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Open airvapor compression refrigeration system for airconditioning and hot water cooled by cool water

Shaobo Hou a,b,*, Huacong Li a, Hefei Zhang a

a School of Power and Energy, Northwestern Polytechnical University, Xian, Shaanxi 710072, PR Chinab College of Engineering, Guangdong Ocean University, East Jiefang Road, No. 40, Xiashan, Zhanjiang, Guangdong 524006, PR China

Received 25 January 2005; received in revised form 22 September 2006; accepted 9 May 2007Available online 18 June 2007

Abstract

This paper presents an open airvapor compression refrigeration system for air conditioning and hot water cooled by cool water andproves its feasibility through performance simulation. Pinch technology is used in analysis of heat exchange in the surface heat exchan-ger, and the temperature difference at the pinch point is selected as 6 C. Its refrigeration depends mainly on both air and vapor, moreefficient than a conventional air cycle, and the use of turbo-machinery makes this possible. This system could use the cool in the coolwater, which could not be used to cool air directly. Also, the heat rejected from this system could be used to heat cool water to 3340 C. The sensitivity analysis of COP to gc and gt and the simulated results T4, T7, T8, q1, q2 and Wm of the cycle are given. The sim-ulations show that the COP of this system depends mainly on T7, gc and gt and varies with T3 or Twet and that this cycle is feasible insome regions, although the COP is sensitive to the efficiencies of the axial compressor and turbine. The optimum pressure ratio in thissystem could be lower, and this results in a fewer number of stages of the axial compressor. Adjusting the rotation speed of the axialcompressor can easily control the pressure ratio, mass flow rate and the refrigerating capacity. The adoption of this cycle will makethe air conditioned room more comfortable and reduce the initial investment cost because of the obtained very low temperature air.Humid air is a perfect working fluid for central air conditioning and no cost to the user. The system is more efficient because of usingcool water to cool the air before the turbine. In addition, pinch technology is a good method to analyze the wet air heat exchange withwater. 2007 Elsevier Ltd. All rights reserved.

Keywords: Turbo-machinery; Air cycle; Air conditioning units; Natural working fluid; Refrigeration; Pinch technology

1. Introduction

The outdoor air temperature can reach 3840 C in mid-

dle China in summer, while the temperature of the waterfrom the water supply system and underground is about17 C and that of the refilled underground water could belower. These waters are too cool for people to bath directlyat the hotel, and the bath water is usually heated by a boi-ler. If these waters were used to cool air for air conditioning

directly, it is not efficient because of the small temperaturedifference and large amount of consumed water.

The air compression refrigeration cycle was studied long

ago. Several disadvantages prevented air from being usedas a working fluid in refrigeration. These included low vol-umetric refrigerating effect, which may result in a largecompressor, and low COP due to low efficiencies of thecompressor and expander. After CFCs invention in the1930s, people paid little attention to air compressionrefrigeration.

Recently, as a result of the destruction of the ozono-sphere by chlorofluorocarbons (CFC) and the pressure ofenvironmental protection, research upon air refrigerationcycles has received more attention [13]. Optimizations of

0196-8904/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2007.05.005

* Corresponding author. Address: School of Power and Energy,Northwestern Polytechnical University, Xian, Shaanxi 710072, PR China.Tel.: +86 759 2886705.

E-mail addresses: [email protected], [email protected] (S. Hou).

www.elsevier.com/locate/enconman

Energy Conversion and Management 48 (2007) 22552260
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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air cycles are also conducted using finite time thermody-namics (FTT) or entropy generation minimization

(EGM) [47].Chen et al. investigated the cooling load versus COP

characteristics of a simple [8] and a regenerated [9,10] airrefrigeration cycle with heat transfer loss and/or otherirreversibilities. Luo et al. [11] optimized the cooling loadand the COP of a simple irreversible air refrigeration cycleby searching for the optimum pressure ratio of the com-pressor and the optimum distribution of heat conductanceof the hot and cold side heat exchangers for fixed total heatexchanger inventory.

With the development of the aeronautical industry,highly efficient axial compressors and turbines have

become a reality. At present, the stagnation isentropic effi-ciencies of a single stage axial compressor and a turbine canreach 0.880.91 [12]. High speed fans have been used inordinary air conditioning systems nowadays.

However, the water vapor in the working fluid was notconsidered in all the above researches [111] on the aircompression refrigeration cycle, and the used equipmentswere a centrifugal compressor and a centripetal turbine,which have lower efficiencies than axial compressors andturbines. The amount of water extracted from high pres-sure wet air can reach 1830 g/kg(d.a.), and the amountof latent heat discharged from the vapor condensed isabout 4575 kJ/kg(d.a.), exceeding the sensible heat fromthe air of 3050 kJ/kg(d.a.).

Hou and Li presented an axial flow airvapor compres-sion refrigerating system for air conditioning in 1992 [13] inwhich wet air is the working fluid and an axial compressorand turbine were used, but these have not yet attractedpeoples attention so far.

Hou and Zhang presented an axial flow airvapor com-pression refrigerating system for air conditioning cooled bycirculating water in Ref. [14] (2004) in which wet air is theworking fluid, an axial compressor and turbine were usedand the wet air is cooled by circulating water. The paperproves its feasibility through performance simulation and

also indicates its advantages. These include the possibility

to simplify air conditioning systems, to reduce the amountof initial investment of an air conditioning system and to

make air conditioned rooms more comfortable.The aim of this paper is to present an open system,

which is an open airvapor compression refrigeration sys-tem for air conditioning and hot water cooled by coolwater and its performance from simulation. In this openairvapor refrigeration cycle, water from the water supplysystem or underground is used. Thus, we could get a lowerwet air temperature before the turbine. In addition, thecool water is heated for the bath.

2. System

Representation on enthalpyentropy coordinates and acircuit diagram of an open air-compression refrigerationsystem for air conditioning and hot water cooled by coolwater are shown in Fig. 1.

The outdoor air at 2 is drawn into the atomizing cham-ber, cooled to saturated air at 3 with some fine water drop-lets and then compressed by an axial compressor. A flow ofcompressed air at 4 with higher temperature, T4, and highpressure, P4, is obtained. Then, the compressed air at 4 iscooled to saturated air at 7 with a temperature T7 by coolwater/underground water in a surface heat exchanger after

Nomenclature

B wet air pressure, PaD humidity ratio of wet air, g/kg(d.a.)H enthalpy of wet air, kJ/kg(d.a.)

P pressure, PaT temperature, K or CTwet wet bulb temperature, K or CWc ideal input work of compressor, kJ/kg(d.a.)Wt ideal output work of turbine, kJ/kg(d.a.)Wm practical work consumed by system, kJ/kg(d.a.)gc efficiency of compressorgt efficiency of turbineratio mass flow rate ratio of cooling water to dry air

R gas constant, kJ/kg Kn exponent

Subscriptsair dry airvapor water vapor in moist airs saturatedlast last timehot hot streamcold cold streamw water

Fig. 1. Representation on enthalpyentropy coordinates and circuitdiagram of an open air-compression refrigeration system for air condi-

tioning and hot water cooled by cool water.

2256 S. Hou et al. / Energy Conversion and Management 48 (2007) 22552260


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the axial compressor outlet. Some vapor is condensed,and the latent heat of the vapor is discharged from 4 to7. Then, the saturated air at 7 is expanded and cooled tothe cool air at 8 in the turbine. The cool air at 8 is thenducted to the air conditioned rooms. The cool water isheated in the surface heat exchanger.

Water injection before the axial compressor aims todecrease both the temperature of the working fluid andthe polytropic exponent in the compression process. Thus,we can save some compression work. This method has beenused in a jet engine when a fighter plane increases its speed.However, the difference is that what is injected in a jetengine is water, alcohol, etc. [12].

The water vapor in the compressed air can easily beextracted by a surface heat exchanger. With the same tem-perature, the humidity ratio of the saturated wet air at highpressure P4 is only about P3/P4 of that at pressure P3. Themethod of using compressed air to acquire dry air has beenused in some workshops in southern China.

The system above differs from a conventional air cyclesystem. There are many characteristics in this airvaporrefrigeration circle.

Firstly, an axial compressor and a turbine are used inthe above system. The characteristics of turbo-machinesare large mass flow rate and high efficiency. The other typesof compressor and expander have none of the aboveadvantages.

Secondly, this refrigeration system intakes precooledwet air with fine water droplets, and some vapor is con-densed during the air cooling from 4 to 7. The amount ofwater extracted from the high pressure wet air can reach

1830 g/kg(d.a.), and the amount of latent heat dischargedfrom the vapor condensed, about 4575 kJ/kg(d.a.),exceeds the sensible heat from the air, 3050 kJ/kg(d.a.).For this reason, the refrigeration load in this airvaporrefrigeration system depends on a combination of the sen-sible heat of air and the latent heat of vapor.

Lastly, the cool from the cooling water was used. Usu-ally, it cannot be used.

3. Performance simulation

3.1. Wet air

The humidity ratio of wet air, D, is obtained from

D 621:98Pvap

B Pvap1

The enthalpy of wet air, H, is calculated from

H 1:006t 0:001D2501 1:805t 2

The adopted relation for water vapor between satura-tion pressure and saturation temperature, Ps = f(ts), isselected from Ref. [15].

To calculate the saturated temperature of the wet airfrom the saturated enthalpy, Eqs. (1) and (2) and Ps = f(ts)

are used.

3.2. Axial compressor

During the compression process of the wet air, the finewater droplets in the air may evaporate. Because the evap-oration of water takes in heat, we can regard the ideal com-pression process of the wet air in the compressor as a

polytropic process. Therefore, we can obtain the ideal workof the compressor per kilogram dry air, Wc, from

Wc n

n 1Rair 0:001DRvaporT3 1

p4

p3

n1n

" #3

in which n is the polytropic exponent for the compressionprocess.

The practical work consumed by the axial compressor isWc/gc in which gc is the thermal efficiency of thecompressor.

3.3. Turbine

The saturated air with a pressure of P7 and a tempera-ture of T7 before the turbine has been dehumidifed in thesurface heat exchanger by cooling water. At point 7, theamount of vapor included in the saturated air is very small,about P3/P7 of the amount included in saturated air at P3.Thus, the water condensed in the air is in fog. Nevertheless,expansion of the saturated air in the turbine cannot beregarded as an adiabatic expansion of an ideal gas. Withthe decrease of the wet air pressure in the turbine, the tem-perature of the wet air decreases, and some heat is dis-charged during the condensation of some water vapor.The heat discharged may cause increases in both the tem-perature of the turbine outlet and the work done in theexpansion.

For this problem, we can imagine that no phase changeexists and that there is some heat added to the wet air dur-ing the expansion process when we calculate the work doneby the expansion process. According to the above assump-tion, this problem can be simplified to a problem of thepolytropic expansion of an ideal gas. Consequently, wecan obtain the ideal work done by the expansion, Wt,through iteration and then obtain the real work generatedby the turbine and the temperature of the turbine outlet.

The following are the steps to calculate Wt and T8.

1. Determine Ps7 from T7, D7 from Ps7 and P7, and H7from D7 and T7.

2. Get gas constant R for the saturated air at 7 by usingthe formula:

R 0:001287 0:461D7: 4

3. Calculate the initial Wt to iterate according to an adi-abatic expansion of an ideal gas.

4. Calculate the enthalpy of the saturated air at 8 by usingH8 = H7 Wt.

5. Determine T8, Ps8 D8 using H8 and P8.

S. Hou et al. / Energy Conversion and Management 48 (2007) 22552260 2257


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6. Obtain nn1

logP7=P8logT7=T8

: 5

7. Wt;new n

n1RT7 T8: 6

8. Wt WtWt;new

2.

9. H8 = H7 Wt.10. T8last = T8.

11. Determine T8, Ps8, D8 using H8 and P8.12. If abs (T8last T8) < 0.1 go to step 13. otherwise, go to

step 5.13. Obtain the enthalpy using the formula, H8 = H7

Wt gt.14. Determine T8, Ps8, D8 using H8 and P8.

3.4. Surface heat exchanger

Pinch technology is used in the analysis of heat exchangein the surface heat exchanger, and the temperature differ-ence at the pinch point is 6 C. Pinch technology is a graph-

ical method of identifying technically and economicallyinteresting energy efficiency measures. The minimum cool-ing and heating demands in the system can thereby bedetermined, together with the net heat for each tempera-ture level. The concepts and methodology of pinch technol-ogy are well explained in the works of Linhoff et al. [16],Eastop and Croft [17], Linnhoff [18] and Mubarak Ebra-him [19].

The cold stream is cooling water, and the enthalpy isdetermined by Hcold = cpwt.

The hot stream is hot wet air with a pressure ofP4, andthe enthalpy is determined by

Hhot 1:006t 0:001Ds2501 1:805t 7in which

Ds 621:98Pst

P4 Pst8

The optimum mass flow rate ratio of cool water to dryair can be obtained from the hot and cold curves accordingto pinch technology.

4. Performance

The refrigerating capacity per kilogram dry air, q2, can

be determined by the enthalpy difference between the inletof the compressor and the outlet of the turbine by using thefollowing formula

q2 H3 H8 9

The work consumed by the refrigeration cycle is calcu-lated by

Wm Wc

gc

Wt gt 10

The COP of this refrigeration system is calculated by thefollowing formula. (The work consumed by the cool water

system is not included in Wm.)

COP q2

Wm11

5. Results

There are many factors that may influence the COP of

this refrigerating system for air conditioning and hot watercooled by cool water. These include the pressure ratio ofthe axial compressor, P4/P3, the efficiencies of the axialcompressor and turbine, the wet bulb temperature of theatmosphere Twet and the cool water temperature.

During simulation, the pressure ratio of the axial com-pressor was varied from 1.6 to 2.5, the wet bulb tempera-ture of the outdoor air from 20 to 30 C and the coolingwater temperature from 15 to 25 C.

There is 300 Pa pressure loss before the axial compres-sor, 300 Pa between the axial compressor and turbine and600 Pa after the turbine.

Some encouraging results are illustrated in Figs. 26.The results are calculated from a lower pressure ratio ofthe axial compressor than that in Ref. [14].

The sensitivity of COP to the efficiencies of the axialcompressor and turbine is illustrated in Fig. 2. The linesin Fig. 2 are the COP lines of an open air compressionrefrigeration system for air conditioning and hot watercooled by cool water when T7 = 296 K and the efficienciesof the axial compressor and turbine are 90%, 88%, 86% and80%, respectively. From Fig. 2, the efficiencies of the axialcompressor and the turbine influence the COP heavily. TheCOP is higher because of a lower T7 than we can get thanin Ref. [14]. Also, the optimum pressure ratio of the axial

compressor declines. That is, the number of stages couldbe decreased to two.

Although the COP is sensitive to the efficiencies of theaxial compressor and turbine, these cycles are feasible.Firstly, this new turbo-machinery works near the designpoint, and the efficiencies of axial compressors and turbines

20 22 24 26 28

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

nt=nc=0.9

nt=nc=0.88

nt=nc=0.86

nt=nc=0.80

COP

T3/ Twet,oC

Fig. 2. The sensitivity of COP to efficiencies of the axial compressor and

turbine when T7 = 23 C.



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are very high at the design point, about 0.890.91. Sec-ondly, the intake air is clean and without dust, and there-fore, the efficiencies of the axial compressor and turbinewill not drop greatly while working. Thirdly, there is nogreat complexity of the combustion chamber and high tem-perature turbine in the turbo-machinery. Consequently, itis much easier to accomplish than many people imagine.Lastly, the efficiencies of the axial compressor and turbinehave room for improvement with additional designmeasures.

The simulations of an open airvapor compressionrefrigeration system for air conditioning and hot watercooled by cool water when gc = 0.90 and gt = 0.90 andT7 = 296 K are illustrated in Figs. 3 and 4. Fig. 3 givesthe relations of the temperature after the compressor, T4,the temperature before the turbine, T7, and the tempera-

ture after the turbine, T8 to the temperature before the

compressor, T3 (Twet). Fig. 4 gives the relations of therefrigerating capacity per kilogram dry air, q2, the dis-charging heat per kilogram dry air, q1, and the work con-sumed by the refrigeration system, Wm, to thetemperature before the compressor, T3 (Twet). The relationof COP to the temperature before the compressor, T3 (Twet)can be located in Fig. 2.

The simulations of an open airvapor compressionrefrigeration system for air conditioning and hot watercooled by cool water when gc = 0.90and gt = 0.90 andT7 = 2025 C are illustrated in Figs. 5 and 6. Fig. 5 givesthe variation of COP with T3 (Twet) and T7. Fig. 6 gives thevariation of refrigerating capacity per kilogram dry air, q2,

with T3(Twet) and T7.

20 22 24 26 28

0

20

40

60

80

100

q1, kJ/kg(d.a)

q2, kJ/kg(d.a)

wm, kJ/kg(d.a)

T3/ Twet,oC

Fig. 4. The simulated q1, q2, Wm of an open air-compression refrigerationsystem for air conditioning and hot water cooled by cool water whengc = 0.90 and gt = 0.90 and T7 = 23 C.

2021

2223

2425

3.0

3.5

4.0

4.5

20

2122

2324

2526

2728

COP

T 3/ T

wet,

o C

T7 ,

oC

Fig. 5. The COP of an open air-compression refrigeration system for airconditioning and hot water cooled by cool water when gc = 0.90 and

gt = 0.90.

T 3/ T

wet,

o C

T7 ,

oC

2021

2223

24

25

50

60

70

80

90

20

2122

2324

2526

2728

q2,

kJ/(d.a

)

Fig. 6. The q2 of an open air-compression refrigeration system for airconditioning and hot water cooled by cool water when gc = 0.90 andgt = 0.90.

T3/ Twet,oC

20 22 24 26 28

-10

0

10

20

30

40

50

60

T4

T7

T8

T,o

C

Fig. 3. The simulated temperatures of an open air-compression refriger-ation system for air conditioning and hot water cooled by cool water whengc = 0.90 and gt = 0.90 and T7 = 23 C.

S. Hou et al. / Energy Conversion and Management 48 (2007) 22552260 2259


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Fig. 7 gives the hot and cold curves of the cooling anddehumidification process in the surface heat exchangerwhen the mass flow rate ratio of cool water to dry airis one. In Fig. 7, the temperature difference at the pinchpoint is 6 C, and there are two hot lines, which is whenT3 = 20 C and T3 = 27 C, respectively. From Fig. 7, wecan get hot water with a temperature of about 3340 Cand the same mass flow rate as the dry air mass flowrate.

6. Conclusions

This study shows the feasibility of an open airvaporcompression refrigeration system for air conditioning andhot water cooled by cool water. The calculation resultsshow:

1. The open airvapor compression refrigeration systemfor air conditioning and hot water cooled by cool watergiven in this paper could use the cool in cool water,which could not be used to cool air directly. Also, theheat rejected by this system could be used to heat water.

2. Humid air is a perfect working fluid for refrigeration incentral air conditioning.

3. To compare with airvapor compression refrigeration

system for air conditioning cooled by circulating water[14], this system is suitable to be employed in the placewhere we could get cold water easily.

4. According to the axial compressor performance charac-teristics among rotation speed, mass flow rate and pres-sure ratio, we can easily control its mass flow rate andpressure ratio and the refrigerating capacity by adjustingthe rotation speed of the axial compressor.

5. The COP of this refrigeration air conditioning circle var-ies with the wet bulb temperature of the atmosphere.The higher the wet bulb temperature of the atmosphere,the higher is the COP of this refrigerating air condition-

ing cycle.

6. The COP of this refrigeration air conditioning systemrests mainly on gc and gt. The temperature of cool waterwill also affect it heavily. Although the COP is sensitiveto gc and gt, an open airvapor compression refrigera-tion cycle for air conditioning and hot water cooled bycool water is still feasible.

7. The optimum pressure ratio of the axial compressorcould be lower in this system. This results in a fewernumber of stages of the axial compressor.

8. Pinch technology is a good method to analyze the heatexchange of wet air cooling.

References

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[8] Chen L, Wu C, Sun F. Cooling-load versus COP characteristics for

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[15] ASHRAE Handbook fundamentals volume, 2001, ASHRAE 1791Tulie Circle, Atlanta, GA 30329, United States of America.

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[18] Linnhoff B. Pinch analysis a state-of-the-art overview. Trans IchemePart A 1993;71(September):50322.

[19] Mubarak Ebrahim A. Pinch technology: an efficient tool for chemical

plant energy and capital cost saving. Appl Energy 2000;65(14):459.

40 60 80 100 120 140 16 0

10

20

30

40

50

60

70

4': T3=20

oC

4: T3=27

oC

4'

7

4

Outlet

Cool Water

Inlet

T,

oC

H, kJ/kg

Fig. 7. The hot and cold curves of dehumidification process in surfaceheat exchanger when ratio = 1.


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