APPLICATION NOTE - 1018
MULTI-ZONE Power Control Systems
Author – George A. Sites, Director of Engineering,


INTRODUCTION

The use of Multi-Zone Power Control Systems can improve the overall effectiveness of the heating system. This is especially true in large furnaces or heating systems. By using multiple zones of heat control, you put the heat where it is needed most.

Figure 1 – Electrically Heated Pusher Plate Furnace
(Photo courtesy of Harrop Industries, Inc.)

Figure 1 is a 19 zone SCR controlled Pusher Plate Furnace used for manufacturing high efficiency filters used in cell phones. It is approximately 38 feet long and uses top and bottom radiant heaters to effectively distribute the heat as required. The first seven zones called “Pre-Heat” provide as much heat as possible to bring the work piece up to temperature quickly. The next ten zones called Fire Zone or High Heat regulate the heat within 2 degrees Fahrenheit. The last two zones called the Cooling Zones provide less heat to slow the cooling process. In this particular application allowing the work piece to cool too quickly could cause it to crack.

Heating elements such as Nichrome, Kanthal, Silicon Carbide, Molybdenum Disilicide or Tungsten are used depending on the required furnace temperature. The type of heating element usually determines the firing method of the SCR Power Control to be used.

Figure 2 – Cross-Sectional drawing of a Multi-Zone Furnace
(Drawing courtesy of Harrop Industries, Inc.)

Figure 2 is an example of top and bottom heating elements. According to Dan O’Brien, Vice President of Harrop Industries, “The use of Multi-Zone control can sculpt the thermal profile thereby minimizing hot spots, improving efficiency and reducing the overall heating costs”. He goes on to say “SCR Power Controls provide consistency, precision and reliability to the process”.

PRACTICAL ISSUES
The length of the heating system may determine the most practical mechanical configuration for the Power Control System. Typically, Multi-Zone Power Control Systems costs less to build than the same number of Single-Zone systems. It usually costs less to install and requires less floor and/or wall space. As shown in Figure 3, Multi-Zone Power Control Systems can be contained in one enclosure or each Zone could be in it’s own enclosure. In this particular case, the single enclosure has a main incoming section and eight zones of control. Each zone contains it’s own disconnect switch, meters, lights and auto/manual controls. By utilizing one enclosure, the field installation work is minimized. The input power connections are connected only to the input disconnect switch compared to making the connection seventeen times to individual enclosures.

For example, it may make more sense to use a Multi-Zone Power Control System on a furnace that is 10 feet in length as compared to one that is 100 feet long. The installed cost of the Multi- Zone Power Control System on a 100-foot long furnace may be higher due to the requirements

Figure 3 – A Typical Multi-Zone Power Control System
(Photo courtesy of AMETEK HDR Power Systems)

of additional cabling, conduit, space requirements, labor, etc. for the load connections. In this case individual enclosures may be the better choice located closer to the heat zone. The overall cost and space availability needs to be considered to make the correct decision.

WHAT’S IN THE FUTURE FOR MULTI-ZONE SCR CONTROL

Digital control with communications is the future. For example, AMETEK HDR’s MP1 series of SCR Power Controls (see Figure 4) feature communication via DeviceNet and utilizes an HMI on the enclosure door. The MP1 allows the user to daisy-chain the control signals of all zones with one DeviceNet cable and communicate with a host computer else where in the plant. Each MP1 in the chain is individually addressable. The command signal as well as the following is communicated over the cable: output voltage, current and power, alarms such as overtemperature, blown fuse and/or over-current. A fault log is also maintained and is available to the user. All of this information can be used in a database to provide charts or whatever other type of display and/or log you may want. Never before has this been possible without the addition of many transducers and miscellaneous other pieces of equipment in addition to the SCR Power Control.

Figure 4 – AMETEK HDR Power System’s MP1 SCR Power Controller
(Photo courtesy of AMETEK HDR Power Systems)

The use of an HMI (see Figure 5) on the door of the enclosure allows the user to operate the SCR locally and/or monitor many details. It also allows for checking the status of the SCR without opening the enclosure door. You can even change from Zero-Fired to Phase-Fired. Zero-Firing into a transformer is also 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, requiring the engineer/technician to “suit up” and risk flash. This method puts safety first!

Figure 5 – AMETEK HDR Power System’s MP1 HMI Display
(Photo courtesy of AMETEK HDR Power Systems)

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. The use of an HMI on the enclosure door also provides a safe method for reviewing zone information. This is true regardless of whether it’s a Multi-Zone or Single-Zone enclosure. SAFETY FIRST!

© 2009