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Evaluation of Seasonal Performance Improvements in a 3-Ton Air-Conditioning Heat Pump System Using a Novel Design of Integrated Electronic Expansion Valves and Distributors.

INTRODUCTION

The need to reverse the refrigeration cycle in heat pump systems means that both indoor and outdoor units require both an expansion device and check valve mechanism to bypass this device during the mode of operation in which it is not needed. This assembly then needs to be integrated with the distributor in multi-circuited round tube heat exchangers. Such an assembly in the outdoor unit of a high efficiency system tested in this study is shown in Figure la. One note worthy feature of the system tested was that it used electronically controlled expansion devices on both the indoor and outdoor side of the system. The use of electronically controlled expansion devices is becoming more popular, especially in higher efficiency units as this allows for programmable control of the superheat in many operating modes. While the use of the electronic expansion valve allows for energy saving control strategies, the complexity of this subassembly requires a larger bill of materials and extra effort in fabrication when compared to a simple piston expansion device integrated into a check valve. With these difficulties in mind, an expansion valve manufacturer developed a proprietary and novel electronic expansion valve with both the check valve and distributor integrated into the body of the valve. An example of this valve, referred to as an integrated electronic expansion valve distributor, is shown installed in the outdoor unit of the test system in Figure lb. One direct difference with the integrated electronic expansion valve distributor is the simplicity of the valve subassembly packaging; another key difference is the proximity of the distributor to the expansion volume this can be seen in the exploded view of the integrated electronic expansion valve distributor shown in Figure 1c. In the integration of the distributor with the valve, the distributor is placed directly at the outlet of the orifice. This is noticeable different from conventional valves where the distributor is brazed some length downstream of the orifice. The additional length can provide opportunity for the two phases of the refrigerant to separate and unequally distribute among the circuits. In some cases, an additional orifice is put between the expansion device and the distributor to offset this effect.

The importance of proper refrigerant flow distribution through a multi-circuited evaporator is well known. Choi et al., 2003, showed that as much as 30% capacity degradation can come from refrigerant flow maldistribution, even at the same superheat setting. Kaem et al., 2013, indicated that better matched individual circuit outlet superheat in an interlaced evaporator can lead to as much as 7% improvement in overall UA and 2.4% improvement in system COP. Fay and Hrnjak, 2011, indicated that a 4% and 5% gain in COP and capacity, respectively, is possible when moving from a refrigerant flow distribution where some circuits have two-phase exit, to a matched superheat condition. To realize these kinds of improvements, active individual circuit superheat control is typically implemented. While a few attempts have been made to automate this control scheme in a single device, this is most often done experimentally through the use of a throttling device placed on each circuit, which is not practical in a production application. Another way to approach uniform superheat distribution, assuming uniform circuit loading, is to more uniformly distribute the flow inside of the distributor

The objective of the current study was to demonstrate that, in addition to the above described advantages for system integration, this technology can improve system level performance metrics such as capacity and coefficient of performance (COP), as well as secondary metrics such as refrigerant distribution. To quantify these effects, a prototype valve with integrated check valve and distributor was provided by the valve manufacturer and installed in both the indoor and outdoor units of a 3-ton high efficiency reversible heat pump system. The indoor unit had a three row six circuit symmetric V-shaped coil, with interlaced circuiting on each side of the coil. Circuits 1-3 were on the left side of the coil and 4-6 on the right. The outdoor unit had 5 separate circuits. Nominal efficiencies of this system, as provided by the manufacturer were a SEER of 18 and an HSPF of 9.8. For configurations with both the original electronic valves and the integrated electronic expansion valve distributor's, a full series of SEER and HSPF tests were performed according to AHRI 210/240-2008 at various superheat settings for a system having a two-capacity compressor. These results were then compared. In addition to recording all the required measurements for capacity and efficiency, both the indoor and outdoor coils were equipped with temperature measurements on all of the circuits to determine the uniformity of superheat among the circuits and several additional pressure transducers to quantify pressure drop in various components. The instrumentation used on the refrigerant side of the system is shown schematically in Figure 2. In addition to instrumentation installed on the refrigerant side of the system sufficient instrumentation was installed in the air stream of the indoor unit to obtain a capacity determination on this stream in compliance with ASHRAE 37-2009 and ASHRAE 116-2010.

RESULTS & DISCUSSION

Cooling Performance

Cooling mode tests were performed on the system in configurations using both valve subassembly options according to AHRI 210/240-2008 Table 7. Steady state data was acquired at the four required wet coil tests conditions, [A.sub.2], [B.sup.2], [B.sub.1] and F1, as well as the two steady dry coil conditions, [C.sub.2] and [C.sub.1]. Cycling tests, [D.sub.2] and [D.sub.1], were also performed to determine cycling degradation coefficient ([C.sub.D]). Testing on the system in the baseline configuration was performed with no modifications to any of the control devices. As the system configuration with the integrated electronic expansion valve distributor could not utilize the original control signal provided to the valve by the manufacturer of the system, a separate control algorithm, developed by the valve's manufacturer, was implemented and a superheat optimization was performed on the system with the integrated electronic expansion valve distributor installed. These superheat optimization consisted of varying the superheat control set point from 4R to 8R over all required conditions and performing the rating tests at the lowest superheat set point that showed stability over all test conditions. It was found that a set point of 6R was the optimal setting, based upon these criteria; this was only slightly lower than the baseline configuration. While the use of different control algorithms makes the comparison between the baseline and integrated expansion valve and distributor systems less direct in cycling tests, the steady state testing should allow for direct comparisons.

The cooling capacity and EER as determined by these test results are shown in Figure 3a and Figure 3b, respectively. In all conditions, both cooling capacity and EER are higher for the system using the integrated electronic expansion valve distributor configuration. The increase in cooling capacity ranged from 3.6%, at condition [C.sub.2], to 6.0%, at condition [B.sub.2]. Overall efficiency increases varied from 1%, at condition [C.sub.2], to 8.1% at condition [B.sub.2]. Improvements in EER over the baseline were not directly proportional to the improvement seen in cooling capacity due to changes in system power at the different test conditions. In some cases the power increased and in others it decreased. The largest increase in system power, compared to the baseline occurred at condition [C.sub.2], with the integrated electronic expansion valve distributor configuration requiring 2.5% more power. This explains why, at condition [C.sub.2], the EER improvement is actually smaller than the capacity gain when using the integrated electronic expansion valve distributor. Conversely, at condition B2, the EER improvement is two percentage points larger than the gain in capacity because the system power was reduced by nearly 2%. The results from the cycling tests yielded cycling degradation coefficients at both low and high capacity, of 0.0. Using the calculation guidelines set forth for calculation SEER, it was determined that the baseline system achieved an SEER of 18.00, while the system with the integrated electronic expansion valve distributor improved on this by nearly 5%, achieving an SEER of 18.83. This indicates, that in addition to providing a solution that reduces complexity in manufacturing, the integration of this novel expansion valve, check valve, and distributor may also provide significant performance improvement in cooling mode.

As mentioned earlier, one possible "unseen" advantage of integrating the distributor with the expansion valve is improved refrigerant flow distribution among the circuits of the heat exchanger, leading to better matched circuit superheats. This secondary indicator of performance improvement was also seen as a result of integrating the integrated electronic expansion valve distribute) in the system during cooling mode. The graphs shown in Figure 4a and Figure 4b present the exit superheat in the individual circuits of the indoor coil during the 30 minutes of testing at the [A.sub.2] condition in the baseline configuration and with the integrated electronic expansion valve distribute, respectively. In the baseline configuration, Circuit 4 has an exit condition that is not superheated over the majority of the test period. This indicates that this circuit either receives much more liquid refrigerant or is loaded considerably less on the air side of the coil. With Circuit 3 being the symmetric circuit on the other side of the coil, it appears that loading on the air side may not be the primary cause of this maldistribution. As a result of this one circuit with a wet exit, the other circuits must have higher superheats to bring the average superheat up to the set point. The other five circuits have exit superheats that average between 8R and 10R. The average exit superheat, as determined through a time average of an immersion thermocouple at the exit of the common suction was just over 5R.

Using the integrated valve and distributor, none of the circuits appear to have a wet exit and the maximum superheat deviation among circuits is reduced from 10R to 6R. Assuming that the loading on the air side of the coil is the same between these two tests, these results indicate that the implementation of this technology, led to better refrigerant distribution among the circuits of the coil. While it may be difficult to quantify, it also appears that the superheat in all of the circuits is more stable with the integrated electronic expansion valve distributor installed. This increased stability could be caused by a variety of things; but is most likely attributable to the controller, the refrigerant distribution, or a combination thereof. While the different control algorithms used to control superheat may have differences that lead to better or worse stability, the act of combining the two phase flow leaving one or more circuits with the superheat vapor leaving the others may contribute to stability as well. This would be a strong function of the amount of liquid present, the degree of superheating present and required, and the length available for these flows to mix prior to measuring the bulk temperature.

Heating Performance

Heating mode tests were performed on the system in configurations using both valve subassembly options according to AHRI 210/240-2008 Table 8. One maximum temperature test ([H0.sub.1]), two high temperature tests ([H1.sub.2] and [H1.sub.1]), two frost accumulation tests ([H2.sub.2] and [H2.sub.21]), and two low temperature test ([H3.sub.2] and [H3.sub.2]) were conducted. The optional two cycling tests (HI[C.sub.1] and HI[C.sub.2]) were also performed. As with the cooling mode tests, an optimization on superheat control point setting was performed at each steady test condition. The superheat set point was varied from 7R to 14R and the lowest stable superheat overall conditions was chosen as the set point for the performance evaluation tests, in this case 10R. It should be noted, that at the low capacity conditions, the stable superheat setting was consistently lower 7-8R. This indicates that further efficiency improvements may be possible by implementing a control strategy that constantly "looks" for the lowest stable superheat as opposed to the fixed superheat set point logic used in this study.

The heating capacity and EER as determined by these tests are shown in Figure 5a and Figure 5b, respectively. In all but the high capacity low temperature condition, the system with the integrated electronic expansion valve distributor installed achieved equal or higher capacity and EER. The largest improvement in both capacity and EER was at condition [H1.sub.1] where both capacity and EER were increased by 8%. In the single condition in which performance was degraded, capacity was only reduced by 1.5%, while EER was reduced by 3%. The results from the cycling tests yielded cycling degradation coefficients at both low and high capacity, of 0.0. Using the procedures outlined in AHRI 210/240, the HSPF was calculated for both system configurations, assuming the system would be running in climatic region IV as described in AHRI 210/240 Table 17. The system in the baseline configuration achieved an HSPF of 9.79. Integrating the integrated electronic expansion valve distributor assembly into the system resulted an increase in HSPF of approximately 3% to 10.06.

The largest noticeable difference in superheat between cooling and heating mode is that no single circuit appeared to have liquid present at the exit. This is illustrated in the comparison of individual circuit superheats shown in Figure 6a and Figure 6b. In the baseline configuration, the exit of circuits 1, 2, 3, and 5 is superheated by 10-12R, and stable. While the superheat at the exit of circuit 4 is approximately 7R, on average with much less stability. The highest deviation between circuit superheats is approximately 7R, which is significantly less than the baseline indoor coil during cooling operation. With the implementation of the integrated electronic expansion valve distributor valve assembly, Figure 6b, the deviation between the highest and lowest superheat is reduced from 7R to 5R and superheat, on average, is reduced. In addition, it does not appear that any single circuit has the instability noted in the baseline results. This again, is a secondary indicator that integrating the distributor into the expansion device may provide improvements in refrigerant distribution. Another indicator of improved refrigerant flow distribution is an increase in evaporation pressure. Typically, evaporation temperature (pressure) will increase as refrigerant is more uniformly distributed because the heat exchanger area is used more effectively. In all conditions tested, the evaporation pressure was increased when the integrated electronic expansion valve distributor was implemented. The largest increase in evaporation pressure, from 160.7psi to 167.7psi, was seen at condition H01. The smallest increase, from 74.4psi to 76.3psi, was at condition H32.

CONCLUSIONS

Managing the refrigerant flow distribution in a multi-circuited evaporator in a way that optimizes the air side loading of the coil is crucial to obtaining the best performance. The work presented here demonstrated the possibility for performance improvements using a novel electronic expansion device for reversible systems that integrates the electronic expansion valve, the check valve, and the distributor. While such a device has advantages from a manufacturing and packaging point of view, the present work shows that appreciable gains in both system capacity and efficiency can be achieved through the integration of this technology. In the high efficiency 3-ton reversible system investigated, integration of this technology led to increases in SEER and HSPF of nearly 5% and 3%, respectively. For all but one test condition, system capacity (both cooling and heating) was also increased. In addition to the improvement in these primary metrics, secondary metric improvements were also observed. In both modes of operation, the spread of individual circuit superheats were reduced, superheat stability was improved, and in some cases evaporation pressure was also increased. All of these secondary metrics point towards improved refrigerant flow distribution through integration of the integrated electronic expansion valve distributor technology.

ACKNOWLEDGMENTS

The authors would like to acknowledge Parker Hannifin--Sporlan Division for their development of the integrated electronic expansion valve distributor concept and their support of this work

NOMENCLATURE
[A.sup.2]    =  AHRI 210/240 cooling mode condition [A.sub.2],
                steady- wet coil
[B.sup.1]    =  AHRI 210/240 cooling mode condition [B.sup.1],
                steady- wet coil
[B.sub.2]    =  AHRI 210/240 cooling mode condition [B.sub.2],
                steady- wet coil
[C.sub.1]    =  AHRI 210/240 cooling mode condition [C.sub.1],
                steady- dry coil
[C.sub.2]    =  AHRI 210/240 cooling mode condition [C.sub.2],
                steady- dry coil
CD           =  Cycling Degradation Coefficient (-)
COP          =  Coefficient of Performance (-)
[D.sub.1]    =  AHRI 210/240 cooling mode condition [D.sub.1],
                cyclic- dry coil
[D.sub.2]    =  AHRI 210/240 cooling mode condition [D.sub.2],
                cyclic- dry coil
EER          =  Energy Efficiency Ratio (BTU/W-h)
[F.sub.1]    =  AHRI 210/240 cooling mode condition [F.sub.1],
                steady- wet coil
[H0.sub.1]   =  AHRI 210/240 heating mode condition [H0.sub.1], steady
[H1.sub.1],  =  AHRI 210/240 heating mode condition [H1.sub.1], steady
[H1.sub.2]   =  AHRI 210/240 heating mode condition [H1.sub.2], steady
H1[C.sub.1]  =  AHRI 210/240 heating mode condition HI[C.sub.1], cyclic
H1[C.sub.2]  =  AHRI 210/240 heating mode condition H1[C.sub.2], cyclic
[H2.sub.1]   =  AHRI 210/240 heating mode condition [H2.sub.1], frost
                accumulation
[H2.sub.2]   =  AHRI 210/240 heating mode condition [H2.sub.2], frost
                accumulation
[H3.sub.1],  =  AHRI 210/240 heating mode condition [H3.sub.1], steady
[H3.sub.2]   =  AHRI 210/240 heating mode condition [H3.sub.2], steady
HPF          =  Heating Performance Factor (BTU/W-h)
HSPF         =  Heating Seasonal Performance Factor (BTU/W-h)
SEER         =  Seasonal Energy Efficiency Ratio (BTU/W-h)


REFERENCES

AHRI Standard 2140/240-2008, Performance Rating of Unitary Air Conditioning & Air-Source Heat Pump Equipment, 2008, Air-Conditioning, Heating, and Refrigeration Institute, 2008, 2111 Wilson Boulevard, Suite 500, Arlington, VA 22201, U.S.A.

ASHRAE Standard 37-2009, Methods of Testing for Rating Unitary Air-Conditioning and Heat Pump Equipment, 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle N.E., Atlanta, GA 30329, U.S.A.

ASHRAE Standard 116-2010, Methods of Testing for Rating Seasonal Efficiency of Unitary Air-Conditioners and Heat Pumps, 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle N.E., Atlanta, GA 30329, U.S.A.

Choi, J.M., Payne, W.V., Domanski, P.A.. 2003 Effects of Non-Uniform Refrigerant and Air Flow Distributions on Finned-Tube Evaporator Performance, International Congress of Refrigeration, ICR0040

Fay, M.A., Hrnjak, P.S., 2011, Effect of Conical Distributors on Evaporator and System Performance, University of Illinois Air Conditioning and Refrigeration Center, Technical Report 284

Kaem, M.R., Elmegaard, B., Larsen, L.F.S, 2013, Comparison of fin-and-tube interlaced and face split evaporators with flow maldistribution and compensation. International Journal of Refrigeration, 36:203-214.

Chad D. Bowers, PhD

Associate Member ASHRAE

Predrag Hrnjak, PhD

Fellow ASHRAE

Dave Wrocklage

Associate Member ASHRAE

Stefan Elbel, PhD

Member ASHRAE

Dr. Bowers is a Senior Research Engineer at Creative Thermal Solutions, Inc in Urbana, Illinois. Mr. Wrocklage is an Engineering Manager at Parker-Sporlan Division in Washington, Missouri. Dr. Elbel is the Chief Engineer at Creative Thermal Solutions, Inc in Urbana, Illinois. Dr. Hrnjak is President of Creative Thermal Solutions, Inc. in Urbana, Illinois.
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Author:Bowers, Chad D.; Hrnjak, Predrag; Wrocklage, Dave; Elbel, Stefan
Publication:ASHRAE Conference Papers
Date:Dec 22, 2014
Words:3236
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