Rapid cooling the kiln

Rapid cooling the kiln is a legitimate kilnforming process which is widely practiced where sensible to do so. Some kiln designs facilitate it, whilst others present risks to either the glass or to the operator.

One may wish to rapidly remove heat from the kiln for two reasons;

• To lower the temperature so that the glass is frozen in a particular shape or form. This will usually be at the end of top soak during complex slumping operations.
• To speed up the firing process. This may be either at the end of top soak or during final cooling.

In either case it involves the introduction of large quantities of cold air to displace the hot. This must be done in such a way that it does not impinge directly onto the glass at all, and with minimum direct impingement on the kiln lining and furniture; all of which may be prone to thermal shock to some degree.

There are a number of ways of producing this air movement, but the one available to any kilnformer involves the creation of a ‘chimney effect’, where cold air is allowed to enter low down and the hot air exits at a higher level.

The other methods involve kiln modification, or incorporation at the design stage, so is discussed in the All about kilns premium content area.

What is the ‘Chimney Effect’?

Air expands and becomes lighter as it is heated. At 800°C it has about 3.7 times the volume it had at room temperature. This is called the ‘air volume expansion ratio’.

At 450°C, the bottom of anneal, it is 2.5 : 1 and at 100°C the number has dropped to about 1.3 : 1.

It is this expansion which induces the upward movement of the lighter air and the drawing in of cold air.

The higher the number the lighter the air, so it is easy to see why kiln cooling by this method becomes increasingly slow as the temperature drops. The heat content of the exiting air is becoming lower at the same time as the force creating the air movement is decreasing.

Rapid cooling: techniques and pitfalls

The procedures to be followed, and the potential hazards to both glass and operator, differ with the type of kiln; so we  will consider them separately for each of the types of kiln in common use.

Top loaders

The chimney effect can be induced by allowing cold air to enter through the spyhole near the bottom and  hot air to exit at the top by slightly raising the lid.

Figure: 23-01

The spy hole is usually small in diameter, so the amount of cold air which can enter is limited.

Even with the increased volume of the hot air, it requires only a small gap between lid and body to allow this volume to escape. The spyhole is for looking in, not letting lots of cold air in.  The quantity of heat removed won’t be great.

The taller, pottery kiln form of top loader, often has additional spyholes vertically above the first. They will admit additional volumes of cold air but will have little effect on the rate of cooling as there’s lots more heat stored in there. The lid may need to be lifted a little higher.

The glass is normally positioned slightly below the spyhole level: so that one can look down onto it. The cold air will flow straight in and could impinge on the part of the glass closest to the spyhole.

The tendency will be for the cold air to rise up inside the front of the kiln and lower the temperature there much more than toward the back.

The thermocouple is usually mounted toward the back and away from where the cold air enters, so it can give little indication of the effectiveness of the cooling nor the difference in temperature across the glass.

It is essential that power to the element be turned off during crash cooling. For kilns without a lid interlock, be sure to do whatever is necessary to achieve this condition.

Procedure:

• Avoid rapid cooling at soak wherever possible.
• To speed up cooling after the completion of anneal, prop the lid open a small amount with a piece of kiln shelf or a piece of fibreboard. It must be a refractory material as the air exiting can be dangerously hot.

As to safety of the operator:

• Hot air, hot enough to give third degree burns and sear meat, exits the kiln almost exactly where the lid handle is positioned.
• It is ESSENTIAL that GAUNTLETS capable of protecting the operator BE WORN whenever the kiln is being opened during a firing. 

It is not being unduly critical to say that top loading kilns aren’t designed for rapid cooling.

Front loaders

These are generally pottery kilns and Australian made, although some imported units are appearing. As mentioned elsewhere, kilns for other crafts without elements on door and all walls aren’t suitable for glass firing. Like the top loader, these kilns have one or more spyholes in the door. Unlike that design they also have one or more vents in the roof.

Figure: 23-02

For a nominal amount of cooling the roof vents can be uncovered and the spy holes removed.

Figure: 23-03

To increase  cooling it is necessary to crack open the door to create a gap between door face and door jambs so that a vertical current of air can flow through the gap.

Cold air enters at the bottom and expels hot air at the top. The cooling effect of this rising column of air is confined largely to the front of the kiln. Again, it has the potential to create temperature gradients across large pieces of glass, and across the kiln shelves.

The thermocouple on these kilns is usually on one side and about midway between front and back, so it is slightly less insulated against the cooling effect than that device in the top loader.

The kiln shell is often made of mild steel and is folded around the front of the lining. Rising hot air and radiant heat from the door both impinge on the metal faces and does severe heat damage to the metal.

To achieve more rapid and thorough purging, it is a not un-common practice for those with the need, to swing the door open and closed boisterously so that much turbulence is created and hot air is expelled from deep inside the kiln.

With the air at up to 800°C (1480°F) and going in any direction, that’s hardly the safest operation to contemplate.

If following this procedure to speed up the cooling from soak to anneal, the installed temperature indicator will give only a very rough indication of how close is the glass to anneal. The indicated temperature can bounce up by unpredictable amounts when the door is closed, depending on how the cool air circulated.

If doing this with kilns fitted with programmable controllers, care should be taken to keep the temperature within the limits of the current step; otherwise the controller could advance to the next step prematurely.

The likelihood of damaging the glass is ever present and the risk to the operator can be considerable. To be safe, wear a fireman’s suit.

Clamshell

Figure: 23-04

In this style kiln the walls generally form part of the hood and the glass is level with, or even some distance above, the base.

In kilns with vent holes in the roof and spyholes in the walls, a small cooling effect can be achieved with the hood closed without damaging the glass. The effect will not be great, as both spyholes and vent holes are usually only token gestures.

With the air expansion ratio of 3.7 : 1 at 800°, the cross sectional area of the vent needs to be at least four times that of the spyhole to be effective.

If the hood is lifted slightly it may allow some hot air to be spilled, but there is also the possibility that cold air can impinge on the edge of the glass. This would particularly be the case should there be a slight breeze which could blow cold air in one side and hot air out the other.

Top Hat

Figure: 23-05

Unlike the other types of kiln, top hat kilns with properly designed hood/hearth interface are ideally suited for rapid cooling.

As shown in Fig.5, opening the roof vents and  slightly raising the hood allows:

• a gentle stream of cold air to enter all around the base of the hood,
• to flow across the walls and absorb heat from the lining, and to

Some method of shielding the edge of the glass from direct impingement of cold air is essential if rapid cooling is to be performed.

There have been cases where cold air impinging on the edge of hot glass has built in stress which later caused damage costing thousands of dollars to rectify.

Figure: 23-06

In large kilns such as the Riley commercial units, the upward deflection of cold air is achieved by the design of the hearth/hood interface, as shown in Fig 6.

Here the air, which enters through the gap when the hood is raised by 6–10mm (3/8”), impinges on the side of the 35mm (1 ½”) high step in the hearth, as shown in Fig 6.

Figure: 23-07

The same principle is embodied in smaller kilns such as the Riley GS series which allow the flexibility of using kiln shelves. As shown in Fig 7, the kiln shelf itself creates the lip so that the air has to flow upwards and clear of the glass.

In practice, glass can be placed within 10mm (3/8”) of the edge of a shelf or hearth without being stressed by rapid cooling. In normal operation the hood seals intimately onto the hearth. There is no difference in rapid cooling procedure whether cooling from 800°C (1480°F) or from low temperatures.

Forced ventilation systems

Fan ventilation

Numerous efforts have been made over the years to rapidly cool kilns and furnaces by blowing in cool air.

Where done, it is normal practice to blow in cold air, rather than sucking out hot air. This latter creates enormous design problems overcoming heat damage to fans and drive motors.  

Glass presents a particular problem, due to the risk of contamination of the surface by blown dust or refractory particles. This is particularly the case below top soak, where blown materials could stick or become permanently embedded. At lower temperatures it won’t bond and can be brushed off the cool glass.

Blown air should:

• Be filtered at entry point to remove dust or grit
• Have low velocity to prevent erosion of refractory linings.
• Have laminar flow and enter and exit high in the kiln. This to minimise the risk of creating temperature differences across the glass. 

Figure: 23-08

Fig 8 shows a typical arrangement in a clam shell kiln. It uses a plenum chamber into which cold air is blown so that it can enter under slight pressure into the hood, flow across and exit through vent holes in either the opposite wall or the roof.

Amongst points to be considered are;

• Both the entry and exit holes must be sealed when cooling is not taking place. To not to do so would allow loss of heat right through the firing.
• Because of the air volume expansion ratio, the area of vent holes should be many times that of the entry holes.
• Exiting air can be at dangerously high temperatures, so precautions must be taken to deflect it clear of operators or others. Discharging it vertically is the safest.

Venturi extraction

This method is sometimes used to extract fumes from pottery kilns. Because of the small volume of air extracted, it has no practical application for glass cooling; but is mentioned here in case it is thought it had been overlooked.