Requirements
From the points of view of the furnace designer and operator there are four main areas which must be considered:

• batch melting behaviour
• glass quality
• refractory corrosion
• doghouse design
  

Batch melting behaviour
The “melting” process actually comprises a complex combination of melting and chemical reactions, during which some components are dissolved rather than melted. These reactions take place at high temperatures.

Unmolten batch is a good thermal insulator, and therefore heat transfer within a pile of batch is relatively poor. It can be difficult to raise the temperature in the centre of a pile to that required for the processes to take place, and as a result unmolten batch can be transported a long way into the furnace.

The best way to avoid this problem is to limit the thickness of batch piles, maximise the surface area available for heat transfer, and to cover as much as possible of the bath surface of the melting end with batch. These factors are determined largely by the batch charging technology used. Large unbroken strings of batch should be avoided, as should an overall blanket without gaps.

Glass quality
Batch charging can affect glass quality when, for example, unmelted batch is propelled towards the front of the furnace and into the refining area. It may then be pulled into the extraction current and appear in the product as stones or cords.

This risk is increased if batch “logs” are produced which can be easily pushed along the furnace. Charging in more than one direction can help to alleviate this problem.

Although not directly connected with the batch charging function, the flame and booster and bubbler systems can also exert an important influence on the movement of batch in the furnace.

Refractory corrosion
Batch is both cold and mechanically abrasive, and contact between batch and refractories can result in substantial wear on the refractories.

Chargers that do not use a doghouse, such as screw chargers, often produce a large pile of batch directly in front of the charging position, which is in contact with both superstructure and tank refractories. In end-fired furnaces with a single doghouse attempts to avoid long batch logs moving quickly down the furnace can result in the tendency to charge slightly backwards, towards the rear wall. This can lead to the batch piles literally “bouncing” off the rear wall at a shallow angle and then going on to contact the side wall opposite the doghouse, resulting in accelerated wear of the tank refractories at both contact points.

Doghouse design
The installation of a doghouse on a furnace produces several disadvantages.

During the heating up it is difficult to control the refractory expansion in the doghouse area. During operation the doghouse increases energy losses, in many cases it allows the uncontrolled entry of induced air, and it is often subject to heavier refractory wear so that it may become a weak point in the furnace structure during the later part of the furnace campaign.

Nevertheless, a doghouse allows the use of those charging systems that can influence the pattern of the batch distribution on the surface of the glass bath, which in turn can have a positive effect on achievable melting rate and on glass quality. It also provides a relatively “quiet” area in the superstructure where pick-up of fine batch components is limited, thus reducing carryover and subsequent build-up of solid material in regenerators and waste gas channels.

The design of the doghouse must suit the characteristics of the batch charger to be used, and should minimise contact areas between cold batch and the tank refractories in order to limit wear. Radiation losses should be reduced as far as possible by limitation of the size, and the superstructure should be sealed as effectively as possible to restrict the ingress of cold air.

Types of charger
A number of different types of charger are used to fulfil these requirements. The main types are described below using the chargers supplied by EME Maschinenfabrik Clasen as examples.

Screw charger
This design comprises a horizontal metallic screw located within a cylinder, which is inserted through a hole in the superstructure side wall. Batch falls into the screw by gravity and is transported into the furnace by the movement of the screw. Charging quantity variation is made by varying the rotation speed of the screw.

The construction does not need a doghouse, and can be installed in a simple opening in the superstructure. This means that it is easy to seal around the charger and prevent the uncontrolled ingress of cold air.

However, the principle has a range of disadvantages. A water-cooled mantle is required around the cylinder to protect the material from the high temperatures in the furnace. This can cause damage to the superstructure refractory in the charging area.

The charger pushes batch into the furnace, but it does not have any influence on the form or size of batch piles in the furnace, or on their movement.

As the batch is not moved away from the charging area by the action of the charger there is a high level of contact between cold batch and the refractory adjacent to the charger opening, both in the superstructure and the substructure, and this can lead to extensive damage to these refractories.

Finally, the use of cullet is limited, as it results in high wear levels on both the screw and the jacket.


Figure 1 – screw charger

Piston charger
The piston charger is similar in many ways to the screw charger, but the screw is replaced by an oscillating piston which is used to push the batch into the furnace. The advantages and disadvantages are also similar to those of the screw charger. However, the piston charger is only suitable for very fine batch materials, and cannot be used when cullet is included in the batch.

Blanket charger (Type SB)
This type of charger requires a doghouse, as the material is fed onto the glass bath surface.

The basis of this type of charger is an inclined tray situated below the outlet of the batch hopper. The tray is moved backwards and forwards more or less parallel to the plane of the tray bottom. As it moves forward the batch contained in the tray also moves forwards, as does the batch floating on the glass bath in front of the charger. As the tray moves backwards the batch in the tray is prevented from moving backwards by the downward slope of the tray and the column of batch emerging from the hopper. The space created at the rear of the tray by the backwards movement is filled by material dropping by gravity from the hopper. The tray then moves forward again, and the cycle is repeated.

The basic principle is shown in figure 2 below.



Figure 2 – basic principle of the blanket batch charger

The rate of charging is changed by varying the speed of movement of the tray or by switching the movement on and off. The stroke of the tray movement is a further variable which can be used to influence the charging pattern, but this cannot be adjusted on the run.

This type of charger does influence the movement of the batch away from the doghouse, in that the forward movement of the tray pushes the batch piles outwards. However, because the feed and forward pushing effect are combined, this effect stops whenever the feed is interrupted. When there is no feed there is no movement of the batch, except that which occurs as a result of the extraction current in the glass, and this is very slow. Therefore, this type of charger does not break up the batch into individual piles, but, as the name suggests, it produces a closed batch blanket.

Side-port furnaces are charged from the rear side and the doghouse can be made almost as wide as the furnace itself. A number of blanket chargers can be installed alongside one another, and the combination provides very good coverage of almost the complete glass bath surface. In this case, the fact that the batch piles tend not to be broken into smaller individual piles is compensated by the large area which is covered. This is a typical arrangement in large float tanks.

The large doghouse required by the blanket charger is particularly difficult to seal.


Figure 3 – the EME type BS blanket batch charger

Enclosed doghouse pusher charger (Type ESE)

This type of charger comprises a water cooled pusher located underneath a small feed hopper. The pusher is curved and is pivoted so that it moves in an arc through an angle of approximately 45°. When the pusher is in the rear position the opening of the feed hopper is open and batch can fall onto the surface of the glass bath.

As the pusher moves forward this batch is moved into the furnace by the front face of the pusher, whilst the outlet of the feed hopper is closed by the pusher body. As the pusher moves back the feed hopper outlet is opened again, fresh batch falls into the space left by the batch which has just been moved away and the cycle is repeated.

The charging rate is varied by altering the pusher speed, or by switching the movement on and off. Manual adjustment of the pusher stroke is available and the flow rate from the feed hopper can be varied by means of a slide.

The feed and pushing action are combined, and therefore there is no forwards movement of the batch when the feed is stopped. As a result it is difficult to break up the batch into individual piles, especially at higher loads.

An improvement can be obtained when the charger is swivelled to the left and right by about 5 -10° to enable charging to take place in different directions.

This type of charger is very compact, and actually sits on top of a small doghouse. The charger is surrounded by water cooled boxes which are in contact with the refractory and require a large volume of cooling water.

The design offers a completely sealed doghouse.


Figure 4 – the EME type ESE closed doghouse pusher batch charger

Pusher charger (Type CPO)
In this design of charger the feeding and pushing operations are separated. Batch is fed onto the surface of the glass in the doghouse by a vibrating chute located below the feed hopper. The batch is then pushed forwards by a water cooled pushing arm, which traces an elliptical path in front of the end of the vibrating chute.

The basic principle is shown in figure 5 below.


Figure 5 – basic principle of the pusher batch charger

The charging rate is determined by the vibration speed of the chute, which is independent of the movement of the pusher. It is therefore possible to continue the pushing action after the feed has stopped, and in this way individual batch piles can be created.

The pusher movement offers a number of variables that can be used to influence the charging pattern. These are the stroke, speed and depth of immersion.

This type of charger is often provided with a swivelling action so that batch is charged in two or three different directions. This also helps to distribute the batch over more of the glass bath surface and is especially useful in the case of large end-fired furnaces.

The introduction of the two mechanical elements – the chute and the pusher arm – into the doghouse superstructure, and the shape of the movement of the pusher, make it difficult to seal the doghouse. A removable hood seems to offer the best solution, but the sealing is incomplete.


Figure 6 – the EME type CPO pusher batch charger

Conclusions
A number of factors must be taken into account when the best type of batch charger is chosen for a particular application. The choice may be complicated by the fact that some of
the requirements are contradictory.

Doghouse sealing is an important aspect, especially in the case of oxy-fuel furnaces. Oxy-fuel firing is often used for special glasses, where low specific melting rates mean that the charging pattern may be less critical than in other applications. In addition, the cullet ratio used is often low. Therefore the screw charger may be a very suitable solution.

Side-port furnaces offer the advantage that the doghouse can be installed on the rear wall and, if needed, it can be made almost as wide as the tank, thus allowing charging over the complete width of the furnace. The blanket charger is well suited for this type of application, as demonstrated by the typical arrangement found on float furnaces.

Large, high capacity end-fired furnaces, especially those with high specific melting rates, are particularly dependent on the batch charging technology used. In such cases the charging flexibility offered by the pusher design cannot be bettered, and today, this design of charger is the type most commonly used on such furnaces.

Richard Sims - Nikolaus Sorg GmbH & Co KG

www.sorg.de