LIGHTWEIGHT SETTING SUPPORT SYSTEM
ITS INFLUENCE ON THE PERFORMANCE OF THE KILN CAR
The setting supports have a vital role in the overall performance of a kiln car when firing ceramic materials. This is even more important whenever the ceramic materials are directly set on the kiln car in green state.
Only the air and the setting supports are in direct contact with the ware during the ceramic firing process. That’s why the behaviour of the setting supports has an enormous influence on the heating, firing and cooling of the ceramic ware supported by
Additionally to supporting the ware, the setting supports are asked to provide for air passage underneath. A good balance is required, for an optimum firing, between the air flow underneath the setting, the air flow between the settig and the roof, and the flow between the setting and the walls. Correctly balanced flows allow for a more uniform firing, lower scrap and better possibilities for a shorter firing cycle.
In the plants with direct green setting on kiln cars, the behaviour of the setting supports is, additionally, influencing the drying process. More precisely, the temperature of the setting surface should be as low as possible at the moment when the green ware is set. A high temperature, and this is more probable in summer time or when a kiln car is loaded again shortly after leaving the kiln, produces an accelerated drying of the first layers, with the corresponding damage.
We are going to analyse the last development of Forgestal - Refractarios Campo in setting supports and their advantages as related to the classic designs.
NEW SETTING SUPPORT DESIGN
Forgestal-Campo have designed a lightweight setting support (U 200102335(9)) based on their well known design of a isoresisting bridge type support, developed and produced for the first time in 1994.
This design joins three factors, all of them critically positive for the performance of the setting supports:
More surface exposed to the air, with the corresponding improvement in the heat exchange air- setting support.
Less mass to be heated and cooled.
All those factors favour the fast heating and cooling, with following consequences in practice:
Better follow-up of the temperature curve by the setting support, with lower delay because of reduced thermal inertia.
The setting support exits the kiln at a lower temperature.
Lower heat retained at the kiln exit, because of both the lower temperature and the lower mass.
The final cooling of the setting support is also faster when out of the kiln.
The temperature of the setting support is lower at the moment when the new ware is set. This is very important in any case of direct green setting.
This behaviour makes those setting supports especially interesting in the following applications:
Direct setting of green ware on kiln car. The low temperature of the setting supports at the moment of setting is critical to avoid breakage because of premature drying in the lower layers.
Firing of very low density ware (hollow bricks) in fast cycles. When the setting supports must follow the firing curve as fast as the ware, to avoid breakage in the lower courses. This is even more important when thin hollow bricks are set on edge, the
lower course meaning a very significant % of the total set ware.
In any case of fast cycle, when the classic solid setting supports can impair the fast heating-cooling of the lower courses of
We have analyzed, by means of the finite elements method, the relative behaviours of three different setting support solutions for the firing of hollow thin bricks:
Lightweight settings supports, as proposed by Forgestal, Solid setting supports, as proposed by Forgestal, Another solution from the market.
The material to be fired is, in this case, Carrobrick. The considered section is corresponding to the center of the setting table, where the influence of the kiln wall and the firing lane can be neglected.
The analysis concerns the rapid cooling between 950 ºC (soaking temperature) and 650 ºC. As a consequence, both the ware and the setting supports are assumed to be uniformly at 950 ºC when the simulation starts.
First of all, the air flow is calculated through the ware and through the setting supports. The air velocity is calculated for every possible duct, with the condition that the head loss will be the same for all of them. As a consequence, the velocity through the bigger sections under the setting supports will be quite higher than the velocities through the smaller sections, inside the setting supports or inside the ware.
According to the calculated air velocities and temperatures, the surface convection coefficients are calculated for each duct. The surface convection coefficient gives a measure of the heat flow between the ware and the surrounding cooling air.
When the preceding scheme is applied by means of the finite elements method, the evolution of the temperatures at each point can be calculated. The color scale gives a visual indication of the temperatures in º C. Please note that the correspondence color-temperature is not the same for the different successive pictures corresponding to successive states.
For a correct analysis of the results, let us put our attention on the temperature differences that we can see inside the Carrobrick part which is in direct contact with the setting support. Those differences produce different thermal expansion/shrinkages, with the corresponding thermally generated internal stresses. The ware will break when the stresses are bigger than the material strength. It is important to see that the temperature of the Carrobrick wall directly in contact with the setting support is almost
the same as the temperature of the setting support at the contact point.
State 1 shows the initial conditions, when all parts are at the same initial temperature.
States 2, 3 and 4 show how the thinner walls cool down faster than the thicker ones.
State 5 shows that all the ware is at a basically homogeneous temperature from the second layer on. But, as shown by detail pictures of State 5, the temperature differences inside the first layer differ a lot depending on the different setting supports underneath.
In the first case, with the lightweight setting support proposed by Forgestal, the Carrobrick in the first layer is at a basically homogeneous temperature, i. e. without significant internal stresses.
In the second case, with the solid setting support by Forgestal, small temperature differences are visible inside the first layer.
But the big temperature differences appear in the third case, with the corresponding probability of breakage.
It is obvious, from this analysis, that the first solution is clearly superior, the setting support following a cooling curve quite similar to the one of the ware, with, as a consequence, much less or no breakage in the first layer.
As it is customary in any thermal calculation-simulation, results can not be taken as a strict approach in absolute values, but they are very significant when comparing the behaviour of different solutions under the same conditions.
The lightweight setting supports offer substantial advantages in the case of firing hollow bricks or fast cycles, and whenever the ware is directly green set. Advantages consist in a faster answer to changes of temperature within the kiln, avoiding or limiting the breakage in the first courses, and in a lower energy consumption because of the lower heat retained when exiting the kiln.
In the case of direct green setting, the main advantage is based on the fact of a lower temperature of the setting supports when the green ware is set.
All the mentioned advantages have been confirmed in real practice, in most cases with impressive reductions of the breakage during drying and/or firing.