By John Chiles
In a glass making studio often one of the largest operating costs is energy, and the largest energy user is the furnace. There are two types of furnaces commonly used in small studios: day tanks and free standing crucible furnaces. (For a number of reasons, an invested crucible furnace is considered a day tank). In most of the world, the most common type of furnace used in art glass studios and factories is a freestanding crucible furnace.
Here are a few points to consider when making a decision about what type of furnace to use:
- Insulation of tank furnace walls fail progressively over the life of the furnace. In a day tank or invested crucible furnace, the refractory lining that is in contact with the glass is also the furnace wall. A typical day tank insulation schematic starts with the glass contact refractory, backed up with super duty castable and insulation bricks on the outside. Eventually the lining cracks and glass starts to migrate through the cracks and into the backup insulation, washing stones and cords back into the tank every time the furnace is worked down. Because hot glass is so corrosive, cracks get bigger and the problem steadily gets worse. The glass seeping through the cracks also eats away the brick behind the lining and will eventually saturate the entire insulation layer. As this happens, the insulation layer stops working as insulation and becomes a heat sink. The brick behind the glass contact lining in every day tank I have ever taken apart has been completely saturated with glass.
- Free standing crucible furnaces can be insulated to a greater extent than day tank furnaces. In a free standing crucible furnace, the glass is not in contact with the furnace wall and therefore it is possible to insulate furnace walls to the maximum.
- The expected life of a free standing crucible furnace is much longer than that of a day tank. The typical life of a day tank is only five to seven years. Over a day tank’s lifespan the glass quality steadily gets worse and the energy bill steadily climbs higher. In the end the combustion system and furnace get torn down and most of it is thrown away. With routine maintenance and repair the life of a crucible furnace is measured in tens of years. Many of the furnaces we have built are over 20 years old and still producing glass every day.
- Maintenaning a free standing crucible furnace can be simple and easy. In a well designed furnace the floor or sump under the crucible is sloped to the front for easy draining and cleaning while the furnace is hot. Traditionally the front face opens for easy access to the inside of the furnace and for changing the crucible. It should never be necessary to pound out the glass, or dismantle the burner system.
- Free standing crucible furnaces use less energy and are quieter. Because it is possible to use more insulation and the crucible is heated from all sides, a smaller capacity burner will melt the same amount of glass in a shorter period of time. In Europe, where energy costs are typically two times higher than in the US, most glass factories and schools use free standing crucibles. This practice is driven by a need to be as efficient as possible. A smaller burner will also reduce the noise level in a shop significantly.
- Free standing crucible furnaces consistently provide better quality glass. Because the crucible is heated more evenly there is better glass circulation in the furnace, which leads to a reduction of cords. Replacing the crucible when it wears out eliminates the problems associated with a cracked lining – fewer cords and stones.
- Round bottom crucibles have more accessible glass than square sided day tanks. Because the shape of the crucible allows more complete access to the glass, a 500 lb. crucible is similar in useable volume to a 700 lb. day tank. For instance a 700 pound day tank with three inches of glass in the bottom will contain 175 pounds of glass. A 500 pound crucible with three inches of glass in the bottom will contain less than 30 pounds.
Fifty years ago in Toledo it was demonstrated that it was possible with very little knowledge to put a small day tank furnace together with bricks, stick a burner in it, and melt glass. With the development of fused cast refractories, building a day tank got even simpler and more accessible. These are technologies that were originally developed for very large furnaces and scaled down so that they could utilized by artists with very little technical experience. There was very little information available about crucible furnace design and operation, and initial attempts at using crucible furnaces in the small studio were frustrating.
The most common concern about crucible furnaces is that the crucible will break in the middle of a busy production schedule or school session. This fear of unexpected down time has lead to the use of day tanks in this country. During the last 50 years the cost of energy has been low enough in this country that the higher operating cost of a day tank was justified by the fear of a crucible breaking. Our experience is that crucibles rarely crack. In 30 years of building furnaces I’ve seen crucibles fail from cracking very rarely, and in most cases the failure can be traced back to the initial heat up. If a crucible is not heated slowly and evenly stress will be introduced as the temperature is raised from room temperature to red heat. This introduced stress may cause the crucible to fail right away or after several months.
In a properly designed furnace the crucible can be heated evenly and slowly. Recommended heat up for a crucible is 25 degrees F and hour to red heat. Once it is above red heat the crucible is relatively ductile, not brittle. I have tried to break an old crucible while it is still hot in the furnace and it took a pretty hard hit with a ladle before it failed. If you leave a crucible in the furnace long enough, it will develop pinholes or wear through. We recommend changing the crucible yearly but we regularly hear from customers with crucibles that are 2 or 3 years old and still going strong.
The second most common concern is “We need a lot of glass–a day tank or continuous melt furnace is the only type that will fill our needs.” Large volumes of available glass can be achieved by using multiple furnaces. Using multiple furnaces also allows flexibility to meet demand, as furnaces can be turned off when not needed. This is the way that it is currently done everywhere except in the United States.
Here are some additional suggestions for reducing energy usage with your furnace:
- Turn the furnace down when not in use, weekends and nights. During longer turn downs of more than a week turn the furnace off. Our practice is to turn the furnace down to 1600 if we are not using it for a couple of days and 1800 overnight if we are not charging.
- Use a stopper and insulated cover on the furnace door when not in use.
- Install a damper on the flue and use it to control the pressure inside the furnace. Glass furnaces should always be operated with a slight positive internal pressure. A flue damper does not have to be more complicated than a piece of kiln shelf on top of the flue.
- Use a temperature controller or at the very least use a temperature indicator.
- If the furnace has a forced air burner, use a variable speed blower and turn the glory hole blower off when not in use.
- Use a recuperator to preheat combustion air with exhaust gas after it has left the furnace.
- Examine your melting cycle. In many cases it is possible to melt at lower temperatures for a shorter period of time without affecting glass quality–in fact it may be possible to increase quality because you decrease the likelihood of cords caused by the fluxes in the glass batch attacking the glass contact refractory.
- Install an operating system on doors so that they are kept closed unless being used. We installed an additional door operated by a foot pedal on a 6 1/2” diameter opening in a furnace front. The result is that the furnace was able to maintain the same operating temperature with a lower input equal to 3 gallons of propane per day. That’s 660 gallons operating 5 days a week for 44 weeks.
- Keep track of your energy usage. If you are using electric or natural gas you already have a meter. If you are using propane, have a meter installed.
The chart below summarizes a section of a report on the business practices of a mid sized glassblowing studio. Since equipment and energy are major costs of running a glassblowing studio this comparison was central to the report
From an economic stand point there is no doubt: a Free Standing Crucible (FSC) Furnace will save you money over a Day Tank (DT) furnace. Further a FSC furnace fitted with a recuperator promises a further fuel reduction of 10-20%.
At the time the report was written this studio ran a 700# tank furnace and a 350# Invested pot tank, and seven glory holes. The equipment section of the report focused on furnace comparison cost estimates of the existing 700# tank to a 500# FSC furnace. Using a conservative estimate for FSC furnace life (10 years) and a conservative figure for gas saved over a day tank of similar size (20%) one can see that the FSC furnace provides significant savings over the life of the equipment.
This chart is based on projections from a specific scenario; it is shown here to illustrate the potential savings from building a Free Standing Crucible furnace.
| 10 year equipment cost |
FSC Furnace |
Day tank Furnace |
| Purchase cost* |
$27,800 |
$15,000 |
| Life Estimate |
10 yrs** |
3 yrs |
| Crucible cost 1 per yr 10 yrs |
$15,000 |
|
| Cost after 10 years |
$42,800 |
$50,000 |
| Natural gas costs |
FSC Furnace |
Day tank Furnace |
| Per Year |
$44,479*** |
$55,598**** |
| Cost after 10 years |
$444,787 |
$555,984 |
| 10 yr cost equipment and energy |
FSC Furnace |
Day Tank Furnace |
| 10 Year subtotal |
$487,587 |
$605,984 |
| Yearly Cost |
$48,759 |
$60,598 |
| Monthly Cost |
$4,063 |
$5,050 |
Total yearly savings = $11,842
Total savings after 10 years = $118,397
The payback period for a new FSC furnace is 2.35 years.
*Purchase cost includes recuperator, Burner, and crucible.
**With proper maintenance a FSC furnace can last twice this long or longer.
***Twenty percent saving is based on proven fuel savings with FSC furnaces. This does not consider the saving of 10-20% gained by recuperation.
****Based on actual natural gas used in 2000 @ $1 per therm.
John Chiles runs Hub Consolidated in Orwell, Vermont. You can email John at john@hubglass.com, or visit Hub’s website at http://www.hubglass.com
