APPLICATION NOTE - 1015
BETTER FOREHEARTH CONTROL FOR ELECTRIC GLASS MAKING
Author – George A. Sites, Director of Engineering, AMETEK HDR Power Systems

INTRODUCTION

As we all know, glassmaking has been around for years and years. Originally using only fossil fuels for heat generation, engineers have added electric heating for boosting the performance of an existing fossil fuel fired furnace and/or in some cases to provide all of the heating. The elimination of fossil fuels minimizes many of the environmental issues and, electrical control of the melting process is normally more efficient. In addition, the cost of electrical energy is more stable than fossil fuels.

Figure 1- Typical Multi-Zone Forehearth Power Control System

It’s been believed for many years that if you could control the resistivity of the molten glass you would have better results in glass temperature uniformity. The more uniform the glass temperature, the better the overall production results would be. Glass Company engineers have experimented with resistance control using their own “black boxes” or by measuring voltage and current and calculating resistance using a PLC or computer. The problem with this approach was the overall resistance control loop was just too slow. It was thought by most of the SCR Power Control manufacturers that it was impossible to control glass resistance using SCR Power Controllers.

HDR’S SOLUTION

Several years ago, HDR developed what has now become an industry standard – the VIPR (pronounced viper) circuit. The VIPR circuit allows the user to select RMS Voltage, RMS Current, True Power or Resistance as their control or feedback means and includes RMS Current Limit. This allowed the engineer to have his/her normal control method and still have the flexibility to experiment with other means of control including the much desired resistance control.

Figure 2 - HDR’s VIPR Circuit
(fits inside of HDR’s power controllers)

WHY DOES HDR’S SOLUTION WORK?

First of all, there is no risk to the engineer for trying a new control method. HDR’s VIPR circuit has all of the normal controls built in so the user can fall back to their normal method of control if resistance control doesn’t perform to their satisfaction. In addition, the cost of the VIPR circuit is only slightly more than a standard control circuits.

Results from earlier attempts at resistance control were marginal at best and most engineers continued using RMS current control as a result. Although the glass melting process is relatively slow, the VIPR circuit controls the resistance very quickly. This coupled with any changes in furnace design and/or furnace operation has resulted in significant improvements in operation and efficiency.

MECHANICAL PACKAGING

Figure 3 – MCC Style System

HDR’s VIPR circuit is small-especially when compared to the competition’s rack assemblies. The small size allows HDR to package these systems in a variety of different ways. Two of the most popular methods are shown in Figure 1 and 3. Figure 1 is a typical Multi-Zone Forehearth Power Control System. In this case it has 17 zones and is housed in a modified NEMA 12 enclosure. It features a main incoming section and multiple zones. Each zone has it’s own circuit breaker, meters and controls for local control. Each of these zones will feed an isolation transformer. Each zone controls a portion of the Forehearth with electrodes placed directly in the glass. Figure 3 shows a Motor Control Center (MCC) that has been modified using HDR’s Power Controllers with VIPR circuits. This type of construction is best if the process requires a very quick change out of electronics in case of a failure.

WHAT ARE THE BENEFITS?

HDR has received reports of glass temperature uniformity being an order of magnitude better with the VIPR circuit than before using it. In addition to this, the use of HDR’s VIPR circuit along with changes to the Forehearth design have lowered electrical power requirements to less than a third of that originally needed on a per zone basis. HDR’s VIPR circuit is clearly the best control method for Forehearth control!

NOTE:
This Application Note is intended for informational and illustration purposes only. Actual applications may differ. Contact the HDR factory for information on your exact requirements.

© 2005