INDUSTRIAL AND COMMERCIAL INFRARED SYSTEMS DESIGNED FOR YOUR APPLICATION
Eurolinia offers custom designed control systems from small single heater controls to large multi-zone systems for reliable, economical, and efficient heater performance to ensure precise process temperatures.
Some clients, in order to cut costs, underestimate the importance of using appropriate control systems and, as a result, cannot get effective uniform heating and desired quality of the final product. When we develop a control unit for our infrared heating system – we guarantee that the final product will meet all requirements and expectations. We usually use client’s product samples to test and adjust our infrared system, so that production can be started without any delays with minimum setup work.
The important factor in infrared heating is that power density of the heating energy absorbed by the material, and hence the temperature on the surface of the material being heated, depends on the temperature of the radiating surface of the heating elements. Infrared heating devices are predominantly open systems that are subject to uncontrollable environmental exposure. This effect is expressed in the convection of heated air and redistribution of reflected ray energy. Because of such exposure, the temperature of the heating elements in different parts of the heater can vary significantly, which leads to uneven heating of the product.
Unregulated infrared heating is acceptable when temperature gradient on the surface of the products does not exceed allowed values, or when thermal conductivity of the material being heated is high enough to level out the uneven temperature distribution.
However, in the majority of cases, in order to obtain high-quality products, uniform surface heating is required, which is possible only under constant automatic compensation of external influences on infrared elements. In our heating devices, this is achieved by dividing the entire radiating surface into zones and automatically controlling the temperature of each zone.
Infrared heating power control is carried out depending on the temperature of the radiating surface of each zonal electric heater. Temperature data from each heated zone is passed to a multi-channel temperature controller, which regulates the supply voltage of the zonal infrared heaters. This allows to maintain the preset temperature of the heating elements for each zone and provides protection against overheating.
Built-in automatics constantly monitors the efficiency of the heating elements and prevents emergencies. In case of incorrect heating system operation or an emergency, light and sound alarms are given and the corresponding problem area is displayed on the control unit (failure of the heating element, short circuit, or open circuit).
Infrared systems manufactured by EUROLINIA can be equipped with an automated control system (ACS) that allows adjusting the temperature of the heating elements using a specially developed mathematical algorithm called - "To the target from the first step". The capabilities of this algorithm allow to quickly minimize deviations of the current temperature of the heated product surface from the set target temperature.
Programmable logic controller together with the visual control system provides online adjustment, control, and monitoring of the heating modes from the control panel or from an external computer. The controller also allows starting the heating system, technological and emergency shutdowns of heating in automatic mode with simultaneous activation of the light and sound alarms.
The operational temperature of the heating elements for each zone is selected from a preset or entered manually from the control panel, or from an external computer connected via a built-in RS-485 interface. Monitoring is carried out visually on the control panel’s screen or from a remote computer. The SCADA system program allows you to control and manually change the heating temperature in online mode, enter heating zone settings from the stored preset, calculate the current infrared power density and its ratio for each zone.
Product quality and productivity have always been the primary goals of manufacturing systems. Because infrared heat treatment systems can provide high energy transfer efficiency, this technology is a viable choice for drying, curing, baking, and other process heating applications. Proper process control is critical for consistent, repeatable results.
Two types of control schemes are commonly used:
Examples of open-loop control include:
A heater is connected directly to a power source; Heaters connected to a manually regulated power source such as a silicon controlled rectifier (SCR); Any device that requires the operator to manually adjust the system based on operator judgment without using process information.
The advantage of open-loop control is simplicity. The disadvantage is that the system power supply voltage changes frequently throughout the day, week, month, or season. For this reason, power levels in the process also change unless voltage stabilization is built into the power supply. In addition, the ambient indoor temperature will affect the process, and changes in product inlet temperature can also affect the result. And, of course, operator error can be a concern.
Closed-loop control systems rely on a sensor to verify actual process operation. Sensors can be used to monitor variables such as:
For a stable process, closed-loop control of the heaters is accomplished with a thermocouple that measures the source temperature. This control scheme will ensure consistent performance day in and day out. As the product moves through the system, the thermocouple picks up the drop in heater temperature due to energy absorbed by the parts.
The system then compensates for the energy consumption by increasing the energy supply to the heater, thereby maintaining the set temperature. The placement of the thermocouple is critical to maintaining a close connection between the heater and the process. The thermocouple must quickly monitor the actual heater temperature.
An improvement to the closed-loop thermocouple control is to use a device to measure the actual temperature of the product at the oven outlet, as well as intermediate positions in the process if necessary. Some products absorb more infrared energy at certain wavelengths. In these situations, the initial zones can be set to the heater temperature that is most closely related to the energy transfer to the product, set by the thermocouple in the heater. In the end zone, the controller modulates the power to achieve the desired final product temperature. Similar circuits can be used to measure other process parameters such as product moisture, gloss, or color matching.
Zone control comes into play for two reasons. For many products, the process involves bringing the product to the desired temperature and then holding it at that temperature for a preset time. Other processes may require several intermediate steps to achieve the desired results. Both of these desired temperature profiles can be achieved using zone heating. Zone control provides the ability to change the power/temperature of the zones in the direction of the product or, if it is in a fixed position, to change the temperature during the cycle to achieve the desired profile.
In continuous web applications, edge effects can cause the center of the web material to be hotter than the edges. In these situations, closed-loop control can be used to raise the temperature in the edge zones and lower the temperature in the center zones to improve the uniformity of the final product.
Zoning is also used to correct for imbalances in the network that may have been introduced into the product by a previous process. For example, moisture levels are often unequal on a roll of coated or coated paper. Selective control of the heating zones throughout the product helps to increase uniformity, thereby equalizing moisture content.
Sensors for measuring heater temperature used in closed-loop control include:
Thermocouples, which generate small voltages that vary with temperature. RTDs, whose resistance varies with temperature. Non-contact temperature devices such as optical pyrometers or thermopiles, which measure product temperature directly and optically.
Infrared heat scanners can continuously and automatically scan multiple points in a continuous network of materials, creating a temperature map.
In addition, moisture measurement systems can detect high and low moisture content in a target and send the information back to the heater control system. The control system uses this moisture level information to provide power where it is needed.
Power controllers provide the heaters with electrical power. They can be simple switches or contactors to turn the power on or off mechanically. They can use contacts that can spark and wear out, solid-state relays/contactors that can switch power electronically, or SCR power regulators.
Solid-state relays/contactors are available that switch from no power to full power when a sine wave crosses the zero point. Alternating current provides power in such a way that the power changes (varies) from plus to minus 60 times per minute (in North America). If the power is switched mid-cycle (other than the zero points), it can cause heat, possibly creating line noise that can affect other devices and reduce component life. If the power is switched at the point where the sine wave crosses the zero point (see illustration), the power is not switched and little noise is generated on the line.
SCR power regulators are available as zero voltage or phase angle starting units. Zero voltage regulators turn each full cycle on and off proportionally: heater fully on or fully off. Phase angle firing units "chop" the upper part of the sine wave, providing a uniform power level to the heaters. (Phase angle firing is similar to the way a wall dimmer works on your home lighting.) Without proper filtering, this type of control can also create linear noise that can affect other equipment.
In addition, smart SCR power controllers are available to improve communication with control system think tanks such as PLCs.
Variables that affect the choice of power controller type include:
For example, a product with low mass will be sensitive to changes in heater temperature. Thus, such a product needs a stable power supply. This can be achieved by using an SCR with phase angle triggering or a zero voltage triggering device that includes a very fast cycle. This type of control can also be useful for a heater with low mass and fast response.
Products of higher mass or slower response heaters are less likely to be affected by changes that can be obtained from a power controller. The brain of the system - whether a stand-alone temperature controller, a PLC, or a computer system - will be in either on/off mode, proportional time mode, direct power mode, or similar circuitry.
The on/off mode provides the least amount of process control. With this control scheme, if the heater or process temperature drops below the setpoint, the heater will turn on. When the process temperature exceeds the setpoint, the heater is turned off. Often there is a small bandwidth or temperature range that allows the heater to stay on instead of just switching quickly. This allows the process to level out somewhat. A home thermostat works similarly to an on/off control circuit.
A proportional time control circuit will set the bandwidth depending on the speed of the heater. The circuit is designed so that when the process temperature is below the lower limit, the heater is on at 100 percent. Similarly, when the process temperature exceeds the upper limit, the heater is 100 percent off. Within the bandwidth, however, the device turns on and off impulsively, perhaps very quickly to maintain the desired process temperature. Both solid-state relays (SSRs) and SCRs with zero-voltage startups work this way. A phase angle SCR will continuously modulate the power of the heater without an on/off cycle.
Low mass, fast response time heaters include tungsten halogen lamps, quartz tube carbon fiber heaters, and foil and tape heaters. Standard quartz tube heaters are also relatively fast, but not as responsive as the heaters above. Flat heaters tend to have more mass, resulting in slower response times both up and down. They can use simpler control circuitry to maintain uniform output. Each heater is different, and your supplier can help you choose the proper sensors and power controls.
As your machine operator performs or interrupts the process, facilitating their work helps you. Manual controls that simply set power levels or turn heaters on/off rely heavily on the operator to constantly monitor the process and manually make necessary adjustments. Switching to closed-loop controls and self-contained temperature controllers that display the set temperature and the actual temperature works well.
However, if the system has multiple zones and many different products are running on the line, the operator must correctly enter information into each temperature controller. Programmable logic controllers (PLCs) are cost-effective, and they allow individual recipes to be set up for each product. These recipes can include temperature settings as well as other process variables such as line speed and airflow. The PLC can accept multiple sensors, and this feature can be used to create a circuit for each product. Error information can be displayed and recorded. The operator only needs to enter the recipe number to set up the entire line. Graphical displays can be provided on the PLC for this process. Process records can be stored and downloaded for quality control history, and PLCs can be set up to communicate with each other over a network with data available to management in their offices. Supervisory Control and Data Acquisition (SCADA) systems can communicate with PLCs for overall plant process control.
In this way, a properly controlled electrical infrared system can provide a more efficient operation. Benefits include energy savings, accurate and repeatable process control, improved product quality and consistency, and easy operator setup. The control system can include alarms that alert the operator if a part of the process is out of specification, and maintain process records for quality assurance purposes. Overall, with these improvements, operating costs will be reduced, scrap rates will be reduced, and customers will be satisfied with the quality and consistency of your products.
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