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.

System Considerations

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 differently),
• 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 be used, or if electronic control with indication is needed a simple On/Off controller with a contactor is necessary.
• Process speed: 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 selected with the surrounding area in mind. Wet, dry, and explosion hazard areas must be considered, as well as the ambient temperature range the equipment will operate in. Mechanical controls should not be exposed to temperatures above their stated range. Electronic controls are 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 control failure, or interruption of flow in moving systems. If the power control is an SCR, a contactor or shunt trip should be provided 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.

System Components

These parameters will help you determine the system components you need:

Sensors

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 the signal from RTD’s and thermocouples to a temperature reading.

Thermocouples

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 voltage, 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 a temperature.

Advantages: Lots of choices, rugged, inexpensive.

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 wire 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 thermocouple color-coding.

RTD’S

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 than thermocouples.

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.

Transmitters

A transmitter is an electronic circuit that converts the low level signal of a thermocouple, RTD, or other device or parameter (like 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 interference.

Disadvantage: Increased cost of installation.

Infrared Sensor

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 applications.

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.

Sensor Placement

Placement is very important for a good control result. The temperature control, no matter how smart its PID loop is, can only process the 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.

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, and 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

Electronic Controllers receive a signal from a thermocouple or RTD and determine how much heat is needed to control the process. These 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 controls.

Bulb & Capillary and Bi-Metal Thermostats

Mechanical thermostats depend on expanding liquids or metals to open or close contacts in response to temperature changes. Usually, no temperature is displayed, and a calibrated knob is provided on some models. In mechanical controllers, the sensor is part of the controller.

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.

Control Modes

Manual: (switch or circuit breaker) For some applications, such as water pipe freeze protection, circuit breakers are turned 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

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

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.

Power Controls

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

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.


Snubbers

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.

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