Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 112 (2017) 563 – 570
Sustainable Solutions for Energy and Environment, EENVIRO 2016, 26-28 October 2016, Bucharest, Romania
Parametrical functioning study for a small boiler condensing unit Nicolae Antonescu*, Paul-Dan Stănescu UTCB – Building Services Engineering Faculty – Pache Protopopescu 66, Bucharest s2 , Romania
Abstract Considering the actual concern for reducing the fuel consumption along with pollution reduction, the condensing boilers get a major importance for the manufacturers, mainly for the household heating domain. In the context, more and more small boilers producers shift their non-condensing units to condensing units. Because of economical constraints linked to manufacturing costs it is of outmost importance for the producers to study a range of constructive solutions in the design stage. We elaborated a physical model for finned tubes small boilers that we implemented in a computational program. The aim was to generate alternative solutions for the basic design, in order to be able to perform an economical optimization. In the present paper we present an example of such computational modeling and optimization study for a finned tubes aqua-tubular boiler. The computational model gives solid information not only for the general functioning of the boiler, but also gives detailed information about the thermal and physical processes that occur on each individual heat and mass transfer surface along the flue gas flow, permitting an even deeper phenomenological analyze with significant results for the boiler constructive and functional optimization. © 2017 2017The TheAuthors. Authors. Published by Elsevier © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2016. and Environment Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy and Environment 2016 Keywords: Heat and mass transfer, condensing boiler, parametrical study.
1. General context Considering the actual concern for reducing the fuel consumption [1] along with pollution reduction, the condensing boilers get a major importance for the manufacturers, mainly for the household heating domain. * Corresponding author. Tel.: +4-074-201-2987 E-mail address:
[email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy and Environment 2016 doi:10.1016/j.egypro.2017.03.1117
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The temperature levels in modern heating installations meet the requirements of condensing boilers [2] and even more, new European regulations state seasonal efficiencies that can be reached only by using condensing boilers. In the context, more and more small boilers producers shift their non-condensing units to condensing units. In order to do these it is necessary to provide extra heat exchange surface that, under required secondary agent temperature conditions (boiler inlet temperature lower than approx. 55 oC) become also condensing surfaces. Because of economical constraints linked to manufacturing costs it is of outmost importance for the producers to study a range of constructive solutions in the design stage [3],[4] in order to choose the most economical solution for a given production situation but nevertheless meeting the required performance parameters. Considering those, we elaborated a physical model [5],[6] for finned tubes small boilers that we implemented in a computational program. The choice for finned tubes heat exchanger as main part of the heat transfer surface was dictated by the large majority of existing non-condensing small boilers that use this solution. In order to determine the heat transfer from the flue gases to the pipes it is determined as an equivalent convective heat transfer coefficient generated by radiation and also a purely convective heat transfer coefficient. The final heat transfer is dictated by the sum of those two coefficients [7]: ª § T · 3,6 º «1 ¨ p ¸ » « ¨ Tmg ¸ » ap 1 ª W º (1) ¹ » 3 a g Tmg « © D gr C o 10 8 » « 2 2 « § Tp · » ¬m K ¼ ¨ ¸ « 1 » ¨ Tmg ¸ » « © ¹ ¬ ¼
D gc
D gc
were:
2 · § ¨ § lc · 3 ¸ ¨ ¸ for Re t 2300 , 0,024 ¨1 ¨ ¸ ¸ Re 0,8 Pr 0,33 H Re lc ¨ © l1 ¹ ¸ ¹ © 6 105 H Re 1 for 10000 ! Re t 2300 with Re1,8 H Re 1 and for Re ! 10000
O
3,65
O lc
0,0668
O l1
Re Pr
§ ¨ § lc · ¨1 0,04 ¨¨ l Re Pr ¸¸ © 1 ¹ ¨ ©
2 3
· ¸ ¸ ¸ ¹
for
Re 2300
ª W º «¬ m 2 K »¼
(2) ª W º «¬ m 2 K »¼
αgr and αgc are the radiative and convective surface heat transfer coefficients Tp is the wall temperature (metallic pipe or insulation) [K] Tmg is the geometrical mean temperature of the flue gases [K] ; T mg = (Tgi • Tge) 0,5 ap and ag are the radiation absobtion coefficients for the wall and for the flue gases Co•10 – 8 is the Stefan – Boltzmann constant for radiation λ is the thermal conductivity; lc is the feature-length of the flow and l1 is the duct length.
The program was validated after a comprehensive comparison of computational results with experimental results obtained in the Thermal Testing Laboratory belonging to the Technical University of Construction - Building Services Engineering Faculty, for a range of thermal loads (combustible debits) and for different secondary agent temperature values (for both condensing and non-condensing conditions). The testing facility measuring error is less then 1% and the individual errors for the measuring elements is 0,5%. After program validation, the aim was to generate alternative solutions for the basic design, in order to be able to perform an economical optimization. In the
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present paper we present an example of such computational modeling and optimization study for a finned tubes aqua-tubular boiler. The extended surface made out of fins on cylindrical or oval pipes is affected by a thermal heat transfer efficiency calculated as following [7]:
XD
Dnerv de
§ 2 D *aer · ¸ H nerv ¨ ¨ Onerv Bnerv ¸ © ¹
XI
0,5
(3) Dnerv = outer diameter of the finn; de = inner diameter of the finn; Hnerv = finn height; Bnerv = finn thickness; λnerv = thermal conductivity of the finn material; α* = convective heat transfer coefficient for the finn surface
x
for XI > 0.8 :
AK
0,8 - 0,05 X D 2
BK
0,363 0,14 X D 2
CK
0,525 - 0,071 X D 2
K nerv
AK BK X I 0,8
0,797 -0,631 2,359
CK
(4)
x for XI < 0.8 : AK
1
BK
0,295 0,066 X D 2
CK
1,737 - 0,093 X D 2
K nerv
0,71
1,13
AK BK X I
CK
D CORR
D*
K nerv S nerv S base S nerv S base
ª W º «¬ m 2 K »¼
αCORR = corrected with finn efficiency heat transfer coefficient for the extended surface; Snerv = finn heat transfer surface; Sbase = base pipe (on witch the finns are mounted) heat transfer surface. 2. Computational results The “basic solution” considered is the boiler we tested in the Laboratory for the computational program validation. The boiler has two main heat transfer surfaces, one represented by a pair of finned tubes rows and the other represented by a smooth pipes package. The basic design is with aluminum pipes for both finned and smooth pipes and with aluminum fins. The finned tubes are not round but oval. In first place it was studied the effect of three major modifications (in order to reduce the production cost): x switching the pipes and fins material from aluminum to stainless steel; x using round section pipes instead of oval section pipes;
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x eliminating the smooth pipes heat exchanger by increasing the surface extension coefficient of the finned pipes (possible decision from the manufacturer point of view because of the use of stainless steel instead of aluminum). The two general constructive solutions are presented in Fig.1 and in table 1 are presented the modeling results for the “basic” solution and for the alternative solutions.
Figure 1 : general constructive scheme for the “basic” (left) and for the alternative solution (right)
Table 1 : modeling results for the “basic” solution and for the alternative solutions
Reference boiler (aluminium)
Steel boiler version 1
Steel boiler version 2
Number of pipes on 1-st row / diameter
[mm]
6 / Ф 17,2
6 / Ф 19
Surface extension coefficient
row 1
5,6
8,8
Number of pipes on 2-nd row / diameter
[mm]
5 / Ф 17,2
5 / Ф 19
Surface extension coefficient
row 2
5,6
8,8
Number of pipes on 3-rd row / diameter
[mm]
9 / Ф 12
0
Surface extension coefficient
row 3
1
---
[-]
8 / Ф 12
0
row 4
1
---
Number of pipes on 4-th row / diameter Surface extension coefficient Finns thickness
---
Boiler width (perpendicular to pipes)
[mm]
Boiler length (along pipes axes)
[mm]
Functioning parameters Inlet water temperature Outlet water temperature Combustible debit (gas network) Excess air at the burner
thin
thick
thin
175 155
200
(Values in brackets are input values for the computational program) [C]
Steel boiler version 3
( 30 )
[C]
( 50 )
[m3N /h]
( 2,571 )
[-]
( 1,25 )
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Table 1 (continuation) : modeling results for the “basic” solution and for the alternative solutions Flue gas temperature (exhaust)
Reference boiler (aluminium)
Steel boiler version 1
Steel boiler version 2
Steel boiler version 3
[C]
55,5
88,8
63,8
61,8
Total useful thermal load
[kW]
27,09
26,43
27,06
26,87
Sensible useful thermal load
[kW]
25,08
24,65
24,98
25,00
[kW]
2,01
1,78
2,08
1,87
[%]
105,9
103,4
105,9
105,1
Condensing useful thermal load Thermal efficiency (by respect to Hi – lower calorific value)
From the data presented in table 1 some major conclusions can be drawn relatively to the opportunity of choosing an alternative solution for the aluminum boiler. The overall thermal efficiency yields in an acceptable range between 103 % and 106 % so all the alternative solutions can be taken into consideration. If there is set a 105 % thermal efficiency threshold established for technical or more likely commercial reasons, only Version 2 and Version 3 will comply. The main constructive difference between Version 1 and Version 2 refer to the fins thickness. It is interesting to consider the effect of thickening the fins by maintaining the fin pace : x the flow speed (for the same overall section of the heat exchanger) increases due to the decrease of the free flow section and consequently the convective surface heat transfer coefficient will increase; x because the pace is maintained constant and the thickness of the fin increases, it results a decrease of the featurelength for the convective heat transfer and consequently an increase of the convective surface heat transfer coefficient; x the fin thermal efficiency, defined as the report between real heat flow exchanged with the gaseous flow considering the temperature variation along the fin and the maximum heat flow that would have been exchanged if the fin temperature would have been constant and equal to the base temperature, increases with the fin thickness, in both sensible and combined (sensible and latent) heat exchange regimes; in table 2 are presented the characteristic values for the fin thermal efficiency. Table 2 : fin thermal efficiency surface extension efficiency (calculated values)
if only sensible heat transfer occur
74,8 % thin fins
if both sensible and latent heat transfer occur
68,4 %
if only sensible heat transfer occur
82,0 % thick fins
if both sensible and latent heat transfer occur
73,6 %
x the previously stated facts are generating an increase of the global convective heat transfer and consequently an increase of the boiler thermal efficiency, despite the same length for both Version 1 and Version 2 solutions, from 103,4 % to 105,9 % (increase of 2,5 %). The solution for Version 3 was calculated because of the technological difficulties in constructing dense and thick fins. Version 3 is in fact the same constructive solution as Version 1 but with an increased pipe length, in order to match the thermal efficiency values characteristic for the base model or Version 2. It is important to observe that the increased length (of Version 3 versus Version 1) induces, in the context of maintaining the extended surface geometry, a decrease of the superficial heat transfer coefficient, due to the decrease of the flow speed. It was interesting to determine witch of the two tendencies (increase of the heat flux due to the increase of the heat exchange surface or decrease of the heat flux due to flow speed decrease) will prevail when the convective pipes length will change from 155 mm to 200 mm. As presented in table 1, the effect was positive, the thermal efficiency increasing from 103,4 % to 105,1 % (increase of 1,7 %).
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The computational model demonstrated that, for the stainless steel solution, the same thermal efficiency as for the “basic” aluminum solution can also be reached (in the calculated design) by using only the finned pipes heat exchanger, without the presence of the smooth pipes package. To complete the design possibilities for the case of the producer that can easily use smooth pipes and desires to diminish the use of finned pipes, a new set of constructive solutions was computed. In Fig.2 are presented the considered constructive solutions. In fact, the calculus started with a single row of finned pipes and gradually added smooth pipes packages in order to determine their thermal effect and finally picking an optimal solution for the design. The two rows finned pipes heat exchanger was considered as reference for the comparison. There were considered 1 to 4 packages of smooth pipes who replaced the second row of finned pipes. The general computational results are presented in table 3.
one single row of finned pipes heat exchanger
two rows of finned pipes heat exchanger
one row of finned pipes and one package of smooth pipes heat exchanger
one row of finned pipes and two packages of smooth pipes heat exchanger
Figure 2 : considered constructive solutions
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Table 3 : general computational results for the reference solution and for the design variants Number of pipes on 1-st row / diameter Surface extension coefficient Number of pipes on 2-nd row / diameter Surface extension coefficient Number of pipes for package nr 1 / diameter Surface extension coefficient Number of pipes for package nr 2 / diameter Surface extension coefficient Number of pipes for package nr 3 / diameter Surface extension coefficient Number of pipes for package nr 4 / diameter Surface extension coefficient Boiler width (perpendicular to pipes) Boiler length (along pipes axes)
Finned pipes condensing section
Smooth pipes condensing section
[mm]
6 / Ф 19
6 / Ф 19
6 / Ф 19
6 / Ф 19
6 / Ф 19
row 1
8,8
8,8
8,8
8,8
8,8
[mm]
5 / Ф 19
0
0
0
0
row 2
8,8
[mm]
0
pac1
---
--(9+8)/Ф 12 1
[mm]
0
0
pack 2
---
---
--(9+8)/Ф 12 1 (9+8)/Ф 12 1
[-]
0
0
0
pack 3
---
---
---
--(9+8)/ Ф 12 1 (9+8)/ Ф 12 1 (9+8)/ Ф 12 1
[-]
0
0
0
0
pack 4
---
---
---
---
--(9+8)/ Ф 12 1 (9+8)/ Ф 12 1 (9+8)/ Ф 12 1 (9+8)/ Ф 12 1
[mm]
175
175
175
175
175
[mm]
200
200
200
200
200
Functioning parameters
(Values in brackets are input values for the computational program)
Infeed water temperature
[C]
Outlet water temperature
[C]
( 50 )
[m3N /h]
( 2,571 )
[-]
( 1,25 )
Combustible debit (gas network) Excess air at the burner Flue gas temperature at exhaust section
( 30 )
[C]
61,8
144,8
77,3
63,7
58,0
Total useful thermal load
[kW]
26,87
24,54
25,82
26,29
26,58
Sensible useful thermal load
[kW]
25,00
23,94
24,81
24,98
25,05
Condensing useful therm. load Thermal efficiency for sensible heat (by respect to Hi) Total thermal efficiency (by respect to Hi)
[kW]
1,87
0,60
1,01
1,31
1,53
[%]
97,8
93,6
97,0
97,7
98,0
[%]
105,1
96,0
101,0
102,8
104,0
Analyzing the results from the table, the following conclusions can be drawn: x The thermal efficiency for the design variants with less then 3 packages of smooth pipes is less then 102 %, with more then 3 % less than the reference solution, and so they are not recommended as an alternative; x Using 3 or 4 smooth pipes packages increases the thermal efficiency to the threshold of 103 % , but still, even with 4 smooth pipes packages, the thermal efficiency does not match the reference solution’s value of 105 %. x It is interesting to notice that the important difference in the thermal efficiency is generated not by the sensible heat transfer (for witch the sensible heat thermal efficiency ranges from 97 % to 98 % for all the solutions, reference and alternatives with more than 1 smooth pipes package), but by the condensing heat transfer. Considering the facts previously stated the recommendation is not to use the single range finned pipes heat exchanger because the necessary completion smooth pipes packages will make it inefficient from economical point of view, a minimum of 3 packages being required.
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3. Final conclusions As a general conclusion, it can be stated that, prior to production, design optimization is possible with the presented model and computational program. The large array of constructive solutions make the optimization process absolutely necessary and, because of the relative ease of computational modeling, a big number of solutions can be studied. The computational model gives solid information not only for the general functioning of the boiler, but also gives detailed information about the thermal and physical processes that occur on each individual heat and mass transfer surface along the flue gas flow, permitting an even deeper phenomenological analyze with significant results for the boiler constructive and functional optimization. References [1] SR-EN – harmonized standards for boiler construction, testing and labelling; performance parameters determination [2] Danielle Makaire, Philippe Ngendakumana - Modelling the thermal efficiency of condensing boilers working in steady-state conditions Thermodynamics Laboratory – Univeristy of LIEGE, Belgium [3] Harish Satyavada, Simone Baldi - A Novel Modelling Approach for Condensing Boilers Based on Hybrid Dynamical Systems Delft Center for Systems and Control, Delft University of Technology, Mekelweg 2, Delft 2628CD, The Netherlands; [4] João Barros - Condenser boiler modeling - Instituto Superior de Técnico; Technical University of Lisbon; Avenida Rovisco Pais, 1-1049-001 Lisboa; [5] Stanescu P.D., Antonescu N.N.– Aparate Termice - Curs - Editura MATRIX– Bucuresti 2013-978-973-755-878-7– 432 pag. [6] Antonescu N.N.- Instalaţii de ardere şi cazane cu eficienţă energetică ridicată şi poluare redusă – Complemente de curs - Editura MATRIX ROM – Bucuresti 2011-ISBN 978-973- 755-699-8– 269 pag. [7] Nicolae Antonescu, Dan-Paul Stănescu - Wall temperature spectrum determination for high temperature heat exchangers (HTHE) with extended surfaces - 3rd EENVIRO and 6th YRC Conference
