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Investigations on heat loss through the kiln shell in magnesite dead burning process: a case study
Applied Thermal Engineering 22 (2002) 1339–1345 www.elsevier.com/locate/apthermeng
Investigations on heat loss through the kiln shell in magnesite dead burning process: a case study B.K. Chakrabarti
*
Department of Materials and Metallurgical Engineering, National Institute of Foundry and Forge Technology, Hatia, Ranchi 834003, India Received 16 October 2001; received in revised form 23 February 2002; accepted 23 March 2002
Abstract Dead burning of magnesite in rotary kiln is an energy intensive process. Besides the energy consumed by the magnesite, the heat expenditures are through different areas of the process, such as kiln shell, exit gases, clinker exit, dust etc. The heat loss through the kiln shell by radiation and convection amounts to be a great proportion of the total heat loss in the process. The present work was devoted to the evaluation and assessment of the heat loss through radiation and convection in the process. The work comprises of data acquisition and utilization of the same for the evaluation and assessment of the shell heat loss of the kiln which was running for 128th day of its operation. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Rotary kiln; Radiation and convection; Magnesite; Economy
1. Tasks and targets The present work had been limited to: • measurement of heat loss through radiation, • measurement of heat loss through (i) forced air convection (ii) free air convection, • calculate and evaluate the recoverable heat loss through the burning zone (0–30 m) of rotary kiln, • investigate the pattern of the heat loss along the kiln length.
*
Tel.: +91-651-291140; fax: +91-651-290860. E-mail address: [email protected] (B.K. Chakrabarti).
1359-4311/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 2 ) 0 0 0 5 1 - 0
1340
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Nomenclature Ts Ta Tav hr hfc B, n hc r C2 k a qr qfc qc qtf qtc Qtf Qtc A Re V q l D
kiln surface temperature, K atmospheric temperature, K average of Ts and Ta , K radiative heat transfer coefficient, W/m2 K ðhr ¼ erðTs4 Ta4 Þ=ðTs Ta ÞÞ forced convective heat transfer coefficient, W/m2 K ðhfc ¼ kB Ren =DÞ constants varies with ranges of Re values free convective heat transfer coefficient, W/m2 K emissivity Stefan–Boltzmann constant, 5:676 108 W/m2 K4 dimensionless factor that evaluates the effect of solid surface configuration thermal conductivity of atmospheric air, W/m K free convection modulus, 1/m3 K radiative heat loss, W/m2 ðqr ¼ hr ðTs Ta ÞÞ forced convective heat flux, W/m2 free convection heat flux, W/m2 radiative and forced convective heat loss over 21 m the kiln surface, W radiative and free convective heat loss over 21 m of the kiln surface, W total radiative and forced convective heat loss over the kiln surface, W total radiative and free convective heat loss over the kiln, W surface area (pdl) along the kiln length, m2 Reynolds number, Re ¼ V qD=l velocity of air, m/s density of air, kg/m3 viscosity of air, Pa s diameter of the kiln, m
1.1. Description of the kiln system and data acquisition Process Type Kiln length Kiln diameter Rotational speed Inclination Fuel type Calorific value of the fuel Fuel consumptions Raw magnesite input Dead burnt magnesite (DBM) output
Dry process Rotary 84 m OD ¼ 2:575 m 1 rpm 3% LSHS (Low sulfur heavy stock) preheated to 423 K 45.22 MJ/kg 947.92 kg/h 9679.59 kg/h 5129.17 kg/h
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Exit gas temperature Atmospheric temperature Dead burning temperature
1341
603 K 303 K 1923 K
Fig. 1. Profile of surface temperature along the kiln length.
Data acquisition and utilization of the data needs proper attention, since there are certain factors such as, emissivity of the surface, velocity of air etc. influences the ultimate results, therefore mention of these data are important. The kiln shell temperatures were measured by a infrared thermometer. Fig. 1 shows the profile of the average surface temperature along the kiln length at each 21 m length of the kiln.
2. Evaluation of heat loss through the kiln shell 2.1. Heat loss through radiation As per the work of Kuhle [1] the emissivity capacity varies with temperature. However, for simplicity of calculations an approximation was made and an average value of e ¼ 0:8 had been utilized. The radiative heat transfer coefficient (hr ), heat loss (qr ) were evaluated using the standard equations and data [2]. 2.2. Heat loss through forced convection In the calculation and evaluation of heat loss by convection some uncertainty was encountered. As a general guideline if the kiln is housed in an enclosed hall the coefficient for free convection is usually applied, and if it is in the open air the coefficient for forced convection is applied.
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B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Table 1 Results of hr , hfc and heat loss (Qtf ) at different length of the kiln Length of kiln (m)
Ts (K)
Ts Ta (K)
hr (W/m2 K)
hfc (W/m2 K)
h ¼ hr þ hfc (W/m2 K)
A (m2 )
qtf (MW)
%Qtf
Qtf (MW)
0–21 21–42 42–63 63–84
594 549 490 425
291 246 187 122
18.2 15.26 12.25 9.09
12.733 13.737 14.859 16.58
30.933 28.997 27.109 25.67
169 169 169 169
1.5285 1.2112 0.8607 0.5318
36.98 29.31 20.84 12.87
4.1319
48.89
4.1319
Q (burn) (MW) Loss through burning zone (0–30 m) of the kiln 0–30 578 275 17.169 13.115
30.284
242.6
2.0202
The forced convective heat transfer coefficient (hfc ) have been determined using the average temperature of the kiln surface and air film. Air velocity plays an important role on the convection of heat. Since the periodic air velocity data were not available, an approximate value of 5 m/s was considered for air velocity for the calculation of forced convection loss. The Reynolds number (Re), heat transfer coefficient (hfc ) and heat loss (qfc ) were calculated using the standard values and equations [2]. Table 1 shows the evaluated results of hr , hfc , qtf , Qtf etc. at different kiln length. 2.3. Calculation of heat loss through free convection The factor of wind velocity in this particular calculation was not considered. As air is often involved in free convection calculations, simplified equations have been developed [2] for air to facilitate the determination of convective heat transfer coefficient (hc ). Convective heat transfer coefficient (hc ) for air at atmospheric pressure, in contact with the surface of horizontal cylinder of diameter more than 0.6 m is expressed [2] as, hc ¼ ðC2 ka1=3 ÞDT 1=3
W=m2 K
ð1Þ
Details of the notations are available in the nomenclature. If we consider C2 ka1=3 ¼ f , then the Eq. (1) stands as, hc ¼ f DT 1=3
W=m2 K
ð2Þ
f depends on the temperature Tav , where Tav ¼ ðTs þ Ta Þ=2 (in K). The values of f for horizontal cylinder/rotary kiln, dia more than 0.6 m can be obtained by multiplying the values in Table 2 (Panel A) by 0.8125 as stated by Todd and Ellis [2]. Thus the values of f at various Tav values stands as in Table 2 (Panel B). The values of Table 2 (Panel B) had been utilized for making a graph and fitted into MS Excel package. Fig. 2 shows the trend of the change of f with Tav . With the help of MS Excel package an equation could be formulated which is linear in nature, as represented below, f ¼ 0:0026Tav þ 1:341
ð3Þ
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
1343
Table 2 Dependance of f on Tav Tav (C)
f (W/m2 K1:33 )
Panel A: Large vertical flat plate in air 20 0 20 40 60 80 100 120
1.711 1.649 1.587 1.525 1.463 1.400 1.338 1.269
Panel B: Horizontal cylinder of diameter over 0.6 m in air 20 0 20 40 60 80 100 120
1.3902 1.3398 1.2894 1.2391 1.1887 1.1375 1.0871 1.0311
Fig. 2. f vs Tav .
Utilizing the Eq. (3), f had been calculated at different Tav values, corresponding to various kiln shell temperatures. The f values so obtained had been utilized for further calculations to determine hc , qtc and Qtc . The results of calculations are presented in Table 3.
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B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Table 3 Results of hr , hc and heat loss (Qtc ) at different length of the kiln (on free convection) Length of kiln (m)
Ts (K)
Ts Ta (K)
hr (W/m2 K)
hc (W/m2 K)
h ¼ hr þ hc (W/m2 K)
A (m2 )
qtc (MW)
Qtc (MW)
0–21 21–42 42–63 63–84
594 549 490 425
291 246 187 122
18.2 15.26 12.25 9.09
5.9314 5.8972 5.8506 5.4265
24.1314 21.1572 18.1006 14.5165
169 169 169 169
1.1923 0.8837 0.5747 0.3007
2.9514
3. Results and discussion The evaluation of heat loss (qtf , qtc ), heat transfer coefficients (hr , hfc , hc ) were found out at each 21 m length of the kiln. Tables 1 and 3 describes the details of the results. Most important aspect of this type of work is to assign appropriate boundary conditions. In this work heat loss were found out in two different conditions, either the kiln was in air or if the kiln was housed or enclosed within structures. The kiln was in the air, therefore the mode of convection was likely to be of forced nature and the results calculated are given in the Table 1. However, if the kiln would have been housed in a hall then the convection would be free convection and the results calculated in such case was given in Table 3. Fig. 3 shows the change of hr , hc and hfc with surface temperature. Fig. 4 shows the change in heat loss (Qtf ) along the kiln length and describes that Qtf decreases along the kiln length, results are given in Table 1. The heat loss through the burning zone (0–30 m) in the case of forced convection was also shown in Table 1. It had been observed that the loss through burning zone was about 49% of the total loss through the kiln. Results given in Table 3 shows the heat loss pattern of the kiln when the heat transfer was in free convection mode.
Fig. 3. Variation of hfc , hr and hc with surface temperature.
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
1345
Fig. 4. Heat loss (qtf ) along the kiln length.
4. Conclusions The present study reveals that a good amount of heat was lost through the kiln shell. However, while comparing the results it has to be kept in mind that the kiln can be judged only after considering the approximations and boundary conditions which were applied in calculating the results. This type of work provides the data and appropriate informations to the management of the company about how the units have been functioning. Results indicate that loss through forced convection was about 29% more than in the free mode of convection. It was also observed that the loss of heat in the burning zone (0–30 m) was about 49% of the total heat loss through the kiln as shown in Table 1. Therefore, special attention needs to be given on the quality of the brick lining in the burning zone, kiln feed rate, kiln rpm etc to arrest the heat loss.
Acknowledgement Author is thankful to Dr. A.K. Roy of RDCIS, Ranchi for valuable suggestions and help.
References [1] V.W. Kuhle, Zement-Kalk-Gips 23 (1970) 263–268. [2] J.P. Todd, H.B. Ellis, Applied Heat Transfer, Herper & Row, New York, 1982, 131, 140–141.
Investigations on heat loss through the kiln shell in magnesite dead burning process: a case study B.K. Chakrabarti
*
Department of Materials and Metallurgical Engineering, National Institute of Foundry and Forge Technology, Hatia, Ranchi 834003, India Received 16 October 2001; received in revised form 23 February 2002; accepted 23 March 2002
Abstract Dead burning of magnesite in rotary kiln is an energy intensive process. Besides the energy consumed by the magnesite, the heat expenditures are through different areas of the process, such as kiln shell, exit gases, clinker exit, dust etc. The heat loss through the kiln shell by radiation and convection amounts to be a great proportion of the total heat loss in the process. The present work was devoted to the evaluation and assessment of the heat loss through radiation and convection in the process. The work comprises of data acquisition and utilization of the same for the evaluation and assessment of the shell heat loss of the kiln which was running for 128th day of its operation. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Rotary kiln; Radiation and convection; Magnesite; Economy
1. Tasks and targets The present work had been limited to: • measurement of heat loss through radiation, • measurement of heat loss through (i) forced air convection (ii) free air convection, • calculate and evaluate the recoverable heat loss through the burning zone (0–30 m) of rotary kiln, • investigate the pattern of the heat loss along the kiln length.
*
Tel.: +91-651-291140; fax: +91-651-290860. E-mail address: [email protected] (B.K. Chakrabarti).
1359-4311/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 2 ) 0 0 0 5 1 - 0
1340
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Nomenclature Ts Ta Tav hr hfc B, n hc r C2 k a qr qfc qc qtf qtc Qtf Qtc A Re V q l D
kiln surface temperature, K atmospheric temperature, K average of Ts and Ta , K radiative heat transfer coefficient, W/m2 K ðhr ¼ erðTs4 Ta4 Þ=ðTs Ta ÞÞ forced convective heat transfer coefficient, W/m2 K ðhfc ¼ kB Ren =DÞ constants varies with ranges of Re values free convective heat transfer coefficient, W/m2 K emissivity Stefan–Boltzmann constant, 5:676 108 W/m2 K4 dimensionless factor that evaluates the effect of solid surface configuration thermal conductivity of atmospheric air, W/m K free convection modulus, 1/m3 K radiative heat loss, W/m2 ðqr ¼ hr ðTs Ta ÞÞ forced convective heat flux, W/m2 free convection heat flux, W/m2 radiative and forced convective heat loss over 21 m the kiln surface, W radiative and free convective heat loss over 21 m of the kiln surface, W total radiative and forced convective heat loss over the kiln surface, W total radiative and free convective heat loss over the kiln, W surface area (pdl) along the kiln length, m2 Reynolds number, Re ¼ V qD=l velocity of air, m/s density of air, kg/m3 viscosity of air, Pa s diameter of the kiln, m
1.1. Description of the kiln system and data acquisition Process Type Kiln length Kiln diameter Rotational speed Inclination Fuel type Calorific value of the fuel Fuel consumptions Raw magnesite input Dead burnt magnesite (DBM) output
Dry process Rotary 84 m OD ¼ 2:575 m 1 rpm 3% LSHS (Low sulfur heavy stock) preheated to 423 K 45.22 MJ/kg 947.92 kg/h 9679.59 kg/h 5129.17 kg/h
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Exit gas temperature Atmospheric temperature Dead burning temperature
1341
603 K 303 K 1923 K
Fig. 1. Profile of surface temperature along the kiln length.
Data acquisition and utilization of the data needs proper attention, since there are certain factors such as, emissivity of the surface, velocity of air etc. influences the ultimate results, therefore mention of these data are important. The kiln shell temperatures were measured by a infrared thermometer. Fig. 1 shows the profile of the average surface temperature along the kiln length at each 21 m length of the kiln.
2. Evaluation of heat loss through the kiln shell 2.1. Heat loss through radiation As per the work of Kuhle [1] the emissivity capacity varies with temperature. However, for simplicity of calculations an approximation was made and an average value of e ¼ 0:8 had been utilized. The radiative heat transfer coefficient (hr ), heat loss (qr ) were evaluated using the standard equations and data [2]. 2.2. Heat loss through forced convection In the calculation and evaluation of heat loss by convection some uncertainty was encountered. As a general guideline if the kiln is housed in an enclosed hall the coefficient for free convection is usually applied, and if it is in the open air the coefficient for forced convection is applied.
1342
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Table 1 Results of hr , hfc and heat loss (Qtf ) at different length of the kiln Length of kiln (m)
Ts (K)
Ts Ta (K)
hr (W/m2 K)
hfc (W/m2 K)
h ¼ hr þ hfc (W/m2 K)
A (m2 )
qtf (MW)
%Qtf
Qtf (MW)
0–21 21–42 42–63 63–84
594 549 490 425
291 246 187 122
18.2 15.26 12.25 9.09
12.733 13.737 14.859 16.58
30.933 28.997 27.109 25.67
169 169 169 169
1.5285 1.2112 0.8607 0.5318
36.98 29.31 20.84 12.87
4.1319
48.89
4.1319
Q (burn) (MW) Loss through burning zone (0–30 m) of the kiln 0–30 578 275 17.169 13.115
30.284
242.6
2.0202
The forced convective heat transfer coefficient (hfc ) have been determined using the average temperature of the kiln surface and air film. Air velocity plays an important role on the convection of heat. Since the periodic air velocity data were not available, an approximate value of 5 m/s was considered for air velocity for the calculation of forced convection loss. The Reynolds number (Re), heat transfer coefficient (hfc ) and heat loss (qfc ) were calculated using the standard values and equations [2]. Table 1 shows the evaluated results of hr , hfc , qtf , Qtf etc. at different kiln length. 2.3. Calculation of heat loss through free convection The factor of wind velocity in this particular calculation was not considered. As air is often involved in free convection calculations, simplified equations have been developed [2] for air to facilitate the determination of convective heat transfer coefficient (hc ). Convective heat transfer coefficient (hc ) for air at atmospheric pressure, in contact with the surface of horizontal cylinder of diameter more than 0.6 m is expressed [2] as, hc ¼ ðC2 ka1=3 ÞDT 1=3
W=m2 K
ð1Þ
Details of the notations are available in the nomenclature. If we consider C2 ka1=3 ¼ f , then the Eq. (1) stands as, hc ¼ f DT 1=3
W=m2 K
ð2Þ
f depends on the temperature Tav , where Tav ¼ ðTs þ Ta Þ=2 (in K). The values of f for horizontal cylinder/rotary kiln, dia more than 0.6 m can be obtained by multiplying the values in Table 2 (Panel A) by 0.8125 as stated by Todd and Ellis [2]. Thus the values of f at various Tav values stands as in Table 2 (Panel B). The values of Table 2 (Panel B) had been utilized for making a graph and fitted into MS Excel package. Fig. 2 shows the trend of the change of f with Tav . With the help of MS Excel package an equation could be formulated which is linear in nature, as represented below, f ¼ 0:0026Tav þ 1:341
ð3Þ
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
1343
Table 2 Dependance of f on Tav Tav (C)
f (W/m2 K1:33 )
Panel A: Large vertical flat plate in air 20 0 20 40 60 80 100 120
1.711 1.649 1.587 1.525 1.463 1.400 1.338 1.269
Panel B: Horizontal cylinder of diameter over 0.6 m in air 20 0 20 40 60 80 100 120
1.3902 1.3398 1.2894 1.2391 1.1887 1.1375 1.0871 1.0311
Fig. 2. f vs Tav .
Utilizing the Eq. (3), f had been calculated at different Tav values, corresponding to various kiln shell temperatures. The f values so obtained had been utilized for further calculations to determine hc , qtc and Qtc . The results of calculations are presented in Table 3.
1344
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
Table 3 Results of hr , hc and heat loss (Qtc ) at different length of the kiln (on free convection) Length of kiln (m)
Ts (K)
Ts Ta (K)
hr (W/m2 K)
hc (W/m2 K)
h ¼ hr þ hc (W/m2 K)
A (m2 )
qtc (MW)
Qtc (MW)
0–21 21–42 42–63 63–84
594 549 490 425
291 246 187 122
18.2 15.26 12.25 9.09
5.9314 5.8972 5.8506 5.4265
24.1314 21.1572 18.1006 14.5165
169 169 169 169
1.1923 0.8837 0.5747 0.3007
2.9514
3. Results and discussion The evaluation of heat loss (qtf , qtc ), heat transfer coefficients (hr , hfc , hc ) were found out at each 21 m length of the kiln. Tables 1 and 3 describes the details of the results. Most important aspect of this type of work is to assign appropriate boundary conditions. In this work heat loss were found out in two different conditions, either the kiln was in air or if the kiln was housed or enclosed within structures. The kiln was in the air, therefore the mode of convection was likely to be of forced nature and the results calculated are given in the Table 1. However, if the kiln would have been housed in a hall then the convection would be free convection and the results calculated in such case was given in Table 3. Fig. 3 shows the change of hr , hc and hfc with surface temperature. Fig. 4 shows the change in heat loss (Qtf ) along the kiln length and describes that Qtf decreases along the kiln length, results are given in Table 1. The heat loss through the burning zone (0–30 m) in the case of forced convection was also shown in Table 1. It had been observed that the loss through burning zone was about 49% of the total loss through the kiln. Results given in Table 3 shows the heat loss pattern of the kiln when the heat transfer was in free convection mode.
Fig. 3. Variation of hfc , hr and hc with surface temperature.
B.K. Chakrabarti / Applied Thermal Engineering 22 (2002) 1339–1345
1345
Fig. 4. Heat loss (qtf ) along the kiln length.
4. Conclusions The present study reveals that a good amount of heat was lost through the kiln shell. However, while comparing the results it has to be kept in mind that the kiln can be judged only after considering the approximations and boundary conditions which were applied in calculating the results. This type of work provides the data and appropriate informations to the management of the company about how the units have been functioning. Results indicate that loss through forced convection was about 29% more than in the free mode of convection. It was also observed that the loss of heat in the burning zone (0–30 m) was about 49% of the total heat loss through the kiln as shown in Table 1. Therefore, special attention needs to be given on the quality of the brick lining in the burning zone, kiln feed rate, kiln rpm etc to arrest the heat loss.
Acknowledgement Author is thankful to Dr. A.K. Roy of RDCIS, Ranchi for valuable suggestions and help.
References [1] V.W. Kuhle, Zement-Kalk-Gips 23 (1970) 263–268. [2] J.P. Todd, H.B. Ellis, Applied Heat Transfer, Herper & Row, New York, 1982, 131, 140–141.