Considering the importance of optimizing all process phases to achieve the best results is so well known that it is surprising that the raw materials charging techniques have always been considered so little promising from this point of view, and have often remained unchanged for decades. It is worth remembering that the function of the charging machine is not only the feeding of the raw materials into the tank but also spreading them onto the glass surface in the shape of islands, which should expose a wide surface to the flame radiation and at the same time remain detached from each other, thus enabling the heat to penetrate deeply into the molten glass through the empty space between adjacent islands. It is equally important to avoid that raw materials reach the melting tank walls when entering the furnace as this contact could cause premature corrosion of the refractory material, thus shortening its life. Last but not least, it is very important that the charging machine keeps the inside...
Considering the importance of optimizing all process phases to achieve the best results is so well known that it is surprising that the raw materials charging techniques have always been considered so little promising from this point of view, and have often remained unchanged for decades. It is worth remembering that the function of the charging machine is not only the feeding of the raw materials into the tank but also spreading them onto the glass surface in the shape of islands, which should expose a wide surface to the flame radiation and at the same time remain detached from each other, thus enabling the heat to penetrate deeply into the molten glass through the empty space between adjacent islands. It is equally important to avoid that raw materials reach the melting tank walls when entering the furnace as this contact could cause premature corrosion of the refractory material, thus shortening its life. Last but not least, it is very important that the charging machine keeps the inside of the furnace well insulated from the outside so that no parasite air can leak into and no flames or hot gas can go out.
Even though all of the above is well known, it is easy to verify that many of the existing furnaces now in operation and designed according to modern concepts adopt raw materials charging systems ofan open type, i.e. systems which do not prevent the air leakage into the furnace. The consequences are a negative impact on energy consumptions, Nox emissions, along with easier exhaust of dust, all of which lead to uncomfortable and unhealthy conditions around the charging area. Moreover, most of the time, such systems are of a fixed or semi-fixed type. Afixed system cannot push the materials charged in more than one direction and therefore it cannot distribute them onto a large area of the melting surface. A semi-fixed system enables a modest rotation of the turret around the vertical axis with a limited angle, normally inferior to 20°, which cannot permit an effective distribution of the raw materials onto the molten glass.
There are other furnaceswhich are equipped with closed charging systems, having an oscillating turret with wide rotation angles, up to almost 45°. Such systems have undoubtedly many advantages if compared with open dog-house systems, either fixed or semi-fixed, but the control technology of such systems has always been limited to an increase of the turret positions from 3 to 5, never approaching pusher kinematics control and therefore reducing dramatically the possibility of changing the shape and distribution of the batch islands on the molten glass surface inside the tank.
It should be mentioned, however, that the adjustment of the charging dynamics parameters requires some mechanical regulations to be carried out on the machine itself after it has been stopped. This procedure affects melting process stability and forces operators to work in a hot and dusty environment.
It is therefore easy to understand that the adjusting operations, whose efficiency is already reduced because of the technological limits of the kinematics mechanics, are greatly discouraged by the need of working in a difficult environment. Well aware of all above described problems and wishing to give furnace operators the best instruments to optimize the charging process without any of the above limitations and hindrances, Glass Technologies (GT) has developed an innovative control process of the charging phase for sealed dog-house machines, enabling the control of the entire charging process via operator panel, with the machine in operation and without the need for mechanical intervention on the machine itself.
THE ELECTRONIC OSCILLATING BATCH CHARGER (PATENT PENDING)
The electronic batch charger conceived by GT is a sealed doghouse charging machine: it can maintain the inside of the furnace closely separated from the outside and make the turret turn with a rotation angle up to 45°, so that the raw materials are effectively distributed onto the molten glass surface inside the tank. Such capabilities have been known and appreciated for some time now.
GT’s innovation consists of a new control method of the mechanical movements, which free the pusher action from the intrinsic limits of the mechanical kinematic motion. Pusher action The sealed doghouse charging machines are equipped with a pusher, which literally pushes the raw materials into the furnace with a forward/backward movement controlled by a crankshaft-rod mechanism. This mechanism is, in turn, controlled by a ceaselessly rotating driving motor. While the pusher is in a backward position, the raw materials drop onto the molten glass surface and are pushed into the furnace by the same pusher during its forward movement. The pistonhandle kinematics determines the moving speed, which varies according to a sinusoidal law in which the minimum speed corresponds to the stroke extremities, i.e. to the maximum extraction and maximum insertion points of the pusher. From the point of view of the batch islands and the possibility to shape them, the above-described system suffers some significant limitations and offers few possibilities of intervention. During the pushing forward phase, the pusher reaches its maximum speed at halfway of the stroke, diminishing it in the second half until it stops and then starts the backward stroke.
The raw materials, pushed by the pusher, lean onto those loaded with the previous stroke and it is not possible to create a division between two subsequent loads. As a result, as is often observed, the batch creates a series of cords or one wide blanket, composed of various subsequent loads resting or leaning on each other and floating on the molten glass surface for quite a long time. The pusher stroke adjustment can sometimes lead to some improvements but to do so it is necessary to stop the machine and carry out inconvenient on board regulation.
ADVANCED PUSHER CONTROL
The principle of GT’s innovation is based on pusher movement control, now free from the constraints of kinematic mechanic motion. This is possible thanks to the servo motor used to drive the pusher and the partial rotation of the crankshaftrod mechanism within a maximum angle corresponding to the pusher maximum stroke, as shown in the picture below.
The characteristics of the servomotor enable to manage pusher movement speed according to a series of parameters which can be key-entered by the operator without the need of mechanical adjustments on the machine.
It is therefore possible to divide pusher backward and forward strokes in various segments so as to define the acceleration and deceleration laws for each of them and to determine stop positions:
• P4 maximum possible pusher stroke serviceable angle, adjustable by the operator. The sum of P4 and P6 corresponds to the total 180° rotation of the crankshaft-rod axis;
• P5 maximum serviceable angle pusher movement corresponding to the loading method (see later);
• P6 pusher final stroke angle corresponding to the maximum speed necessary for the final push.
The result is that the pushing impulse given to the raw materials to load them into the furnace has two main effects. First of all, due to the pressure exerted the load tends to form a high and narrow
pile; and, secondly, thanks to the acceleration imparted, the materials are pushed deeply into the furnace so that two sub-sequent loads do not pile onto each other and some distance remains between them.
The pusher cycle can be better described if divided into phases:
• Pushing Phase
- the pusher get started and accelerated up to maximum speed;
- the pusher continues its stroke at the maximum speed until it reaches the forward stroke limit;
- having reached the stroke limit, the pusher stops for a pre-set period of time;
• Returning Phase
- once the stopping time is finished, the pusher returns to its original position at a pre-set speed until it reaches the backward stroke limit;
- having reached the stroke limit, the pusher stops for a pre-set period of time.
All above described parameters, represented with letter “P” on the control panel, can be set by the operator thanks to the menu shown in video.
TURRET ROTATION CONTROL
The turret rotation angle can reach 45° depending on the doghouse design.
Rotation control is based on the on a rod-handle mechanism prevent ing the turret from exceeding the maximum angle designed. This solution is intrinsically safe: it makes it impossible to exceed the maximum rotation angle and useless to install the limit switches which are usually installed on other machines present on the market. Thanks to a digital encoder, it is possible to set all intermediate halting positions of the machine directly from the control panel and within the maximum rotation angle. The basic programme includes a maximum of five but it is also possible to increase this number upon request.
The control software makes it possible to set:
• the angle of each arresting position of the turret;
• the positioning sequence of the turret, from one position to the following one;
• the stopping time for each position or, as an alternative, the number of the pusher strokes.
It is also possible to save a different menu of the pusher movement for each position of the turret, with different parameters for each different position.
RAW MATERIALS LOADING METHODS
In order to keep the glass level constant inside the furnace tank, the batch charging machine has to be able to vary the quantity of the raw materials to charge according to the output signal coming from the glass level measuring and control signal.
Depending on the type of batch charging machine, this signal acts either on the vibration intensity of the vibrating feeder (open dog-house) or on the pusher motor number of rounds (sealed dog-house). GT’s electronic batch charger offers two different methods of raw materials loading which can be selected from the operator panel:
• Frequency Mode
- the quantity of the raw materials charged into the furnace is adjusted by controlling the number of machine-cycles in time according to the glass level regulating signal, similar to the regulation of the pusher motor number of rounds used on conventional machines.
• Stroke Span Mode
- the quantity of machine cycles remains constant and the glass level control signal acts on the pusher stroke;
- the pusher action can be compared to a slide valve and its opening is controlled by the glass-level measuring system;
- this mode enables to control the pusher stroke keeping it as limited as necessary for the raw materials to drop onto the molten glass surface.
The possibility to choose between the two modes combined with the possibility of controlling the pusher stroke as described above offers an unparalleled level of flexibility in the charging of raw materials with many possible advantages:
• high constancy of the glass level;
• lower emissions of dust in the dog-house area;
• cut down of air leakage through the dog-house;
• wide range of possible interventions on the batch island shape and distribution on the molten glass;
• easy adjustment to different batch humidity and smoothness conditions;
• increased effectiveness in preventing the melting tank refractory structure.