Thermodynamic Analysis of a Cascade Refrigeration System Based On Carbon Dioxide and Ammonia

Satyananda Tripathy et al Int. Journal of Engineering Research and Applications www.ijera.com
ISSN : 2248-9622, Vol. 4, Issue 7( Version 1), July 2014, pp.24-29
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Thermodynamic Analysis of a Cascade Refrigeration System Based On Carbon Dioxide and Ammonia Satyananda Tripathy1, Jibanananda Jena2, Dillip K. Padhiary3, Manmatha K. Roul4 1Department of Mech Engg., Suddhananda Engineering & Research Center(SERC), Odisha, India 2,3Department of Mech. Engg., Bhadrak Institute of Engineering & Technology, Odisha, India 4Department of Mech. Engg., Gandhi Institute for Technological Advancement(GITA), Bhubaneswar, India Abstract Thermodynamic analysis of a cascade refrigeration system that uses carbon dioxide-ammonia (R744-R717) as refrigerant is presented in this paper to determine the optimum condensing temperature of the cascade condenser at given design parameters, to maximize the COP of the system. The design and operating parameters considered in this study include (1) condensing, sub cooling, evaporating and super heating temperatures in the ammonia (R717) high-temperature circuit, (2) temperature difference in the cascade heat exchanger, and (3) evaporating, superheating, condensing and sub cooling in the carbon dioxide (R744) low-temperature circuit. A multilinear regression analysis was employed in order to develop two useful correlations for maximum COP, and optimum condensing temperature. Keywords: Cascade refrigeration system, Optimization, Coefficient of performance, Operating parameters, Correlation,
I. INTRODUCTION
In low temperature applications, including rapid freezing and the storage of frozen food, the required evaporating temperature of the refrigeration system ranges from -40°C to -55°C, so a single stage vapor- compression refrigeration system is insufficient. Two stage or cascade refrigeration systems are used for low temperature applications. The high and low pressure sides of a two stage refrigeration system are filled same refrigerant but the high and low temperature circuits of a cascade system are filled separately with appropriate refrigerants with respect to global environmental protection, the use of natural refrigerant in refrigeration systems has been demonstrated to be a complete solution to permanent alternative fluorocarbon-based refrigerant [1,2].Therefore, using natural refrigerants in both two stage and cascade refrigeration system helps to satisfy the obligation of environmental treaties. Ammonia (R717) is a natural refrigerant that is most commonly adopted in low-temperature two stage refrigeration systems, but it has disadvantages. For instance, ammonia has a pungent smell; it is toxic and moderately flammable.
Additionally, the evaporating pressure of ammonia system is below atmospheric pressure, when the evaporating temperature is below -35°C, causing air to leak in to the system, leading to short- term inefficiency and the long term unreliability of the system. Hence, a non toxic, non flammable and dense refrigerant gas with a positive evaporating pressure should be chosen for evaporation below - 35°C.A cascade refrigeration system with natural refrigerants CO2 and NH3 meets these requirements. Hence, a non toxic, non flammable and dense refrigerant gas with a positive evaporating pressure should be chosen for evaporation below - 35°C. A cascade refrigeration system with natural refrigerants CO2 and NH3 meets these requirements. A CO2/NH3 cascade refrigeration system uses carbon dioxide and ammonia as refrigerants in low and high temperature circuits, respectively. Some of the characteristics of CO2 make it a good alternative to ammonia for use in large - scale refrigeration plants operated at low temperature. The most obvious advantages of carbon dioxide are that it is non toxic, incombustible and has no odour. More over as compared with ammonia two stage refrigeration- systems, the CO2/NH3 cascade refrigeration system has significantly lower amount of ammonia, and the COP of the cascade system exceeds that of a two stage system at low – temperatures [3,5,7]. Therefore CO2/NH3 cascade refrigeration systems are attracting attention all over the world [3,6-10]. In the design phase of a CO2/NH3 cascade refrigeration system, an important issue is the means of determining the optimum condensing temperature of a cascade –condenser under particular design conditions, such as condensing temperature, evaporating temperature and the temperature difference between the high and low temperature circuits in cascade condenser.
RESEARCH ARTICLE OPEN ACCESS

A small concern with cascade refrigeration system is the initial installation cost which is about 10% higher than the traditional direct expansion systems(Wilson and Maier,2006) . But this cost can be negated with less refrigerant charge requirements and environmental advantage of the cascade system due to less direct emissions as compared to single stage system. An improvement, which can also be observed in cascade system is the reduced amount of superheat in the discharge temperature of the high temperature circuit that results in a reduced capacity of the high temperature condenser and an increased refrigeration effect (Ratts and Brown,2000). The isentropic efficiency and volumetric efficiency are regarded here as a function of the pressure ratio of the compressor. Two correlations that are useful for determining the optimum condensing temperature of a cascade condenser and its corresponding maximum COP are presented, to support the development of CO2/NH3 cascade refrigeration systems and related equipment in the design phase.
II. System description
Fig.1 schematically depicts a CO2/NH3 cascade refrigeration system. Fig.2 presents the corresponding T-s and P-h diagrams. This refrigeration system comprises of two separate refrigeration circuits, the high temperature circuit (HTC) and the low temperature circuit (LTC). Ammonia is the refrigerant in HTC, where as carbon dioxide is the refrigerant in LTC. The circuits are thermally connected to each other through a cascade condenser, which acts as an evaporator for the HTC & as a condenser for LTC. Fig.2. Shows that the condensing and evaporating pressure in the NH3 circuit are both lower than those in the CO2 circuit. So the NH3 circuit is called HTC rather than the high pressure circuit, and CO2 circuit is called the LTC rather than the low pressure circuit.
Fig.1.Schematic diagram of a CO2/NH3 cascade refrigeration system
Fig.2 (a): The T-s and (b) the p-h diagrams of a CO2/NH3 cascade refrigeration system. Fig.1.indicates that the condenser in the cascade refrigeration system rejects a heat of QH at condensing temperature of Tc, to its warm coolant or environment at temperature To. The evaporator of this cascade system absorbs a refrigerated load QL from the cold refrigerated space at TcL to the evaporating temperature TE .The heat absorbed by the evaporator of the LTC plus the work to the LTC compressor equals the heat absorbed by the evaporator of the HTC. TCC and TME represent the condensing and evaporating temperature of the cascade-condenser, respectively. ΔT= (TCC-TME) represents the difference between the condensing temperature of LTC and the evaporating temperature of HTC. The evaporating temperature TE, the condensing temperature Tc, and the temperature difference in the cascade condenser are three important design parameters of a CO2/NH3 cascade refrigeration system.
III. Thermodynamic analysis
A parametric study with fixed cooling capacity, and various condensing temperatures, evaporating temperatures and temperature differences in the cascade-condenser has been conducted to determine the optimum condensing temperature of a cascade-

condenser in a CO2/NH3 cascade refrigeration
system operating at low temperatures. The
condensing temperatures used in the parametric study
are 35°C, 40°C and 45°C.The evaporating
temperatures are -45°C, -50 °C, and -55°C. The
temperature differences in the cascade-condenser are
3°C, 4°C and 5°C.Each component in the cascade
refrigeration system, shown in fig.1 can be treated as
a control volume.
Assumptions
The following assumptions are made to simplify
the thermodynamic analysis.
1. All components are assumed to operate at a
steady state .The changes in the potential and the
kinetic energy of the working fluids across each
components are negligible.
2. The high and low temperature circuit
compressors are adiabatic but non isentropic, and
their isentropic efficiency can be expressed as a
function of pressure ratio.
3. Combined motor and mechanical efficiency of
each compressor is assumed to be 0.93.
4. The heat loss and pressure drops in the piping
connecting the components are negligible.
5. All throttling devices are isenthalpic.
6. The outlet states of the condenser and the
cascade condenser are at sub cooled state and
that of the evaporator is at superheated state.
Based on the assumptions described above, the
balanced equations are applied to find the mass flow
rate of each cycle, the work input to the compressor,
the heat transfer rates of condenser, cascade
condenser and evaporator.
Mass balance
in out
m m
(1)
Energy balance
0
in out
Q W m h mh 
(2)
Table-1. Mass and energy balance equations
of different components.
Component mass balance
Energy balance
HTC compressor 5 6 H m  m  m
6 5 6 5 ( ) ( ) H s H
H
s m e m e
m h h m h h
W
    
 
 
  
Condenser 6 7 H m  m  m
6 7 ( ) H H Q  m h  h
HTC throttling 7 8 H m  m  m
7 8 h  h
Device
Cascade condenser
2 3
8 5
L
H
m m m
m m m
 
 
  
  
5 8 2 3 ( ) ( ) M H L Q  m h  h  m h  h
LTC compressor 1 2 L m  m  m
2 1 2 1 ( ) ( ) L s L
L
s m e m e
m h h m h h
W
    
 
 
  
LTC throttling device 3 4 L m  m  m
3 4 h  h
Evaporator 4 1 L m  m  m
1 4
1 4
( )
( )
L L
L
L
Q m h h
Q
m
h h
 


 


Compressor Efficiency
The isentropic and volumetric efficiencies of
ammonia and carbon dioxide compressors can be
expressed in terms of compression ratio, as
3 NH
Compressor [20] s 
= -0.00097
2
p R
-0.01026
p R
+0.83955 (3)
V 
= -0.00076
2
p R
-0.05080 p R
+1.03231 (4)
CO2 Compressor [21]
s 
= -0.00476
2
p R
-0.0923 p R
+0.89810 (5)
V 
= -0.00816
2
p R
-0.15293 p R
+1.13413 (6)
System performances
The overall coefficient of performance, or the
first law efficiency, of the cascade refrigeration
system is given by,

COP=
L
H L
Q
W W

 
=
( )( )
1
LTC HTC
LTC HTC
COP COP
COP COP
,
where (7)
LTC COP
=
L
L
Q
W


(8)
M
HTC
H
Q
COP
W



(9)
The refrigeration capacity L Q 
, the heat transfer
rate in the cascade condenser M Q 
, the work in put to
the HTC compressor H W 
and the work input to the
LTC compressor L W 
can all be determined using the
relationship given in the table.
IV. Results and discussion
The thermodynamic properties of ammonia and
carbon dioxide used here such as specific volume,
enthalpy and entropy, are determined using the
software EES.
Fig-3 Effect of TCC on the COP of HTC and LTC
Fig.3 plots the curves of COP versus TCC at
TC =30°C, TE = -50°C and ΔT= 3K .It gives the
effect of TCC on the COP of HTC and LTC, as
determined by equations (8) and (9).The COP of
HTC increases with TCC where as that the COP of
LTC decreases as TCC increases. Hence an optimum
TCC and its corresponding maximum COP exist,
similar to a two stage system with a single
refrigerant. It gives that the COP is maximum 2.01
at the optimum Tcc of -17°C.
Fig. 4. plots the curves of overall COP versus Tcc at
different design parameters
Fig. 5 The influence of Tc on the Tcc,opt of a
CO2/NH3 cascade refrigeration system.
Fig-6 The influence of Tc on the COPmax of a
CO2/NH3 cascade refrigeration system.

Fig - 6 and 7 show the effect of the condensing
temperature Tc on the TCC,OPT and the
corresponding COPmax at various evaporating
temperatures TE and various temperature differences
in the cascade condenser ΔT. The figures show that
increasing TC increases TCCOPT and reduces
COPmax.
Fig-6 gives that TCCOPT is linearly related to
the parameters of TC,TE, and ΔT. Fig-7 plots the
same linear relationships between COPmax and the
parameters of TC, TE, and ΔT. The following
regression equations are obtained from the above
data.
, 41.48 0.4 0.4 0.78 CC OPT C E T   T  T  T
(10)
2.083 0.0231 0.0312 0.03167 MAX C E COP   T  T  T
(11)
The unit used in Equations (10) and (11) is Kelvin
(K).
V. Conclusions
This paper presents the optimum condensing
temperature CC,OPT T
,and the corresponding
maximum coefficient of performance max COP
for
CO2/NH3 cascade refrigeration systems reference to
three design parameters – condensing temperature
Tc, evaporating temperature TE, and the temperature
difference in the cascade condenser ΔT.
1. The optimum condensing temperature of a
cascade condenser increases Tc, TE and ΔT,
where as the maximum COP increases with only
TE, but decreases as Tc or ΔT increases.
2. Two correlations used to determine the optimum
condensing temperature of a cascade –condenser
and the corresponding maximum COP are
obtained with reference to three design
parameters.
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Thermodynamic Analysis of a Cascade Refrigeration System Based On Carbon Dioxide and Ammonia

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