Control Systems Selection Guidelines
Process controls, over temperature controls, level controls,
sensors, power controls, and panels. Now that you have selected
the heater(s) for your process, it is time to choose control components,
panels, and sensors, to provide the desired results.
In order to assemble a complete control system, you
will need the following information:
Voltage, wattage, current (calculated from
voltage and wattage),
Number of zones: (different sections controlled
Area location or classification: (indoor, outdoor,
explosion hazard), and
The desired process temperature range,
as well as permitted deviations should be
specified. Close control and/or control of one pass heating
of gas or liquids will probably require electronic control.
Process accuracy issues: For large mass processes
(big tanks, large blocks of metal) where the temperature won’t or can’t
move quickly, and the temperature requirement is not
critical, mechanical bulb and capillary thermostats can usually
used, or if electronic control with indication is needed a simple
controller with a contactor is necessary.
For processes, having low mass, fast, accurate control is important.
A proportional or PID controller with an SCR power
controller would be a good choice.
Process upset: If
the process is subject to upset, (oven door opened for new batch,
for instance), a PID control will be required
for good results. This is also the case if heating liquid or gas (air)
in one pass. An SCR will be needed as well.
Environmental (ambient conditions): Process
controls, over temperature controls, and accessories must be
the surrounding area in mind. Wet, dry, and explosion hazard
areas must be considered, as well as the ambient temperature
equipment will operate in. Mechanical controls should not be
exposed to temperatures above their stated range. Electronic
designed to operate in an ambient Temperature of above 32°F,
and below a stated maximum, usually 120 or 140°F.
Safety: An over temperature control should
be included to protect process, area, heater(s), and/or product
in the event of a primary
failure, or interruption of flow in moving systems. If the
power control is an SCR, a contactor or shunt trip should be
so the load can be shut down, even if the SCR’s are shorted.
If heating confined liquid or gas, an approved mechanical temperature/pressure
relief valve is also required. For some areas, ASME certification
may be required on pressure vessels.
These parameters will help you determine the system
components you need:
- Sensor: This can be a bulb and capillary,
thermocouple, RTD or non-contact IR sensors.
- Temperature Controller:
This can be a mechanical bulb & capillary
controller or an electronic controller to accurately
control the process.
- Over temperature Controller (Limits): For protection
of the process and/or the heater sheath, an over temperature
always be used to ensure safe operation in the event of process
control failure and/ or interruption of flow in dynamic systems.
Controller: In order to switch the heater load, either
mechanical contactors or SCR’s are needed.
The sensor is the device measuring the temperature or other variable
of a system. It is usually in direct contact with the
heated medium and must be specified to handle the temperature
and conditions of the process. Electronic controllers convert
signal from RTD’s and thermocouples to a temperature
Rugged and versatile, with many selections for various temperature
ranges, thermocouples consist of two different material
wires welded together. These devices produce a very small DC
depending on temperature and thermocouple type. The controller
or over temperature controller, interprets this voltage,
and compares it with internal standards, displaying and/or controlling
Advantages: Lots of choices, rugged,
Disadvantages: Output is not linear with temperature
when new thermocouples are within 2 to 3°F accuracy. Thermocouple
alloys age, which affects accuracy further. Microprocessor
controls are best at interpreting TC voltage curves. Thermocouple
of the same type as the thermocouple (i.e. type J for J),must
be used to connect the thermocouple to the controller.
Note: The red lead is always the negative lead in USA
RTD’s or Resistance Temperature Detectors, provide a resistance
change linearly related to a temperature change. The most common
is the 100-ohm platinum. The controller measures the change of
resistance, and relates it to temperature. Advantages: RTD’s
are much more accurate and more linear than thermocouples. Standard
copper wire can be used to connect the sensor to the control. Since
the signal is larger than a thermocouple signal, it is immune to
electrical noise. Three wire RTD's can also be run longer distances
Disadvantages: RTD’s are more costly
than thermocouples, and less rugged. In addition, they should not
be exposed to a temperature higher than their rated operating temperature.
Don’t weld or braze them.
A transmitter is an electronic circuit that converts the low
level signal of a thermocouple, RTD, or other device or parameter
humidity) to a current loop, typically a 4 to 20mA signal. This
produces better immunity to noise than the low-level signal by
itself. Advantage: Longer control signal runs are possible without
Disadvantage: Increased cost of installation.
IR (non-contact) sensors provide a control signal related to
the temperature of an object, without touching the object. The
IR sensor “looks” at
the process, and adds or reduces heat as required. They are often
used in continuous processes where material is passing through
a convection oven or under radiant heaters. Advantages: Provides
good closed loop control for flowing processes or surface drying
Disadvantages: More expensive than contact sensors. Does not
work well for shiny objects. A temperature control is still required
to interpret the output of an
IR sensor, compare it to the set point, and operate a power controller.
Placement is very important for a good control result. The temperature
control, no matter how smart its PID loop is, can only process
Information supplied to it. Where possible, in a block type system
(like a platen) the heater, sensor and load (die) should be as
close together as possible. This minimizes thermal lag, and provides
good response to changes. (See Figure 1) In a stable system, where
the heater is separated from the load, the sensor can be placed
near the heater to provide for close heater control. The load will
be cooler than the sensed temperature by the drop through the heat
transfer path from the heater to the load. This is not good for
changing condition systems. (See Figure 2) A compromise may be
provided for by placing the sensor between the heater and the load.
This is good for fairly stable systems where the heat demand may
be alternately constant or variable.(See Figure 3) For changing
systems, the sensor can be placed closer to the load to respond
to changing load requirements. The sensor farther from the heater
increases the thermal load. This will cause overshoots and undershoots.
A PID controller is required to minimize the temperature cycling.
(See Figure 4) In conclusion, it is important that the heater,
sensor and load be as close as possible. The sensor should always
be between the heater and the load.
Upper Temperatures for Protected Thermocouples
Sheath Diameters & Wire
Sizes for Single Elements
Depending upon the maximum Temperatures,
the Thermocouple has to survive. The thermocouple gauge has
to be selected. For other types of thermocouples, consult
Temperature or Process Controllers
Electric heat, while clean, efficient and manageable, can cause damage
to product and / or equipment if the temperature is not known,
corrections applied as required. Best results will be obtained when
the maximum and minimum allowable temperatures for a given process
are known, and controls selected to achieve these results.
Types of Controllers:
Electronic Controllers receive a signal from a thermocouple or
RTD and determine how much heat is needed to control the process.
controllers can range from very simple dial controllers to complex
multi loop PID controllers.
Advantages: Very accurate control, digital displays
and flexibility for many applications
Disadvantage: More expensive than some mechanical
Bulb & Capillary and Bi-Metal
Mechanical thermostats depend on expanding liquids or metals to
open or close contacts in response to temperature changes. Usually,
temperature is displayed, and a calibrated knob is provided on
some models. In mechanical controllers, the sensor is part of the
Advantages: Relatively inexpensive. Some bulb and capillary controls
can switch large amounts of current for one or more poles (conductors).
Easy to set up, just turn the knob for the desired temperature.
Disadvantages: On-off controls sometimes have a large differential
or dead band. This is the difference in degrees between turn off
and turn on. Your process variation will be greater than the dead
band. Bulb and capillary controls do not fail safely. If the capillary
tube with the fluid in it becomes pinched or broken, the thermostat
will fail in a heat-on condition, which is a hazard. Bi-metal thermostats,
which have no bulb or capillary, typically have smaller dead bands,
and can control more closely. Some will not operate a contactor,
which may be needed to switch the higher currents and voltages
needed by the heater. They are often appropriate only for small
120-240V single-phase heaters. Temperature accuracy is inferior
to electronic controllers.
Manual: (switch or circuit breaker) For some
applications, such as water pipe freeze protection, circuit breakers
on in the Fall and off in the Spring. Advantages: Low cost, easy
operation. Disadvantages: Possibility of not remembering to turn
on equipment in the fall. Energy is wasted when equipment is
on if it is not required. Consider an ambient temperature control
to switch the equipment on if the temperature is below 40°F.
Open Loop (Intensity or duty-cycle control):
Includes motor driven timers, infinite control bi-metal relays,
and SCR controllers with knobs for setting power percentage. Open
loop control does not use a sensor to determine the amount of heat
needed. The control device is set to a specific percent output
and switches the output on and off to approximate the percentage
of available heater wattage. Typically used for radiant heat. Advantages:
Low cost, ease of operation. Disadvantage: Does not compensate
for variations in ambient temperatures or incoming product temperatures.
Must, in many cases be reset, often after operator observation
of poor process results. On Off (bulb & capillary,
bi-metal, or electronic) (See Figure 5)
The dead band (Hysteresis) represents an area about set point in
which no control action takes place, and determines at what temperature
the output switches ON and OFF. Narrow deadband settings give control
that is more accurate but result in more frequent output switching,
which can cause early failure of electromechanical contactors.
On-Off control is available in electronic, bulb and capillary,
and bi-metal controls.
Disadvantage: The control is only as accurate as the dead band.
Large overshoots will occur with systems with significant lag.
Proportional controls reduce the heat output gradually (within
the Proportional Band), as the process approaches the set point.
Advantage: More accurate control than On-off
control. In stable conditions (constant load), proportional control
can maintain a
specific temperature. Since they are electronic, with wired sensors,
such as thermocouples, the control can sense an open sensor and
shutdown the process, resulting in a safer control system than
mechanical on-off controls.
Disadvantage: Proportional controls work best
on stable processes. They have trouble maintaining temperature
during process upsets.
Some proportional controls can switch significant loads with
optional high current relays and solid state switching devices.
PID (Proportional, Integral, and Derivative) controls, when properly
set up (tuned) can manage most situations, including process
upsets. Like a Proportional control, the heat output is gradually
reduced while approaching set point, but also with the integral
and derivative action can control processes with varying loads
at set point. A wide variety of sensors and parameters ensure
a good match of control to process. Many PID controllers have
auto tuning functions that automatically tune to the process.
Advantages: Good overall control. Since they
are electronic, with wired sensors, such as thermocouples,
the control can sense an
open sensor and shut down the process, resulting in a safer control
system than mechanical on-off control.
Disadvantages: More costly; more set-up required
because of greater flexibility. Requires external power controller
to switch the load.
Overtemperature Controls(High Limit Controls):
(Bulb & capillary, electronic non-indicating, and electronic
indicating). Overtemperature controls provide a safety backup for
the primary control and/or the heaters in case of a problem. The
over temperature controller's function is to protect the process
or heater. In an over temperature condition the over temperature
controller will shut down the process. The over temperature controller
cannot be cleared until the process cools and an operator manually
resets the controller. It is important to use over temperature
controllers with a shutdown device such as a contactor to protect
the heater process and personnel from damage or injury.
For small loads (less than 20 amps) some bulb and capillary and
electronic controllers can switch the heater directly. For larger
loads it is
necessary to use an external power controller. There are various
mechanical and solid state power controllers available.
Mechanical contactors are similar to motor starters. They are capable
of switching large amounts of power on an infrequent basis. If
turned on and off at a fast rate (more than 1 or 2 times a minute),
mechanical wear and contact erosion will require frequent replacement.
Advantages: Low cost. High switching currents. They do not produce
much heat from their operation.
Disadvantages: Contactors are subject to mechanical wear, and
produce electrical and mechanical noise.
To minimize electrical noise, snubbers should be connected across
each contactor coil minimizing arcing of control relay contacts.
A Snubber is an electronic circuit, which absorbs the inductive
kick back of the contactor coil when it turns off.