Toward energy efficient kiln operation

Introduction

Visualise two pieces of 3mm glass about 250mm square. They weigh about 1kg. Based on the present (2010) cost of electricity of about 20 cents per kilowatt hour (kWh), the energy cost to raise their temperature from ambient to 800°C is about six cents. Those pieces can represent the entire glass mass of a firing done in small hobby kilns, or even in larger kilns by glass artists working on a special piece. (Energy costs mid 2011)

At another level of operation, an architectural piece of one square metre of 6mm thick glass weighs about 15kg. The energy cost to heat it to 800°C would be about 80 cents.

Most will know that those numbers bear absolutely no resemblance to the actual cost of a firing.

There are two types of firing processes, continuous and periodic. A continuous process is one where a kiln furnace or other heat enclosure is heated to a working temperature and the material to be processed is passed through in a continuous stream, often for months or years at a time.

A periodic process is where the material is placed in the kiln and the whole kiln and load are heated and cooled together. A common example is the kitchen oven, where it is heated to cooking temperature and then cooled down when the job is done. Not all that different to the kilnforming process, except that a number of dishes can be cooked successively without wasting the heat stored in the oven; something the glassie cannot do.

Of all kiln firing processes, the kilnforming process is among the most inefficient known. Unfortunately the nature of glass allows no other.

To help us progress toward more efficient kiln operation we can consider it in three broad areas;

• Operating existing kilns more efficiently.
• Modifying existing kilns to operate more efficiently.
• Developing more efficient kilns.

Operating existing kilns more efficiently  

There are no magic bullets to improve the energy efficiency of firings, but rather a series of small steps which can be taken; each of which can achieve marginal savings.

Some of you may know the Riley FS-1 kiln. A popular configuration is 16Amp 3ph 11.5kW. Maximum power cost is $2.30 per hour. Hearth measures 1300 x 670. One owner uses that model to sometimes fire 72 glass coasters, each 100 x 100 x 4mm. The mass of glass is about 7.5kg and the cost of power to heat that glass is about 40 cents.  Using a fast firing pattern appropriate to single colour glass, time to end of soak is under 3 hours, power cost under $6.00. That equates to 8 cents each coaster.

This can be compared to a  recent firing, in a similar kiln, in which the operator used a pattern that had a 5 ½ hour step to 550°C with a correspondingly overly long anneal soak. This firing pattern was prompted by the recommendations of art glass makers who advise the most gentle treatment of their products.
That is not a criticism of art glass makers. By recommending the kindest conditions they minimise complaints from the users of their products.

The load in the kiln was about 3 kg, or less than half of what it could comfortably hold.  Estimated cost, about $10. Cost of the heat actually absorbed by the glass, about 12 cents. Much more glass could have been fitted in the kiln, at an additional energy cost, or marginal cost, of less than 20 cents.

• Maximise the load in the kiln

• Shorten the firing time. A shorter firing reduces energy consumption.

Can firing times be shortened? By how much? There are various pages on this website that discuss the firing times and variations for float glass, rolled glass and annealing. Here I will mention just a few points;

Float is a single colour glass. It has been shown that it can withstand rapid temperature change. In test firings on 10mm float I have reached 250°C in 5 minutes; an average of 50°C per minute in the most sensitive stage of a firing, and the glass was successfully slumped.

All glass is coloured, by minerals. Clear float is coloured green by iron, whilst other minerals produce other colours. Why the difference between COE88 float and single colour COE90/96 art glass. If float glass makers had to process their glass at rates implicit in art glass recommendations they would never get any glass made. It must be stressed that I am NOT advocating the treating of all art glass as if it were float.

Different colours of glass are created by incorporating different minerals into the basic mix, and these additions can produce melts having slight variation in viscosity curves. When different melts are mixed together to create multi coloured sheet, these variations can produce stress between the colours which can best be negotiated at slower rates of change of temperature; rates which may be much slower than those which are safe for a single colour glass.

What IS suggested is that there may be opportunity to shorten the firing time of many single colour glasses; and possibly of some multi coloured glass also.

In the manual supplied with all Riley hobby kilns, users were encouraged to make their first firings using float. A pattern considered suitable for float glass firing was loaded as pattern 1 and took the glass to 540°C (1000°F)  in 54 minutes.

So, if only six cents of energy per kg goes into the glass, where goes the rest of the heat. It goes into three general areas;

1. Into the rest of the load. Into moulds, shelves etc. These are generally silica based, with a similar specific heat, so it costs about the same to heat a kg of glass as it does a kg of kiln shelf or mould.  Metals have a lower specific heat, so cost much less per kg to heat.
2. Into the insulation.
3. Becomes lost to the atmosphere.  

1 and 2 above absorb heat only once, so represent a single cost per firing, irrespective of the total firing time. It is referred to as heat stored.

3 above is continuous, and varies with the temperature gradient across the insulation. It is stated as ‘the rate of heat loss per hour per unit surface area’.

The heat is lost from all exposed surfaces, but the rate differs with the orientation. Per unit area of kiln shell or case, loss from the top is higher than from the walls, with that from the  bottom being lower again.

Fig 1
The rate at which heat is lost from a kiln is not constant at all process temperatures. Rather, it increases at an ever faster rate the higher the kiln temperature, as shown in fig 1. Firing at a lower temperature will not only reduce the energy used in heating the kiln to soak, it will also reduce the ongoing heat loss for the time the kiln is at soak.

The rate at which heat flows through the insulation varies with the difference in temperature between hot face and cold face. Winds or draughts will lower the temperature of the cold face and remove the heat more rapidly. A kiln version of the chill effect.
From the above we can propose three more small steps;

Lower the total mass of the load; be it shelves, props, moulds, whatever.

Protect the kiln from winds and draughts.

Fire at the lowest temperature for the task being performed.

Hot air rises. It makes the fire draw, the smoke go up the chimney. All that is needed is an air entry lower down and a point of escape higher up. Called the chimney or stovepipe effect. Doesn’t have to be at bottom and top. Can be two spyholes, one above the other, both loose fitting. Cold air enters the bottom one, hot air exits the top one. Leaks in the shell, badly fitting roof vents, spyhole plugs, doors, lids, hoods. All can give rise to stovepipe effect losses.

Minimise stovepipe effect losses

Of course, one shouldn’t seal things up too tight. Air expands as it is heated, and needs to escape. Between ambient and 800°C (1470°F) the air in a kiln will expand to 3.7 times its original volume. It is called the Air volume expansion ratio. The hotter the air gets the more it expands.

A single vent of some kind is necessary. It should be as high up in the kiln as possible; to allow the expanding air to escape but also to allow purging of emissions from the load.

Generally, there are not a lot of emissions from glass firing, but they can sometimes come from paints, minerals such as kaolin in battwash or from moulds. Glassies may sometimes fire their own clay moulds, and they can give off nasty substances.

So, we have our six small steps for Improving operational efficiency.

1. Maximise the load in the kiln.
2. Keep the firing time as short as possible.
3. Lower the total mass of kiln furniture and moulds.
4. Protect the kiln from winds and draughts.
5. Fire at the lowest temperature for the task being performed.
6. Minimise stovepipe effect losses.

Modifying existing kilns to operate more efficiency

As has been seen, heat storage and heat loss are inherent in the design of, and the insulating materials used in, a particular kiln. In brick lined kilns, most kiln builders will have already used the best quality available, and nothing short of making the lining thicker will reduce the heat loss. That won’t be an upgrade; it will be a complete rebuild.

With a fibre lined kiln, additional insulation would have to go on the outside of the existing, and that would require a larger shell or case; again as with the brick unit, a major rebuild.

One area where older kilns can be upgraded is by the fitting of an efficient programmable controller to allow more precise temperature and time control.

Sadly, other than steps already mentioned, such as stopping air flow through the kiln, there isn’t a lot which can be done.

It should be mentioned here that putting a layer of ceramic fibre insulation (CF) on the outside of an existing brick or fibre kiln is NOT a viable proposition.

Fig  2
Whilst the temperature profile through a lining is often drawn as a straight line, it is not so at all. Consider the profile across a multi layer ceramic fibre lining shown in the figure above. A substantial part of the temperature drop occurs across the outer layer, with that across inner layers becoming successively less.

Were a layer of CF to be wrapped around an existing kiln, then the metal shell could be subject to excessively high temperatures. As an example, were a 25mm layer of CF to be put on the outside of an American top loader with side elements, such as a coffin kiln, then the temperature of the embedded metal could go as high as 400 °C. The stainless steel may tolerate this, but some of the metal fittings most certainly would not. 

Developing more efficient kilns

Let’s consider this from various points of view:

• More efficient materials.
• More efficient designs.
  

More efficient materials

First, let’s consider materials available or being developed to improve efficiency. Ceramic fibre has been around for 50 years, IFB for much longer. There are no new refractory insulating materials being developed which are likely to lower energy use at high temperatures, largely because there has not been any great inducement for industry to do so. Glass kilns are but a cup of water in the ocean. Pottery kilns not much more. Both just ride on the coat tails of general industry, and they can operate quite effectively with their continuous processes and existing materials. 
Whatever is achieved in improved glass kiln efficiency must be done by the better combination of existing materials, not by waiting for scientists to produce new materials.

More efficient designs

There ARE more efficient designs than those presently widely available, but what inducement do major kiln builders have to change when they can continue to sell the old.
Other than being cut down and having an element in the lid, imported top loader glass kilns today differ little from what was a pottery kiln in 1960. The major change has been in controls, and that has not been done by kiln builders. Few manufacturers have such a dream run with little need for product improvement. 

When a major consideration is price, economies of scale play a significant role. Demand in Australia will ever be small, so local manufacture of efficient kilns is unlikely to be viable.

The hobby market will continue to be met by imported products, until the exchange rate makes them too expensive for other than the most indulgent. What then?

For the serious glassie, they have the choice of a kiln which falls short of their desires, or building their own. Possibly, from among the ranks of glassie owner-builders will come someone who will give local enthusiasts what they desire.

This paper is based on a talk presented by Peter Riley at the Ausglass Conference in Hobart,  January 2009