INTRODUCTION
Recent developments in cooling systems show a growing interest in the
application of absorption systems. Absorption refrigeration systems provide
opportunities for energy saving because they can use heat energy to produce
cooling, instead of electricity used by conventional vapour compression
chillers. Furthermore, nonconventional sources of energy such as solar,
waste heat and geothermal can be used as their primary energy input. In
addition, absorption units use environmentally friendly working fluid
pairs instead of CFCs and HCFCs, which deplete the ozone layer of the
atmosphere (Chua et al., 2000). The first law analysis of double
effect and triple effect lithium bromidewater absorption system has been
done recently by some researchers (Xu et al., 1996; Arun et
al., 2001; Kaita, 2002). Also Exergy analysis of single and double
effect absorption refrigeration cycles with lithium bromide/water was
carried out by some researchers (Talbi and Agnew, 2000; Arzu Sencan et
al., 2005). However, there is a lack of data in the second law analysis
of triple effect absorption refrigeration systems albeit of their higher
COPs. In this study energy and exergy analysis is carried out for two
types of Triple effect absorption refrigeration cycles, parallel and series
flow, with lithium bromide/water as the working fluid pair. The study
was performed for different operating conditions by means of the computer
program.
MATERIALS AND METHODS
The series flow and parallel flow kinds of triple effect absorption refrigeration
cycles shown schematically in Fig. 1 and 2
have been analyzed.

Fig. 1: 
Series flow cycle 

Fig. 2: 
Parallel flow cycle 
In the seriesflow cycle, Fig. 1, the weak solution
from the absorber is sent directly to High Temperature Generator (HTG)
and returned to the absorber through Middle Temperature Generator (MTG)
and Low Temperature Generator (LTG). In the parallel flow cycle in Fig.
2, the weak solution is sent first to LTG and then to MTG and at the
end to HTG. For thermodynamic analysis of the absorption system the principles
of mass conservation, first and second laws of thermodynamics are applied
to each component of the system. Each component can be treated as a control
volume with inlet and outlet streams, heat transfer and work interactions.
In the system, mass conservation includes the mass balance of total mass
and each component of the solution.
where, is
the mass flow rate and x is mass concentration of LiBr in the solution
and the subscripts i and o are inlet and outlet flow in the control volume.
The first law of thermodynamics yields the energy balance of each component
of the absorption system as follows:
The exergy of a fluid stream can be defined as:
where, ψ is the exergy of the fluid at temperature T and pressure P. h_{0}
and s_{0} are the enthalpy and entropy of the fluid at environmental
temperature and pressure. Irreversibility in each component is calculated by:
I_{cv} is irreversibility that occurred in the process.
COP of the absorption system is defined as the ratio of control volume
heat transfer in the evaporator to that of the generator.
The exergetic efficiency, E, is defined as the ratio of the exergy gained
at the evaporator to the exergy gained at the generator:
For simplification purposes, the work input to the solution pump and
the frictional losses inside the system are neglected.
RESULTS AND DISCUSSION
The initial conditions introduced to the program include the ambient
conditions, heat exchanger effectiveness and temperatures of HTG, evaporator
and condenser and also absorber exit mass flow rate. With the given parameters,
the program calculates all the thermodynamic properties at any point of
the cycle. The results are presented in Table 1 and 2
for parallel and series flow, respectively.
Both tables were obtained with
T_{gen} = 200°C, T_{e} = 5°C and T_{c} = 30°C.
A parametric study was performed for the performance of both systems,
varying the T_{g} and T_{e}. All these figures indicate
and influence of above mentioned temperatures on first and second law
efficiencies (Fig. 38). Figure 3 shows
the variation of the COP of the parallel flow system. As, it can be shown
from Fig. 3, in general the COP increases with an increase
of T_{g} and/or T_{e}. However, this increase is highly
pronounced at lower T_{e} and T_{g}. This can be explained
by Fig. 4 and 5 that show the variation
of HTG and Evaporator heat loud (Q_{htg} ,Q_{e}) with
increasing T_{g} and T_{e} for parallel flow and take
attention to the Eq. 6.
As can be seen from Fig. 4 and 5 the
rate of increase in Q_{e} is faster than Q_{htg} with
increasing T_{g} and T_{e}.
Figure 6 shows the variation of the second law efficiency
of the parallel flow type with T_{g} and T_{e}.
Table 1: 
State point data for tripleeffect parallel flow cycle 

Table 2: 
State point data for tripleeffect series flow cycle 

As, it shows, the exergetic efficiency (E) of the system decreases with increasing
T_{g} and T_{e}. This can be explained by the fact that
an increase of T_{g} causes an increase of exergy entering the
generator and also an increase of T_{e} causes a decrease of exergy
entering the evaporator. According to Eq. 7 these changes
of input exergies both result in the exergy efficiency to decrease.

Fig. 3: 
Variation of the COP of the parallel flow type with
the T_{g} and T_{e} 

Fig. 4: 
Variation of Q_{e} and Q_{htg} with
T_{g} for parallel flow type 

Fig. 5: 
Variation of Q_{e} and Q_{htg} with
T_{e} for parallel flow type 

Fig. 6: 
Variation of the exergetic efficiency (E) of the parallel
flow type with the T_{g} and T_{e} 

Fig. 7: 
Variation of the COP of the series flow type with the
T_{g} and T_{e} 
The results for series flow type (Fig. 7, 8)
are similar to parallel flow type qualitatively. However, a relatively
higher amount of COP and E is found for parallel system.
Irreversibilities in the absorber and generators are more than that of
the other components of the system (Fig. 9, 10).
This is due to the heat of mixing in the solution, which is not present
in pure fluids.
Figure 11 shows the variation of the COP of triple effect
parallel flow and series flow, double effect series flow and single effect systems
with T_{g} when T_{c} = 30°C and T_{e} = 5°C. As it
can be seen from this figure, for the triple effect systems there is a good
agreement between present results and those reported in the literature.

Fig. 8: 
Variation of the exergetic efficiency (E) of the series
flow type with the T_{g} and T_{e} 

Fig. 9: 
:Irreversibility at the various components of the parallel
system 

Fig. 10: 
Irreversibility at the various components of the series
system 

Fig. 11: 
Variation of COP with generator temperature (T_{g})
in single, double and triple effect absorption systems 

Fig. 12: 
Variation of exergetic efficiency (E) with generator
temperature (T_{g}) in single, double and triple effect absorption
systems 
The results also show that the COP of triple effect systems is about 50% more
than that of the double effect series flow that obtained from present results
and other researchers (Arun et al., 2000) work and is about 2.2 times
more than that of the single effect systems. As it shows, exergetic efficiency
(E) of triple effect parallel flow type is more than that of the series type
(Fig. 12). Also in triple effect types the rate of decrease
in E is lower in comparison with those of double and single effect systems.
CONCLUSION
For both triple effect series and parallel flow types there is an increase
of COP and a decrease of E as HTG temperature (T_{g}) and/or evaporator
temperature (T_{e}) increases.
Considering both the first law and second law efficiencies, there is
a relative preference of triple effect parallel flow type in comparison
with series type.
The COP of triple effect systems is about 50% more than that of the double
effect systems and it is about 2.2 times that of the single effect systems.
For a given value of ΔT_{g} the variation of E for triple effect systems
is less than that for double effect systems. This variation is the most for
single effect systems.
Most of the irreversibilities occur in generators and absorber due to
mixing losses in series and parallel flow types.
NOMENCLATURE
ABS 
= 
Absorber 
COND 
= 
Condenser 
E 
= 
Exergetic efficiency 
EVAP 
= 
Evaporator 
H 
= 
Enthalpy (kJ kg^{1}) 
HHX 
= 
Hightemperature heat exchanger 
HTC 
= 
Hightemperature condenser 
HTG 
= 
Hightemperature generator 
LHX 
= 
Lowtemperature heat exchanger 
LTG 
= 
Lowtemperature generator 
MHX 
= 
Middletemperature heat exchanger 
MTC 
= 
Middletemperature condenser 
MTG 
= 
Middletemperature generator 

= 
Mass flow rate (kg sec^{1}) 

= 
Heat load (kW) 
s 
= 
Entropy (kJ kg^{1} K) 
T 
= 
Temperature (K) 
x 
= 
Mass fraction of lithium bromide (%) 
COP 
= 
Coefficient of performance 

= 
Work (kW) 
ψ 
= 
Exergy (kJ kg^{1}) 
¤ 
= 
Irreversibility (kW) 
Subscripts
c 
= 
Condenser 
e, evap 
= 
Evaporator 
g, gen 
= 
Generator 
I 
= 
Inlet stream 
o 
= 
Outlet stream 