Section 3.3: Furnace Operations

After leaving the batch house, the mix is fed continuously into the furnace or tank where it is melted into glass.

  • The batch is charged into the furnace at the same rate as glass is being pulled out so that the amount of glass in the furnace is kept constant at all times.
  • Glass depth must be controlled to within ±0.01 inch for proper forming machine operation.
  • Furnaces consist of three main parts:
    • 1. Melter
    2. Refiner
    3. Regenerators or Checkers
  • Most furnaces are designed to use natural gas but are capable of using alternate fuels-oil, propane and electricity-if necessary.
  • Furnaces range in size from about 450 to more than 1,400 square feet of melter surface.
  • Glass depth is between 4 to 5 feet.
    • SidePortFurn1cap.jpg
  • A properly operated and well-maintained furnace will last for 10 years or more with just one partial repair and will produce over 1,000 tons of glass per each square foot of melter surface over the life of the furnace.
  • Energy use is about 4 million BTU per ton of glass.
  • Both furnace life and energy use have dramatically improved over the past 20 years.
  • Computers have the ability to tie together furnace, refiner and forehearth operations as never before.
  • Statistical process control techniques have also revolutionized furnace operations, leading to improved efficiency and glass container manufacturing quality.
  • Among the more recent developments is the use of oxygen boosting or oxy-fuel melting.
  • With oxygen boosting, oxygen and gas are injected into the combustion air to improve flame control and allow higher pull rates or lower temperatures.
  • With oxy-fuel firing, oxygen replaces all of the combustion air, eliminating the need for regenerators and combustion air fans. With this technique, manufacturers are better able to melt the glass and deliver it to the forehearth.


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Melter and Regenerator


  • The melter is a rectangular basin in which the actual melting and fining (seed removal) takes place.
  • In a side-fired furnace, the batch is charged into the furnace through the doghouse, which is an extension of the melter, protruding from the back wall.
  • Along each side of the melter, above glass level, are three to seven ports, which contain the natural gas burners and direct the combustion air and exhaust gases.
  • An end-fired furnace differs from the side-fired machine insofar as the doghouses are along the side and the firing is done through a single port and exhausting is done through an adjacent port on the end of the furnace.
  • The furnace is fired alternately in one direction and then in the other in 15- to 30- minute intervals.
  • The ports on the firing side direct the combustion air from the regenerators on that side into the melter.
  • The exhaust gases exit through the opposite ports and pass down through the regenerators on the opposite side.
  • Since the exhaust gases are hot, they heat the brick stacked in a checkerboard pattern in the regenerator.
  • When firing is reversed, the combustion air passes up through the hot bricks and is preheated, thus recovering energy that would otherwise go out the exhaust stack.
  • The melter basin is separated from the refiner by the bridge wall (throat end wall). Glass passes from the melter to the refiner through the throat, which is a water-cooled tunnel that extends through the bridge wall.


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Refiner

  • In the past, the refiner was the region in which fining was accomplished, but with the higher pull rates in recent years, the glass must now be seed free by the time it leaves the melter.
  • The refiner now acts as a holding basin where the glass is allowed to cool to a uniform temperature before entering the forehearths.
  • The melter and refiner are covered by crowns to contain the heat.
  • The throughput load or pull on a furnace is commonly expressed as square feet of melter surface area per ton of glass pulled in 24 hours. This allows furnaces of different sizes to be compared on an equal basis.
  • Most flint furnaces are designed to pull at a rate of 3 to 3.5 square feet per ton of melter surface area depending on the type of glassware being made and the machine speed.

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Forehearth

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  • In the forehearth, heat from natural gas burners and cool fan air are carefully applied to the surface of the flowing hot glass.
  • Constructed as a long ceramic "bathtub" glass temperature is reduced as hot glass flows through the forehearth.
  • Glass is cooled from 2350 degrees F° at the entrance to 2150 degrees F° at the exit.



continue to Section 3.4: Forming Process


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