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:
These parameters will help you determine the system components you need:
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.
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 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.
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.
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.
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.
| Recommended Upper Temperatures for Protected Thermocouples | |||||
| Thermocouple Type |
Sheath Diameters & Wire Sizes for Single Elements | Maximum Element Temperature |
|||
| 1/16 OD | 1/8 OD | 3/16 OD | 1/4 OD | ||
| 28 Gauge | 22 Gauge | 19 Gauge | 16 Gauge | ||
| J | 400° C | 400° C | 470° C | 470° C | 680° C |
| K | 740° C | 740° C | 800° C | 800° C | 960° C |
Depending upon the maximum Temperatures, the Thermocouple has to survive. The thermocouple gauge has to be selected. For other types of thermocouples, consult factory.
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.
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.
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.
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 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.
(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.
