button (9). The setpoint controller begins to
heat the process to 80 o C where it will maintain it indefinitely. The only time the limit controller comes
into play is if, for whatever reason, the temperature of the process rises to 85 o C at which point power is
removed from outlets 8. Power remains disconnected until being manually reset. The operation of the
limit controller can be verified at any time by entering a temperature lower than ambient which should
activate the over temperature protection circuit indicated by light 6 coming on.
The Model HCC derives its’ unique safety advantages from it’s design in which the temperature
sensing ability and action of the two controllers are completely independent of the other. By considering
the way the controller responds to different types of heating failures demonstrates the safety benefits of
this construction. If the limit controller fails closed, the over temperature protection circuit is activated
disconnecting all power to the heater. If it fails open, this has no effect on the temperature regulation of
the setpoint controller, which continues to maintain the desired temperature of the process. If the setpoint
controller fails closed (The most dangerous failure mode. (i.e. uncontrolled full power to the heater)) The
limit controller activates the over temperature protection circuit permanently disconnecting all power to
the heater. If the setpoint controller fails open the process simply cools to room temperature. Since each
controller has its’ own thermocouple a failure with either thermocouple results in the same scenarios as
above. The Model HCC provides complete, 100% redundant control of the heating process for maximum
safety of unattended operations.