STUDY OF THE ENERGY SAVINGS
WITH LIGHT LININGS
The aim of the present study is to quantify the energy savings obtainable with a new light slabs and setting supports set on the kiln car linings. This calculation is based on a real experiment carried out in an existing brick plant. There it has been possible to
compare two different lining designs in real conditions:
Kiln car lining made of dry pressed solid slabs and setting supports, originally used to equip the full fleet. From now on, we will call it SOLID KILN CAR.
Kiln car lining made of extruded hollow slabs and hollow setting supports made by Forgestal/Campo. Few kiln cars have been equipped with this kind of pieces.
The real experiment is used to figure out the temperatures of the refractory at the entry and at the exit of the kiln.
The basic data of the installation is the following:
• Kiln cycle: 14 hours
• Kiln pace: 30 min/car
• Output: 48 cars/day
• Car size: 4,3 m Wide x 3,98 m Long
• Product: Hollow bricks
In a brick kiln, the fuel is used to heat up the ware to a certain temperature to get it fired. In order to achieve a better thermal efficiency, once the ware is on its firing temperature, it is cooled down, recovering this energy which will be used to warm
the ongoing ware.
In the cooling section of the kiln, it should be recovered the energy from the fired ware but also from the kiln car lining. The kiln car is been heated as it is travelling through the first section of the kiln, accumulating energy. In the cooling section of the kiln,
part of this accumulated energy is recovered but not totally. In order to increase the thermal efficiency, it should be recovered as much energy as possible.
Once the kiln car has come out the kiln, the energy accumulated on the lining will be transferred to the workshop atmosphere. Therefore this energy cannot be used again, it is a LOST ENERGY.
There are two main strategies to reduce this amount of energy that literally the cars are taking out from the kiln to the workshop:
Improving the cooling performance of the kiln. By means of increasing the cooling duration (making this section longer) or increasing the heat exchange effectiveness (convection rate) increasing the air speed, the turbulence etc.
Using an optimized kiln car design in a way that:
Accumulates the minimum amount of energy along the heating section.
Be easy to cool it down
Have a low average heat capacity (this is very linked to the weight/density of the lining).
Therefore, at the same temperature it will have a lower amount of stored energy.
The point #1 is only concerning to the kiln configurations. Therefore we will concentrate on #2 where the kiln car design is playing a role. The best way to measure the efficiency related to the described aspects, is MEASURING THE REMAINING HEAT in the car at the exit of the kiln.
Comparing the figures of two different designs, working in the same kiln, at the same conditions, we will be able to quantify the advantage of one design over the other, concerning energy saving.(When we talk about accumulated heat, it is understood that we take as a reference point, the accumulated energy at the atmosphere temperature).
CALCULATION OF THE HEAT LOST TO THE WORKSHOP AMBIENCE
The heat lost to the workshop ambience from the kiln cars is the dierence or the balance between the remaining heat at the exit of the kiln and the remaining heat at the later entry.
The remaining heat of any physic part depends on the physic parameters:
The specific heat (kcal/kg ºC), indicates the capacity to store heat of a certain material. The specific heat multiplied by the mass yield the thermal inertia (kcal/ºC), which means the heat necessary to (kcal) to rise one Celsius degree the temperature of a part.
In the present study, we compare two different car designs. The parameters 1 and 2 depend only on the materials and the design of the car and therefore are analytically able to be calculated. However, to know the temperatures, though it could be made analytically (Forgestal has developed a Finite-Element-based application to do it), we will measure real temperatures of the kiln car when is coming out the kiln.
The temperatures were measured with an infrared device which gives the temperatures on the surface immediately. This way we measured the temperatures on several slabs at different points in order to get an average figure.
To simplify the calculations, it is considered that the entire slab (in depth) is at the same temperature of the one on the upper face. This shortcut is admissible because actually the lower part of the piece will be warmer than the upper surface (the cooling is done on the upper face); therefore the results of the calculation will be on the safe side. In other words the real savings will be higher than the ones calculated.
Once got the data, we apply the following formula:
∆Q remain= ∑ [ (Tfi – Tii) · ci · mi]
∆Q remain = Remaining heat
T fi = Final temperature of element i
T ii = Initial temperature of inicial i
C i = Specific heat capacity of material de i
m i = Mass of the element
Below, we compare these parameters between the two kiln car designs:
The table below shows that the temperatures at the exit of the kiln are very different depending on the kiln car design. In case of Forgestal design, the temperature is 140º C on the core slabs and 111ºC on the perimeter, while the temperatures in the Solid design are 225ºC and 121ºC respectively.
In the last table, the temperature of the kiln car at the entry of the kiln has been assumed to 35ºc in both designs. This situation would represent a complete cooling down of the lining while travelling outside the kiln. This would happen when the kiln cars are out of the kiln for relatively long time.
On the other hand, can also happen that the kiln cars which are just came out the kiln, are de-hacked, loaded and introduced again into the kiln with very short delay of time. This relatively short time would not allow a complete cooling down, just a partial cooling down. The temperature of these kiln cars was measured just before the entry of the kiln: 65ºC:
The amount of mass to be heated up is fundamental because the energy involved it will be in proportion
to it. In the present study, only the heat accumulated on the slabs is taken into account, discarding all the
head accumulated in deeper layers.
There are two reasons: In one hand because it is obviously complicated to know and measure the temperature at deeper layers. In the other hand, the variations of the temperature in such layers are lower, and also their density is much lower. Therefore its
contribution to the overall values is much lower. The comparisons of Weight/mass in both slabs are shown in the next table:
The same value has been taken for both designs. The reason is that variations within materials of the same nature are very little. The value adopted is 0,29 kcal/kg ºC.
HEAT LOST TO THE WORKSHOP AMBIANCE
Applying the mentioned formula, with the values describe, the following results are obtained:
In the case of complete cooling down, there is a saving of 167.743 - 74.602 = 93.141 kcal per kiln car.
Assuming a cycle in the kiln yielding 52 cars/day, the diary saving would be 5.632 kWh meaning an annual saving of 2.055.590 kWh.
In the case of partial cooling down the calculation is analog.
To calculate the economic figures, we have taken the following data:
Gas Price: 3,46 c€/kWh
Installation efficiency: 80%
Effective energy cost: 5,029 x 10E-05 €/kcal
With this dada, we obtain the following results:
The final Annual Saving will be a figure between 80.441 € and 88.904 € depending on the proportion of kiln cars with complete cooling down and partial cooling down (cars which remain some time on the tracks or cars which go again into the kiln quickly)
PERFORATED SETTING SUPPORTS
Similarly to the calculations on the slabs, we have calculated also the differences between the solid setting supports against the perforated ones from Forgestal.
In this study, we have considered only the remaining heat accumulated in the setting supports, discarding the heat accumulated on the ware in contact with the setting supports which will be at similar temperatures.
This simplification makes that the real saving will be bigger of the one calculated theoretically.
The calculation table for the setting supports is the following:
The energy saving is relatively less in case of the slabs. Although there is an important difference in weight, the temperatures of the setting supports at the exit of the kiln are notably lower because the rate mass/surface in contact with the air is much lower
and therefore they cool down faster in the last section of the kiln.
The enormous advantages of the hollow setting supports must be found in their features in relation to make easier the firing of the ware, rather than they accumulate less energy when they come out the kiln. In this study it is not valued the less broken material in the first layers (or making possible faster cycles) or its longer life, achievable with this kind of pieces.
The energy saved obtainable from the change the slabs to a Forgestal hollow slabs is considerable, of about 85.000 € annually.
In case of changing also the setting supports, there will be an additional annual saving of 13.154 €.
If we have in mind that the investment necessary for the complete change of 65 kiln car lining would be 476.000 €, it makes sense to think in the energy saving, as a motivation for justifying the change.