Influence of the expansion device on the performance of aheat pump using R407C under a range of charging conditions

Jongmin Choia, Yongchan Kimb,*aDepartment of Mechanical Engineering, Hanbat National University, Duckmyung-Dong, Yusung-Gu, Daejeon, 305-719, South Korea

bDepartment of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-701, South Korea

Received 27 August 2003; received in revised form 5 December 2003; accepted 9 December 2003

Abstract

The objective of this study is to investigate the effects of the expansion device on the performance of a water-to-waterheat pump using R407C, which has been considered as one of the alternative refrigerants to replace R22 with ‘‘soft-optimization’’, at various charging conditions. The heat pump applying the expansion devices of a capillary tube and

an EEV was tested by varying refrigerant charge amount from �20% to +20% of full charge and changing watertemperature entering the condenser from 30 �C to 42 �C, while maintaining water temperature entering the evaporatorat 25 �C. The R22 capillary tube system is utilized as a baseline unit for the performance comparison with the R407C

system. The performance of the capillary tube system is more sensitive to off-design charge than that of the EEV sys-tem. As the refrigerant charge deviates from the full charge, the R407C EEV system shows a much lower degradationof capacity and COP as compared to the R22 and R407C capillary tube systems due to an optimum control of super-heat by electronically adjusting the EEV opening. In addition, the R407C EEV system shows more a stable compressor

discharge temperature at off-design charge than the R407C capillary tube system.# 2004 Elsevier Ltd and IIR. All rights reserved.

Keywords: Experiment; Electric expansion device; Capillary; Heat pump; R407C; Performance

Influence du dispositif de detente sur la performance d’unepompe a chaleur utilisant le R407C dans un eventail de

conditions de charge

Mots cles : Experimentation ; Detendeur electrique ; Capillaire ; Pompe a chaleur ; R407C ; Performance

1. Introduction

Due to the phaseout of CFCs (chlorofluorocarbon)

and HCFCs (hydrochlorofluorocarbon), the systemsmust be redesigned to satisfy design requirements for

alternative refrigerants. System modifications can beclassified into three categories: ‘‘drop-in’’ with nohardware change, ‘‘soft-optimization’’ with moderate

hardware changes, and ‘‘hard-optimization’’ withmajor hardware changes. R407C has the similar ther-modynamic properties as those of R22 with an excep-tion of the temperature gliding during the phase change

process at a constant pressure [1]. Therefore, R407C hasbeen considered as one of the alternatives with ‘‘soft-optimization’’.

0140-7007/$35.00 # 2004 Elsevier Ltd and IIR. All rights reserved.

doi:10.1016/j.ijrefrig.2003.12.002

International Journal of Refrigeration 27 (2004) 378–384

www.elsevier.com/locate/ijrefrig

* Corresponding author. Tel.: +82-2-3290-3366; fax: +82-

2-921-5439.

E-mail address: yongckim@korea.ac.kr (Y. Kim).

An expansion device controls refrigerant flow andbalances the system pressures in heat pumps and refrig-eration systems. Conventional expansion devices, suchas capillary tubes, short tube orifices, and thermostatic

expansion valves (TXV) are being gradually replacedwith electronic expansion valves (EEVs) due to increas-ing focus on comfort, energy conservation, and appli-

cation of a variable speed compressor [2,3]. Therefore,the performance of the system with a conventionalexpansion device and an EEV has to be investigated

with new alternative refrigerants. In addition, theamount of refrigerant charge in a heat pump is anotherprimary parameter influencing energy consumption. The

effects of refrigerant charge are closely related withthe expansion device. Undercharge or overcharge of therefrigerant into a heat pump will degrade system per-formance and deteriorate system reliability [4,5]. There-

fore, an investigation on the optimum amount ofrefrigerant charge with respect to expansion devicesmust be conducted for new alternative refrigerants.

The performance of a heat pump with alternativerefrigerants has been reported by many investigators [6–9]. Jung et al. [6] assessed the performance of R407C

along with other refrigerant mixtures in a breadboardtype heat pump. Devotta et al. [7] executed experimentalwork to compare the performance of R22 and R407C

for a window air conditioner. Spatz [8], and Motta andDomanski [9] conducted simulation studies on an air-conditioner using alternative refrigerants. In addition,the effects of charge amount on the performance of a

heat pump have been investigated with conventionalexpansion devices [4,5,10,11]. Houcek and Thedford [4]conducted the experiments at three charging conditions:

�23%, nominal, and +23% of nominal charge.Stoecker et al. [5] compared the performance of an airconditioner with a capillary tube and TXV when the

system was charged based on manufacturer’s guideline.Farzard and O’Neal [10,11] also studied refrigerantcharging effects on the performance of a heat pump witha capillary tube, short tube orifice, and TXV. They

observed that the TXV system showed a small variationof the COP with respect to refrigerant charge. Recently,Choi and Kim [12] experimentally investigated the per-

formance of a heat pump with a capillary tube and EEVusing R22 under various charging conditions.Most previous studies regarding the effects of refrig-

erant charge were focused on a heat pump with a capil-lary tube, a short tube orifice, and a TXV using R22.However, a study on the characteristics of the system

with an EEV using alternative refrigerants under var-ious charging conditions is very limited. The objective ofthis study is to investigate the effects of the expansiondevice on the performance of a heat pump using R407C

under various charging conditions, providing informa-tion on the design of an expansion device and refriger-ant charge. A water-to-water heat pump using R407C is

tested in steady state, cooling mode operation byapplying the expansion devices of a capillary tube andan EEV under various charging conditions. The perfor-mance of the R407C system is compared with that of

the R22 system, which is the baseline unit using a capil-lary tube, at various charging conditions.

2. Experimental setup and test procedure

2.1. Experimental setup

An experimental setup was designed to measure the

performance of the water-to-water heat pump undervariable operating conditions. As shown in Fig. 1, thetest rig includes the heat pump and water flow loops.The nominal cooling capacity of the heat pump is 3.5

kW using R22 as a working fluid. The heat pump con-sists of a scroll compressor, two heat exchangers (con-denser and evaporator), and an expansion device. The

detailed specifications of the major components of thetested heat pump are given in Table 1. The condenserand evaporator are double tube type heat exchangers

with a counter flow pattern between the refrigerant andwater. Water is selected as a heat source and sink for theheat pump system because of their simplicity in capacity

measurements. Water flow loops (secondary flow loops)for the evaporator and condenser include a magnetic

Table 1

Specifications of the main components of the system

Components

Specifications

Compressor

Manufacturer Daikin Co.

Model No.

JT32C-VZ

Type

Horizontal scroll

Rated capacity (kW)

3.5

Condenser

Manufacturer Donghwa Co.

Model No.

DDH-1/2

Type

Double tube

Diameter of inner tube (mm)

9.52

Diameter of outer tube (mm)

15.88

Length of tube (mm)

4787

Evaporator

Manufacturer Donghwa Co.

Model No.

DDH-1/3

Type

Double tube

Diameter of inner tube (mm)

9.52

Diameter of outer tube (mm)

15.88

Length of tube (mm)

3564

Capillary tube

Manufacturer Donghwa Co.

Diameter (mm)

1.2

Length (mm)

900

EEV

Manufacturer Saginomiya Co.

Model No.

DKV-14D13

Step (pulse)

0–480

Orifice diameter (mm)

1.4

Orifice length (mm)

3

J. Choi, Y. Kim / International Journal of Refrigeration 27 (2004) 378–384 379

pump and a constant temperature bath. A variablespeed pump and a manual needle valve control thewater flow rate supplied to the condenser and evaporatorto establish test conditions based on Refs. [13–15].

The R22 heat pump system with a capillary tube wasused as a baseline unit for the performance comparisonof the R407C system. Either an EEV or a capillary tube

was utilized as an expansion device in the R407C heatpump system. Control system for driving EEV unitincludes an A/D card, a stepping motor driver, and a

computer.Temperatures in the test setup were monitored at the

selected locations using thermocouples according to

ASHRAE Standard 41.1 [16], and refrigerant pressureswere also measured according to ASHRAE Standard41.3 [17]. A mass flow meter to measure refrigerant flowrate was installed between the condenser and expansion

device. The pressure drop across the mass flow meterwas approximately 3.92 kPa, which was less than thevalue of 82.7 kPa allowed in ASHRAE Standard 116

[18] at full charge condition. A volumetric flow meterwas installed to measure water flow rate in the second-

ary flow loop. Each sensor was calibrated to reduceexperimental uncertainties. The specifications anduncertainties of sensors are summarized in Table 2.

2.2. Test procedure

The first step of the test procedure was to determine

full charge under a standard condition, water tempera-tures of 34 �C and 25 �C entering the condenser andevaporator, respectively. The full charge condition was

selected to have a maximum COP. For the heat pumpwith the capillary tube using R22 and R407C (called as‘‘the R22 capillary tube system’’ and ‘‘the R407C capil-

lary tube system’’, respectively), the refrigerant wasadded into the heat pump in a 50 g increment until themaximum COP was obtained for each system. Oncethe full charge was determined, the heat pump was evac-

uated and then the refrigerant charge was varied from�20% to +20% of full charge for each refrigerant. Inthis study, the tests for the heat pump with the EEV

using R407C (called as ‘‘the R407C EEV system’’) wereperformed by setting the same full charge as the capil-lary tube system in order to compare characteristics of

R407C at the same basis of charge amount. During thetests, the EEV opening was manually changed to max-imize the COP at each operating condition and charge

amount.Since the heat pump is normally charged in the cool-

ing mode [10], the tests in this study were carried out inthe cooling mode only. Water temperature entering

the evaporator was kept at 25 �C, and that entering thecondenser was varied at 30 �C, 34 �C, 38 �C, and 42 �C.Water flow rates through the condenser and evaporator

was kept constant at 9 lpm and 7 lpm, respectively.Temperature, pressure, mass flow rate, and power input

Fig. 1. Schematic of the experimental setup.

Table 2

Specifications and uncertainties of sensors

Parameter

Uncertainty Full scale

Temperature

(T type thermocouple)

�0.1 �C

�270–400 �C

Pressure transducer

�0.2% of full scale 3447 kPa

Mass flow meter

(Coriolis meter)

�0.2% of reading

5 kg/min

Turbine flow meter

�0.5% of reading 57 LPM

Power meter

�0.01% of full scale 20 kW

Electronic balance weight

�0.5 g 41 kg

380 J. Choi, Y. Kim / International Journal of Refrigeration 27 (2004) 378–384

of the heat pump were monitored using a data acquisi-tion system. The test data were recorded continuouslyfor 40 min with 2-s intervals.Cooling capacity was calculated by using water flow

rate and temperature difference between evaporatorinlet and outlet. To confirm the calculation, the capacitywas also determined by refrigerant flow rate and

enthalpy difference between evaporator inlet and outlet[13–15]. The maximum difference between the water-side and the refrigerant-side capacity was less than 5%

which was consistent with ARI Standard 320 [14].Ninety percent of the data was within 2.7%. Theuncertainties of cooling capacity and COP estimated by

the single-sample analysis according to ASHRAEGuideline 2 [19] were approximately 3.1% and 3.2%,respectively.

3. Results and discussion

Fig. 2 shows the variations of cooling capacity for theR22 and R407C capillary tube systems as a function ofrefrigerant charge. The full charge amount of the R407C

capillary tube system was lower than that of the R22capillary tube system due to a lower density of R407C ata given condition. In this experiment, the full charge

amount of the R407C and R22 systems were 1250 g and1350 g, respectively. The cooling capacity of the R407Ccapillary tube system is from 89.2% to 101.6% of theR22 capillary tube system with a variation of refrigerant

charge.For both the R22 and R407C capillary tube systems,

the slope of the capacity with a charge amount is much

steeper at undercharged conditions than that at over-charged conditions. For overcharged conditions, thecapacity drops due to a decrease of the temperature

difference between the refrigerant and the water in

the evaporator with increasing refrigerant charge. Forundercharged conditions, the capacity rapidly decreaseswith a reduction of refrigerant charge due to a drop ofrefrigerant flow rate and compressor efficiency. Since the

superheat at undercharged conditions is higher thanthat at overcharged conditions, a more significantdegradation of evaporator efficiency occurs at under-

charged conditions.Normally, the capacity of a heat pump is expected to

drop at off-design refrigerant charge. However, the

capacity of the R407C capillary tube system remainsnearly constant with a rise of charge amount beyond fullcharge while the COP drops. For a non-azeotropic

mixture of R407C, both temperature gliding and pres-sure drop decrease saturation temperature in the con-denser. Besides, the pressure drop of R407C shows moreinfluence on the reduction of saturation temperature

than that of R22 [20]. Due to a lower saturation temp-erature for the R407C system at the end section of thecondenser, the reduction of heat transfer coefficient for

R407C would be more severe than that for R22. Thecapacity reduction for the R407C system at overchargedconditions from the full charge becomes nearly negli-

gible because the overcharged refrigerant wouldenhance heat transfer performance at the end section ofthe condenser and reduce the drop of temperature dif-

ference between the refrigerant and the water in thecondenser.Fig. 3 shows the capacity ratio for the R407C capil-

lary tube and EEV systems based on the capacity of the

R407C capillary tube system at the standard condition,water temperatures of 34 �C and 25 �C entering thecondenser and the evaporator, respectively. For all

water temperatures entering the condenser, the R407CEEV system shows much lower degradation of capacityas compared with the R22 and R407C capillary tube

systems as the refrigerant charge is altered from �20%

Fig. 2. Capacity variations of the R22 and R407C capillary

tube systems.

Fig. 3. Capacity ratios of the R407C capillary tube and

EEV systems to the R407C capillary tube system at standard

condition.

J. Choi, Y. Kim / International Journal of Refrigeration 27 (2004) 378–384 381

to +20% of full charge. When the deviation of chargeamount from full charge increases, the EEV systemshows much more refrigerant flow rate at underchargedconditions and lower evaporator inlet temperature at

overcharged conditions than the capillary tube system(Fig. 4). The capacity for the R407C EEV system ishigher by 5.2% to 21% at a �20% charge condition

than that for the R22 capillary tube system.Fig. 5 shows the COP of the R22 and R407C capillary

tube systems as a function of water temperature entering

the condenser and refrigerant charge. For the capillarytube system, as the charge amount deviates from the fullcharge, the reduction of COP is a little more significant

at undercharged conditions than that at overchargedconditions. This trend was also reported by Farzard andO’Neal [10] for a R22 system. The COP decreases by16.1% and 4.8% at �20% and +20% of full charge,

respectively, at a water temperature of 34 �C. For over-charged conditions, even though the capacity of theR407C capillary tube system remains nearly constant

(Fig. 2), the COP is gradually reduced with an increaseof charge amount due to a rapid increase of powerconsumption, which is caused by a rise of pressure ratio

and a higher friction loss in the compressor. However,the COP degradation of the R407C capillary tube sys-tem with respect to charge amount at overcharged con-

ditions is a little less pronounced than that for the R22system because the rise of the evaporator inlet temper-ature in the R407C capillary tube system is lower thanthat in the R22 capillary tube system.

The COP of the R407C EEV system is relativelyinsensitive to refrigerant charge compared with thecapillary tube system as shown in Fig. 6. When the water

temperature entering the condenser is 34 �C, the max-imum reduction in COP for the R407C EEV systemfrom the full charge is 3.6%, but that for the R407C

capillary tube system is 16.5%. The COP reduction ofthe EEV system is much less than that of the capillary

tube system at undercharged conditions because theEEV system shows higher refrigerant flow rate thanthe capillary tube system. For overcharged conditions,the COP degradation in the EEV system is minimized

by lowering evaporating temperature.Fig. 7 shows the subcooling of the R22 and R407C

capillary tube systems, and the R407C EEV system. The

condensing pressure increases with an addition ofrefrigerant charge due to an accumulation of the refrig-erant in the high-pressure side, increasing subcooling.

This trend was also observed by Stoecker et al. [5] andFarzard and O’Neal [10]. The subcooling decreaseswhen the water temperature entering the condenser

increases at all charge conditions except for at �20% offull charge. When the water temperature entering thecondenser increases, the mean temperature differencebetween the water and the refrigerant drops and less

heat is rejected in the condenser. For +20% of fullcharge, the subcooling decreases from 10.4 �C to 7.7 �Cas the water temperature entering the condenser increases

from 30 �C to 42 �C in the R22 capillary tube system.

Fig. 4. Evaporator inlet temperatures of the R407C capillary

tube and EEV systems.

Fig. 6. COP ratios of the R407C capillary tube and EEV sys-

tems to the R407C capillary tube system at standard condition.

Fig. 5. COPs of the R22 and R407C capillary tube systems.

382 J. Choi, Y. Kim / International Journal of Refrigeration 27 (2004) 378–384

The subcooling of the R407C capillary tube system islower than that of the R22 capillary tube system due toa temperature gliding in the condenser even though the

condensing pressure of R407C is higher than that ofR22. The temperature difference between dew pointtemperature corresponding to the condenser inlet pres-

sure and bubble point temperature corresponding to thecondenser exit pressure in the R407C system is higherthan that in the R22 system. Therefore, the R407C

capillary tube system represents lower subcooling thanthe R22 capillary tube system.Generally, the subcooling of a heat pump with a

capillary tube or a short tube tends to drop as the water

temperature entering the condenser increases [5,10].However, the variation of the subcooling for the R407CEEV system is nearly negligible with an increase of

water temperature entering the condenser at under-charged conditions. As the water temperature enteringthe condenser increased, the EEV opening was reduced

to maximize the evaporator effectiveness. Therefore, themass flow rate through EEV did not vary much with anincrease of condensing pressure due to an active controlof the EEV opening to maximize the system perfor-

mance. In addition, the mean temperature differencebetween the refrigerant and the water in the condenserwas nearly constant with an increase in water temper-

ature due to a rise of condensing temperature resultedfrom an reduction of the EEV opening.Fig. 8 represents the superheat for the R22 and

R407C capillary tube systems, and the R407C EEVsystem as a function of refrigerant charge. Forthe capillary tube systems, the superheat is reduced

as the refrigerant charge increases due to a rise ofthe refrigerant flow rate through the evaporator. For thecapillary tube systems, the superheat is kept at nearlyzero for +10% and +20% of full charge, causing wet

compression. For the R407C EEV system, the superheatis nearly constant even though both the watertemperature entering the condenser and the amount of

refrigerant charge increase due to an active control of

EEV opening.Fig. 9 shows the discharge temperature of the R407C

capillary tube system and the R407C EEV system with avariation of refrigerant charge. The discharge temper-

ature in the R407C capillary tube system increases con-tinuously with a reduction of charge amount due to arise of superheat and a decrease of mass flow rate.

However, the EEV system represents relatively stabledischarge temperature at all charge conditions. There-fore, it can be concluded that maintaining a constant

superheat can optimize the performance of a heat pumpand enhance the reliability of the system at off-designconditions.

4. Conclusions

The effects of refrigerant charge on the performanceof the water-to-water heat pump using R407C weremeasured by applying the capillary tube and the EEV as

Fig. 7. Variations of subcooling with charge amount.

Fig. 8. Variations of superheat with charge amount.

Fig. 9. Variations of discharge temperature for the R407C

capillary tube and EEV systems.

J. Choi, Y. Kim / International Journal of Refrigeration 27 (2004) 378–384 383

an expansion device under various charging conditions.The R22 capillary tube system was utilized as a baselineunit for the performance comparison with the R407Csystem. The full charge amount of the R407C capillary

tube system is less by 8% than that of the R22 capillarytube system. Generally, the performance of the R22 andR407C capillary tube systems is very sensitive to refrig-

erant charge and the degradation of the performance ishigher at undercharged conditions than that at over-charged conditions due to a relatively higher superheat

at undercharged conditions. However, the capacity andCOP of the R407C EEV system have little dependenceon the refrigerant charge due to an optimum control of

superheat by electronically adjusting the EEV opening.The R407C EEV system shows much lower reduction ofcapacity and COP as compared with the R22 andR407C capillary tube systems as the refrigerant charge

deviates from full charge. When the water temperatureentering the condenser is 34 �C, the maximum reductionsin COP for the R407C EEV and capillary tube systems are

3.6% and 16.5%, respectively. The discharge temperatureof the R407C capillary tube system increases with areduction of charge amount due to a rise of superheat

and a drop of mass flow rate. However, the EEV systemshows relatively stable discharge temperature at allcharge conditions. Generally, the EEV system showed

much higher system performance and reliability ascompared with the capillary tube system due to anactive electronic control of refrigerant flow rate throughthe EEV to obtain an optimum superheat level.

Acknowledgements

This work was supported by the Carbon DioxideReduction & Sequestration Center, one of the 21st

Century Frontier R&D Programs of the Ministry ofScience and Technology of Korea.

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