THERMAL PERFORMANCE OF KILN CARS

IN REAL FIRING CYCLES

Forgestal, S.L. has developed a computer program by finite elements, specifically adapted for kiln cars. It calculates the evolution of the heat inputs and outputs in the kiln car as well as the evolution of the temperatures in all the nodes of a predefined
internal network, as a function of both the kiln car properties and the evolution of conditions of temperature and gas velocity along the real firing cycle.
Modern kiln cars, made of fired refractory pieces, can look so similar between them that it is not easy to distinguish between different designs, both on the drawing and sometimes even on real new kiln cars.
But its performance in operation, regarding the cycles and real firing conditions, can be very different depending on some important design details. The calculations indicate and the experience confirms that kiln cars very similar in appearance can show
radically different performances regarding mechanical impact, thermal shock, chemical attack, vacuum cleaning possibilities, resistance to chipping penetration and fuel consumption in real cycle.

Each one of these aspects, and even more the whole of them, is important for life and profitability of a kiln car system, but we will focus this article on the last listed one: Fuel consumption.
The intuitive concept that heavy kiln cars consume more heat than light ones is basically true (1).

It is a fact that a better insulated kiln car will transmit, in steady heat flow conditions, less heat than a worse insulated kiln car.

But it would be wrong to deduce from the prior statements that two kiln cars with identical weights and identical global coefficients of transmission will have the same consumption in real cycle. In fact consumptions in real cycle can be very different even in kiln cars with identical weights and identical global coefficients of transmission. Real consumption is very little depending on global weight and global heat transmission coefficient, and much more depending on the individual densities, individual transmission coefficients and physical disposition of each one of the individual components.
The reason of this apparent contradiction is very simple: Kiln car working conditions in real cycle are very far from the ideal conditions in which the calculation of steady flow heat transmission is based (2).
In steady heat flow situation,100% of the lost heat (consumption) correspond to the transmission heat losses.

But kiln car conditions, especially in modern kilns with cycles seldom higher than 36 hours and sometimes lower than18 hours, are radically different, to such an extent that the transmission heat loss of a kiln car inside the kiln can be less than 20% of the heat retained by the same kiln car at the kiln exit.
Whilst calculation of heat loss in steady flow conditions is relatively simple, the calculation of the heat retained in the kiln car is rather more complex and requires both an accurate preparation and a powerful calculation program.

Forgestal, S.L. has developed a calculation program by finite elements (3), specifically conceived to be applicable to kiln cars. This program, calculates the evolution of the heat inputs and outputs in the kiln car as well as the evolution of the temperatures in all
the nodes of a predefined internal network, as a function of both the kiln car properties and the evolution of conditions of temperature and gas velocity along the real firing cycle.

The points of this network are usually situated at a distance between 1 and 10 mm, so that the information obtained is very vast for each kiln car section. This vast information should be excessive for presentation and study, so the program includes several
possibilities to gather and condensate it.

The results are displayed here gathered to the global level of a kiln car, but other displays much more detailed are used for the optimisation of the designs. Calculations, independently of the gathering level for display, are always done for each one of the section types constituting the kiln car.

As application examples, we next show the results obtained about:

  • Three alternative possibilities of kiln car design for firing facing bricks at high temperature.

  • Two alternative possibilities of kiln car design for firing tiles in U-cassettes.

For each one we display a fragment of schematic section and the graphic results of the calculation for the average of the whole of the kiln, including the evolution of the average temperatures at different levels and the heat contained in each instant in the whole of the kiln car.

Facing bricks 1

Facing bricks 2

Facing bricks 3

U-Cassettes 1    

U-Cassettes 2    

AVERAGE KILN CAR HEAT RETENTION AND TEMPERATURE

                Kiln car U-Cassettes 2          

TIME, in kiln car modules 

(1-84) inside kiln 1 module=1.050 seg. - (85-106) outside kiln 1 module=6000 seg.

TEMPERATURE ºC

HEAT Kcal/kiln car

AVERAGE KILN CAR HEAT RETENTION AND TEMPERATURE

Kiln car Facing bricks 1 (with setting supports)

TIME, in kiln car modules 

(1-70) inside kiln 1 module=1.350 seg. - (71-106) outside kiln 1 module=6000 seg.

TEMPERATURE ºC

HEAT Kcal/kiln car

AVERAGE KILN CAR HEAT RETENTION AND TEMPERATURE

Kiln car Facing bricks 2 (with setting supports)

TIME, in kiln car modules 

(1-70) inside kiln 1 module=1.350 seg. - (71-106) outside kiln 1 module=6000 seg.

TEMPERATURE ºC

HEAT Kcal/kiln car

AVERAGE KILN CAR HEAT RETENTION AND TEMPERATURE

Kiln car Facing bricks 3 (with setting supports)

TIME, in kiln car modules 

(1-70) inside kiln 1 module=1.350 seg. - (71-106) outside kiln 1 module=6000 seg.

TEMPERATURE ºC

HEAT Kcal/kiln car

AVERAGE KILN CAR HEAT RETENTION AND TEMPERATURE

                Kiln car U-Cassettes 1            

TIME, in kiln car modules 

(1-84) inside kiln 1 module=1.050 seg. - (85-106) outside kiln 1 module=6000 seg.

TEMPERATURE ºC

HEAT Kcal/kiln car

AVERAGE KILN CAR RETENTION AND TEMPERATURES

Kiln car Facing bricks 1 (with setting supports)

Steady heat flow conditions

     TIME, in kiln car modules      

(1-70) 1 module=1.350 seg. - (71-106)  1 module=6000 seg.

TEMPERATURE ºC

To notice that, due to space reasons, the time scales in the inside of the kiln are different from the ones on the outside of it. The scales used are indicated in each graphic.
For one of them, the Facing brick kiln car 1, the temperature evolution in “almost-steady heat flow” has also been calculated. The evolution of the temperatures has been calculated in a virtual cycle that begins with a heating identical to the normal
cycle, but all the temperatures are then maintained during 3 days with the same values they had at the end of the firing zone. At the end of this time the temperatures are almost stabilised and the difference is less than one grade with the ones that the calculation should give us in steady flow. Comparison of this graphic with the one of a normal cycle shows us the big difference between the temperatures of the kiln cars in real cycle and the theoretical temperatures in steady heat flow situation.

A summary table is also displayed with some of the characteristics and significant variables individualized for each of the two groups: Facing bricks, U-cassette.

SUMMARY TABLE

Facing br.1

Firing temperature

Cycle time

Total kiln car weight

ºC

1.180

hours

26,25

Kg

7.550

Lower steel plate (steady flow)

Heat transmission (steady flow)

ºC

92

Kcal/h kiln c.

16.600

Maximum lower steel temp. (real cycle)

ºC

66

Transmission loss inside kiln car (real cycle)

Kcal/kiln car

58.900

Retained heat at kiln exit (real cycle)

Kcal/kiln car

467.500

Kiln Car

Facing br.2

Facing br.3

U-Cassette 1

U-Cassette 2

1.180

26,25

8.250

1.180

26,25

9.100

1.040

24,50

4.800

1.040

24,50

7.500

98

20.500

92

16.200

87

10.400

108

19.700

77

111.200

614.700

66

49.400

715.400

69

43.100

248.600

73

69.900

489.900

Kiln Car Face-Brick type 1

Kiln Car U-Cassettes type 1

Analysing the data of this table it is evident that the optimal design, with very important advantages in the consumption, corresponds to the types “Kiln car Facing bricks 1” and “Kiln car U-cassettes 1”. In the photographs some of the kiln cars designed and built by Forgestal and Refractarios Campo following these designs are shown.
In some cases this important difference in consumed heat (transmitted + retained) corresponds comparably with the theoretical transmissions in steady heat flow (“Kiln car Facing bricks 1” compared with “Kiln car Facing bricks 2” and “Kiln car U-cassettes 1” compared with “Kiln car U-cassettes 2”) but in other cases (“Kiln car Facing bricks 1” compared with “Kiln car Facing bricks 3”), the dierence of heat retained by the second is a 53% superior to the first one even though its transmissions in steady heat flow are almost the same.

CONCLUSIONS

  1. The calculation of the transmission steady flow situation, essential for the correct design of a kiln car, is not useful at all for the calculation of the heat retained by the kiln car at the kiln exit. In fact, it is not even useful for the calculation of the transmission loss inside the kiln.

  2. Both the loss due to transmission and the loss due to retained heat at the kiln exit require a calculation program that simulates the kiln car heat and temperature evolution according to the variable conditions along the firing cycle.

  3. Intuition, not verified by appropriate calculations, could cause serious confusions when prejudging the thermal performance
    of a kiln car.

  4. The calculation program by finite elements, developed and used by Forgestal, S.L. since 1999, has shown to be a very useful tool to optimize the kiln car design in order to improve the firing conditions and to reduce the heat consumption.

NOTES

  1. Thanks to the fact that the specific heats (calorific capacity for unit of material mass) of the usual composing materials of the kiln cars, light or heavy, are very similar.

  2. Reminder: In the steady heat flow theoretical conditions of transmission:

    1. Temperatures are constant at both upper and lower faces of the kiln car.

    2. The heat flow crossing all the layers of the kiln car is the same in all of them at any time.

    3. The heat contained in the kiln car is constant, without accumulation or loss.

  3. Reminder: This computing method, applying physical laws to material elements being very small but of finite dimension, allows to do
    engineering calculations that would not be possible to solve by means of classic dierential calculation. Developed in parallel with the
    computers that made it possible, it is widely used now in calculations related to strength of materials, fluids, heat transmission, weather
    forecasts, space navigation and any kind of simulation calculations.

© 2020 FORGESTAL S.L.   Camí Ral, 104 | Polígon Industrial Sud | 08292 ESPARREGUERA | Tel. +34 937 778 707 | Fax. +34 937 787 714 | forgestal@forgestal.com

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