The use of multi-zone power control systems can improve the overall effectiveness of the heating system. This is especially true in large heating systems or furnaces. By using multiple zones of heat control, you put the heat where it is needed most.
For example, consider a 19-zone SCR-controlled pusher plate furnace used for manufacturing high efficiency filters used in cell phones (figure 1). It is approximately 38' long and uses top and bottom radiant heaters to effectively distribute the heat as required. The first seven zones, called the preheat section, provide as much heat as possible to bring the work piece up to temperature quickly. The next 10 zones, called the fire zone or high heat, regulate the heat within 2°F (1.1°C). The last two zones, called the cooling zones, provide less heat to slow the cooling process. In this application, allowing the work piece to cool too quickly could cause it to crack.
Depending on the required oven or furnace temperature, heating elements such as Nichrome, Kanthal, silicon carbide, molybdenum disilicide or tungsten are used. The type of heating element usually determines the firing method of the SCR power control to be used.
Figure 2 shows a cross-sectional drawing of a multi-zone oven to illustrate top and bottom heating elements. Oven and furnace manufacturers that design complex heating systems use multi-zone control to optimize the process. For example, Dan O’Brien, vice president of Harrop Industries, said that using multi-zone control allowed his company to sculpt the thermal profile, thereby minimizing hot spots, improving efficiency and reducing the overall heating costs for its oven and furnace customers. O’Brien also noted that SCR power controls provide consistency, precision and reliability to the process.
Practical IssuesThe length of the heating system may determine the most practical mechanical configuration for the power control system. Typically, multi-zone power control systems cost less to build and install than the same number of single-zone systems. Moreover, a multi-zone system may require less floor or wall space.
Multi-zone power control systems can be contained in one enclosure, or each zone could be in its own enclosure. In the example shown in figure 3, the single enclosure has a main incoming section and 10 zones of control. Each zone contains its own disconnect switch, meters, lights and auto/manual controls. By utilizing one enclosure, field installation work is minimized. The input power connections are connected only to the input-disconnect switch rather than making the connection 10 times to individual enclosures.
The physical size of the heating system also influences when multi-zone control should be used. For example, it may make more sense to use a multi-zone power control system on a 10' oven or furnace than it would for one 100' long. The installed cost of the multi-zone power control system on a 100' long oven or furnace would likely be higher than several single systems due to the requirements of additional cabling, conduit, space requirements, labor, etc., for the load connections. In cases such as a 100' long system, individual enclosures located closer to the heat zone may be the better choice. The overall cost and space availability need to be considered to make the correct decision.
What's Next for Multi-Zone Control?Expect multi-zone controls to continue to incorporate digital control with networked communications. Some SCR power controls already offer communication via DeviceNet (figure 4) and utilize an human-machine interface (HMI) on the enclosure door (figure 5). The multi-zone control allows the user to daisy-chain the control signals of all zones with one DeviceNet cable and communicate with a host computer elsewhere in the plant. Some controls allow each device in the chain to be individually addressable. Command signals - as well as information about output voltage, current and power, and alarms such as overtemperature, blown fuse or overcurrent - are communicated over the cable. Some systems also incorporate a fault log that is user accessible and controlled. With the advent of digital control, all of this information can be used in a database to provide charts or whatever other type of display or log needed. This brings a much greater level of control than was ever allowed without the addition of many transducers and miscellaneous other pieces of equipment in addition to the SCR power control.
The use of an HMI on the of the enclosure’s door allows the user to operate the SCR locally and monitor many details. It also allows for checking the status of the SCR without opening the enclosure door. In some cases, users can update the firmware in the power control from outside the enclosure should it be necessary, or even change from zero-fired to phase-fired control. Zero-firing into a transformer may also be an option. This is especially important with the new NFPA 70E-2005 Arc Flash requirements for industrial control panels designed and built to the National Electric Code and UL 508A. Without the HMI or the communications features, you may have to open the door of the enclosure to check status of the SCR. Including an HMI allows users to put safety first.
In conclusion, the proper use of multi-zone SCR power control can help improve the overall heating system, improve production and help reduce the user’s electric utility costs. In addition, the use of digital communications helps to reduce the initial installed cost of multi-zone SCR controls and provides the user with information never before available with SCR power controls. Finally, the use of an HMI on the enclosure door also provides a safe method for reviewing zone information.