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Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles
Applied Thermal Engineering xxx (2014) 1e8
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Research paper
Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles Shuangfeng Wang a, b, *, Hongfeng Ke a, Xuanyou Li b, Song Cheng a a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Industrial Energy Conservation Center of Shandong Academy Science, Jinan 250103, China
h i g h l i g h t s Two kinds of 5 mm finned tubes with different helix angles are proposed. Comparisons are made between systems based on the proposed and conventional tubes. The cycle of the outdoor heat exchanger is adjusted. Material cost of the adjusted air conditioner system is proved less.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2014 Accepted 1 December 2014 Available online xxx
The cycle performance of the novel 5 mm outer diameter (OD) finned tube heat exchangers (FTHX) with different helix angles (1, 6 ) were investigated as indoor unit in split air conditioning systems to be compared with those of the conventional indoor ones (5 mm and 7 mm FTHX with helix angle of 18 ). Conventional T-splitter of the outdoor heat exchanger (HX) was afterward replaced by a Y-splitter and numbers of U-tubes in the two bypasses were adjusted to balance the heat exchange amount of two different bypasses. At first, experimental results showed that the 5 mm system with helix angle of 6 was superior to those 5 mm systems with helix angles of 1 and 18 but still inferior to the conventional 7 mm system, in aspects of capacity and coefficient of performance (COP) both in cooling and heating mode. But after the adjustments, performance of the 5 mm system with helix angle of 6 experienced a 1.5% and 0.7% higher capacity both in cooling and heating mode, respectively, and a 1.7% higher heating COP in spite of a 0.7% lower cooling COP. The system adopting the 5 mm tubes with the helix angle of 6 after adjustments is demonstrated that it can commendably ease material pressure and meanwhile promote the cycle performance. © 2014 Elsevier Ltd. All rights reserved.
Keywords: 5 mm TFHX Helix angle Splitter Capacity COP
1. Introduction With the increasingly rapid development of daily life and urgent demand from human, the refrigerating industry has become more and more eye-catching. Meanwhile, rising demand for copper resources is also forecasted to drive growth in copper consumption in the upcoming years.
* Corresponding author. Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China. Tel.: þ86 020 22236929. E-mail address: [email protected] (S. Wang).
As necessary products in refrigerating industry, air conditioners are utterly dependent on mineral resources due to their excessive material consumption in the air conditioning system, such as joints between critical components in the heat exchanger (HX). With the drastic increase of the complexity of air conditioners, there is no doubt that this dependence on mineral resources will be aggravated [1]. For example, as the shortage risks and prices of copper resources gradually rise, cost pressure from air conditioning enterprises is increasing. To ease the contradiction between supply and demand, the miniaturization of air conditioners has also become a trend to achieve less material consumption. As the top important component of the air conditioner, the HX occupies most room of the air conditioning system. As a result, the main measure to miniaturize the air conditioner is adopting tubes with smaller diameters in the HX.
http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010 1359-4311/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
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To overcome the material problems mentioned above, microchannel HX composed of enhanced tubes with smaller diameters become popular today in the air conditioning industry [2e5]. Among them, many researchers have investigated the cycle performance of the air conditioners with harmonica-shaped tubes [6,7]. However, this kind of HX can be performed well only as evaporator at high temperature environment and would frost in the low temperature in spite of their high efficiency with less material cost and refrigerant charge than the traditional finned tubes [8e10]. In consideration of their serious frosting in the low temperature environment, harmonica-shaped tubes with smaller diameters can hardly replace the conventional tubes in the air conditioning system at present. In contrast with the HX based on the harmonica-shaped tubes, the finned tube heat exchangers (FTHX) can be performed well in both cooling and heating modes. Nowadays, the traditional 7 mm finned tubes have been the most widely applied in the air conditioners as a result of far higher efficiency of heat exchange compared with the traditional smooth tubes [11,12]. However, more attractions have been concentrated on tubes with diameters below 7 mm [13e16], such as the 5 mm tubes which have been investigated considerably at present and are expected to replace the 7 mm in the air conditioning system to achieve less material cost. Kim et al. [17] selected vertical smooth and micro-fin tubes with the outer diameter of 5 mm to measure the evaporative heat transfer coefficient. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). The average evaporation heat transfer coefficients for the micro-fin tube were approximately 111e207% higher than those for the smooth tube at the same test conditions, and the PF was increased from 106 to 123%. Cho et al. [18] measured the heat transfer coefficient data during evaporation process of carbon dioxide for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. They concluded that average evaporation heat transfer coefficients for a microfin tube were approximately 150e200% for 9.52 mm tube and 170e210% for 5 mm tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2e1.35 was smaller than that of heat transfer enhancement. Therefore, developing the 5 mm finned tubes can reduce the usage of raw materials and meanwhile can perform well in different temperature environment. But in recent years, there have been some bottleneck problems in the process of the 5 mm tube application as well. Owing to the bigger frictional resistances of the 5 mm tube compared with the 7 mm one, the performance of the air conditioning system based on 5 mm tubes often has a certain degree of attenuation. The performance decrease principally manifests in the following two aspects. Firstly, in the indoor unit, it is easy to find the reduction of refrigeration capacity when using the 5 mm heat exchanger tube directly to replace the 7 mm one. Secondly, in the outdoor unit, replacing the conventional 7 mm tubes with 5 mm tubes will contribute to insufficient coefficient of performance (COP) or lack of heating capacity. According to the phenomena discussed above, the primary reason why the refrigerating capacity decreases when replacing the 7 mm tubes with the 5 mm is that the mass flow rate of the working fluid will be reduced as a consequence of the greater internal frictional resistance and the smaller heat exchange areas in the HX. In the meantime, the greater flow resistance will result in uneven distribution or premature evaporation of refrigerant, thus the evaporator heat transfer area is not made adequately use of. In brief, several factors included above lead to the decrease of refrigerating capacity. Therefore, it is extraordinarily crucial to solve the problems above and to promote the performance of 5 mm HX tubes simultaneously.
To decrease the performance reduction when using 5 mm finned tubes, many institutes concentrated their attention on the study concerning the internal spiral-grooved tubes. K. Aroonrat et al. [19] investigated heat transfer and flow characteristics of water flowing through horizontal internal spiral-grooved tubes. Their test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. The results showed that the thermal enhancement factor obtained from groove tubes was about 1.4e2.2 for a pitch of 0.5 in.; 1.1e1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove. M.A. Akhavan-Behabadi et al. [20] carried out an experiment to study the heat transfer characteristics during the evaporation of R134a inside a single helical microfin tube with different tube inclination angles. The results demonstrated that the tube inclination angle affected the boiling heat-transfer coefficient in a significant manner. Ding et al. [21] studied two-phase frictional data for R410A-oil mixture flow boiling in an internal spiral grooved microfin tube with outside diameter of 5 mm. In their investigations included above, although the performance of HX made of internal spiral-grooved tubes has been promoted to a certain extent, it is still lower than that of the conventional 7 finned tubes. To further overcome the obstacle of lower performance for HX with internal thread, this paper first considers reducing the internal frictional resistance of the 5 mm internal spiral-grooved tubes through adjusting the parameters of internal thread. Then HX composed of the 5 mm tubes with the internal thread of different helix angles are designed. The performance of the air conditioners based on different internal spiral-grooved tubes and the conventional finned tubes diameters of 7 mm and 5 mm are meanwhile studied as comparisons. Afterwards, the outdoor cycle is adjusted to make sure that two bypasses in the outdoor air conditioning system exchange heat uniformly. This paper can provide authentic information for the development of the new enhanced 5 mm tubes applied in air conditioning systems.
2. Experimental system 2.1. Design of the 5 mm enhanced copper tubes As is shown in Table 1, structural parameters of 5 mm evaluated tube 1, 5 mm evaluated tube 2, the conventional 5 mm and 7 mm tubes are displayed, respectively. Apex angles and helix angles are reduced compared with those of conventional tubes in the experiments, making the internal flow resistance of the HX tubes smaller. From Table 1, it can be seen that helix angles of the 5 mm evaluatedtube 1 and the 5 mm evaluated-tube 2 are 1 and 6 , respectively, which are far less than the traditional 5 mm 18 and the 7 mm 18 . As to addendum angle, the values of two evaluated-tubes are also far smaller than those of the traditional 5 mm and 7 mm ones. Table 1 Structural parameters of different copper tubes. Parameters
Value
Tube Outer diameter (mm) Inner diameter (mm) Bottom wall thickness (mm) Tooth height (mm) Addendum angle ( ) Helix Angle ( ) Article thread number (N) Weight per meter (g/m)
1 2 3 4 5.05 ± 0.05 5.05 ± 0.05 5.05 ± 0.03 7.00 ± 0.03 4.37 ± 0.03 4.37 ± 0.03 4.37 ± 0.03 6.30 ± 0.03 0.20 ± 0.03 0.20 ± 0.03 0.20 ± 0.03 0.25 ± 0.03 0.14 ± 0.02 20 ± 10 1 ± 2 40 34.5 ± 2
0.14 ± 0.02 20 ± 10 6 ± 2 40 34.5 ± 2
0.14 ± 0.02 40 ± 5 18 ± 3 40 34.5 ± 2
0.10 ± 0.02 40 ± 5 16 ± 2 65 52.6 ± 2
1：5 mm evaluated tube 12：5 mm evaluated tube 2. 3：5 mm conventional tube 4：7 mm conventional tube.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
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Fig. 1. The 5 mm indoor heat exchanger: (a) Structure; (b) Physical map.
2.2. Design of the HX with the 5 mm enhanced tubes The structure map and physical map concerning the 5 mm HX used in this experiment are also exhibited in Fig. 1. The 5 mm HX has three flow cycles, in the second of which a U-tube is extracted. In the cooling mode, the refrigerant after throttling flows to each bypass and transfers heat in the bypass indirectly with the air flowing through the fins around the tube. The way of heat exchange between the air and the refrigerant is cross-flow. Refrigerant in the evaporator is evaporated after absorbing heat from the air directly, which contribute to the condensation of the water from the air. Condensed water flows along the fins and the HX tubes and finally down to the water collector under the action of wind. 2.3. Air conditioning system with the designed HX In order to compare the performance differences between the 5 mm and the 7 mm HX, the indoor 5 mm HX and the indoor 7 mm HX both with the same 7 mm outdoor HX are investigated in the experiments. Structure and physical map of the 7 mm indoor HX are shown in Fig. 2. Two bypasses in the 7 mm indoor HX adopted are both composed of 5 U-tubes. In the refrigerating mode,
refrigerant is evaporated in the evaporator after absorbing heat from the air through fins. Water in the air after being condensed flow through evaporator, along fins, then across HX and finally to the water collector by the action of wind. As is exhibited in Fig. 3, the outdoor HX used in the experiment is the 7 mm round finned tube HX, which has two bypasses. The two bypasses will meet at the confluence behind the middle section of the 7 mm outdoor HX. The numbers of U-tubes in the two bypasses in front of the confluence joint are 3, 4, respectively. Behind the confluence joint, the number of U-tubes is 1. The structure sizes and types of fins of indoor and outdoor HX based on different tube diameters are listed in Table 2. All fins are louver fins and the fin height is 1.2 mm. Fins of the indoor HX are double row of which widths are larger than those of the outdoor HX. The indoor HX is in arc shape and its size is measured according to its expanded length, width and height. Joints connecting air conditioners, HX, four-way valves and compressors are all copper tubes. The diameters of liquid line and vapor line are 6 mm and 12 mm, respectively. Connecting tubes and air conditioning components are attached by welding. Compressor in the air conditioning system is model Ruizhi 44R273AK-FESC, of which specifications are presented in Table 3.
Fig. 2. The 7 mm indoor heat exchanger: (a) Structure; (b) Physical map.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
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Fig. 3. The 7 mm outdoor heat exchanger: (a) Structure; (b) Bypass.
Table 2 Structural parameters of heat exchangers and fin types. Parameters
Types of heat exchangers Indoor 5 mm
Indoor 7 mm
Outdoor 7 mm
Size (mm) Tube pitch (mm) Fin width (mm) Types of fins Louver fin height (mm) Fins pitch (mm)
574 266 22 19 22 Louver 1.2
574 299 27.2 21 27.2 Louver 1.65
751 462 18.19 21 18.19 Louver 1.65
1.4
1.4
1.4
3. Testing process The testing process in this experiment is completed in the air enthalpy potential lab according to Chinese standard GB/T7725 [22], which is presented in Fig. 4 [23]. The measurement conditions are exhibited in Table 4. The air enthalpy potential lab includes two rooms, namely the indoor and outdoor conditions. In each room, a set of air conditioning system is fixed to adjust the air conditions. Temperatures controlling the indoor and outdoor conditions are measured by katathermometer. Evaporating temperatures, condensing temperatures and other temperatures in the air conditioning are detected by thermocouples that connect to the computers to acquire data systematically. Air inlet and outlet wet and dry bulb temperature of the indoor and outdoor environment, air outlet temperature after the nozzle were measured by Pt100 with an accuracy of 0.02 C. The air pressure before and behind the nozzle, indoor and outdoor air inlet and outlet pressure are measured by pressure sensors with an accuracy of 0.5%. The discharge temperature, suction gas temperature, evaporating and
4. Analysis of experimental results 4.1. Comparisons of performance in the nominal refrigerating condition
Table 3 Specifications of compressor. Specifications Output volume (cc) Inlet diameters (mm) Outlet diameters (mm) Nominal voltage (V) Frequency (Hz) Oil volume (cc) Refrigerant tolerance (g)
condensing temperature and other temperatures around the heat transfer loops are detected by thermocouples with an accuracy of 0.1 C. To study the performances with different refrigerant charges, a set of precise charging tools is utilized which includes a weighing device with an accuracy of 1 g. Additionally, a power meter with an accuracy of 0.5% were used to measure the power consumption. The uncertainties of capacities and COP in the heating and cooling systems are all less than 5%. Environmental temperatures are controlled by the wet and dry bulb thermometers according to the ASHRAE test condition. The outdoor temperature in the nominal cooling and heating modes is controlled at a dry and wet bulb temperature of 35/24 C and 7/6 C, respectively. The indoor temperature in the nominal cooling and heating mode is controlled at a dry and wet bulb temperature of 27/ 19 C and 20/15 C, respectively. During the experiment, indoor HX are selected three. To begin with, the air conditioning systems assembled with three HX based on 5 mm conventional tubes is installed in the air enthalpy potential lab in the nominal condition T1. Then the charge amount of refrigerant is gradually adjusted to obtain the ideal refrigerating cycle. Afterwards, in the heating mode, similarly, the charge amount of refrigerant is gradually adjusted to obtain the ideal heating cycle in the nominal temperature T2. Finally, other components in the air conditioners are kept unchanged and the indoor HX are substituted by 5 mm evaluated-tubes 1 to get the ideal cycles in the refrigerating and heating modes, respectively. The same procedures are done for the 5 mm HX based on evaluatedtubes 2 and 7 mm conventional one. The data acquisition process is conducted after the system has maintained stability over an hour.
10.8 12.8 8.1 220e240 50 270 ± 10 850
From Figs. 5e7, it can be seen that performance trends of air conditioning systems assembled with different indoor HX are almost similar. With the increase of the refrigerant charge, cooling capacity and COP firstly increase, then continue increasing with a smaller trend and finally decrease. Power consumptions of compressors in all systems increase with the increase of refrigerant charge.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
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5
Fig. 4. Schematic diagram of the test rig.
Table 4 Experimental test conditions. Conditions
Air temperature indoor ( C) Dry bulb
Wet bulb
Dry bulb
Wet bulb
Nominal cooling (T1) Nominal heating (T2)
27 20
19 15
35 7
24 6
Air temperature outdoor ( C)
2850
1220
2800
1210
2750
1200
Power consumption (W)
Capacity (W)
From Figs. 5 and 7, it can also be found that for the system with the 5 mm indoor HX, its refrigerating capacity and COP peaks both appear at the refrigerant charge of 600 g, while the refrigerating capacity and COP both reach their maximums at the charge of 660 g for the 7 mm indoor HX. Reasons contributing to the difference above can be ascribed to the varying conditions inside the indoor HX. When the 7 mm HX is used as the indoor HX in the air conditioning system, the space in the evaporator for the refrigerant to evaporate is larger and the pressure drop is much smaller than that of the 5 mm indoor HX. Therefore, the refrigerant can be evaporated more easily in the 7 mm evaporator. However, when the 5 mm HX is used as the evaporator in the indoor system, the space for the refrigerant to be evaporated is smaller, leading to bigger evaporation pressure and larger flow resistance for the refrigerant. As a result, it's harder for the refrigerant vapor to evaporate and
then refrigerant gas superfluous refrigerant liquid accumulates in the condenser because of the pressure loss from the accumulation of refrigerant vapor in the evaporator. Furthermore, the mass of the evaporated refrigerant in the evaporator decreases drastically and too little refrigerant vapor circulates back to the compressor as well, which contributes to increase of the power consumption of the compressor and in turn reduces the refrigerating capacity in the air conditioning system. Therefore, for the 5 mm air conditioning system, when the refrigerant charge is 660 g, the cycle performance is instead lower compared with the traditional 7 mm one. When refrigerant charge is decreased to 600 g, the refrigerant does not accumulate in the condenser excessively and can be evaporated adequately in the evaporator after throttling. Therefore, 600 g is the most experimentally appropriate charge value to acquire the best performance parameters for the 5 mm system. As exhibited in Figs. 5 and 7, it can be seen that the peak of capacity curve and COP curve corresponds with the best cycle of the air conditioning system. The best performance parameters of every HX in the refrigerating mode are presented in Table 5. For the cycle performance in the cooling mode, the lowest is the 5 mm evaluated-tube 1 of which capacity and COP are 2.9% and 2.2% lower than the conventional 5 mm with angle of 18 , respectively. For the evaluated-tube 2, its best performance parameters are better than the 5 mm conventional system in small extent, which is 0.6% higher in capacity and 1.2% higher in COP. Compared with the
2700 2650 2600 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2550 2500 2450 2400
500
550
600
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
650
Charge (g)
700
Fig. 5. Cooling capacity of different refrigerating systems.
1190 1180 1170 1160
5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
1150 1140 1130
750
500
550
600
650
Charge (g)
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
700
750
Fig. 6. Power consumption of different refrigerating systems.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
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2950
2.40
2900
2.35
Capacity (W)
2850
COP
2.30 2.25 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2.20 2.15 500
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
550
600
Charge (g)
2800 2750 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2700 2650 2600
650
500
550
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
600
Charge (g)
650
700
700 Fig. 8. Capacity of different heating system.
Fig. 7. COP of different refrigerating systems.
4.2. Comparisons of performance in the nominal heating condition As are shown in Figs. 8e10, in the heating mode, with the increase of the refrigerant charge, the COP and capacity of the system firstly increase, then continue to increase at a smaller extent and finally begin to decrease. The trends of COP curves and capacity curves are almost similar to those in the refrigerating mode. However, power consumption in the heating mode keeps rising with the increment of the charge. For the 5 mm system, the most efficient cycle appears at the charge of 600 g. But for the 7 mm system, the most efficient cycle appears when the charge is 660 g.
Table 5 Best performance parameters in the refrigerating mode.
5 5 5 7
mm mm mm mm
1 6 18 18
1
2
3
4
5
6
7
8
9
2731 2832 2813 2839
2.32 2.40 2.37 2.41
1176 1182 1187 1177
85.7 86 84.6 83.6
7 12.7 14.6 12.7
78.7/77.7 81.5/81.8 80.7/80.1 81.5/81.8
49.7/38.9 49.0/39.3 48.9/38.9 49.0/39.3
38.4 38.4 37.5 37.8
600 600 600 660
1- Capacity (W); 2-COP; 3-Power (W); 4-Temperature of vapor outside ( C); 5Temperature of vapor inside ( C); 6-Inlet temperature of condenser ( C); 7-Outlet temperature of branch ( C); 8-Outlet temperature of condenser ( C); 9-Mass of refrigerant (g).
5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
1040
Power consumption (W)
conventional 5 mm, even though the 5 mm evaluated-tube 2 shows improvement in system performance, it still has disadvantage performance compared with the traditional 7 mm (capacity 0.3% lower, COP 0.4% lower). From Table 5, it can be found that power consumption, vapor temperature and condensation temperature are all lower in the 5 mm system with angle of 1 than those in the other systems included. The reason is that the pressure drop in the HX with helix angle of 1 is larger than that with other angles. Compared with the 5 mm system with helix angle of 1, the 7 mm system with 18 has almost equal power consumption due to the pressure drop from the decrease of the tube diameter. What can be inferred from analysis above is that for the 5 mm HX with angle of 1, refrigerant fails to stay in the HX for long due to the low pressure drop, which finally leads to the lower capacity for the system. However, when the helix angle of the 5 mm is 6 , the pressure drop of refrigerant decreases partially while the capacity of the system with moderate amount of charge does not decrease, leading to the improvement of COP.
Parameters corresponding with the best cycle performance in air conditioning systems with different HX are recorded in Table 6. From the experimental data in Table 6, it can be seen that cycle performance of the 5 mm system with helix angle of 1 is the worst. In the heating mode, the capacity and COP of the 5 mm 6 system based on the 5 mm HX with angle of 6 are respectively slightly 0.2% and 0.7% higher compared with the conventional 5 mm 18 system but are still 1.2% and 0.7% lower than those of the conventional 7 mm 18 one. Additionally, the outlet temperature of the 5 mm 1 system is lower than those of the other systems in the heating mode. By contrast, outlet temperature of the 7 mm is the highest in the heating mode. For the 5 mm 1 air conditioning system, internal pressure drop is the lowest and heat exchange ability of the indoor HX as condenser is also the lowest of all. Refrigerant not condensed totally in the 5 mm 1 system flows to the evaporator and results in weaker heat exchange ability of the evaporator, finally contributing to the lower performance of the system. The power consumption of the 7 mm is the highest in the heating mode instead, which shows obvious difference from that in the cooling mode. The reason contributing to the phenomena above is as follows: In the heating mode, the indoor system is used as condenser. The pressure drop of the refrigerant vapor that flows along the cycle to the condenser is far greater than that of the refrigerant liquid in the refrigerating mode. Nonetheless, the
1020
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
1000
980
960
940
500
550
600
650
Charge (g)
700
750
Fig. 9. Power consumption of different heating system.
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7
3.00 2.96 2.92 2.88
COP
2.84 2.80 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2.76 2.72 2.68 500
550
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
600
650
Charge (g)
700
750
Fig. 10. COP of different heating system.
refrigerant charge of the 7 mm HX is more than that of the 5 mm, leading to larger pressure drop in the 7 mm system. Finally, the power consumption is far larger than that of the 5 mm system. According to the performance of the system both in the cooling and heating mode, it can be concluded that the 5 mm evaluatedtube 2 is superior to the 5 mm 18 one but still inferior to the 7 mm both in the capacity and COP. From Tables 5 and 6, it is easy to notice that temperature changes are evident after refrigerant flows past two cycles through the condenser or evaporator in the outdoor system, reaching 10 C in the refrigerating mode and 5 C in the heating mode. The reason is that uneven heat exchange in the two bypasses results in the reduction of the system performance. For example, in the refrigerating mode, the refrigerant temperatures in the inlet and outlet are 81.5 C and 49 C in the 3-U bypass for the 5 mm evaluated-tube 2, respectively. However, refrigerant temperatures in the inlet and outlet are 81.8 C and 39.3 C in another 4-U bypass, which shows nearly 10 C difference in the outlet in these two bypasses. In the 4 U-tubes bypass, refrigerant has been totally condensed to the state of being undercooled. After the undercooled refrigerant mixes with the refrigerant not undercooled from another bypass, the degree of undercooling decreases. Though the refrigerant from two bypasses converges and flows through another U-tube, the best cycle performance of the system does not appear. Furthermore, the refrigerant lightly undercooled flows to the evaporator, leading to the higher evaporating temperature of the refrigerant. The evaporating pressure of the refrigerant decreases due to higher evaporating temperature, which meanwhile results to the lower cycle performance of the system. Accordingly, temperatures of the refrigerant meeting from two bypasses must be controlled to guarantee the least power consumption and the best cycle performance.
Fig. 11. Bypasses of the outdoor heat exchanger: (a) Before adjustment; (b) After adjustment.
4.3. Experimental study on the adjusted cycle of the outdoor condenser When the tube diameter is changed from 7 mm to 5 mm, pressure drop will increase resulting from the geometry change and it becomes more difficult for the refrigerant inside the tube to be evaporated. Furthermore, the excessive pressure drop will reduce the flow rate of refrigerant and finally lead to a decline of refrigerating capacity. Therefore, when the 5 mm HX with the same cycle of the 7 mm HX is used, refrigeration capacity will inevitably fall sharply. Accordingly, the number of bypasses in this 5 mm HX is increased to proper extent, which will decrease the refrigerant flow rate correspondingly and shorten the process of every bypass. The reduction of frictional resistance and pressure drop resulting from the decrease of refrigerant flow rate will solve the problem to a certain extent appropriately when using 5 mm copper tubes. In order to guarantee uniform heat exchange of two bypasses in the outdoor system, the cycle of the outdoor HX after adjustment are investigated experimentally. As is presented in Fig. 11, T-splitter in the outdoor HX is changed to the Y-splitter. Besides, the numbers of bypasses before confluence and U-tubes after confluence are adjusted to 3, 3 and 2, respectively. Heating and refrigerating experiments based on the system including the 5 mm indoor HX and the adjusted outdoor HX are carried out in the air enthalpy potential lab. Capillary charge and refrigerant charge are both
Table 6 Best performance parameters in the heating mode.
5 5 5 7
mm mm mm mm
1 6 18 18
1
2
3
4
5
6
7
8
9
2871 2922 2915 2957
2.93 2.96 2.94 2.98
980 987 990 992
51.8 59.7 57.2 66.7
2.2 2.8 2.7 2
0.5/6.7 0.2/6.3 0.0/6.3 0.5/6.8
2.5/2.0 1.2/0.9 1.2/1.0 1.1/1.1
3.2 1.7 1.7 1.5
600 600 600 660
1- Capacity (W); 2-COP; 3-Power (W); 4-Temperature of vapor outside( C); 5Temperature of vapor inside ( C); 6-Inlet temperature of condenser ( C); 7-Outlet temperature of branch ( C); 8-Outlet temperature of condenser ( C); 9-Mass of refrigerant (g).
Table 7 Performance parameters after the adjustment.
Cooling Heating
1
2
3
4
5
6
7
8
9
2882 2978
2.45 2.96
1177 1007
84.3 62.9
13.6 1.9
79.1/79.3 0.7/1.0
49.2/46.8 2.1/2.0
37 4.2
600 600
1- Capacity (W); 2-COP; 3-Power (W); 4-Temperature of vapor outside( C); 5Temperature of vapor inside ( C); 6-Inlet temperature of condenser ( C); 7-Outlet temperature of branch ( C); 8-Outlet temperature of condenser ( C); 9-Mass of refrigerant (g).
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according to the original parameters with the maximum performance. Experimental results are exhibited in Table 7. As can be seen from the data in Table 7, the performance of air conditioning system with the 5 mm evaluated-tube 2 after adjustment increases in all aspects compared with the original 5 mm evaluated-tube 2. In the refrigerating mode, the capacity and COP increase by 1.7% and 2.1%, respectively. In the heating mode, the COP is almost equal to that of the original system. However, the capacity increases by 1.9%. Furthermore, compared with the conventional 7 mm indoor system, it can be found that the capacity increases by 1.5% and 0.7% in the cooling and heating mode, respectively. Even though the COP is slightly 0.7% lower than the conventional 7 mm in the heating mode, the COP is 1.7% higher in the refrigerating mode. From the experiment, it can be also seen that temperature difference between inlets and outlets decreases in the two bypasses, indicating that adjustments on the cycle not only narrow the difference of heat exchange in the two bypasses but also improve the performance of the system. 5. Conclusions In this paper, the 5 mm FTHX with different internal thread parameters and traditional 7 mm FTHX are investigated as indoor unit of air conditioners with a same outdoor unit to evaluate their performance. Appropriate adjustments to the outdoor HX are also afterward carried out to study the performance changes. Based on the experimental investigations, the following conclusions could be drawn. In the refrigerating mode, compared with the traditional 5 mm 18 system, the capacity and COP of the 5 mm evaluated-tube 1 system is the lowest (2.9% lower and 2.2% lower than that of the 5 mm 18 system, respectively). The 5 mm evaluated-tube 2 system acquires slightly better performance than the 5 mm 18 , namely 0.6 higher% capacity and 1.2% higher COP, however, which is still 0.3% lower in capacity and 0.4% lower in COP comparing with the conventional 7 mm system. In the heating mode, the system performance of the 5 mm evaluated-tube 1 is also the lowest. As to the 5 mm evaluated-tube 2 system, compared with the performance of the conventional 5 mm 18 HX system, it is 0.2% and 0.7% higher, respectively, in capacity and COP but is still 1.2% and 0.7% lower than the conventional 7 mm 18 . For the 5 mm evaluated-tube 2 system, after the adjustment changing T-splitter to the Y-splitter and adjusting the numbers of bypasses and U-tubes of the outdoor HX, the cycle performance is obviously improved. In the refrigerating mode, it is improved by 1.7% in capacity and 2.1% in COP compared with the original 5 mm evaluated-tube 2. Though its COP is close to the original 5 mm, the capacity has an increase by 1.9% in heating. Compared with the conventional 7 mm indoor air system, the capacity is 1.5% higher in the refrigerating mode and 0.7% higher in the heating mode, respectively. Even though the COP of the adjusted system in the heating mode is slightly 0.7% lower than the conventional 7 mm, the COP in the cooling mode is obvious 1.7% higher instead. Conclusively, the air conditioning system based on the HX composed of the 5 mm 6 tubes after appropriate adjustments is experimentally proved that it can save the material cost and meanwhile guarantee the cycle performance of the air conditioning system to improve the performance price ratio.
Acknowledgements The authors gratefully acknowledge the financial support for this project from National Natural Science Foundation of China (Granted No. 51476059 and No. 51276068) and International Cooperation and Exchange Program from the Ministry of Science and Technology of China (Grant No. 2013DFG60080). References [1] Mineral Commodity Summaries, United States Geological Survey, GPO, Washington, DC, 2011. [2] R. Yun, Y.C. Kim, C. Park, Numerical analysis on a microchannel evaporator designed for CO2 air-conditioning systems, Appl. Therm. Eng. 27 (2007) 1320e1326. [3] L.L. Shao, L. Yang, C.L. Zhang, Comparison of heat pump performance using finand-tube and microchannel heat exchangers under frost conditions, Appl. Energy 87 (2010) 1187e1197. [4] H.L. Wang, R.B. Peterson, Performance enhancement of a thermally activated cooling system using microchannel heat exchangers, Appl. Therm. Eng. 31 (2011) 2951e2962. [5] K. Kim, M.H. Kim, D.R. Kim, K.S. Lee, Thermal performance of microchannel heat exchangers according to the design parameters under the frosting conditions, Int. J. Heat Mass Transfer 71 (2014) 626e632. [6] L. Chen, Z. Li, Z.Y. Guo, Experimental investigation of plastic finned-tube heat exchangers, with emphasis on material thermal conductivity, Exp. Therm. Fluid Sci. 33 (2009) 922e928. [7] A.A. Sertkaya, K. Altınısık, K. Dincer, Experimental investigation of thermal performance of aluminum finned heat exchangers and open-cell aluminum foam heat exchangers, Exp. Therm. Fluid Sci. 36 (2012) 86e92. [8] W.M. Yan, H.Y. Li, Performance of finned tube heat exchangers operating under frosting conditions, Int. J. Heat Mass Transfer 46 (2003) 871e877. [9] X.M. Guo, Y.G. Chen, Experimental study on frost growth and dynamic performance of air source heat pump system, Appl. Therm. Eng. 28 (2008) 2267e2278. [10] G.B. Zhou, Y.F. Zhang, Performance of a split-type air conditioner matched with coiled adiabatic capillary tubes using HCFC22 and HC290, Appl. Energy 87 (2010) 1522e1528. [11] J.H. Masliyah, K. Nandakumar, Heat Transfer in internally finned tubes, J. Heat Transfer 98 (2) (1976) 257e261. [12] D. Han, K.J. Lee, Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes, Int. J. Heat. Mass Transfer 48 (2005) 3804e3816. [13] Y.Y. Yan, T.F. Lin, Evaporation heat transfer and pressure drop of refrigerant R134a in a small pipe, Int. J. Heat Mass Transfer (1998) 4183e4194. [14] X.C. Huang, G.L. Ding, Condensation heat transfer characteristics of R410Aeoil mixture in 5 mm and 4 mm outside diameter horizontal microfin tubes, Exp. Therm. Fluid Sci. 34 (2010) 845e856. [15] S. Mancin, A. Diani, L. Rossetto, R134a flow boiling heat Transfer and pressure drop inside a 3.4 mm ID MicrofinTube, Energy Proc. 45 (2014) 608e615. [16] S. Cheng, S.F. Wang, Z.M. Liu, Cycle performance of alternative refrigerants for domestic air-conditioning system based on a small finned tube heat exchanger, Appl. Therm. Eng. 64 (2014) 83e92. [17] Y.J. Kim, J.M. Cho, M.S. Kim, Experimental study on the evaporative heat transfer and pressure drop of CO2 flowing upward in vertical smooth and micro-fin tubes with the diameter of 5 mm, Int. J. Refrig. 31 (2008) 771e779. [18] J.M. Cho, M.S. Kim, Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm, Int. J. Refrig. 30 (2007) 986e994. [19] K. Aroonrat, C. Jumpholkul, R. Leelaprachakul, A.S. Dalkilic, O. Mahian, S. Wongwises, Heat transfer and single-phase flow in internally grooved tubes, Int. Commun. Heat Mass Transfer 42 (2013) 62e68. [20] M.A. Akhavan-Behabadi, S.G. Mohseni, S.M. Razavinasab, Evaporation heat transfer of R-134a inside a microfin tube with different tube inclinations, Exp. Therm. Fluid Sci. 35 (2011) 996e1001. [21] G.L. Ding, H.T. Hu, X.C. Huang, B. Deng, Y.F. Gao, Experimental investigation and correlation of two-phase frictional pressure drop of R410Aeoil mixture flow boiling in a 5 mm microfin tube, Int. J. Refrig. 32 (2009) 150e161. [22] Chinese Standard, GB/T 7725, Room Air Conditioners, General Administration of Quality and Supervision, Inspection and Quarantine of the People's Republic of China and Standardization Administration of the People's Republic of China, Beijing, China, 2004. [23] S.F. Wang, S. Cheng, H.M. Yu, Z.H. Rao, Z.M. Liu, Experimental investigation of AleCu composed tubeefin heat exchangers for air conditioner, Exp. Therm. Fluid Sci. 51 (2013) 264e270.
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Research paper
Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles Shuangfeng Wang a, b, *, Hongfeng Ke a, Xuanyou Li b, Song Cheng a a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Industrial Energy Conservation Center of Shandong Academy Science, Jinan 250103, China
h i g h l i g h t s Two kinds of 5 mm finned tubes with different helix angles are proposed. Comparisons are made between systems based on the proposed and conventional tubes. The cycle of the outdoor heat exchanger is adjusted. Material cost of the adjusted air conditioner system is proved less.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2014 Accepted 1 December 2014 Available online xxx
The cycle performance of the novel 5 mm outer diameter (OD) finned tube heat exchangers (FTHX) with different helix angles (1, 6 ) were investigated as indoor unit in split air conditioning systems to be compared with those of the conventional indoor ones (5 mm and 7 mm FTHX with helix angle of 18 ). Conventional T-splitter of the outdoor heat exchanger (HX) was afterward replaced by a Y-splitter and numbers of U-tubes in the two bypasses were adjusted to balance the heat exchange amount of two different bypasses. At first, experimental results showed that the 5 mm system with helix angle of 6 was superior to those 5 mm systems with helix angles of 1 and 18 but still inferior to the conventional 7 mm system, in aspects of capacity and coefficient of performance (COP) both in cooling and heating mode. But after the adjustments, performance of the 5 mm system with helix angle of 6 experienced a 1.5% and 0.7% higher capacity both in cooling and heating mode, respectively, and a 1.7% higher heating COP in spite of a 0.7% lower cooling COP. The system adopting the 5 mm tubes with the helix angle of 6 after adjustments is demonstrated that it can commendably ease material pressure and meanwhile promote the cycle performance. © 2014 Elsevier Ltd. All rights reserved.
Keywords: 5 mm TFHX Helix angle Splitter Capacity COP
1. Introduction With the increasingly rapid development of daily life and urgent demand from human, the refrigerating industry has become more and more eye-catching. Meanwhile, rising demand for copper resources is also forecasted to drive growth in copper consumption in the upcoming years.
* Corresponding author. Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China. Tel.: þ86 020 22236929. E-mail address: [email protected] (S. Wang).
As necessary products in refrigerating industry, air conditioners are utterly dependent on mineral resources due to their excessive material consumption in the air conditioning system, such as joints between critical components in the heat exchanger (HX). With the drastic increase of the complexity of air conditioners, there is no doubt that this dependence on mineral resources will be aggravated [1]. For example, as the shortage risks and prices of copper resources gradually rise, cost pressure from air conditioning enterprises is increasing. To ease the contradiction between supply and demand, the miniaturization of air conditioners has also become a trend to achieve less material consumption. As the top important component of the air conditioner, the HX occupies most room of the air conditioning system. As a result, the main measure to miniaturize the air conditioner is adopting tubes with smaller diameters in the HX.
http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010 1359-4311/© 2014 Elsevier Ltd. All rights reserved.
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To overcome the material problems mentioned above, microchannel HX composed of enhanced tubes with smaller diameters become popular today in the air conditioning industry [2e5]. Among them, many researchers have investigated the cycle performance of the air conditioners with harmonica-shaped tubes [6,7]. However, this kind of HX can be performed well only as evaporator at high temperature environment and would frost in the low temperature in spite of their high efficiency with less material cost and refrigerant charge than the traditional finned tubes [8e10]. In consideration of their serious frosting in the low temperature environment, harmonica-shaped tubes with smaller diameters can hardly replace the conventional tubes in the air conditioning system at present. In contrast with the HX based on the harmonica-shaped tubes, the finned tube heat exchangers (FTHX) can be performed well in both cooling and heating modes. Nowadays, the traditional 7 mm finned tubes have been the most widely applied in the air conditioners as a result of far higher efficiency of heat exchange compared with the traditional smooth tubes [11,12]. However, more attractions have been concentrated on tubes with diameters below 7 mm [13e16], such as the 5 mm tubes which have been investigated considerably at present and are expected to replace the 7 mm in the air conditioning system to achieve less material cost. Kim et al. [17] selected vertical smooth and micro-fin tubes with the outer diameter of 5 mm to measure the evaporative heat transfer coefficient. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). The average evaporation heat transfer coefficients for the micro-fin tube were approximately 111e207% higher than those for the smooth tube at the same test conditions, and the PF was increased from 106 to 123%. Cho et al. [18] measured the heat transfer coefficient data during evaporation process of carbon dioxide for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. They concluded that average evaporation heat transfer coefficients for a microfin tube were approximately 150e200% for 9.52 mm tube and 170e210% for 5 mm tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2e1.35 was smaller than that of heat transfer enhancement. Therefore, developing the 5 mm finned tubes can reduce the usage of raw materials and meanwhile can perform well in different temperature environment. But in recent years, there have been some bottleneck problems in the process of the 5 mm tube application as well. Owing to the bigger frictional resistances of the 5 mm tube compared with the 7 mm one, the performance of the air conditioning system based on 5 mm tubes often has a certain degree of attenuation. The performance decrease principally manifests in the following two aspects. Firstly, in the indoor unit, it is easy to find the reduction of refrigeration capacity when using the 5 mm heat exchanger tube directly to replace the 7 mm one. Secondly, in the outdoor unit, replacing the conventional 7 mm tubes with 5 mm tubes will contribute to insufficient coefficient of performance (COP) or lack of heating capacity. According to the phenomena discussed above, the primary reason why the refrigerating capacity decreases when replacing the 7 mm tubes with the 5 mm is that the mass flow rate of the working fluid will be reduced as a consequence of the greater internal frictional resistance and the smaller heat exchange areas in the HX. In the meantime, the greater flow resistance will result in uneven distribution or premature evaporation of refrigerant, thus the evaporator heat transfer area is not made adequately use of. In brief, several factors included above lead to the decrease of refrigerating capacity. Therefore, it is extraordinarily crucial to solve the problems above and to promote the performance of 5 mm HX tubes simultaneously.
To decrease the performance reduction when using 5 mm finned tubes, many institutes concentrated their attention on the study concerning the internal spiral-grooved tubes. K. Aroonrat et al. [19] investigated heat transfer and flow characteristics of water flowing through horizontal internal spiral-grooved tubes. Their test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. The results showed that the thermal enhancement factor obtained from groove tubes was about 1.4e2.2 for a pitch of 0.5 in.; 1.1e1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove. M.A. Akhavan-Behabadi et al. [20] carried out an experiment to study the heat transfer characteristics during the evaporation of R134a inside a single helical microfin tube with different tube inclination angles. The results demonstrated that the tube inclination angle affected the boiling heat-transfer coefficient in a significant manner. Ding et al. [21] studied two-phase frictional data for R410A-oil mixture flow boiling in an internal spiral grooved microfin tube with outside diameter of 5 mm. In their investigations included above, although the performance of HX made of internal spiral-grooved tubes has been promoted to a certain extent, it is still lower than that of the conventional 7 finned tubes. To further overcome the obstacle of lower performance for HX with internal thread, this paper first considers reducing the internal frictional resistance of the 5 mm internal spiral-grooved tubes through adjusting the parameters of internal thread. Then HX composed of the 5 mm tubes with the internal thread of different helix angles are designed. The performance of the air conditioners based on different internal spiral-grooved tubes and the conventional finned tubes diameters of 7 mm and 5 mm are meanwhile studied as comparisons. Afterwards, the outdoor cycle is adjusted to make sure that two bypasses in the outdoor air conditioning system exchange heat uniformly. This paper can provide authentic information for the development of the new enhanced 5 mm tubes applied in air conditioning systems.
2. Experimental system 2.1. Design of the 5 mm enhanced copper tubes As is shown in Table 1, structural parameters of 5 mm evaluated tube 1, 5 mm evaluated tube 2, the conventional 5 mm and 7 mm tubes are displayed, respectively. Apex angles and helix angles are reduced compared with those of conventional tubes in the experiments, making the internal flow resistance of the HX tubes smaller. From Table 1, it can be seen that helix angles of the 5 mm evaluatedtube 1 and the 5 mm evaluated-tube 2 are 1 and 6 , respectively, which are far less than the traditional 5 mm 18 and the 7 mm 18 . As to addendum angle, the values of two evaluated-tubes are also far smaller than those of the traditional 5 mm and 7 mm ones. Table 1 Structural parameters of different copper tubes. Parameters
Value
Tube Outer diameter (mm) Inner diameter (mm) Bottom wall thickness (mm) Tooth height (mm) Addendum angle ( ) Helix Angle ( ) Article thread number (N) Weight per meter (g/m)
1 2 3 4 5.05 ± 0.05 5.05 ± 0.05 5.05 ± 0.03 7.00 ± 0.03 4.37 ± 0.03 4.37 ± 0.03 4.37 ± 0.03 6.30 ± 0.03 0.20 ± 0.03 0.20 ± 0.03 0.20 ± 0.03 0.25 ± 0.03 0.14 ± 0.02 20 ± 10 1 ± 2 40 34.5 ± 2
0.14 ± 0.02 20 ± 10 6 ± 2 40 34.5 ± 2
0.14 ± 0.02 40 ± 5 18 ± 3 40 34.5 ± 2
0.10 ± 0.02 40 ± 5 16 ± 2 65 52.6 ± 2
1：5 mm evaluated tube 12：5 mm evaluated tube 2. 3：5 mm conventional tube 4：7 mm conventional tube.
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Fig. 1. The 5 mm indoor heat exchanger: (a) Structure; (b) Physical map.
2.2. Design of the HX with the 5 mm enhanced tubes The structure map and physical map concerning the 5 mm HX used in this experiment are also exhibited in Fig. 1. The 5 mm HX has three flow cycles, in the second of which a U-tube is extracted. In the cooling mode, the refrigerant after throttling flows to each bypass and transfers heat in the bypass indirectly with the air flowing through the fins around the tube. The way of heat exchange between the air and the refrigerant is cross-flow. Refrigerant in the evaporator is evaporated after absorbing heat from the air directly, which contribute to the condensation of the water from the air. Condensed water flows along the fins and the HX tubes and finally down to the water collector under the action of wind. 2.3. Air conditioning system with the designed HX In order to compare the performance differences between the 5 mm and the 7 mm HX, the indoor 5 mm HX and the indoor 7 mm HX both with the same 7 mm outdoor HX are investigated in the experiments. Structure and physical map of the 7 mm indoor HX are shown in Fig. 2. Two bypasses in the 7 mm indoor HX adopted are both composed of 5 U-tubes. In the refrigerating mode,
refrigerant is evaporated in the evaporator after absorbing heat from the air through fins. Water in the air after being condensed flow through evaporator, along fins, then across HX and finally to the water collector by the action of wind. As is exhibited in Fig. 3, the outdoor HX used in the experiment is the 7 mm round finned tube HX, which has two bypasses. The two bypasses will meet at the confluence behind the middle section of the 7 mm outdoor HX. The numbers of U-tubes in the two bypasses in front of the confluence joint are 3, 4, respectively. Behind the confluence joint, the number of U-tubes is 1. The structure sizes and types of fins of indoor and outdoor HX based on different tube diameters are listed in Table 2. All fins are louver fins and the fin height is 1.2 mm. Fins of the indoor HX are double row of which widths are larger than those of the outdoor HX. The indoor HX is in arc shape and its size is measured according to its expanded length, width and height. Joints connecting air conditioners, HX, four-way valves and compressors are all copper tubes. The diameters of liquid line and vapor line are 6 mm and 12 mm, respectively. Connecting tubes and air conditioning components are attached by welding. Compressor in the air conditioning system is model Ruizhi 44R273AK-FESC, of which specifications are presented in Table 3.
Fig. 2. The 7 mm indoor heat exchanger: (a) Structure; (b) Physical map.
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Fig. 3. The 7 mm outdoor heat exchanger: (a) Structure; (b) Bypass.
Table 2 Structural parameters of heat exchangers and fin types. Parameters
Types of heat exchangers Indoor 5 mm
Indoor 7 mm
Outdoor 7 mm
Size (mm) Tube pitch (mm) Fin width (mm) Types of fins Louver fin height (mm) Fins pitch (mm)
574 266 22 19 22 Louver 1.2
574 299 27.2 21 27.2 Louver 1.65
751 462 18.19 21 18.19 Louver 1.65
1.4
1.4
1.4
3. Testing process The testing process in this experiment is completed in the air enthalpy potential lab according to Chinese standard GB/T7725 [22], which is presented in Fig. 4 [23]. The measurement conditions are exhibited in Table 4. The air enthalpy potential lab includes two rooms, namely the indoor and outdoor conditions. In each room, a set of air conditioning system is fixed to adjust the air conditions. Temperatures controlling the indoor and outdoor conditions are measured by katathermometer. Evaporating temperatures, condensing temperatures and other temperatures in the air conditioning are detected by thermocouples that connect to the computers to acquire data systematically. Air inlet and outlet wet and dry bulb temperature of the indoor and outdoor environment, air outlet temperature after the nozzle were measured by Pt100 with an accuracy of 0.02 C. The air pressure before and behind the nozzle, indoor and outdoor air inlet and outlet pressure are measured by pressure sensors with an accuracy of 0.5%. The discharge temperature, suction gas temperature, evaporating and
4. Analysis of experimental results 4.1. Comparisons of performance in the nominal refrigerating condition
Table 3 Specifications of compressor. Specifications Output volume (cc) Inlet diameters (mm) Outlet diameters (mm) Nominal voltage (V) Frequency (Hz) Oil volume (cc) Refrigerant tolerance (g)
condensing temperature and other temperatures around the heat transfer loops are detected by thermocouples with an accuracy of 0.1 C. To study the performances with different refrigerant charges, a set of precise charging tools is utilized which includes a weighing device with an accuracy of 1 g. Additionally, a power meter with an accuracy of 0.5% were used to measure the power consumption. The uncertainties of capacities and COP in the heating and cooling systems are all less than 5%. Environmental temperatures are controlled by the wet and dry bulb thermometers according to the ASHRAE test condition. The outdoor temperature in the nominal cooling and heating modes is controlled at a dry and wet bulb temperature of 35/24 C and 7/6 C, respectively. The indoor temperature in the nominal cooling and heating mode is controlled at a dry and wet bulb temperature of 27/ 19 C and 20/15 C, respectively. During the experiment, indoor HX are selected three. To begin with, the air conditioning systems assembled with three HX based on 5 mm conventional tubes is installed in the air enthalpy potential lab in the nominal condition T1. Then the charge amount of refrigerant is gradually adjusted to obtain the ideal refrigerating cycle. Afterwards, in the heating mode, similarly, the charge amount of refrigerant is gradually adjusted to obtain the ideal heating cycle in the nominal temperature T2. Finally, other components in the air conditioners are kept unchanged and the indoor HX are substituted by 5 mm evaluated-tubes 1 to get the ideal cycles in the refrigerating and heating modes, respectively. The same procedures are done for the 5 mm HX based on evaluatedtubes 2 and 7 mm conventional one. The data acquisition process is conducted after the system has maintained stability over an hour.
10.8 12.8 8.1 220e240 50 270 ± 10 850
From Figs. 5e7, it can be seen that performance trends of air conditioning systems assembled with different indoor HX are almost similar. With the increase of the refrigerant charge, cooling capacity and COP firstly increase, then continue increasing with a smaller trend and finally decrease. Power consumptions of compressors in all systems increase with the increase of refrigerant charge.
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5
Fig. 4. Schematic diagram of the test rig.
Table 4 Experimental test conditions. Conditions
Air temperature indoor ( C) Dry bulb
Wet bulb
Dry bulb
Wet bulb
Nominal cooling (T1) Nominal heating (T2)
27 20
19 15
35 7
24 6
Air temperature outdoor ( C)
2850
1220
2800
1210
2750
1200
Power consumption (W)
Capacity (W)
From Figs. 5 and 7, it can also be found that for the system with the 5 mm indoor HX, its refrigerating capacity and COP peaks both appear at the refrigerant charge of 600 g, while the refrigerating capacity and COP both reach their maximums at the charge of 660 g for the 7 mm indoor HX. Reasons contributing to the difference above can be ascribed to the varying conditions inside the indoor HX. When the 7 mm HX is used as the indoor HX in the air conditioning system, the space in the evaporator for the refrigerant to evaporate is larger and the pressure drop is much smaller than that of the 5 mm indoor HX. Therefore, the refrigerant can be evaporated more easily in the 7 mm evaporator. However, when the 5 mm HX is used as the evaporator in the indoor system, the space for the refrigerant to be evaporated is smaller, leading to bigger evaporation pressure and larger flow resistance for the refrigerant. As a result, it's harder for the refrigerant vapor to evaporate and
then refrigerant gas superfluous refrigerant liquid accumulates in the condenser because of the pressure loss from the accumulation of refrigerant vapor in the evaporator. Furthermore, the mass of the evaporated refrigerant in the evaporator decreases drastically and too little refrigerant vapor circulates back to the compressor as well, which contributes to increase of the power consumption of the compressor and in turn reduces the refrigerating capacity in the air conditioning system. Therefore, for the 5 mm air conditioning system, when the refrigerant charge is 660 g, the cycle performance is instead lower compared with the traditional 7 mm one. When refrigerant charge is decreased to 600 g, the refrigerant does not accumulate in the condenser excessively and can be evaporated adequately in the evaporator after throttling. Therefore, 600 g is the most experimentally appropriate charge value to acquire the best performance parameters for the 5 mm system. As exhibited in Figs. 5 and 7, it can be seen that the peak of capacity curve and COP curve corresponds with the best cycle of the air conditioning system. The best performance parameters of every HX in the refrigerating mode are presented in Table 5. For the cycle performance in the cooling mode, the lowest is the 5 mm evaluated-tube 1 of which capacity and COP are 2.9% and 2.2% lower than the conventional 5 mm with angle of 18 , respectively. For the evaluated-tube 2, its best performance parameters are better than the 5 mm conventional system in small extent, which is 0.6% higher in capacity and 1.2% higher in COP. Compared with the
2700 2650 2600 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2550 2500 2450 2400
500
550
600
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
650
Charge (g)
700
Fig. 5. Cooling capacity of different refrigerating systems.
1190 1180 1170 1160
5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
1150 1140 1130
750
500
550
600
650
Charge (g)
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
700
750
Fig. 6. Power consumption of different refrigerating systems.
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2950
2.40
2900
2.35
Capacity (W)
2850
COP
2.30 2.25 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2.20 2.15 500
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
550
600
Charge (g)
2800 2750 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2700 2650 2600
650
500
550
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
600
Charge (g)
650
700
700 Fig. 8. Capacity of different heating system.
Fig. 7. COP of different refrigerating systems.
4.2. Comparisons of performance in the nominal heating condition As are shown in Figs. 8e10, in the heating mode, with the increase of the refrigerant charge, the COP and capacity of the system firstly increase, then continue to increase at a smaller extent and finally begin to decrease. The trends of COP curves and capacity curves are almost similar to those in the refrigerating mode. However, power consumption in the heating mode keeps rising with the increment of the charge. For the 5 mm system, the most efficient cycle appears at the charge of 600 g. But for the 7 mm system, the most efficient cycle appears when the charge is 660 g.
Table 5 Best performance parameters in the refrigerating mode.
5 5 5 7
mm mm mm mm
1 6 18 18
1
2
3
4
5
6
7
8
9
2731 2832 2813 2839
2.32 2.40 2.37 2.41
1176 1182 1187 1177
85.7 86 84.6 83.6
7 12.7 14.6 12.7
78.7/77.7 81.5/81.8 80.7/80.1 81.5/81.8
49.7/38.9 49.0/39.3 48.9/38.9 49.0/39.3
38.4 38.4 37.5 37.8
600 600 600 660
1- Capacity (W); 2-COP; 3-Power (W); 4-Temperature of vapor outside ( C); 5Temperature of vapor inside ( C); 6-Inlet temperature of condenser ( C); 7-Outlet temperature of branch ( C); 8-Outlet temperature of condenser ( C); 9-Mass of refrigerant (g).
5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
1040
Power consumption (W)
conventional 5 mm, even though the 5 mm evaluated-tube 2 shows improvement in system performance, it still has disadvantage performance compared with the traditional 7 mm (capacity 0.3% lower, COP 0.4% lower). From Table 5, it can be found that power consumption, vapor temperature and condensation temperature are all lower in the 5 mm system with angle of 1 than those in the other systems included. The reason is that the pressure drop in the HX with helix angle of 1 is larger than that with other angles. Compared with the 5 mm system with helix angle of 1, the 7 mm system with 18 has almost equal power consumption due to the pressure drop from the decrease of the tube diameter. What can be inferred from analysis above is that for the 5 mm HX with angle of 1, refrigerant fails to stay in the HX for long due to the low pressure drop, which finally leads to the lower capacity for the system. However, when the helix angle of the 5 mm is 6 , the pressure drop of refrigerant decreases partially while the capacity of the system with moderate amount of charge does not decrease, leading to the improvement of COP.
Parameters corresponding with the best cycle performance in air conditioning systems with different HX are recorded in Table 6. From the experimental data in Table 6, it can be seen that cycle performance of the 5 mm system with helix angle of 1 is the worst. In the heating mode, the capacity and COP of the 5 mm 6 system based on the 5 mm HX with angle of 6 are respectively slightly 0.2% and 0.7% higher compared with the conventional 5 mm 18 system but are still 1.2% and 0.7% lower than those of the conventional 7 mm 18 one. Additionally, the outlet temperature of the 5 mm 1 system is lower than those of the other systems in the heating mode. By contrast, outlet temperature of the 7 mm is the highest in the heating mode. For the 5 mm 1 air conditioning system, internal pressure drop is the lowest and heat exchange ability of the indoor HX as condenser is also the lowest of all. Refrigerant not condensed totally in the 5 mm 1 system flows to the evaporator and results in weaker heat exchange ability of the evaporator, finally contributing to the lower performance of the system. The power consumption of the 7 mm is the highest in the heating mode instead, which shows obvious difference from that in the cooling mode. The reason contributing to the phenomena above is as follows: In the heating mode, the indoor system is used as condenser. The pressure drop of the refrigerant vapor that flows along the cycle to the condenser is far greater than that of the refrigerant liquid in the refrigerating mode. Nonetheless, the
1020
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
1000
980
960
940
500
550
600
650
Charge (g)
700
750
Fig. 9. Power consumption of different heating system.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
S. Wang et al. / Applied Thermal Engineering xxx (2014) 1e8
7
3.00 2.96 2.92 2.88
COP
2.84 2.80 5mm 18 1# 5mm 18 2# 5mm 18 3# 5mm 6 1# 5mm 6 2# 5mm 6 3#
2.76 2.72 2.68 500
550
5mm 1 1# 5mm 1 2# 5mm 1 3# 7mm 18 1# 7mm 18 2# 7mm 18 3#
600
650
Charge (g)
700
750
Fig. 10. COP of different heating system.
refrigerant charge of the 7 mm HX is more than that of the 5 mm, leading to larger pressure drop in the 7 mm system. Finally, the power consumption is far larger than that of the 5 mm system. According to the performance of the system both in the cooling and heating mode, it can be concluded that the 5 mm evaluatedtube 2 is superior to the 5 mm 18 one but still inferior to the 7 mm both in the capacity and COP. From Tables 5 and 6, it is easy to notice that temperature changes are evident after refrigerant flows past two cycles through the condenser or evaporator in the outdoor system, reaching 10 C in the refrigerating mode and 5 C in the heating mode. The reason is that uneven heat exchange in the two bypasses results in the reduction of the system performance. For example, in the refrigerating mode, the refrigerant temperatures in the inlet and outlet are 81.5 C and 49 C in the 3-U bypass for the 5 mm evaluated-tube 2, respectively. However, refrigerant temperatures in the inlet and outlet are 81.8 C and 39.3 C in another 4-U bypass, which shows nearly 10 C difference in the outlet in these two bypasses. In the 4 U-tubes bypass, refrigerant has been totally condensed to the state of being undercooled. After the undercooled refrigerant mixes with the refrigerant not undercooled from another bypass, the degree of undercooling decreases. Though the refrigerant from two bypasses converges and flows through another U-tube, the best cycle performance of the system does not appear. Furthermore, the refrigerant lightly undercooled flows to the evaporator, leading to the higher evaporating temperature of the refrigerant. The evaporating pressure of the refrigerant decreases due to higher evaporating temperature, which meanwhile results to the lower cycle performance of the system. Accordingly, temperatures of the refrigerant meeting from two bypasses must be controlled to guarantee the least power consumption and the best cycle performance.
Fig. 11. Bypasses of the outdoor heat exchanger: (a) Before adjustment; (b) After adjustment.
4.3. Experimental study on the adjusted cycle of the outdoor condenser When the tube diameter is changed from 7 mm to 5 mm, pressure drop will increase resulting from the geometry change and it becomes more difficult for the refrigerant inside the tube to be evaporated. Furthermore, the excessive pressure drop will reduce the flow rate of refrigerant and finally lead to a decline of refrigerating capacity. Therefore, when the 5 mm HX with the same cycle of the 7 mm HX is used, refrigeration capacity will inevitably fall sharply. Accordingly, the number of bypasses in this 5 mm HX is increased to proper extent, which will decrease the refrigerant flow rate correspondingly and shorten the process of every bypass. The reduction of frictional resistance and pressure drop resulting from the decrease of refrigerant flow rate will solve the problem to a certain extent appropriately when using 5 mm copper tubes. In order to guarantee uniform heat exchange of two bypasses in the outdoor system, the cycle of the outdoor HX after adjustment are investigated experimentally. As is presented in Fig. 11, T-splitter in the outdoor HX is changed to the Y-splitter. Besides, the numbers of bypasses before confluence and U-tubes after confluence are adjusted to 3, 3 and 2, respectively. Heating and refrigerating experiments based on the system including the 5 mm indoor HX and the adjusted outdoor HX are carried out in the air enthalpy potential lab. Capillary charge and refrigerant charge are both
Table 6 Best performance parameters in the heating mode.
5 5 5 7
mm mm mm mm
1 6 18 18
1
2
3
4
5
6
7
8
9
2871 2922 2915 2957
2.93 2.96 2.94 2.98
980 987 990 992
51.8 59.7 57.2 66.7
2.2 2.8 2.7 2
0.5/6.7 0.2/6.3 0.0/6.3 0.5/6.8
2.5/2.0 1.2/0.9 1.2/1.0 1.1/1.1
3.2 1.7 1.7 1.5
600 600 600 660
1- Capacity (W); 2-COP; 3-Power (W); 4-Temperature of vapor outside( C); 5Temperature of vapor inside ( C); 6-Inlet temperature of condenser ( C); 7-Outlet temperature of branch ( C); 8-Outlet temperature of condenser ( C); 9-Mass of refrigerant (g).
Table 7 Performance parameters after the adjustment.
Cooling Heating
1
2
3
4
5
6
7
8
9
2882 2978
2.45 2.96
1177 1007
84.3 62.9
13.6 1.9
79.1/79.3 0.7/1.0
49.2/46.8 2.1/2.0
37 4.2
600 600
1- Capacity (W); 2-COP; 3-Power (W); 4-Temperature of vapor outside( C); 5Temperature of vapor inside ( C); 6-Inlet temperature of condenser ( C); 7-Outlet temperature of branch ( C); 8-Outlet temperature of condenser ( C); 9-Mass of refrigerant (g).
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010
8
S. Wang et al. / Applied Thermal Engineering xxx (2014) 1e8
according to the original parameters with the maximum performance. Experimental results are exhibited in Table 7. As can be seen from the data in Table 7, the performance of air conditioning system with the 5 mm evaluated-tube 2 after adjustment increases in all aspects compared with the original 5 mm evaluated-tube 2. In the refrigerating mode, the capacity and COP increase by 1.7% and 2.1%, respectively. In the heating mode, the COP is almost equal to that of the original system. However, the capacity increases by 1.9%. Furthermore, compared with the conventional 7 mm indoor system, it can be found that the capacity increases by 1.5% and 0.7% in the cooling and heating mode, respectively. Even though the COP is slightly 0.7% lower than the conventional 7 mm in the heating mode, the COP is 1.7% higher in the refrigerating mode. From the experiment, it can be also seen that temperature difference between inlets and outlets decreases in the two bypasses, indicating that adjustments on the cycle not only narrow the difference of heat exchange in the two bypasses but also improve the performance of the system. 5. Conclusions In this paper, the 5 mm FTHX with different internal thread parameters and traditional 7 mm FTHX are investigated as indoor unit of air conditioners with a same outdoor unit to evaluate their performance. Appropriate adjustments to the outdoor HX are also afterward carried out to study the performance changes. Based on the experimental investigations, the following conclusions could be drawn. In the refrigerating mode, compared with the traditional 5 mm 18 system, the capacity and COP of the 5 mm evaluated-tube 1 system is the lowest (2.9% lower and 2.2% lower than that of the 5 mm 18 system, respectively). The 5 mm evaluated-tube 2 system acquires slightly better performance than the 5 mm 18 , namely 0.6 higher% capacity and 1.2% higher COP, however, which is still 0.3% lower in capacity and 0.4% lower in COP comparing with the conventional 7 mm system. In the heating mode, the system performance of the 5 mm evaluated-tube 1 is also the lowest. As to the 5 mm evaluated-tube 2 system, compared with the performance of the conventional 5 mm 18 HX system, it is 0.2% and 0.7% higher, respectively, in capacity and COP but is still 1.2% and 0.7% lower than the conventional 7 mm 18 . For the 5 mm evaluated-tube 2 system, after the adjustment changing T-splitter to the Y-splitter and adjusting the numbers of bypasses and U-tubes of the outdoor HX, the cycle performance is obviously improved. In the refrigerating mode, it is improved by 1.7% in capacity and 2.1% in COP compared with the original 5 mm evaluated-tube 2. Though its COP is close to the original 5 mm, the capacity has an increase by 1.9% in heating. Compared with the conventional 7 mm indoor air system, the capacity is 1.5% higher in the refrigerating mode and 0.7% higher in the heating mode, respectively. Even though the COP of the adjusted system in the heating mode is slightly 0.7% lower than the conventional 7 mm, the COP in the cooling mode is obvious 1.7% higher instead. Conclusively, the air conditioning system based on the HX composed of the 5 mm 6 tubes after appropriate adjustments is experimentally proved that it can save the material cost and meanwhile guarantee the cycle performance of the air conditioning system to improve the performance price ratio.
Acknowledgements The authors gratefully acknowledge the financial support for this project from National Natural Science Foundation of China (Granted No. 51476059 and No. 51276068) and International Cooperation and Exchange Program from the Ministry of Science and Technology of China (Grant No. 2013DFG60080). References [1] Mineral Commodity Summaries, United States Geological Survey, GPO, Washington, DC, 2011. [2] R. Yun, Y.C. Kim, C. Park, Numerical analysis on a microchannel evaporator designed for CO2 air-conditioning systems, Appl. Therm. Eng. 27 (2007) 1320e1326. [3] L.L. Shao, L. Yang, C.L. Zhang, Comparison of heat pump performance using finand-tube and microchannel heat exchangers under frost conditions, Appl. Energy 87 (2010) 1187e1197. [4] H.L. Wang, R.B. Peterson, Performance enhancement of a thermally activated cooling system using microchannel heat exchangers, Appl. Therm. Eng. 31 (2011) 2951e2962. [5] K. Kim, M.H. Kim, D.R. Kim, K.S. Lee, Thermal performance of microchannel heat exchangers according to the design parameters under the frosting conditions, Int. J. Heat Mass Transfer 71 (2014) 626e632. [6] L. Chen, Z. Li, Z.Y. Guo, Experimental investigation of plastic finned-tube heat exchangers, with emphasis on material thermal conductivity, Exp. Therm. Fluid Sci. 33 (2009) 922e928. [7] A.A. Sertkaya, K. Altınısık, K. Dincer, Experimental investigation of thermal performance of aluminum finned heat exchangers and open-cell aluminum foam heat exchangers, Exp. Therm. Fluid Sci. 36 (2012) 86e92. [8] W.M. Yan, H.Y. Li, Performance of finned tube heat exchangers operating under frosting conditions, Int. J. Heat Mass Transfer 46 (2003) 871e877. [9] X.M. Guo, Y.G. Chen, Experimental study on frost growth and dynamic performance of air source heat pump system, Appl. Therm. Eng. 28 (2008) 2267e2278. [10] G.B. Zhou, Y.F. Zhang, Performance of a split-type air conditioner matched with coiled adiabatic capillary tubes using HCFC22 and HC290, Appl. Energy 87 (2010) 1522e1528. [11] J.H. Masliyah, K. Nandakumar, Heat Transfer in internally finned tubes, J. Heat Transfer 98 (2) (1976) 257e261. [12] D. Han, K.J. Lee, Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes, Int. J. Heat. Mass Transfer 48 (2005) 3804e3816. [13] Y.Y. Yan, T.F. Lin, Evaporation heat transfer and pressure drop of refrigerant R134a in a small pipe, Int. J. Heat Mass Transfer (1998) 4183e4194. [14] X.C. Huang, G.L. Ding, Condensation heat transfer characteristics of R410Aeoil mixture in 5 mm and 4 mm outside diameter horizontal microfin tubes, Exp. Therm. Fluid Sci. 34 (2010) 845e856. [15] S. Mancin, A. Diani, L. Rossetto, R134a flow boiling heat Transfer and pressure drop inside a 3.4 mm ID MicrofinTube, Energy Proc. 45 (2014) 608e615. [16] S. Cheng, S.F. Wang, Z.M. Liu, Cycle performance of alternative refrigerants for domestic air-conditioning system based on a small finned tube heat exchanger, Appl. Therm. Eng. 64 (2014) 83e92. [17] Y.J. Kim, J.M. Cho, M.S. Kim, Experimental study on the evaporative heat transfer and pressure drop of CO2 flowing upward in vertical smooth and micro-fin tubes with the diameter of 5 mm, Int. J. Refrig. 31 (2008) 771e779. [18] J.M. Cho, M.S. Kim, Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm, Int. J. Refrig. 30 (2007) 986e994. [19] K. Aroonrat, C. Jumpholkul, R. Leelaprachakul, A.S. Dalkilic, O. Mahian, S. Wongwises, Heat transfer and single-phase flow in internally grooved tubes, Int. Commun. Heat Mass Transfer 42 (2013) 62e68. [20] M.A. Akhavan-Behabadi, S.G. Mohseni, S.M. Razavinasab, Evaporation heat transfer of R-134a inside a microfin tube with different tube inclinations, Exp. Therm. Fluid Sci. 35 (2011) 996e1001. [21] G.L. Ding, H.T. Hu, X.C. Huang, B. Deng, Y.F. Gao, Experimental investigation and correlation of two-phase frictional pressure drop of R410Aeoil mixture flow boiling in a 5 mm microfin tube, Int. J. Refrig. 32 (2009) 150e161. [22] Chinese Standard, GB/T 7725, Room Air Conditioners, General Administration of Quality and Supervision, Inspection and Quarantine of the People's Republic of China and Standardization Administration of the People's Republic of China, Beijing, China, 2004. [23] S.F. Wang, S. Cheng, H.M. Yu, Z.H. Rao, Z.M. Liu, Experimental investigation of AleCu composed tubeefin heat exchangers for air conditioner, Exp. Therm. Fluid Sci. 51 (2013) 264e270.
Please cite this article in press as: S. Wang, et al., Cycle performance of air conditioning system based on finned tube heat exchangers with different helix angles, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.12.010