Performance of a room air conditioner under varied duration of system evacuation

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Introduction
The demand for air conditioners (AC) in the world tends to increase due to the increasing standard of living and the need for thermal comfort.In addition, global warming is also driving the increasing sales of air conditioners around the world.In developed countries, the amount of energy consumed by buildings is about 40% of the total energy demand [1].Consumption of electrical energy for refrigeration and air conditioning systems reaches 45% in the residential and commercial sectors.This, of course, significantly contributes to emission of greenhouse gas.Due to the rapid growth of the economy and human population, energy consumption is projected to increase by around 71% from 2003 to 2030.For hot regions such as in the Middle East, air conditioning machines can even consume as much as 70% of the total energy demand [2].Therefore, efforts to save energy are needed so that natural resources can be used more efficiently, and CO2 emissions and the issue of climate change can be anticipated better [3].
One of the important processes that affects durability and energy efficiency of an air conditioner is the evacuation process.This process is carried out to remove water vapor and air from the cooling unit.Otherwise, the water vapor will melt, freeze, and block the flow of refrigerant, which can reduce the efficiency of the system or even damage the compressor [4,5].The evacuation process could also minimize the presence of air or non-condensable gas can also reduce the capacity of the cooling unit.
So far, no research publications have been found that discuss the effect of system evacuation duration on AC performance.Instead of system evacuation, the studies of the effect of refrigerant charge have been conducted for decades [6][7][8][9].In textbooks on refrigeration and air conditioning, it is only explained that an evacuation system up to a certain vacuum level is important to ensure the refrigeration system works well.
This paper discusses the effect of the system evacuation duration on the performance of an air conditioner.Three main parameters, i.e., power consumption, cooling capacity, and coefficient of performance (COP) are analyzed on the basis of ISO 16258-1 [10] as well as ISO 5151 [11].

Methodology
A residential air conditioner with a nominal cooling capacity of 9000Btu/hr was used in this test.The indoor unit of the AC was installed in the indoor compartment and the outdoor unit was installed in the outdoor compartment.The tests were carried out based on ISO 5151 and have been applied in previous studies [12][13][14][15][16].The analysis and calculation were performed on the basis of ISO 5151 and ISO 16358-1.
During the tests, the outdoor compartment was kept at a dry-bulb temperature (DBT) of 35C and wet-bulb temperature (WBT) of 24C.Whereas, the DBT and WBT of the indoor compartment were maintained at 27C and 19C, respectively.Detailed method and experimental setup can be found in previous studies [15,16].
The testing of the AC unit was started by installing indoor and outdoor units and checking for leaks.If there is no leakage in the unit, the evacuation process can be carried out.After the evacuation process, the valve on the manifold gauge connected to the vacuum pump is closed, the vacuum pump is turned off, and wait for about 15-30 minutes.If there is no pressure rise, the system is ready to be charged with refrigerant.The amount of refrigerant charged into the system is in accordance with the manufacturer's recommendations.The duration of the evacuation process in this study was varied using 5 variations, i.e., 60 minutes, 45 minutes, 30 minutes, 15 minutes, and 0 minute.
From this test, 32 parameters can be measured and calculated.Three main quantities, namely input power, cooling capacity, and COP can be derived from these data based on ISO 5151.The calculation of Cooling Seasonal Performance Factor (CSPF), Cooling Seasonal Total Load (CSTL), and Cooling Seasonal Energy Consumption (CSEC) were accomplished in accordance with ISO 16358-1.Detailed calculations for CAPF, CSTL, and CSEC have been investigated [17].

Power consumption
Figure 1 shows the effect of evacuation duration on the power required to operate the AC unit.The data were obtained from 7 runs of experiment.When the AC unit is operated without prior evacuation, the power consumption appears to be higher than if it is evacuated first.An average of 760 W and a range of 759 -761 W of power was recorded in this experiment.If the piping system was evacuated for 15-minute duration, the average power decreases to 738 W. Evacuation durations of 30, 45, and 60 minutes result in almost the same power consumption with an average of 736.7 W and a maximum deviation of only about 0.21%.Meanwhile, when compared to conditions without evacuation, the difference in energy consumption can reach 3.25%.This range of power consumption is similar to that of previous study [13].
In the experiment without previous evacuation, it is possible that there is still water vapor left in the piping, causing a disturbance in the refrigerant flow.In the expansion device the water vapor can melt and freeze, so that it can block the flow of refrigerant and create a pressure drop in the piping.Probably, this is what causes the increase in power consumption in the AC unit.

Cooling capacity
The profile of cooling capacity as a function of evacuation duration is shown in Fig. 2. Cooling capacity has an average value of 2648 W with a range of 2645 to 2650 W when the system operates without evacuation.The evacuation process for 15 minutes gave an average cooling capacity of 2658 W with a range between 2654 to 2661 W. Furthermore, the evacuation duration of 30 minutes resulted in an average cooling capacity of 2659 W. The cooling capacities of 2662 and 2665 W were given by experiments with an evacuation duration of 45 and 60 minutes, respectively.This range of cooling capacity is slightly higher than that of reported in previous study in an air conditioner using various distance of condenser air obstruction [13].Detailed examination of Fig. 2 reveals that evacuation duration of 15 minutes to 60 minutes result in almost the same cooling capacity.The maximum deviation cooling capacity is only 0.26%.This indicates that duration of evacuation of 15 minutes can be considered as enough to evacuate water vapor and non condensable gas from the AC system as the difference in cooling capacity is very small.However, this should be validated using longer test duration in order of weeks or months, or even longer.

COP
COP is a parameter of AC performance that depends on the cooling capacity and power required to operate the AC and produce the cooling capacity.It can be expected that the COP increases as the cooling capacity increases and input power decreases.The profile of the COP for different durations of evacuation process is depicted in Fig. 3.
Detailed inspection of Figure 3 divulges that the COP of the AC unit without evacuation (3.48) is 3.94% lower than that of 60 minutes evacuation duration (3.62).The range of COP is similar to that reported in a study using R22, R410A, R290, and R32 [18].Another finding from this study is the insignificant difference in COP between vacuum durations of 15, 30, 45, and 60 minutes.So that from a performance standpoint, it appears that the duration of the evacuation from 15 minutes to 60 minutes does not seem to have a significant impact.Evacuation duration of 15 minutes, for instance, has a COP of 3.60.This is only 0.56% lower than that of the longest evacuation duration.However, again, longer duration of test should be carried out to evaluate its effect on the long-term performance as well as the lifespan of the AC unit.

Performance calculation using ISO 16358-1
In ISO 16358-1, the performance of the air conditioner can be expressed in terms of CSPF (Cooling Seasonal Performance Factor).This quantity is defined as the ratio of Cooling Seasonal Total Load (CSTL) and Cooling Seasonal Energy Consumption (CSEC).Generally, CSPF is expressed as FCSP, while CSTL and CSEC are expressed as LCST and CCSE, respectively, as written in Eq. (1)(2)(3).
LCST and CCSE can be calculated using the following equations: and The complete calculation techniques for LCST and CCSE is available in [17].
By using equations ( 2) and (3), the total load and the power consumption of the air conditioner for 1 year period can be determined.Therefore, the CSPF can be obtained.The profile of the power and cooling capacity calculated by ISO 16358-1 are depicted in Fig. 4 and 5. From Fig. 4, it can be seen that the power calculated by ISO 16358-1 is lower than that of ISO 5151.The reason is that ISO 5151 tests the air conditioner and measures the power consumption at a constant outside air temperature of 35C.Meanwhile, ISO 16358-1 calculates power when the unit operates in a temperature range of 21 to 35C.In general, air conditioners that operate at lower ambient temperatures will also consume less power.Conversely, an air conditioner that operates at a lower ambient temperature will produce a higher cooling capacity.This can be seen from Fig. 5, where the calculation with ISO 16358-1 gives a higher cooling capacity compared to the test results with ISO 5151.With ISO 16358-1, the average cooling capacity during the tests is 2963 W. This is higher than the test results with ISO 5151, which is 2658 W. The results of the CSPF calculation with ISO 16358-1 are higher than the COP resulted by the test with ISO 5151.As shown in Figure 6, the results of the ISO 16358-1 give COP values of about 6.2% higher than that of ISO 5151 due to higher cooling capacity and lower power consumption.The CSPF of this study is similar to that of study using non-inverter air conditioners conducted in four main cities in Indonesia [17].

Conclusion
A test of an AC with a nominal cooling capacity of 9000 Btu/hr or 2.64 kW under 5 different evacuation durations based on ISO 5151 has been accomplished.The highest performance in terms of COP is resulted from the test with longest evacuation duration with an average COP is 3.62.In terms of CSPF, the performance of the AC is 6.2% higher than that of COP calculated by ISO 5151.One of the important findings of this study is the insignificant difference of system performance at evacuation durations of 15, 30, 45, and 60 minutes.This seems to indicate that the evacuation duration of 15 minutes is enough to remove moisture and air from the AC system.However, it is necessary to carry out further studies in a longer test period to determine the effect of evacuation duration on the performance and durability of the system in the long term.

Fig. 1 .
Fig. 1.The power drawn by the AC for various evacuation duration.