Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE
ANALYSIS OF AIR CONDITIONER’S EVAPORATOR FOR ENHANCED PERFORMANCE
ANKIT JAIN1, JAGDEESH SAINI2
1PG Research Scholar, 2Associate Professor Department Of Mechanical Engineering BM College Of Technology, Indore (M.P)
Abstract:- Evaporator in an air conditioning system play very important role in maintaining the cabin’s temperature constant, and therefore needs to be designed properly as it is directly associated with the compressor which determines the electricity bills. Present research work is devoted to the investigations on evaporator of an air conditioner for enhanced performance. For this purpose, a simulation approach is adopted under which different combinations of materials along a specified set of refrigerants are compared and thermal properties, thermal gradient and heat flux, are investigated. The selected tube materials are copper and aluminum whereas refrigerants are R404a, R22, R152a, and R134a. The analysis done in ANSYS 15.0 simulation software. Analysis shows different rankings for thermal gradient and heat fluxes, which is scrutinized with the help of a statistical parameter coefficient of variance. Results of research work show the suitability of combination of aluminum and R022 for the application, but considering harmful environmental effects of refrigerant, second best option, R134a is being recommended for the purpose.
Keywords:- Evaporator, air conditioner, thermal properties.
1. INTRODUCTION
In evaporators, the actual cooling effect takes place in the refrigeration and the air conditioning systems. For several people the evaporator is the main part of the refrigeration system and they think other parts as less useful. The evaporators are heat exchanger surfaces that transfer the heat from the substance to be cooled to the refrigerant, thus remove the heat from the substance. The evaporators were use for large variety of diverse applications in refrigeration and air conditioning processes and hence they are available in wide variety of shapes, sizes and designs.
They are also classify in different manner depending on the method of feeding the refrigerant, construction of the evaporator, direction of air circulation around the evaporator, application and also the refrigerant control. In the domestic refrigerators the evaporators are commonly identified as the freezers since the ice is finished in these compartments.
In case of the window and split air conditioners and other air conditioning systems where the evaporator is straight use for cooling the room air, it is called as the cooling coil. In case of large refrigeration plants and central air conditioning plants the evaporator is also known as the chiller since these systems are first used to chill the water, which then produces the cooling effect.
Present research work is devoted to the investigations on evaporators of air conditioners for enhanced performance.
For this purpose, a simulation approach is adopted under which different combinations of materials (including existing one) is compared along a specified set of refrigerants and thermal properties, thermal gradient and heat flux, are investigated. In present research work tube materials used are copper, and aluminum, whereas refrigerants used are R404a, R022, R152a, and R134a. The analysis is being done in ANSYS simulation software, with k-ε model as the research model.
Following are the objectives of the research:
a) Modeling of evaporator for air conditioning system;
b) Simulation of evaporator by using different tube materials, and refrigerants, and
c) Identification of most appropriate combination of tube materials and refrigerants.
2. LITERATURE REVIEW
Following are the details of research contributions of researchers in the field of evaporators:-
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE a. Todorov et al. (2017) The present
paper aims to assess and to improve existing design of evaporators for household table top refrigeration appliances using Computational Fluid Dynamics (CFD). This result shows the effect of serpentine geometry on evaporator performance as well as demonstrates the benefits of virtual prototyping when targeting optimization and improvement.
b. Shiva and Kumar (2017) In this thesis, different configurations of fin tube evaporator are modeled in 3D modeling software Pro/Engineer. The temperature distribution, heat transfer rate is analyzed by thermal and CFD analysis done in ANSYS. Thermal analysis is done on four different configurations are continuous fin, continuous fins with zig zag tubes, interrupted fin and interrupted fin with zig zag tubes with different materials for evaporator Aluminum, Aluminum alloy 7075 and Copper. CFD analysis is done by varying fluids R134a, R22a and R410a on all the configurations.
c. Mohaisen and Ramjee (2017) Air conditioning evaporator works by absorbing heat from the other fluid such as brine & water or air that is to be cooled. In the present research work, the modeling and thermal analysis of an air- cooled evaporator for 1.5 ton air conditioner are done. The results obtained from the analysis of each model are expressed in the figures for nodal temperature and heat flux for different fin materials and refrigerants. The best material and best fluid for the evaporator of the design are checked by comparing the results. Comparison among the three results obtained from the analysis for each refrigerant are expressed in the figures for nodal temperature and heat flux for different fin materials to see the effect of tube pitch, thickness and pitch of the fin.
d. Shijie et al. (2017) In this paper, the 7mm evaporator with two branches of refrigerant R22 is improved the new evaporator which had 5 branches. And reduced the cooper tube diameter from 7mm to 5mm, meanwhile the refrigerant R22 was replaced to R290.
The minor-caliber evaporator was simulated by software EVAP-COND.
Conclusions are as follows: 1. The heat
transfer efficiency of three kinds of flow path arrangement of R290 minor-caliber fin-tube evaporator is better than that of the7mm R22 system, while the third scheme has the best heat transfer effect.
2. Compared to 7mm evaporator, the heat exchange and heat transfer coefficient of the third scheme of R290 minor-caliber fin-tube evaporator were increased by 34.62%, 24%, and refrigerant flow, pressure drop were reduced by 28.7%, 43.1%.
e. Guduri et al. (2015) In this paper, dissimilar shapes of fins in fin tube evaporator are modeled in 3D modeling software Pro/Engineer. The fins considered are rectangular fin, circular fin a internal finned. The mass flow rate and heat transfer rate are analyzed by CFD analysis completed in ANSYS. CFD analysis is completed by varying fluids R134A, R22 and R410 on all the models.
The inputs of CFD analysis are velocity and pressure and the results determined are Pressure, Velocity, Mass Flow Rate, Heat Transfer Rate and Heat Transfer Coefficient.
f. Mullen and Bullard (1994) According to the researchers, increasingly stringent energy standards and the redesigning of room air conditioners for use with alternative refrigerants have highlighted the need for design and simulation models that are accurate, easy to modify, and flexible enough for a variety of design and simulation tasks.
This report describes the latest version of the Air Conditioning and Refrigeration Center (ACRC) room air conditioner simulation model. The model is being continually improved using heat transfer and pressure drop correlations and other modifications which are added as a result of an ongoing experimental program.
2.1 GAPS IN THE RESEARCH
On the basis of analysis of theoretical considerations, and research contributions made by different researchers, following gaps in the research are being identified.
a) There is almost nil research available which compares the different materials and refrigerants for an evaporator; and
b) There is almost nil research available which suggests the best
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE combination of tube materials,
and refrigerants.
On the basis of gaps of research, objectives the research work are being decided.
3. SOLUTION METHODOLOGY
Present section is devoted to the details of solution model and software used for the research work, the details of which are presented in upcoming sub-sections.
3.1 SOLUTION MODEL FOR THE RESEARCH WORK
The solution model proposed for the research work is presented as follows.
K-epsilon (k-ε) turbulence model is a very famous model used in the field of computational fluid dynamics for simulating mean flow characteristics for turbulent flow conditions. It is a two equation type of model which offers a general description of existing turbulence by means of two transport equations.
Following are the details of variables obtained through k-ε model:
1. The first transported variable is called turbulent kinetic energy (k), which determines the energy in the turbulence; and
2. The second transported variable is used for determining the rate of dissipation of kinetic energy. This variable is called turbulent dissipation (ε).
Details of the model are as follows (Mierka et al., 2006):
In the framework of eddy viscosity models, the hydrodynamic behavior of a turbulent incompressible fluid is governed by the RANS equations for the velocity u and pressure p.
∂u
∂t+ u . ∇u = −∇p + ∇. ((V + VT)[∇u + ∇uT ]), ∇. u = 0
(3.1)
Where ν depends only on the physical properties of the fluid, while VT is the turbulent eddyviscosity which is supposed to emulate the effect of unresolved velocity fluctuations u′.
If the standard k − ε model is employed, then
VT= Cμ
k2 ε
(3.2)
Where k is the turbulent kinetic energy and ε is the dissipation rate. Hence, the above system is to be complemented by two additional convection-diffusion- reaction equations for computation of k and ε.
∂k
∂t+ ∇. (ku −VT σk
) = Pk – ε (3.3)
∂ε
∂t+ ∇. (εu −VT
σε∇ε) =ε
k(C1PK− C2 ε) (3.4)
where
Pk = VT
2 |∇u + ∇uT|² (3.5) And ε is responsible for production and dissipation of turbulent kinetic energy, respectively. The default values of the involved empirical constants are as follows: Cμ = 0.09, C1 = 1.44, C2 = 1.92, σk = 1.0, σ" = 1.3.
3.2 SIMULATION SOFTWARE USED IN THE RESEARCH WORK
Simulation software used in the research work is ANSYS. ANSYS is a very popular analysis tools, developed by ANSYS Inc., USA for simulating problems of structural analysis, thermal analysis, computational fluid dynamics, modal analysis, harmonic analysis, transient dynamics, buckling, and other categories. The software also offers the facility to develop simple models. With the help of inbuilt library, one can find out the properties of materials. ANSYS also include a set of models to solve complex problems of engineering, sciences, and other applications. In present research work ANSYS 14.0 version is being used.
Following are the salient features of the software:
Provides excellent simulation facility;
Offers different types of complex analysis like modal, transient, etc;
Provides different approaches to solve a problem with different inbuilt models;
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE Facilitates in modeling of simple
parts;
Inbuilt library for properties of materials;
Separate modules for different analyses purposes like structural, modal, etc; and
Better graphics facilities.
In present research work ANSYS 15.0 is being used.
4. PROBLEM ANALYSIS AND SOLUTION Present section deals with problem formulation and solution, the details of which are presented in upcoming sub- sections.
4.1 PROBLEM FORMULATION AND SOLUTION
Present research work is focused on performance enhancement of evaporator of an industry (IC Ice Make Refrigeration) based air conditioner. For this purpose a model of evaporator was prepared, the dimensions of which are presented as follows:-
a) Tube Outer Diameter: 9.53 mm;
b) Tube Inner Diameter: 8.53 mm;
c) Tube thickness: 0.5mm;
d) Tube Pitch (Longitudinal and Transverse both): longitudinal pitch=25mm,transverse
pitch=30mm;
e) Tube Material: Copper;
f) No. of Turns: 9;
g) Fin Pitch and thickness:
pitch=4.33mm, fins
thickness=0.25mm;
h) Material of Fin: aluminum;
i) Tube bending Radius: 30mm; and j) Refrigerant: R-22
Figure 4.1 shows the details of arrangements of different parts of evaporator.
(a) (b) (c) Figure 4.1: Model of Evaporator
(Parikh and Patel, 2013)
In next step is computer based model was prepared, as shown in Figure 4.2. Due to complexity in mesh generation, in present research work, analysis is made only on tubing section.
Figure 4.2: Model of Evaporator In next step, meshing of the model was accomplished. The purpose of meshing is to make the body deformable due to which it can withstand applied changes.
Table 4.1 shows the specifications of meshing.
Table 4.1: Specifications of Meshing S.No Entity Details 1. Element type Triangular 2. Number of nodes 332845
3. Number of
elements 1237090
Figure 4.3 shows the meshed model of the evaporator.
Figure 4.3: Meshed model of Evaporator
In next step, values of heat flux and thermal gradient for evaporator were investigated for copper and aluminum as tube materials and R022, R134a, R404a, and R152a as refrigerants in ANSYS 15.0 simulation software, the detailed
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE properties at 40oC as working
temperature, are presented as follows.
Table 4.2: Properties of Refrigerants at 313 k
5. RESULTS AND DISCUSSION
Present section deals with the results and discussion about the research work, the details of which are presented in upcoming sub-sections.
5.1 RESULTS
Figure 5.1 shows the graphical representations for heat flux values for different refrigerants with copper tubes.
(a) R22 (b)
(b) R404a
(c) R152 (d)
(d) R134a
Figure 5.1: Heat Flux Values for different Refrigerants with Cu Tubes Table 5.1 shows the summary of above mentioned results.
Table 5.1: Heat Flux Values for different Refrigerants with Cu Tubes S.No Refrigerant Heat Flux
(W/m2)
1. R022 -3.571E-002
2. R404a -4.205E-002
3. R152a -3.507E-002
4. R134a -3.067E-002
Figure 5.2 shows the graphical representations for thermal gradient values for different refrigerants with copper tubes.
(e) R22 S.
N o
Proper ty
Refrigerant R22 R40
4a
R152 a
R13 4a Densit
y
6.81 kg/
m³
7.71 kg/
m³
2.739 kg/m
³
8.12 kg/
m³ Viscosi
ty 13.3
micr oPa- s
14 micr oPa- s
10.64 micro Pa-s
12.4 micr oPa- s Sp.
Heat 0.64 96 kJ/k g·K
0.88 kJ/k g·K
0.070 6735 kJ/kg
·K
0.83 34 kJ/k g·K Therm
al Condu ctivity
0.01 15 W/
m-K 0.0 181 W/
m-K
0.129 W/m- K
0.01 46 W/
m-K
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE
(f) R404a (g)
(g) R152
(h) R134a
Figure 5.2: Thermal Gradient Values for different Refrigerants with Cu Tubes Table 5.2 shows the summary of above mentioned results.
Table 5.2: Thermal Gradient Values for different Refrigerants with Cu Tubes S.No Refrigerant Thermal Gradient
(K/m)
1. R022 4.357E+000
2. R404a 3.847E+000
3. R152a 3.084E+000
4. R134a 3.458E+000
Figure 5.3 shows the graphical representations for heat flux values for different refrigerants with Al tubes.
(a) R22
(b) R404a
(c) R152
(d) R134a
Figure 5.3: Heat Flux Values for different Refrigerants with Al Tubes
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE Table 5.3 shows the summary of above
mentioned results.
Table 5.3: Heat Flux Values for different Refrigerants with Al Tubes S.No Refrigerant Heat Flux
(W/m2)
5. R022 -4.687E-002
6. R404a -1.587E-002
7. R152a -2.327E-002
8. R134a -2.889E-002
Figure 5.4 shows the graphical representations for thermal gradient values for different refrigerants with Al tubes.
(a) R22
(b) R404a
(c) R152
(d) R134a
Figure 5.4: Thermal Gradient Values for different Refrigerants with Al Tubes Table 5.4 shows the summary of above mentioned results.
Table 5.4: Thermal Gradient Values for different Refrigerants with Al Tubes
S.No Refrigerant Thermal Gradient (K/m)
1. R022 5.750E+000
2. R404a 4.782E+000
3. R152a 5.028E+000
4. R134a 5.318E+000
Table 5.5 shows the combined results of above analysis.
Table 5.5: Combined Results of Analysis
S.N o
Tube Materia l
Refriger ant
Heat Flux (W/m
2)
Therma l Gradie nt (K/m) 1. Copper
R022 -
3.571 E-002
4.357E +000 2. Copper
R404a - 4.205 E-002
3.847E +000 3. Copper
R152a - 3.507 E-002
3.084E +000 4. Copper
R134a - 3.067 E-002
3.458E +000 5. Alumin
um R022 -
4.687 E-002
5.750E +000 6. Alumin
um R404a -
1.587 E-002
4.782E +000 7. Alumin
um R152a -
2.327 E-002
5.028E +000 8. Alumin
um R134a -
2.889 E-002
5.318E +000
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE On graphing above results graphically,
following results obtained.
Figure 5.5: Graphical Representation of Results
5.2 DISCUSSION
On investigating the ranks of combinations of tube material and refrigerants, following ranks obtained.
Table 5.6: Ranks of different Tube materials and Refrigerant
Combinations S.
No Tube Mater ial
Refrig erant
Hea t Flux (W/
m2) Ra nk
Therm al Gradie nt (K/m)
Ra nk
1. Coppe
r R022
- 3.57 1E- 002
3 4.357E +000
5
2. Coppe
r R404a
- 4.20 5E- 002
2 3.847E +000
6
3. Coppe
r R152a
- 3.50 7E- 002
4 3.084E +000
8
4. Coppe
r R134a
- 3.06 7E- 002
5 3.458E +000
7
5. Alumi
num R022 - 4.68 7E- 002
1 5.750E +000
1
6. Alumi
num R404a - 1.58 7E- 002
8 4.782E +000
4
7. Alumi
num R152a - 2.32 7E- 002
7 5.028E +000
3
8. Alumi
num R134a - 2.88 9E- 002
6 5.318E +000
2
Above Table tells that no combination of tube materials and refrigerants gave common ranking, due to which it became difficult to choose the appropriate criteria for ranking of different combinations. In this situation, a popular statistical technique called coefficient of variance was used, which is the ratio of ratio of standard deviations and averages of scores of the alternatives in percentages.
Using this approach, the criterion is chosen which shows minimum value of percentage of coefficient of variance. On computing coefficient of variance for different alternatives for different criteria following results were obtained.
Table 5.7: Coefficient of Variance for Different Criteria
S.
No Criter ia
Stand ard Deviat ion of Scores
Aver age of Scor es
Coeffic ient of Varian ce
Rema rks
1. Heat
Flux
0.0099 46
0.032 3
30.791 65
2. Ther
mal Gradi ent
0.9342
63 4.453 20.980
54 Prefer red criteri a for ranki ng From above table it can be realized that as the value of coefficient of variance for criterion thermal gradient is less as compared to the heat flux criterion, it was used for final ranking of combinations of tube materials and refrigerants. In this manner, following rankings of different combinations was obtained.
Table 5.8: Overall Ranking of different Combinations of Tube materials and
Refrigerant S.
No
Tube Materia l
Refrigera nt
Thermal Gradient (K/m)
Ra nk 1. Copper
R022 4.357E+0
00 5
2. Copper
R404a 3.847E+0
00 6
3. Copper
R152a 3.084E+0
00 8
-1.00E+00 0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00 7.00E+00
Copper-R022 Copper-R152a Aluminum-R022 Aluminum-R152a
Heat Flux Thermal Gradient
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE 4. Copper
R134a 3.458E+0
00 7
5. Aluminu
m R022 5.750E+0
00 1
6. Aluminu
m R404a 4.782E+0
00 4
7. Aluminu
m R152a 5.028E+0
00 3
8. Aluminu
m R134a 5.318E+0
00 2
From Table 5.8, following points may be observed.
a) Combination of aluminum and R022a shows the maximum thermal gradient;
b) Combination of aluminum and R134a shows second rank in thermal gradient; and
c) Combination of copper and R152a shows last rank in thermal gradient ranking.
From above analysis one can find that aluminum secures rank 1 for evaporator’s tube material due to its better heat dissipation rate as compared to copper.
In air conditioners, air flows past the evaporator, due to which heat dissipation becomes a deciding parameter, due to which it has scored such a ranking. Heat dissipation rate of aluminum is much more than that of copper, due to its lighter density (2800 kg/m3) as compared to copper (8940 kg/m3). However, Thermal Conductivity of copper is 401 W/m/k, while its value for aluminum is 237 W/m/k. In the research work R022 (1, 1-Difluoroethane) has proven itself better as compared to the other refrigerants.
For this reason, the refrigerant has scored rank 1. But due to its ozone depletion tendency, now a day, government is planning to restrict its use.
Therefore, it cannot be suggested as the best refrigerant. At its place, second ranked alternative, R134a can be chosen due to its eco-friendly behavior. For rank 2 combination of aluminum and R152a has appeared. Literature also shows that R152a performs well as compared to different refrigerants, and it is widely used as a replacement gas for refrigerants like
R134a, R400a and R152a. R152a offers zero ozone depletion rates, global warming potential and a shorter environment life.
For rank 3 combination of
aluminum and R404a has
appeared. R404a or Pentafluoro ethane (HFC-125) is a popular type for refrigerant, which is widely used for air conditioning applications. On ranks 4, 5, 6, and 7 combinations of copper with R022, R404a, R134a and R152a appear.
All these combinations have their advantages and limitations.
Considering above discussion, following are the following rankings of the combinations can be proposed.
Table 5.9: Modified Ranking of different Combinations of Tube
materials and Refrigerant S
. N o
Tube Materi al
Refrige rant
Therma l Gradie nt (K/m)
Ra nk
Modified Ranking
0 Copper
R022 4.357E
+000 5 4
1 Copper
R404a 3.847E
+000 6 5
2 Copper
R152a 3.084E
+000 8 7
3 Copper
R134a 3.458E
+000 7 6
4 Alumi num
R022 5.750E +000 1
Cannot be recomme nded due to harmful environm ental effects 5 Alumi
num R404a 4.782E
+000 4 3
6 Alumi
num R152a 5.028E
+000 3 2
7 Alumi
num R134a 5.318E
+000 2 1
6. CONCLUSION, LIMITATIONS AND FUTURE SCOPE OF THE RESEARCH Present section tells about conclusion, and limitations and future scope of the research work, the details of which are presented in upcoming sub-sections.
6.1 CONCLUSION
Present research work is targeted to the evaporator of an air conditioner for enhanced performance. For this purpose two types of materials for tubes of
Vol.03, Issue 07, July 2018, Available Online: www.ajeee.co.in/index.php/AJEEE evaporator and four types of refrigerants
are considered. The tube materials are copper and aluminum, while the refrigerants are R22, R152a, R134a and R404a. With different combinations of tube materials and refrigerants, computational fluid dynamics analysis of the system is made, and two important properties, heat flux and thermal gradient, are investigated. On getting different rankings of combinations from different criteria, with the help of coefficient of variance, unique rankings of combinations are investigated.
Following are the conclusion of the research work.
a) Combination of aluminum and R022 shows the maximum thermal gradient and scores rank 1, but due to harmful effects of refrigerant it cannot be proposed;
b) Combination of aluminum and R134a is considered as best combination;
c) Combination of aluminum and R152a shows second rank in heat flux flow and scores rank 2; and d) Combination of copper and R152a
shows last rank in temperature gradient ranking.
6.2 LIMITATIONS AND FUTURE SCOPE OF THE RESEARCH
Following are the limitations of research work.
a) The research work is limited to investigations of a particular component of air conditioner;
b) The research work is also limited to investigations about limited properties of the refrigerants; and c) The research work is also limited
to performance evaluation of a small set of tube materials and refrigerants.
Following points indicate the future scope of research work.
a) A research work considering a complete air conditioner assembly may be initiated;
b) A research work can also be initiated which shall consists of a broader set of properties of refrigerants to be investigate; and c) A research work considering
broader sets of tube materials and refrigerants can also be undertaken.
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