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Performance of cascade cycles working with blendsof CO2 D natural refrigerants

Giovanni Di Nicola a,*, Fabio Polonara a, Roman Stryjek b, Alessia Arteconi c

aDipartimento di Energetica, Universita Politecnica delle Marche, 60100 Ancona, Italyb Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, PolandcUniversita degli Studi e-Campus, Via Isimbardi 10, 22060 Novedrate (CO), Italy

a r t i c l e i n f o

Article history:

Received 9 July 2010

Received in revised form

30 March 2011

Accepted 2 May 2011

Available online 11 May 2011

Keywords:

Carbon dioxide

Hydrocarbon

Cascade system

Low temperature

Modelling

Refrigerating system

* Corresponding author. Tel.: þ39 0712204277E-mail address: g.dinicola@univpm.it (G.

0140-7007/$ e see front matter ª 2011 Elsevdoi:10.1016/j.ijrefrig.2011.05.004

a b s t r a c t

In this paper, an analysis on the performance of a cascade refrigeration cycle operated with

blends of carbon dioxide (or R744) plus hydrocarbons (ethane or R170, propane or R290,

ethylene or R1150, propylene or R1270) and dimethyl ether (or RE170) as the low-temper-

ature working fluid was carried out. The properties of the investigated blends were used to

simulate the behaviour of a cascade cycle using ammonia (or R717) as the high-tempera-

ture-circuit working fluid.

Theaimof thisworkwas to study thepossibility of using carbondioxidemixtures in those

applications where temperatures below its triple point (216.59 K) are needed. The analysis

was carried out by developing a software based on the CarnahaneStarlingeDeSantis (CSD)

Equation of State (EoS) using binary interaction parameters derived from the experimental

data in the literature.

Results show that adding R744 to HCs and dimethyl ether reduces the cycle perfor-

mance, even if acceptable values are always achieved for the COP. Main attractive of the

R744 þ natural refrigerant blends is connected with their GWP, ODP and flammability

properties lower than those of pure fluids.

ª 2011 Elsevier Ltd and IIR. All rights reserved.

Performance des cycles en cascade utilisant des melanges deCO2 et d’autres frigorigenes naturels

Mots cles : Dioxyde de carbone ; Hydrocarbure ; Systeme a cascade ; Basse temperature ; Modelisation ; Systeme frigorifique

1. Introduction

In industrial applications, the cascade refrigeration cycle is

usefulwhen low temperatures (i.e. below 233.15 K) are required

(Stoecker, 1998). The conventional cascade system often relies

; fax: þ39 0712204770.Di Nicola).ier Ltd and IIR. All rights

on R13 (chlorotrifluoromethane) and R23 (trifluoromethane) as

refrigerants. Due to the heavy environmental impact of these

two fluids, however, several natural refrigerants, i.e. carbon

dioxide, ammonia, nitrous oxide and hydrocarbons, and their

blends have recently been considered for use in cascade

reserved.

Nomenclature

a, b dimensional coefficients for the CSD EoS

c specific heat capacity [kJ kmol�1 K�1]

COP coefficient of performance, dimensionless

CSD EoS CarnahaneStarlingeDeSantis Equation of State

GWP Global Warming Potential

h enthalpy [kJ kmol�1]

HC HydroCarbon

HFC HydroFluoroCarbon

HX Heat eXchanger

Ki,j binary interaction parameter in the CSD EoS,

dimensionless_m mass flow rate [kg s�1]

ODP Ozone Depletion Potential

P pressure [kPa] or [MPa]

T temperature [K] or [�C]R universal gas constant [kJ kmol�1 K�1]

x composition, mole fraction of the liquid phase

y composition, mole fraction of the gas phase

X composition, weight fraction

XM ratio of mass flow rate of the low- and high-

temperature circuits in cascade systems,

dimensionless

Dhm enthalpy of freezing [kJ kmol�1]

3 heat exchanger effectiveness, dimensionless

g activity coefficient, dimensionless

u acentric factor, dimensionless

subscripts or superscripts

bp at bubble point

cond condenser

evap evaporator

i,j components

in inlet

int intermediate

is isentropic

H high temperature

L low temperature

liq liquid

m at melting point

max maximum

out outlet

p at constant pressure

subcool sub-cooling

sup superheating

vap vapor

0 at zero pressure0 low-temperature stage

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 5 1437

refrigeration systems (Zha et al., 2002; Liu et al., 2002; Taylor,

2002; Pearson and Cable, 2003; Kruse and Russmann, 2006; Lee

et al., 2006; Niu and Zhang, 2007; Getu and Bansal, 2008; Gong

et al., 2009; Bhattacharyyaa et al., 2009; Dopazo et al., 2009). In

particular, several studies evaluated ammonia as the high-

stage fluid and carbon dioxide as the low-temperature stage

refrigerant (Zha et al., 2002; Liu et al., 2002; Taylor, 2002;

Pearson and Cable, 2003; Lee et al., 2006; Getu and Bansal,

2008; Bhattacharyyaa et al., 2009; Dopazo et al., 2009). In this

way, ammonia (R717) could be confined to a proper machine

room, while the safer carbon dioxide (R744) could be sent to

evaporators around the factory. The main drawback of using

carbon dioxide lies in its high pressures at normal boiling

temperature and its high melting temperature, which result in

a lower temperature limit for its use as a refrigerant. Lower

temperatures needed in some applications can be achieved by

using blends. That is why we recently (Di Nicola et al.,

2003a,b, 2005) turned our attention to R744 þ HFCs mixtures

as potentially suitable working fluids in low-temperature

refrigeration applications. However, many of the chemicals

that show a finite value of the ODP cannot be considered as

Table 1 e Properties of the investigated fluids.

Fluid Meltingpoint (K)

Normalboiling point (K)

Criticaltemperature (K)

R170 90.35 184.55 305.32

R290 85.47 231.11 369.83

R1150 103.99 104.00 282.34

R1270 87.95 225.45 364.85

RE170 131.65 248.31 400.10

a result of the Montreal protocol and its amendments. A

further limitation results from the Kyoto protocol and, in

Europe, from European directives imposing limitations on the

emission of greenhouse gases responsible for global warming

(GWP).

For this reason, in this paper we focused our attention on

other blends of carbon dioxide that contain hydrocarbons and

dimethyl ether, because they have a lower environmental

impact than HFCs. Blends considered are the following ones:

R744 þ R170 (ethane), R744 þ R1150 (ethylene), R744 þ R290

(propane),R744þR1270 (propylene), andR744þRE170 (dimethyl

ether).

System performance with these mixtures was analyzed

using a purpose-built program based on the

CarnahaneStarlingeDe Santis (CSD) Equation of State (EoS)

(De Santis et al., 1976) with the pure compound coefficients

updated in the light of the latest data in the literature.

Binary interaction parameters were derived from the VLE

data available in the literature (Poettman and Katz, 1945;

Haselden et al., 1951; Reamer et al., 1951; Akers et al., 1954;

Fredenslund and Mollerup, 1974; Gugnoni et al., 1974;

Criticalpressure (MPa)

Heat of fusion @Melting point (kJ/mol)

FlammabilityLFL (%)

4.872 2859 3.20

4.248 3524 2.30

5.041 3351 2.70

4.600 2936 2.00

5.370 4937 3.40

y (R744)

0.0 0.2 0.4 0.6 0.8 1.0

T, K

140

160

180

200

220

Fig. 1 e SLE for the R744 D R134a system. Black symbols

denote the experimental points while the lines denote the

Schroder equation (2).

x,y0.0 0.2 0.4 0.6 0.8 1.0

P, k

Pa

0

500

1000

1500

2000

2500

Fig. 3 e VLE behavior for R744 D R170 at T [ 200 K (solid

lines), T [ 225 K (dashed lines), and T [ 250 K (dotted

lines).

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 51438

Hamam and Lu, 1974, 1976; Davalos et al., 1976; Ohgaki and

Katayama, 1977; Tsang and Streett, 1981; Acosta et al., 1984;

Jonasson et al., 1995; Yucelen and Kidnay, 1999).

The investigation was conducted at fixed temperatures, i.e.

(a) Tevap,L ¼ 203 K at the low-stage evaporator, and (b)

Tcond,H ¼ 313 K at the high-stage condenser. The temperature

of the low stage (203 K) is below the normal melting point of

R744 (216.59 K). The influence of intermediate temperatures

and mixture compositions on the COP was investigated. The

main advantages of the blends consideredwere expected to be

the opportunity to use them at temperatures below the

normal melting temperatures of R744 and a drastic reduction

in the flammability risk in comparison with the use of pure

HCs and/or RE170.

All the calculations were done for the simple cycle (deno-

ted here with “no HX”) and for a cycle containing a suction/

liquid heat exchanger in the low stage with a heat exchanger

effectiveness (Kays and London, 1964) of 0.8 (denoted here

“with HX”). The model is very simple and it is used, along

with the refrigerant properties calculated with the CSD EoS,

simply to give us a trend of the situation.

x,y0.0 0.2 0.4 0.6 0.8 1.0

P, k

Pa

0

200

400

600

800

1000

1200

1400

1600

1800

Fig. 2 e VLE behavior for R744 D RE170 at T [ 200 K (solid

lines), T [ 225 K (dashed lines), and T [ 250 K (dotted

lines).

2. Model for predicting blends behaviorbelow the R744 triple point

As pointed out above, in this paper we considered blends

containing R744 that allow to work in the low stage of

a cascade cycle at temperatures below the normal melting

temperatures of R744.

A blend containing R744 and intended for operation at

temperatures below 216.59 Kmust possess the two requisites:

its melting temperature has to be much lower than the R744

triple point and its temperature glide should be not too great.

These requisites are necessary in order to operate at temper-

atures as far as possible below 216.59 K, and to keep temper-

ature glides as low as possible during evaporation and

condensation and avoid fractionation. A good compromise for

the blend behavior would be a quasi-azeotropic mixture, an

azeotrope would be ideal.

Wechosefivefluids (shownwith their properties inTable1),

as potential second components, among hydrocarbons (HCs),

also because of their ready availability for the experimental

work needed to establish the mixture’s basic properties.

x,y0.0 0.2 0.4 0.6 0.8 1.0

P, k

Pa

0

500

1000

1500

2000

2500

3000

Fig. 4 e VLE behavior for R744 D R1150 at T [ 200 K (solid

lines), T [ 225 K (dashed lines), and T [ 250 K (dotted

lines).

x,y0.0 0.2 0.4 0.6 0.8 1.0

P, k

Pa

0

200

400

600

800

1000

1200

1400

1600

1800

Fig. 5 e VLE behavior for R744 D R290 at T[ 200 K (solid

lines), T[ 225 K (dashed lines), and T[ 250 K (dotted lines).

Fig. 7 e Schematic layout of a cascade refrigeration cycle

(“no HX”).

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 5 1439

To evaluate different options among the possible mixtures

without having to perform systematic tests in actual refriger-

ating systems demands a model for estimating the mixture’s

properties and a computer software to simulate their behavior

in a vapor compression cascade cycle. The commercially

available computer packages are not completely reliable in

assessing the properties of R744 mixtures, when they have to

calculate conditions at temperatures below the carbon diox-

ide’s triple point. The only way to overcome this shortcoming

is to construct a predictive model starting from scratch with

data in the literature.

We chose to use a thermal equation of state (EoS), adding

a knowledge of the specific heat at zero pressure, to model

the thermodynamic properties of involved fluids. Particularly

the CarnahaneStarlingeDe Santis (CSD) EoS (De Santis

et al., 1976) was used to obtain the pressureevolumee

temperatureecomposition (PVTx) parameters. The PVTx

parameters were supplemented with the ideal gas heat

capacity, thereby enabling us to describe all the

thermodynamic functions at each point in the cycle.

So, constructing a model for predicting the R744 blend’s

behavior at temperatures below 216.59 K implies three steps:

x,y 0.0 0.2 0.4 0.6 0.8 1.0

P, k

Pa

0

200

400

600

800

1000

1200

1400

1600

1800

Fig. 6 e VLE behavior for R744 D R1270 at T [ 200 K (solid

lines), T [ 225 K (dashed lines), and T [ 250 K (dotted

lines).

1. evaluating the solideliquid equilibrium (SLE) and the

eutectic composition of themixture in order to estimate the

lowest temperature limit to which the blend can be used as

fluid;

2. evaluating the parameters of the EoS for the mixture’s

components;

3. evaluating the behavior of the mixtures by determining

proper interaction parameters of the EoS.

Fig. 8 e Schematic layout of a cascade refrigeration cycle

with a suction-liquid heat exchanger (HX) on the low-

temperature-circuit (“with HX”).

Table 2 e Operating conditions for the cycle calculations.

Tcond

(K)Tevap

(K)DTSUP

(K)DTSUBCOOL

(K)hIS(%)

High-temperature

circuit

313 213e283 10 5 0.7

Low-temperature

circuit

213e283 203 10 5 0.7

Tevap,H ¼ Tcond�L ¼ Tint ¼ 213e283 K.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 51440

The abovementioned steps are described in detail in the

following paragraphs. The same model was used with good

results for other carbon dioxide’s blends in a previous paper

(Di Nicola et al., 2005).

2.1. Solideliquid equilibrium (SLE)

The SLE depends both on the crystals formed in solution and

on the properties of the liquid phase. Most organic systems

form eutectics: in this case, the course of the liquidus is well

described by the Schroder equation (Schroder, 1954), known

since the end of the 19th century. The exact course of the

liquidus for ideal mixtures (i.e. showing a small deviation

from Raoult’s law) depends mainly on the property of the

solute (R744 in our case) and, in the case of non-ideal

systems, on the property of the liquid phase.

Assuming that the studied systems form eutectics, the

solubility of the solid solute (R744) in any solvent can be

described by the Schroder equation; assuming that any differ-

ence between the heat capacity of the subcooled liquid solute

and solid solute can be disregarded, it takes the following form:

ln g2x2 ¼ �Dhm

RT

�1� T

Tm

�(1)

where the subscript 2 denotes the solute and the subscript m

denotes property at melting point. We assumed as a first

approximationthatourblendsbehavealmost ideally, i.e. that the

solute’s activity coefficient, g2¼ 1; thismeans that we canwrite:

ln x2 ¼ �Dhm

RT

�1� T

Tm

�(2)

This simplification leads to the consideration that the

solubility of the solid solute (R744) is independent of the

solvent over a wide range of compositions.

CO

P

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

XHon XHon XHon XHon

PropylenePropaneEthyleneEthaneDME

Fig. 9 e COP for pure fluids for “no

According to our past measurements on the SLE of

R744 þ HFCs (Di Nicola et al., 2006, 2007a,b, 2008, 2009, 2010),

the melting temperatures of R744 þ HC systems are presum-

ably independent of the second component and depend only

on the quantity of the R744, allowing operation down to

T ¼ 170 K for a presence of up to 50% in mass with no risk of

solidification, whatever the second component involved. An

example of the course of the liquidus is reported in Fig. 1,

where the experimental points for the SLE for the

R744 þ R134a system are reported together with the lines

denoting the Schroder equation.

2.2. Properties of the mixture’s components

The CSD EoS (De Santis et al., 1976) can be used to evaluate the

properties of the R744 blends, providing the equation

parameters for the pure fluids and the interaction

parameters for mixtures are known. The temperature

dependence of both EoS parameters and their respective

coefficients, and the equation for the temperature

dependence of the cp0 with the respective coefficients for

carbon dioxide, propane and propylene were taken from

REFPROP 5 (Huber et al., 1996) and for ethane, ethylene and

DME from REFPROP 7 (Lemmon et al., 2002). For the R744, the

parameters were refitted involving the estimated property of

the supercooled liquid. The thermodynamic properties of

liquid R744 along the saturation line are well known down

to its freezing temperature, but not below 216.59 K when

carbon dioxide may remain liquid in the metastable (known

as ‘supercooled’) state. Needed supercooled liquid properties

were recalculated, according to themethods explained below.

First, the saturation pressure of supercooled R744 was

estimated by the LeeeKesler method (Lee and Kesler, 1975),

based on the corresponding state principle, which expresses

the generalization that equilibrium properties are universally

related to the critical properties. To apply this method, the

properties of a reference fluid with an acentric factor similar

to that of R744 (u ¼ 0.2239) had to be explored. Here,

saturation data for R744 were estimated taking n-butane as

the reference fluid (u ¼ 0.2).

Though no data are available, there are numerous tech-

niques for estimating pure liquid molar volumes along satu-

ration. We used the HankinsoneBrobsteThomson (HBT)

method (Hankinson and Thompson, 1979) and the Rackett

equation (Rackett, 1970; Spencer and Danner, 1972), both are

based on the knowledge of the critical parameters and

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

8.0=XH 8.0=XH 8.0=XHwith HX8.0=XH

PropylenePropaneEthyleneEthaneDME

with HX

HX” (left) and “with HX” (right).

Table 3 e COP of the cascade cycle without the suction/liquid heat exchanger (“no HX”) and with the suction/liquid heatexchanger (“with HX”) at Tint [ 253.15 K, working with pure refrigerants or with the considered blends in the low-temperature circuit. The performance difference between the system working with blends and pure fluids (DCOPbl-pf) isreported. X (R744) [ mass fraction, x (R744) [ mole fraction.

Pure fluids COP(“no HX”)

COP(“with HX”)

Blends X (R744) x (R744) COP(“no HX”)

DCOPbl-pf(“no HX”) (%)

COP(“with HX”)

DCOPbl-pf(“with HX”) (%)

R170 (ethane) 0.93 0.97 R744 þ R170 0.50 0.41 0.90 �3 0.93 �4

R290 (propane) 0.97 1.01 R744 þ R290 0.50 0.50 0.86 �11 0.87 �14

R1150 (ethylene) 0.88 0.93 R744 þ R1150 0.50 0.39 0.88 0 0.91 �2

R1270 (propylene) 0.97 1.00 R744 þ R1270 0.50 0.49 0.89 �8 0.91 �9

RE170 (DME) 0.97 0.99 R744 þ RE170 0.50 0.51 0.87 �10 0.88 �11

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 5 1441

acentric factor, and they produce negligible differences in

molar volumes. For the sake of clarity, data from the HBT

equation were used to determine the coefficients for the

CSD EoS.

The coefficients of the expressions describing the

temperature dependence of parameters a and b of the CSD EoS

were found by regression of the pressures and liquid molar

volumes along saturation. The two temperature dependences

of the CSD parameters are:

aðTÞ ¼ a1exp�a2Tþ a3T

2�

(3)

bðTÞ ¼ b1 þ b2Tþ b3T2 (4)

The ideal gas heat capacitywas calculated using theWooley

equation (Wooley, 1954), valid from 210 to 1100 K. This

equation was slightly extended to a lower T (200 K).

The saturation pressure data of supercooled R744, together

with the values for the liquid molar volumes and with the

ideal heat capacity of supercooled R744 were reported else-

where (Di Nicola et al., 2005).

2.3. Properties of the mixtures

For the blends, the following mixing rules were applied:

a ¼XX

xixjaij (5)

where

aij ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffi�aiaj

�q �1� Kij

�(6)

and

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

XHon XHon

CO

P

XHon XHon

Fig. 10 e COP forR744DRE170 indifferent proportions (mass frac

pure RE170e solid line;BdX (R744)[ 0.1;,dX (R744)[ 0.2;6

b ¼X

xibi (7)

The following averaged binary interaction parameters,

Ki,j, were derived from VLE data in the literature (Poettman

and Katz, 1945; Haselden et al., 1951; Reamer et al., 1951;

Akers et al., 1954; Fredenslund and Mollerup, 1974; Gug-

noni et al., 1974; Hamam and Lu, 1974, 1976; Davalos et al.,

1976; Ohgaki and Katayama, 1977; Tsang and Streett, 1981;

Acosta et al., 1984; Jonasson et al., 1995; Yucelen and Kid-

nay, 1999): K1,2(R744þR1270) ¼ 0.0591, K1,2(R744þR290) ¼ 0.1107,

K1,2(R744þR1150) ¼ 0.0561, K1,2(R744þR170) ¼ 0.1205,

K1,2(R744þRE170) ¼ �0.0391. The binary interaction parame-

ters were assumed to be independent of temperature and

their uncertainty was estimated to be within �0.0025.

The VLE at T ¼ 200 K, T ¼ 225 K, and T ¼ 250 K are shown in

Figs. 2e6, as an example of the binary systems’s behavior.

From the figure it is evident that R744þ R170 and R744þ R1150

show an azeotropic behavior, while R744þ R290, R744þ R1270

and R744 þ RE170 show a zeotropic behavior. Using these

parameters, the thermophysical properties of the five binary

systems were calculated along the fluid cycle.

3. Cycle analysis

The cascade systems combine two or more vapor compres-

sion units, each of one is an independent cycle, working on

separate refrigerants. Here we considered a system with two

circuits, where the high- and low-temperature systems have

to be balanced between them. This implies that the heat

absorbed in the high-temperature cascade (evaporator) must

be equal to the heat rejected in the low-temperature cascade

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

8.0=XH 8.0=XH 8.0=XH 8.0=XHwith HX

tion) for the twocycle configuration (“noHX”and“withHX”):

dX (R744)[ 0.3;7dX (R744)[ 0.4; and>dX (R744)[ 0.5.

Tint 210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

Tint 210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

CO

P

with HX 8 . 0 = X H X H o n with HX

Fig. 11 e COP for R744D R170 in different proportions (mass fraction) for the two cycle configuration (“noHX” and “withHX”):

pure R170 e solid line; Bd X (R744)[ 0.1;,d X (R744)[ 0.2;6d X (R744)[ 0.3;7dX (R744)[ 0.4; and>d X (R744)[ 0.5.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 51442

(condenser). In order to simulate the cascade cycle behavior,

we assumed the ideal condition that in the intermediate heat

exchanger (condensereevaporator) the intermediate temper-

ature between the two circuits (coupling temperature) is

Tint¼ Tcond,L¼ Tevap,H, thatmeans that there is no temperature

difference between the two fluids, DT ¼ 0 (infinite heat

exchanger surface). This simplification is in accordance with

the purposes of the present paper.

A schematic layout of a cascade refrigeration cycle is

illustrated in Fig. 7. The layout of the cycle also containing

a suction/liquid Heat Exchanger (HX) in the low stage, here

denoted “with HX”, is illustrated in Fig. 8.

The cycle parameters for the high stage were calculated

first, followed by the low-stage parameters; then the COP of

the cascade cycle was assessed from the resulting quantities.

The high-stage requires the following input: evaporating

temperature (Tevap,H), superheating temperature (Tsup,H),

condensing temperature (Tcond,H), sub-cooling temperature

(Tsubcool,H), and isentropic efficiency (his,H). R744 þ HCs blends

were considered for the low stage. The binary interaction

parameters can be applied as a default option, or they can be

modified by the user. The program enables the cycle analysis

of blends consisting of up to three components in various

proportions. For the low stage, we need to know the blend

composition, Tsup,L, Tcond,L, Tsubcool,L, and his,L. For both stages,

his includes all cycle irreversibilities (friction, electric motor

efficiency, volumetric efficiency, etc.).

CO

P

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

XHon XHon XHon XHon

Fig. 12 e COP for R744 D R1150 in different proportions (mass f

HX”): pure R1150 e solid line; Bd X (R744) [ 0.1; ,d X (R744)

(R744) [ 0.5.

In the case of zeotropic blends, given that the low-stage

condensation must be above the evaporating temperature of

the high-stage at least, we assumed the condition where

Tevap,H ¼ Tbp, cond,L.

As explained above the two systems have been designed

such that:

_m0ðh20 � h30 Þ ¼ _m�h1 � h0

4

�(8)

Introducing the parameter XM, defined as the ratio of the

mass flow in the low-temperature-circuit ( _m0) to the one in the

high-temperature-circuit ( _m), we have:

XM ¼ _m0= _m (9)

Thus, the COP for the cascade system is calculated from:

COP ¼ XMðh10 � h40 Þðh2 � h1Þ þ XMðh20 � h10 Þ (10)

In order to study the performance of the low-temperature

circuit with the addition of a suction/liquid heat exchanger

(Fig. 8), the following equations were considered in the

computer program:

e Energy balance

ðh30 � h40Þ ¼ ðh10 � h60Þ (11)

e Heat exchanger effectiveness, 3

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

8.0=XH 8.0=XH 8.0=XHwith HX8.0=XHwith HX

raction) for the two cycle configuration (“no HX” and “with

[ 0.2; 6 d X (R744) [ 0.3; 7dX (R744) [ 0.4; and >d X

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

8.0=XHXHon 8.0=XHXHon

CO

P

8.0=XHXHon 8.0=XHXHon with HX

Fig. 13 e COP for R744 D R290 in different proportions (mass fraction) for the two cycle configuration (“no HX” and “with

HX”): pure R290 e solid line; Bd X (R744) [ 0.1; ,d X (R744) [ 0.2; 6 d X (R744) [ 0.3; 7dX (R744) [ 0.4; and >d X

(R744) [ 0.5.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 5 1443

3 ¼_Q

_Qmax

¼�Tvap:out � Tvap:in

��Tliq:in � Tvap:in

� ¼ ðT10 � T60 ÞðT30 � T60 Þ (12)

The thermodynamic analysis of the cycle was done using

the parameters given in Table 2. As previously mentioned, in

all the calculations the heat exchanger effectiveness (3) has

been set equal to 0.8.

3.1. Pure refrigerants as the low-temperature stage fluid

The following pure fluids in the low-temperature-circuit were

considered: R170, R290, R1150, R1270, and RE170. The COP of

the cascade cycle was calculated for each refrigerant and the

results are presented in Fig. 9. The influence of the

intermediate temperature (Tint) was investigated, considering

the relevance that this aspect had in literature (Agrawal,

1989; Jeong and Smith, 1994; Bhattacharyya et al., 2007; Getu

and Bansal, 2008). Particularly the applicability of Jeong and

Smith’s rule (1994) was checked. In fact they demonstrated

that in reversible cycles the optimum coupling temperature

is represented by the square-root of the condensing

temperature, Tcond,H, in the high-temperature circuit and

evaporating temperature, Tevap,L, in the low-temperature

circuit, i.e.:

CO

P

Tint210 220 230 240 250 260 270 280 290

1.1

XHon XHon XHon

0.6

0.7

0.8

0.9

1.0XHon

Fig. 14 e COP for R744 D R1270 in different proportions (mass f

HX”): pure R1270 Be solid line; d X (R744) [ 0.1; ,d X (R744)

(R744) [ 0.5.

Tint ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiTcond;HTevap:L

q(13)

We plotted Tint versus COP, as it is shown in Fig. 9. It is

possible to note that for all refrigerants, the COP clearly

depends on the intermediate temperature (Tint). For all

systems with “no HX”, the maximum COP was observed for

temperatures between 240 and 260 K. These values are close

to T ¼ 258 K, which is the geometric mean (Eq. (13)) between

the two temperatures Tcond,H and Tevap,L. For the systems

with “no HX”, the maximum COP was achieved for RE170.

For the systems with the suction/liquid heat exchanger

(“with HX”), the maximum COP is only observed for R170

and R1150. For all the other fluids, the COP increased

systematically with higher values of Tint.

These results confirm that the same ratio between

condensing and evaporative temperature of each cycle of the

cascade system helps to increase the performance of the

system. Instead the analysis is complicated by the presence of

the suction/liquid heat exchanger. In the latter case in fact the

performance’s trend become strictly connected also to the

refrigerant used.

3.2. R744 blends as the low-temperature stage fluid

The following blends were considered: R744 þ R170,

R744 þ R1150, R744 þ R290, R744 þ R1270, R744 þ RE170. The

Tint210 220 230 240 250 260 270 280 290

0.6

0.7

0.8

0.9

1.0

1.1

8.0=XH 8.0=XH 8.0=XH 8.0=XHwith HX

raction) for the two cycle configuration (“no HX” and “with

[ 0.2; 6 d X (R744) [ 0.3; 7dX (R744) [ 0.4; and >d X

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 4 3 6e1 4 4 51444

COP for cascade systems equipped with those blends were

estimated in the same conditions as in Table 3, considering

Tint ¼ Tcond,L ¼ Tevap,H as an independent variable. In

addition, each set of calculations was performed for five

compositions: 0.1, 0.2, 0.3, 0.4 and 0.5 inmass fractions of R744.

The COP is plotted versus Tint in relation to the mixture’s

composition in Figs. 10e14. As the Figures show, the higher

the content of R744, the lower the COP, except for themixtures

of R1150 in “no HX” configuration, where the COP is inde-

pendent of the fluid’s composition.

For the fact that carbon dioxide is the natural refrigerant

with the lower environmental impact and the lower flam-

mability, those blends with a higher concentration of R744 are

considered particularly interesting for the purposes of this

research. Accordingly to findings reported in Section 2.1, the

maximum content of CO2 allowed in the mixtures is 50% in

mass in order to prevent solidification and so particular

evidence was given to the performance of such blends. The

respective COP values are shown in Table 3, where the

composition of the mixtures is expressed both in mass and

mole fractions. All reported data were obtained at

Tint ¼ 253.15 K. These findings show that, in most of the

cascade systems investigated, the COP varied within a range

of 0.85e0.93 and it was lower for blends containing CO2 than

for the pure fluids here considered, particularly in presence

of the suction/liquid heat exchanger. In Table 3 the

performance difference between the system working with

blends and pure fluids (DCOPbl-pf) is also reported: the

difference between the two corresponding configurations

(pure fluid and blend) reaches a maximum using as

refrigerant propane (DCOPbl-pf ¼ �14%), while the lower

value is for ethylene (DCOPbl-pf ¼ �2%).

Comparing these results with those obtained previously by

us with the same model for R744 þ HFCs systems (Di Nicola

et al., 2005) reveals very similar COP values and trends, both

in composition and Tint.

4. Conclusions

This paper presents a thermodynamic analysis on a cascade

refrigeration cycle using R744 þ HCs blends as the low-

temperature fluid with a view to extending the applicability of

carbon dioxide in such systems below its triple point

(216.59 K).

The results show that COP of the cascade cycle with the

studied R744 blends reaches acceptable values, even if the

better performance is achieved using pure HCs refrigerants in

the low stage of cascade systems. Nevertheless the interest of

considering such blends remains and it is mainly related to

the lower environmental impact of R744 and particularly to its

ability to reduce HCs flammability.

Considering the intrinsic simplicity of the model used to

compare different blends, a more realistic model and experi-

mental work are needed to give recommendations on the

most suitable blend, and this work is currently underway.

Even if the inherent uncertainty of the model permits no final

conclusions, there is no doubting that e for all the blends

considered here e the liquid/suction heat exchangers in the

low stage is capable of improving the system’s performance,

and this aspect therefore also deserves further investigation.

The outcome of such further studieswill also be interesting

with a view to confirming whether the R744 content in the

most suitable blends can make them tend towards non-

flammability.

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