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PID Control
PID control utilizes Proportional, Integral, and Derivative
control methods to scale the output from a process controller
(typically 4-20mA) to an SCR Power Controller or Electronic
Multi-Stage Sequencer. Note that reverse acting signals are
used with heating systems where the heater is OFF at 4mA
and fully on at 20mA.
SCR Power Controllers employ heavy duty SCR switching
modules to switch the power. The SCR uses a fixed time
base of typically 4 seconds. Within every 4 second time
base, the SCR closes to energize the heater for a time frame
proportional to the control signal. A 50% signal, or 12mA,
would mean the heater is on for 2 and off for 2 seconds every
4 seconds. The result of cycling the heater frequently, but
proportional with the load requirement, is more accurate
temperature control.
An Electronic Multi-Stage Sequencer, or Step Controller,
accepts the scaled control signal output from a PID Controller
and pulls contactors in and out as required. This method is
similar to ON/OFF control with multiple stages. The
sequencer has an adjustable Cycle Time similar to that used
for ON/OFF process controllers. The default used on
sequencers is a 40 second delay between stages. This
method is effective on high amperage units because the
multiple stages help split the load into manageable circuits.
For further details about PID control, refer to the process
controller instruction manual.
Factors Impacting System Control
Many factors affect the setpoint tolerance and control of
heating systems. Control Method (noted above), Heat Load
Fluctuations, Sensor Location & Thermal Lag, Controller
Tuning, and Fluid Properties are all significant factors.
Heat Load Fluctuation, or changes in the process, can
cause wide temperature fluctuations. Some typical changes
to a heating process loop are:
1. Addition of fluid at a temperature below the process
temperature.
2. Opening or closing tank access covers.
3. Starting or stopping fluid agitators.
4. Ambient temperature changes.
5. Fluid flow rate changes.
6. Insulation thickness.
7. Power available can be affected due to user voltage
fluctuations.
Process Sensor Location / Lag in the tank or piping can
impact temperature control. For flowing systems, the sensor
must be in the flow stream down-stream from the heater. For
stagnant systems, If the sensor is located close to or on the
heater, the controls may short-cycle before the tank is up to
temperature. Locating the sensor away from the heater will
cause a temperature lag and allow fluid temperatures close to
the heater to exceed desired temperatures. Thermal Lag is a
term relating to process control. Lag is typically a slow
reaction by the process sensor to a change in the operating
temperature. This is often caused by thermowells. The mass
of the thermowell requires heat-up time or "time lag" before
the sensor can detect that the operating temperature has
reached the setpoint, thus causing overshoot of the setpoint.
The PID controller must be tuned down to minimize this
impact, which has the result of less accurate temperature
control.
Controller Tuning is necessary on systems with SCR's or
Sequencers and PID control. Tuning allows the proportional,
integral, and derivative values to be set based on the actual
process conditions. Process controllers are provided with an
auto-tuning feature that measures the thermal
responsiveness of the heating system. During auto-tuning,
the process controller drives the system to heat up, hold, heat
up, and finally, hold. If an alarm condition is encountered
during auto-tuning such as a high limit alarm, the cause of the
alarm must be remedied and the auto-tune must be repeated.
Tuning may need to be repeated after any change in the
process which affects the thermal responsiveness of the
system such as the heat load, flow rate, or fluid properties.
Systems which are unsteady may require manual tuning.
Refer to the process controller manual for instructions.
Fluid Thermal Properties can greatly impact temperature
control. Fluids such as water, with high thermal conductivity,
are easy to heat without experiencing large temperature
gradients. Fluids such as wax or tar pitch have such low
thermal conductivity that heaters must be designed with a
much lower sheath watt density than with most fluids.
Temperature gradients can be significantly reduced in tanks
by using an agitator. Solids buildup on the heaters can also
reduce the heat transfer.
To obtain optimum control, the use of PID control, properly
tuned for the application, is recommended.
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