INTRODUCTION
Absorption cooling systems have become increasingly popular in recent years
from the viewpoint of energy and environment. Despite a lower coefficient of
performance (COP) as compared to the vapor compression, absorption refrigeration
systems are attractive for using inexpensive waste heat, solar, geothermal or
biomass energy sources for which the cost of supply is negligible in many cases.
In addition absorption refrigeration use natural substances, which do not contribute
towards ozone depletion and global warming (Manohar et
al., 2006).
Many different thermodynamic cycles, machine types (refrigerating, heat pumps,
heat transformers) and numerous authors all around the world have proposed new
fluids in these last decades. Among these potential technical solutions one
of the most interesting still remains the absorption cycle with water and lithium
bromide, suitable for air conditioning. The refrigerantabsorbent pair H_{2}O
LiBr, in fact, has numerous advantages such as high enthalpy of vaporization,
no need of rectification; it is neither toxic nor dangerous (Asdrubali
and Grignaffini, 2005).
In the last few years, the demand for energy conservation has increased significantly
and there are increasing needs for research and development on absorption chillers
includes their elemental design, reliability improvement, energy saving control
and diagnosis of degradation of performance.
Manohar et al. (2006) developed steady state
model of a double effect absorption chiller using steam as heat input. The model
is based on the artificial neural network (ANN). The model predict the chiller
performance based on chilled water inlet and outlet temperatures, cooling water
inlet and outlet temperatures and steam pressure.
Park et al. (2004) analyzed the performance
characteristics during part load operation and calculated the energy consumption
amount of H_{2}O/LiBr absorption chiller with a capacity of 210 RT and
they found that the performance of absorption system is more sensitive to the
change of inlet cooling water temperature rather than the cooling water flow
rate.
Kim and Ferreira (2008) presented a model capable of
describing the behaviour of absorption cycles with a convenient number of characteristic
constants for quick simulation of absorption systems. Though this model has
been applied to several examples of singleeffect absorption chillers using
various aqueous working fluids, it may not adapt well enough to reproduce the
performance of commercial or experimental chillers.
The design point information and geometrical configuration are the proprietary
of the manufacturer, therefore, these information are not available. The operational
data and well known heat transfer and thermodynamics equations are used to calculate
the design point information.
DESCRIPTION OF DOUBLEEFFECT ABSORPTION CHILLER CYCLE
The main components of a double effect steam absorption chiller are two generators,
a condenser, an evaporator, an absorber, two solutions heat exchangers, solution
pump, refrigerant pump and two refrigerant expansion valves (Fig.
1).

Fig. 1: 
Schematic diagram for a doubleeffect steam absorption chiller 
The solution pump assures the circulation of the solution inside the system.
The dilute solution (in LiBr) leaving the absorber is pumped through the heat
exchangers to the first and second generators. In the first generator heat is
added to the solution from steam circulating in the tube side and water vapor
is given off by the solution. This vapor is in superheated state, due to the
elevation of the boiling point of water caused by the presence of LiBr solute.
The vapor from first generator is used to heat the solution in the second generator.
Thus, the heating coil (tube side) of the second generator (LTG) is also condenser
for the first generator (HTG). The vapor generated by LTG is condensed in the
condenser, which usually is enclosed in the same vessel or section of shell
as in the second generator. The vapor generated in the first generator, after
condensing is throttled to the pressure of the condenser.
The concentrated solutions from the first and second generators are reunited
at the solution heat exchanger, transferring heat to the dilute solution coming
from the absorber and then the united stream inters the absorber where it is
sprayed onto the absorber tubes, thus facilitating the absorption of the refrigerant
vapor from the evaporator. A throttling process occurs in the absorber spraying
nozzles that reduce the pressure of the concentrated solution to the absorber
pressure. The absorber and evaporator are normally operated at the same pressure.
The absorber by absorbing the refrigerant vapor produces low pressure that is
required for the operation of the evaporator (Gordon and Ng,
2001).
ABSORPTION CHILLER MODELING
Applying mass balance, energy balance and equation of state for the LiBrH_{2}O
solution each component of the double effect steam absorption chiller model
is formulated. Some simplifying assumption have been made for the analysis,
these are:
• 
The analysis is made under steady conditions 
• 
The refrigerant (water) at the outlet of the condenser is saturated liquid 
• 
The refrigerant (water) at the outlet of the evaporator is saturated vapour 
• 
The lithium bromide solution at the absorber outlet is a weak solution
and it is at the absorber temperature 
• 
The outlet temperatures from the absorber and from generators correspond
to equilibrium conditions of the mixing and separation respectively 
• 
Pressure losses in the pipelines and all heat exchangers are negligible 
• 
The system rejects heat to cooling water at the condenser and absorber 
• 
The temperature of water vapor and of the solution leaving the generators
is assumed to be the same 
• 
There is no heat loss or gain from the ambient 
• 
Constant pumping rate, the mass flow rate of weak solution from absorber
to generators is constant 
The governing equations of each component:
The mass balance equation:
Concentration balance equation:
The conservation of energy equation is:
The heat transfer equation is:
(UA)_{1} is the overall conductance of the heating coil in the first
generator
The mass balance equation:
Concentration balance equation:
The conservation of energy equation is:
The heat transfer equation is:
The mass balance equation:
Concentration balance equation:
The conservation of energy equations are:
The heat transfer equation is:
The mass balance equation:
The conservation of energy equations are:
The heat transfer equation is:
where, (UA)_{c} is the overall conductance for heat transfer surface
of the condenser
The conservation of energy equations are:
The heat transfer equation is:
(UA)_{e }: The overall conductance for the heat transfer surface of
the evaporator
• 
Solution heat exchangers: 
High temperature solution heat exchanger:
Low temperature solution heat exchanger:
High temperature drain heat exchanger:
Low temperature drain heat exchanger:
The thermal properties of LiBr solutions and steam used in the calculations
are obtained from Abdullagatov and Magomedov (1997),
ASHRAE (1997), Herold et al.
(1996), Lee et al. (1990), Ozisik
(1985) and Rogers and Mayhew (1992).

Fig. 2: 
Flow chart of the simulation of a doubleeffect steam absorption
chiller 
Figure 2 shows simulation flow chart of a doubleeffect steam
absorption chiller.
RESULTS AND DISCUSSION
Using the formulated mathematical equations that govern the operation of the
steam absorption chiller and from partly available design data the rest of the
design parameters are calculated and presented in Table 1.
The simulation program is validated with real data for Ebara Carrier parallelflow
double effect steam absorption chiller model RAW 150LE with capacity of 1250
RT. The chiller is erected in university technology PETRONAS (Gas District Cooling
Plant).
As shown in Fig. 3 the refrigeration effect is increasing
as the heat input increase. The simulation results are compared with the actual
data and the trend replicated with 1.12% deviation.
Table 1: 
Design point values of doubleeffect steam absorption chiller 


Fig. 3: 
Variation refrigeration effect for different heat input 
Figure 4 shows the variation of the coefficient of performance
for different heat inputs. The coefficient of performance is slightly increasing
this is because the increase in heat input is small. The actual is included
for comparison and the deviation is about 1.1%.
Figure 5 shows the load factor variation with heat input.
As indicated in the figure at a given heat input range the load factor is more
than 85%. The maximum is around 91%. As the heat input increase the load factor
increases that is expected. The maximum load factor may be achieved by increasing
the heat input.

Fig. 4: 
Variation coefficient of performance with heat input 

Fig. 5: 
Variation load factor with heat input 
CONCLUSION
The steady state model of double effect steam absorption chiller is developed
to study the performance of the system. Using partly available data the design
parameters are calculated. The model gives a good predicting means for absorption
chiller over a wide range heat input. For thermo physical and thermodynamic
properties for lithium bromidewater solution, set of computationally efficient
formulations are used.
ACKNOWLEDGMENT
The authors wish to thank University Technology PETRONAS for the opportunity
to use the company’s own data to perform the investigations and for the
research grant.
NOMENCLATURE

= 
Mass flow rate (kg sec^{1}) 
x 
= 
Concentration of the solution (x/100) 

= 
Heat transferred rate (kw) 
h 
= 
Enthalpy (KJ kg^{1}) 
C_{p} 
= 
Specific heat(KJ/kg.°C) 
A 
= 
Heat transfer area (m2) 
U 
= 
Overall heat transfer coefficient (KW/m2 °C) 
COP 
= 
Coefficient of performance 
t 
= 
Temperature(°C) 

= 
Heat exchanger effectiveness 
Subscripts
g_{1}, g_{2 } 
= 
First and second generators 
a 
= 
Absorber 
c 
= 
Condenser 
e 
= 
Evaporator 
HTHEX 
= 
High temperature heat exchanger 
LTHEX 
= 
Low temperature heat exchanger 
1,2,3,… 
= 
State points 
r 
= 
Refrigerant 