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An air-conditioning unit with a zeotropic mixture controlled by a distillation column
Pergamon
PII: S0360-5442(97)00111-4
Energy Vol. 23, No. 9, pp. 777–784, 1998 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0360-5442/98 $19.00 + 0.00
AN AIR-CONDITIONING UNIT WITH A ZEOTROPIC MIXTURE CONTROLLED BY A DISTILLATION COLUMN YANG ZHAO,†‡ MA YITAI,† ZHAO SANYUAN§ and LI YIE§ † Thermal Energy Department, Tianjin University, 92 Weijin Road, Nankai, Tianjin, 300072, Peoples Republic of China; §Architectural engineering college, 33 Jianguo Road, Zhangjiakou, 075000, Peoples Republic of China
(Received 23 December 1996)
Abstract—We describe a new air-conditioning and heat-pump system, which may be used to separate a refrigerant mixture efficiently and adjust the composition in the system with coolingor heating-load changes. A dynamic model has been developed to analyze the transient behavior of the capacity-control system. An air conditioner equipped with a small distillation circuit has been tested. Increases in the SEERs (seasonal energy efficiency ratios) for the new compositioncontrolled system are 28.3 and 17.7% and capacity changes 57.3–91.8% and 71.5–101%, respectively, for R22/142b and R23/124. 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Improvements in the energy efficiencies of residential air conditioners and heat pumps are of current interest. For a conventional air-conditioning and heat-pump system, a major cause of inefficiency is mismatch between the household energy demand and capacity for heating and cooling. One of the most effective methods for controlling capacity involves varying the composition of a zeotropic mixture in the refrigerating cycle to match the heating and cooling loads. Past attempts to achieve desirable changes of mixture compositions are described in Refs. [1–3]. The complexity and cost of the separation equipment have prevented wide applications. 2. A NEW CAPACITY-CONTROLLED SYSTEM
The new system is shown in Fig. 1. The system includes a distillation column (1) with a reboiler (4), which stores the less volatile component in a two-phase mixture, a condensing cooler (8), and another storage (3) for the more volatile component of the liquid. The method is described in Table 1. The valves A1, A2 and A3 are closed when the room temperature reaches a given value and the refrigeration system is separated from the distillation system. For room temperatures lower than the prescribed value in summer, valves A1, A2 are opened so that the distillation system provides a mixture with more of the higher boiling-point component to the system. In winter, valves A1, A3 will be opened and the distillation system will provide a mixture with more of the lower boiling-point component to the refrigerating system which then runs as a heat pump. If the room temperature is higher than specified, valves A2 and A3 are opened or closed as appropriate. In summer, valve A will be closed and B opened and conversely in winter. 3. TRANSIENT BEHAVIOR
The equations for mass conservation will now be given. When the duty capacities are decreased, M1( )Xb( ) − M1( )Xs( ) = d[MsXs( )]/d
‡
Author for correspondence. Fax: (022) 27404741, e-mail: [email protected] 777
(1)
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Yang Zhao et al
Fig. 1. A new air-conditioning and heat-pump systems; 1 = distillation column, 2 = packing material, 3 = storage, 4 = reboiler, 5 = compressor, 6 = outdoor heat exchanger, 7 = indoor heat exchanger, 8 = cooler, 9 = heat exchanger, 10 = valve, 11 = reversing valve, 12 = fan, A, B and A1-A3 are magnetic valves, B1, B2 and B3 are valves controlled by the temperature.
Table 1. Illustration of the method of control for magnetic valves. Season and operational procedure
Room-temperature conditions Valves that should be opened
Winter, running as a heatpump
The room temperature is higher than the specified value. The room temperature is lower than the specified value The room temperature is higher than the specified value The room temperature is lower than the specified value.
Summer, running as an air conditioner
Valves that should be closed
A1, A2, A
A3, B
A1, A3, A
A2, B
A1, A3, B
A2, A
A1, A2, B
A3, A
D( )Yt1( ) = d[Md( )Xd( )]/d,
(2)
D( ) = dMd( )/d,
(3)
Xs( )M1( ) − M1( )Xb( ) − D( )Yt1( ) = d[Mb(T)Xb( )]/d,
(4)
− D( ) = dMb( )/d
(5)
M1( )Xd( ) − M1( )Xs( ) = d[MsXs( )]/d,
(6)
D( )Yt1( ) − M1( )Xd( ) = d[Md( )Xd( )]/d,
(7)
D( ) − M1( ) = dMd( )/d,
(8)
Xs( )M1( ) − D( )Yt1( ) = d[Mb( )Xb( )]/d,
(9)
When duty capacities are increased,
An air-conditioning unit with a zeotropic mixture controlled by a distillation column
779
Fig. 2. Compositions in the refrigeration system.
M1( ) − D( ) = dMb( )/d,
(10)
Ms( )Xs( ) + Mb( )Xb( ) + Md( )Xd( ) = constant
(11)
The equations for energy conservation follow (every parameter is a function of time ). For reduced loads, Qb = Vbhb + M1hb − Lhhl + d(Mbub )/d
(12)
Qb + Lt1ht1l + M1h1 = M1hbVt1ht1 + d(Mbub )/d,
(13)
Qc = Vt1(ht1 − ht1l ),
(14)
Fig. 3. Compositions in the storage and reboiler.
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Yang Zhao et al
Fig. 4. Mdr, Mbr as functions of time.
Fig. 5. Tbr, Tfr, Tvr, Tlr as functions of time for an increase of heating or cooling capacity.
Qb − M1hb − Dht1l − Qc + M1h1 = d(Mbub )/d,
(15)
Dht1l = d(Mdud )/d
(16)
Qb = Vbhb − Lhhl + d(Mbub )/d,
(17)
Qb + Lt1ht1l + M1h1 = Vt1ht1 + d(Mbub )/d,
(18)
Qc = Vt1(ht1 − ht1l ),
(19)
For increased loads,
An air-conditioning unit with a zeotropic mixture controlled by a distillation column
781
Fig. 6. Ptr, Pt as functions of time.
Fig. 7. Comparison between Qb and Qx.
Qb − Dht1l − Qc + M1h1 = d(Mbub )/d,
(20)
Dht1l − M1hd = d(Mdud )/d
(21)
Qx = (h5 − h6 )dMs/d
(22)
For variable loads in general,
The equations have been rewritten in finite-difference form and solved numerically. The CSD (Carnhan–Starling–DeSantise) equation of state Eq. (4) was used to calculate the properties of the mixture. Figs 2–7, show the transient behavior of the new systems with the refrigerant mixture R22/142b. We see that the compositions of R22 are changed when the duty capacities are changed.
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Yang Zhao et al
Fig. 8. Test results of capacities as a function of the Tdb.
The highest temperature in the system is at the reboiler 4. Fig. 8 shows a comparison between the discharge superheat Qx in the outlet of a compressor and the heat energy Qb required by the reboiler 4. 4. EXPERIMENTAL STUDIES
The experimental unit is a residential air conditioner equipped with a distillation circuit. A variable opening-size expansion valve controlled by superheat in the outlet of the indoor heat exchanger 7 is utilized. The distillation column 1 is 200 mm long and 20 mm in diameter and is made of copper pipe. The pipe is filled with a packing material. The system operates either with the zeotropic refrigerant mixture R22/142b or with R32/124. The first test results demonstrate that, for the same working conditions, the capacities change 63 and 68% for the mixtures R22/142b and R32/124, respectively. Figs 8–10, show comparisons of the
Fig. 9. Power consumption as a function the Tdb.
An air-conditioning unit with a zeotropic mixture controlled by a distillation column
783
Fig. 10. Test results for the EER as a function of the outdoor temperature.
Table 2. Comparisons between the two manners of the modulation. Manner of control Working fluid SEER Increase %
Condensation control
Condensation control
On-off control
R22/142b 3.04 28.3
R32/124 2.79 17.7
R22 2.37
capacities qssj, power consumption Wssj and EERj at the steady state for the new system and for the earlier on–off system in which the outdoor temperature Tdbj was varied. Table 2 shows test results for SEERs. We see that the new system is effective in controlling the refrigerant compositions. It is evident that compared with the on–off system, increases in SEER for the new composition-controlled system are 28.3 and 17.7% and capacity changes are 57.3–91.8% and 71.5–101% for R22/142b and R32/124, respectively. The new system changes the heating and cooling capacities to match the building load effectively. REFERENCES
1. Cooper, W. D., ASHRAE Trans, 1992, 82, 1159. 2. Zhao, Y., Energy—The International Journal, 1997, 22, 669. 3. Zhao, Y., Yitai, M. and Canren, L., J. Engng. Thermophys., 1997, 18, 141. NOMENCLATURE
D = mass-flow rate (kg/s) of the fluid entering the storage 3 EERj = energy efficiency ratios for the systems h5,h6 = specific enthalpies (kJ/kg) of the compressor discharge and the fluid entering the outdoor exchanger, respectively hbv,hb = steam and liquid specific enthalpies (kJ/kg) stored in the reboiler 4, respectively hd = liquid specific enthalpy (kJ/kg) of the fluid leaving the storage 3
h1 = liquid specific enthalpy (kJ/kg) of the fluid entering the reboiler 4 ht1l,ht1v = specific enthalpies (kJ/kg) of the fluid entering and leaving the cooler 8, respectively Lh, Lt1 = mass-flow rate (kg/s) of the fluid entering the reboiler 4 and column 1, respectively L = mass-flow rate (kg/s) of the fluid entering the reboiler 4 M1 = mass-flow rate (kg/s) from the distillation system to the refrigeration system
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Yang Zhao et al
Ms,Md,Mb = fluid quantities (kg) stored in the storage 3 and reboiler 4, respectively Mdr,Mbr = relative fluid quantities with respect to original fluid quantities in the refrigerating system stored in the refrigeration, storage 3 and reboiler 4, respectively Ptr, = relative pressure of the distillation column 1 with respect to its original value Pr = pressure ratio of the compressor qssj = heating or cooling capacities (kW) at the steady state Qb, Qc = heat flux (kW) required by the reboiler 4 and the condensing heat removed from the cooler 8, respectively Qx = discharge superheat flux (kW) at the outlet of the compressor SEER = seasonal energy efficiency ratio for the system Tbr = relative temperatures of the fluid in the reboiler 4 with respect to its original value Tfr = relative temperatures of the fluid entering the distillation column 1
with respect to the original temperature of the fluid the in the reboiler Tvr,Tlr = relative temperatures of the fluid entering and leaving the cooler 8, respectively, with respect to original temperature of the fluid in the reboiler 4 Tdb = outdoor temperature (°C) in the dry bubble ub, ud = heat energy per unit mass (kJ/kg) of the liquid stored in the reboiler 4 and storage 3, respectively Vb = mass-flow rate (kg/s) of the steam in the outlet of the reboiler 4 Wssj = power consumption (kW) at the steady state Xst,Xbt = mass fraction of R22 (for the mixture R22/142b) or R32 (for R32/134a and R32/124) in the refrigeration circuit and reboiler 4 (0–100%), respectively Xdt,Yt1 = liquid and vapor phase mass fractions of R22 or R32 in the storage 3, respectively = time(s)
PII: S0360-5442(97)00111-4
Energy Vol. 23, No. 9, pp. 777–784, 1998 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0360-5442/98 $19.00 + 0.00
AN AIR-CONDITIONING UNIT WITH A ZEOTROPIC MIXTURE CONTROLLED BY A DISTILLATION COLUMN YANG ZHAO,†‡ MA YITAI,† ZHAO SANYUAN§ and LI YIE§ † Thermal Energy Department, Tianjin University, 92 Weijin Road, Nankai, Tianjin, 300072, Peoples Republic of China; §Architectural engineering college, 33 Jianguo Road, Zhangjiakou, 075000, Peoples Republic of China
(Received 23 December 1996)
Abstract—We describe a new air-conditioning and heat-pump system, which may be used to separate a refrigerant mixture efficiently and adjust the composition in the system with coolingor heating-load changes. A dynamic model has been developed to analyze the transient behavior of the capacity-control system. An air conditioner equipped with a small distillation circuit has been tested. Increases in the SEERs (seasonal energy efficiency ratios) for the new compositioncontrolled system are 28.3 and 17.7% and capacity changes 57.3–91.8% and 71.5–101%, respectively, for R22/142b and R23/124. 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Improvements in the energy efficiencies of residential air conditioners and heat pumps are of current interest. For a conventional air-conditioning and heat-pump system, a major cause of inefficiency is mismatch between the household energy demand and capacity for heating and cooling. One of the most effective methods for controlling capacity involves varying the composition of a zeotropic mixture in the refrigerating cycle to match the heating and cooling loads. Past attempts to achieve desirable changes of mixture compositions are described in Refs. [1–3]. The complexity and cost of the separation equipment have prevented wide applications. 2. A NEW CAPACITY-CONTROLLED SYSTEM
The new system is shown in Fig. 1. The system includes a distillation column (1) with a reboiler (4), which stores the less volatile component in a two-phase mixture, a condensing cooler (8), and another storage (3) for the more volatile component of the liquid. The method is described in Table 1. The valves A1, A2 and A3 are closed when the room temperature reaches a given value and the refrigeration system is separated from the distillation system. For room temperatures lower than the prescribed value in summer, valves A1, A2 are opened so that the distillation system provides a mixture with more of the higher boiling-point component to the system. In winter, valves A1, A3 will be opened and the distillation system will provide a mixture with more of the lower boiling-point component to the refrigerating system which then runs as a heat pump. If the room temperature is higher than specified, valves A2 and A3 are opened or closed as appropriate. In summer, valve A will be closed and B opened and conversely in winter. 3. TRANSIENT BEHAVIOR
The equations for mass conservation will now be given. When the duty capacities are decreased, M1( )Xb( ) − M1( )Xs( ) = d[MsXs( )]/d
‡
Author for correspondence. Fax: (022) 27404741, e-mail: [email protected] 777
(1)
778
Yang Zhao et al
Fig. 1. A new air-conditioning and heat-pump systems; 1 = distillation column, 2 = packing material, 3 = storage, 4 = reboiler, 5 = compressor, 6 = outdoor heat exchanger, 7 = indoor heat exchanger, 8 = cooler, 9 = heat exchanger, 10 = valve, 11 = reversing valve, 12 = fan, A, B and A1-A3 are magnetic valves, B1, B2 and B3 are valves controlled by the temperature.
Table 1. Illustration of the method of control for magnetic valves. Season and operational procedure
Room-temperature conditions Valves that should be opened
Winter, running as a heatpump
The room temperature is higher than the specified value. The room temperature is lower than the specified value The room temperature is higher than the specified value The room temperature is lower than the specified value.
Summer, running as an air conditioner
Valves that should be closed
A1, A2, A
A3, B
A1, A3, A
A2, B
A1, A3, B
A2, A
A1, A2, B
A3, A
D( )Yt1( ) = d[Md( )Xd( )]/d,
(2)
D( ) = dMd( )/d,
(3)
Xs( )M1( ) − M1( )Xb( ) − D( )Yt1( ) = d[Mb(T)Xb( )]/d,
(4)
− D( ) = dMb( )/d
(5)
M1( )Xd( ) − M1( )Xs( ) = d[MsXs( )]/d,
(6)
D( )Yt1( ) − M1( )Xd( ) = d[Md( )Xd( )]/d,
(7)
D( ) − M1( ) = dMd( )/d,
(8)
Xs( )M1( ) − D( )Yt1( ) = d[Mb( )Xb( )]/d,
(9)
When duty capacities are increased,
An air-conditioning unit with a zeotropic mixture controlled by a distillation column
779
Fig. 2. Compositions in the refrigeration system.
M1( ) − D( ) = dMb( )/d,
(10)
Ms( )Xs( ) + Mb( )Xb( ) + Md( )Xd( ) = constant
(11)
The equations for energy conservation follow (every parameter is a function of time ). For reduced loads, Qb = Vbhb + M1hb − Lhhl + d(Mbub )/d
(12)
Qb + Lt1ht1l + M1h1 = M1hbVt1ht1 + d(Mbub )/d,
(13)
Qc = Vt1(ht1 − ht1l ),
(14)
Fig. 3. Compositions in the storage and reboiler.
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Yang Zhao et al
Fig. 4. Mdr, Mbr as functions of time.
Fig. 5. Tbr, Tfr, Tvr, Tlr as functions of time for an increase of heating or cooling capacity.
Qb − M1hb − Dht1l − Qc + M1h1 = d(Mbub )/d,
(15)
Dht1l = d(Mdud )/d
(16)
Qb = Vbhb − Lhhl + d(Mbub )/d,
(17)
Qb + Lt1ht1l + M1h1 = Vt1ht1 + d(Mbub )/d,
(18)
Qc = Vt1(ht1 − ht1l ),
(19)
For increased loads,
An air-conditioning unit with a zeotropic mixture controlled by a distillation column
781
Fig. 6. Ptr, Pt as functions of time.
Fig. 7. Comparison between Qb and Qx.
Qb − Dht1l − Qc + M1h1 = d(Mbub )/d,
(20)
Dht1l − M1hd = d(Mdud )/d
(21)
Qx = (h5 − h6 )dMs/d
(22)
For variable loads in general,
The equations have been rewritten in finite-difference form and solved numerically. The CSD (Carnhan–Starling–DeSantise) equation of state Eq. (4) was used to calculate the properties of the mixture. Figs 2–7, show the transient behavior of the new systems with the refrigerant mixture R22/142b. We see that the compositions of R22 are changed when the duty capacities are changed.
782
Yang Zhao et al
Fig. 8. Test results of capacities as a function of the Tdb.
The highest temperature in the system is at the reboiler 4. Fig. 8 shows a comparison between the discharge superheat Qx in the outlet of a compressor and the heat energy Qb required by the reboiler 4. 4. EXPERIMENTAL STUDIES
The experimental unit is a residential air conditioner equipped with a distillation circuit. A variable opening-size expansion valve controlled by superheat in the outlet of the indoor heat exchanger 7 is utilized. The distillation column 1 is 200 mm long and 20 mm in diameter and is made of copper pipe. The pipe is filled with a packing material. The system operates either with the zeotropic refrigerant mixture R22/142b or with R32/124. The first test results demonstrate that, for the same working conditions, the capacities change 63 and 68% for the mixtures R22/142b and R32/124, respectively. Figs 8–10, show comparisons of the
Fig. 9. Power consumption as a function the Tdb.
An air-conditioning unit with a zeotropic mixture controlled by a distillation column
783
Fig. 10. Test results for the EER as a function of the outdoor temperature.
Table 2. Comparisons between the two manners of the modulation. Manner of control Working fluid SEER Increase %
Condensation control
Condensation control
On-off control
R22/142b 3.04 28.3
R32/124 2.79 17.7
R22 2.37
capacities qssj, power consumption Wssj and EERj at the steady state for the new system and for the earlier on–off system in which the outdoor temperature Tdbj was varied. Table 2 shows test results for SEERs. We see that the new system is effective in controlling the refrigerant compositions. It is evident that compared with the on–off system, increases in SEER for the new composition-controlled system are 28.3 and 17.7% and capacity changes are 57.3–91.8% and 71.5–101% for R22/142b and R32/124, respectively. The new system changes the heating and cooling capacities to match the building load effectively. REFERENCES
1. Cooper, W. D., ASHRAE Trans, 1992, 82, 1159. 2. Zhao, Y., Energy—The International Journal, 1997, 22, 669. 3. Zhao, Y., Yitai, M. and Canren, L., J. Engng. Thermophys., 1997, 18, 141. NOMENCLATURE
D = mass-flow rate (kg/s) of the fluid entering the storage 3 EERj = energy efficiency ratios for the systems h5,h6 = specific enthalpies (kJ/kg) of the compressor discharge and the fluid entering the outdoor exchanger, respectively hbv,hb = steam and liquid specific enthalpies (kJ/kg) stored in the reboiler 4, respectively hd = liquid specific enthalpy (kJ/kg) of the fluid leaving the storage 3
h1 = liquid specific enthalpy (kJ/kg) of the fluid entering the reboiler 4 ht1l,ht1v = specific enthalpies (kJ/kg) of the fluid entering and leaving the cooler 8, respectively Lh, Lt1 = mass-flow rate (kg/s) of the fluid entering the reboiler 4 and column 1, respectively L = mass-flow rate (kg/s) of the fluid entering the reboiler 4 M1 = mass-flow rate (kg/s) from the distillation system to the refrigeration system
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Yang Zhao et al
Ms,Md,Mb = fluid quantities (kg) stored in the storage 3 and reboiler 4, respectively Mdr,Mbr = relative fluid quantities with respect to original fluid quantities in the refrigerating system stored in the refrigeration, storage 3 and reboiler 4, respectively Ptr, = relative pressure of the distillation column 1 with respect to its original value Pr = pressure ratio of the compressor qssj = heating or cooling capacities (kW) at the steady state Qb, Qc = heat flux (kW) required by the reboiler 4 and the condensing heat removed from the cooler 8, respectively Qx = discharge superheat flux (kW) at the outlet of the compressor SEER = seasonal energy efficiency ratio for the system Tbr = relative temperatures of the fluid in the reboiler 4 with respect to its original value Tfr = relative temperatures of the fluid entering the distillation column 1
with respect to the original temperature of the fluid the in the reboiler Tvr,Tlr = relative temperatures of the fluid entering and leaving the cooler 8, respectively, with respect to original temperature of the fluid in the reboiler 4 Tdb = outdoor temperature (°C) in the dry bubble ub, ud = heat energy per unit mass (kJ/kg) of the liquid stored in the reboiler 4 and storage 3, respectively Vb = mass-flow rate (kg/s) of the steam in the outlet of the reboiler 4 Wssj = power consumption (kW) at the steady state Xst,Xbt = mass fraction of R22 (for the mixture R22/142b) or R32 (for R32/134a and R32/124) in the refrigeration circuit and reboiler 4 (0–100%), respectively Xdt,Yt1 = liquid and vapor phase mass fractions of R22 or R32 in the storage 3, respectively = time(s)