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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
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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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