Optimization of operating temperatures in the gas operated single to triple effect vapour absorption refrigeration cycles

Accepted Manuscript Title: Optimization of operating temperatures in the gas operated single to triple effect vapour absorption refrigeration cycles Author: Md. Azhar, M. Altamush Siddiqui PII: DOI: Reference:

S0140-7007(17)30267-0 http://dx.doi.org/doi: 10.1016/j.ijrefrig.2017.06.033 JIJR 3698

To appear in:

International Journal of Refrigeration

Received date: Revised date: Accepted date:

20-10-2016 26-5-2017 27-6-2017

Please cite this article as: Md. Azhar, M. Altamush Siddiqui, Optimization of operating temperatures in the gas operated single to triple effect vapour absorption refrigeration cycles, International Journal of Refrigeration (2017), http://dx.doi.org/doi: 10.1016/j.ijrefrig.2017.06.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title:

Optimization of Operating Temperatures in the Gas Operated Single to Triple Effect Vapour Absorption Refrigeration Cycles Authors: Md. Azhar* and M. Altamush Siddiqui

Affiliation: Computational and Experimental Heat Transfer Research Laboratory Department of Mechanical Engineering Z. H. College of Engineering and Technology Aligarh Muslim University, Aligarh-202002, Uttar Pradesh, India.

*Corresponding author’s contact: Telephone: +917895698621, Email: [email protected], [email protected]

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HIGHLIGHTS



Operating parameters of single effect is optimized in one step for maximum COP



Operating parameters of double effect is optimized in two steps for maximum COP



Operating parameters of triple effect is optimized in three steps for maximum COP



Minimum flow rates of LPG and CNG are found at different conditions



Irreversibility induced in condenser-generator set(s) are investigated

ABSTRACT Thermodynamic analysis of LiBr-H2O Single, Double and Triple Effect vapour absorption cycles have been carried out using LPG and CNG as sources of energy. Optimization of operating temperatures in single to triple effect cycles have been carried out for maximum COP of the system and minimum gas requirement in it at desired temperatures in evaporator, absorber and main condenser using iterative technique. In single effect cycle optimum temperatures in main generator have been obtained. While in double effect cycle, low pressure generator, high pressure condenser and main generator temperatures have been optimized. In triple effect cycle having three condensers and three generators, condenser temperatures (Tc3 and Tc4) and generator temperatures (Tg2, Tg3 and Tg) have been optimized. The maximum COP of triple effect cycle goes up to 1.955 which is around 132% higher than single effect cycle with its gas requirement reduced to around 122% at the same conditions.

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KEYWORDS: Triple Effect; Gas energy sources; LiBr-H2O; Coefficient of performance; Optimum generator temperature; Optimum condenser temperature; Salt concentration

NOMENCLATURE A

absorber

Ap

absorption percentage

C

main condenser

C3

condenser at pressure P3

C4

condenser at pressure P4

COP

coefficient of performance [-]

Cpl

specific heat of liquid refrigerant [kJ kg-1 K-1]

Cpv

specific heat of vapour refrigerant [kJ kg-1 K-1]

CNG

compressed natural gas

E

evaporator

G

main generator

G2

generator at pressure P2

G3

generator at pressure P3

h

specific enthalpy [kJ kg-1]

I

irreversibility generation [kW]

LPG

liquefied petroleum gas

LiBr

lithium bromide salt

m

mass flow rate [kg s-1]

M

molecular weight of gas [kg kmol-1]

P

Pressure [kPa]

PC

precooler 3 Page 3 of 58

PH1, PH2, PH3

preheaters at three levels

Q

rate of heat transfer [kJ s-1]

QC

energy released due to burning of CNG [kJ kg-1 of CNG]

QL

energy released due to burning of LPG [kJ kg-1 of LPG]

S

entropy generation [kW K-1]

SCR

solution circulation rate

T

temperature [oC or K]

TV

throttle valve

v

specific volume of gas [m3 kg-1]

VL

volume flow rate of LPG [m3 s-1]

VC

volume flow rate of CNG [m3 s-1]

Wp

work of pump [kW]

X

mass concentration of lithium bromide salt [%]

Xc

crystallization value of the lithium bromide concentration [%]

Greek Symbols: ΔH

enthalpy of combustion

ɛ

effectiveness of heat exchanger

Subscripts: a

absorber

c

main condenser

c3

condenser at pressure P3

c4

condenser at pressure P4

e

evaporator

eqm

equilibrium

exp

experiment 4 Page 4 of 58

g

main generator

g2

generator at pressure P2

g3

generator at pressure P3

gen

generation

i

inlet

o

outlet

p

product

r

reactant

s

standard

sol

solution

th

theoretical

rel

release

ref

refrigerant

1.

INTRODUCTION

The Absorption cooling system has recently become quite popular despite of low COP. It is one of the best alternatives to the vapour compression cooling system from the viewpoints of energy and environment. The attractive features that an absorption system possess are, it can operate using a low grade energy in the form of heat and uses natural substances as working fluid, which do not cause ozone depletion and global warming. A number of working fluids have been tested for this system. The most popular among them are H2O-NH3 and LiBr-H2O combinations [1]. Out of these two mixtures, the LiBr-H2O system is simpler in design and

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operation though it is operative only in air-conditioning applications since water freezes at 0°C. The system is less noisy as compared to other refrigeration system [2]. The single-effect absorption system utilizes low temperature-heat sources and has relatively low COP due to which it is not competing economically with the conventional vapour compression system, except in case of waste heat applications where the input energy is virtually free of cost. To reduce size of the equipment and operating cost of the absorption cycle, it is desirable to increase the COP. Therefore, multi-effect absorption systems with high temperature heat sources have now been developed [3]. The double effect system was first patented by Loweth [4] and commercialized by TRANE in 1970 which was later modified and improved by Saito and Inoue [5] and by Alefed [6]. Ohuchi et al. [7] firstly invented a triple-stage absorption refrigeration system having three generators where the working solution is supplied from the absorber directly to each generator in parallel. An alternate triple effect cycle, the double condenser coupled (DCC) cycle was patented in 1993 [8]. The first triple effect gas absorption chiller was commercialized in October 5, 2005 by Kawasaki Thermal Engineering Co., Ltd. (KTE). Siddiqui [9], Saghiruddin and Siddiqui [10,11], Malik and Siddiqui [12] have carried thermodynamic and economic analysis of single effect cycle with LiBr-H2O, NH3-H2O, NH3LiNO3 and NH3-NaSCN combinations using different sources of energy like, Biogas, LPG and Solar Collectors. Siddiqui [13], optimized the generator, condenser and absorber temperatures in single effect water cooled absorption refrigeration system and also investigated the optimum pumping rate corresponding to minimum operating cost. Samanta and Basu [14] have optimized the generator temperature of single effect cycle. Lee and Sherif [15] and Sencan et al. [16] carried thermodynamic analysis of single effect absorption system for cooling and heating applications using pressurised hot water as the source of energy. Dincer and Dost [17] carried out the energy analysis of single effect cycle using ammonia 6 Page 6 of 58

and water as the working fluids and verified this system with experimental data. Siddiqui and Riaz [18] and Saghirudddin and Siddiqui [19] optimized the generator temperature in two stage dual fluid absorption cycles. Dehua Cai et al. [20] have performed an air cooled absorption refrigeration system using ammonia-lithium nitrate and ammonia-sodium thiocyanate solution as working fluid. Dehua Cai et al. [21] also carried experimental analysis of single effect vapour absorption refrigeration system using NH3–LiNO3 and NH3–NaSCN as working pair. The experimental results shows that NH3–NaSCN based system has more efficient than NH3-LiNO3 system. Canan Cimsit et al. [22] carried thermo economic optimization of LiBr-water absorption system and R134a based compression system and found the optimum generator temperature, condenser temperature, absorber temperature, condenser temperature of vapour compression system, effectiveness and isentropic efficiency of compressor. Khaliq and Kumar [23] examined a double-effect vapour absorption refrigeration system. The system comprised of a cascade of two single-effect cycles. They [23] computed effects of the generator, absorber, evaporator and condenser temperatures on the system performance and showed that exergy destructions occurred significantly in the generators, absorber, evaporator and heat exchangers. Arora and Kaushik [24, 25] and Marcos et al. [26] performed energy and exergy analysis of single and double effect LiBrH2O absorption refrigeration system. Gomri and Hakimi [27] conducted energy and exergy analysis of double effect vapour absorption system and showed that COP increases on increasing the low pressure generator (LPG) temperature, but decreases on increasing the high pressure generator (HPG) temperature. Ratlamwala et al. [28] carried energy and exergy analysis of triple effect ammonia-water absorption refrigeration system. They [28] have shown the effect of COPs for different operating conditions and reference environment conditions on the system performance. Khaliq et al. [29] conducted the energy and exergy analyses of a new solar driven triple staged refrigerated cycle i.e. the cycle is an integration of 7 Page 7 of 58

absorption refrigeration cycle, ejector refrigeration cycle and ejector expansion JouleThomson refrigeration cryogenic cycles. Gomri [30] conducted energy and exergy analysis of triple effect absorption cycles for the production of chilled water and found that the maximum COP of the triple effect cycle at evaporator temperature of 4oC, condenser temperature of 33oC and high pressure generator temperature of 190oC is 1.766. Gebreslassie et al. [31] have performed the first and second law analysis for half to triple effect water-LiBr absorption system, and concluded that at higher heat source temperature, the COP and exergetic efficiency decreases slowly. The studies carried so far on the double and triple effect cycles, to some extent, present optimization of the main generator temperature. But, optimization of the temperatures in the other components of the system which are at the intermediate stages of the two cycles, like condenser(s) and generator(s), seems to be missing. The present study has therefore been taken up to optimize the operating temperatures in various components of the single, double and triple effect cycles for maximum COP and minimum gas requirement. The COP of the absorption cycle depends upon the cooling load of the system and the heat supplied to the generator. The generator and condenser in the system operate at the same pressure for mass transfer equilibrium. Therefore, the parameters that govern operation of a generator are, temperature (Tg) and concentration (Xg) of the LiBr-H2O solution in it and the saturation temperature (Tc) of the vapour released from it, which then enters the condenser. It thus, means that at-least two of the above three parameters should be known to operate the generator, because the third parameter will automatically get fixed. And, this can be readily determined from the relation: Tc= f (Tg , Xg) which has been derived from the pressure equilibrium equation (Pc=Pg). The single effect absorption cycle has only one generator, therefore variation of the salt concentration Xg at fixed value of the condenser temperature Tc will lead to an optimum generator temperature that will correspond to the maximum COP of 8 Page 8 of 58

the system. The double effect cycle on the other hand, consist of two generators. The main generator (at high pressure) operates by means of some external heat source and the other generator operates with the heat of condensation released from the high pressure condenser. Since, both of these generators simultaneously effect the COP of the absorption system, optimization in the double effect cycle has to be done in two steps. Similarly, the triple effect cycle having three generators requires the optimization process in three steps. This needs systematic procedure of analysing the absorption system. Therefore, the iterative method of optimization has been selected in the present study so that effect of the various parameters on the system may be studied in detail.

2.

SYSTEM DESCRIPTION

The single effect vapour absorption system using lithium bromide-water as a working fluid is shown in Fig. 1. In this system, the diluted solution (low concentration of the LiBr salt, Xa) of LiBr-H2O mixture at state 1 is pumped from the absorber to the generator via a preheater. In the preheater the solution is heated before entering the generator by using the heat rejected from the strong solution (high concentration of LiBr salt, Xg) leaving the generator at state 8. The solution after getting heat in the generator releases pure refrigerant that flows to the condenser. The remaining strong solution flows down to the absorber through the preheater. This improves COP of the system. On the other hand, the refrigerant vapour passing through the condenser condenses after rejecting heat to the sink. The condensate refrigerant is allowed to pass through the throttled valve TV1 to reduce it to the evaporator pressure and temperature. Refrigerant in the evaporator vaporize due to cooling load, there by producing cooling effect. The refrigerant vapour leaving the evaporator is then taken to the absorber where it gets absorbed in the solution coming from the generator. Thus, the cycle gets completed. 9 Page 9 of 58

The schematic diagram of a double effect vapour absorption system is shown in Fig. 2. It consists of two generators. The main generator (G) and the condenser (C3) are at pressure P3 (= Pg = Pc3) while the main condenser (C) and the generator (G2) are at pressure P2 (= Pc = Pg2). Heat is provided to the main generator and heat rejected is from the main condenser. The evaporator and the absorber work at low pressure P1 (= Pe = Pa). In this system, the weak solution at state 1 is pumped from the absorber to the main generator (G) through two heat exchangers (i.e. PH1 and PH2). The solution in the main generator is heated at relatively high temperature to boil out the refrigerant vapour from the solution. The primary vapour, from G goes to the condenser C3. The heat of condensation in C3 is used to heat the solution in the generator G2. The strong solution leaving the generator G at state 8 flows to the generator G2 through PH2 which is cooled by the weak solution. The vapour released in the generator G2 and the refrigerant-condensate from the condenser C3 flow into the main condenser. Thus the total amount of liquid refrigerant leaving the main condenser will be the sum of refrigerant coming from the generators G and G2. The refrigerant liquid from this condenser flows to the evaporator through a throttle valve. After extracting heat from the medium to be cooled, the refrigerant evaporates and then passes to the absorber, and gets absorbed by the strong solution coming from the generator G2 through a preheater PH1. The resulting weak solution in the absorber is then pumped to the main generator and the cycle completes. Figure 3 shows schematic diagram of triple effect vapour absorption system. It consists of three generators and three condensers. The main generator G and the condenser C4 are at pressure P4 (= Pg = Pc4), the condenser C3 and the generator G3 are at pressure P3 (= Pg3 = Pc3) and the condenser C and the generator G2 are at pressure P2 (= Pg2 = Pc), while the evaporator and the absorber are at low pressure P1 (=Pe=Pa). In this system, the weak solution at state 1 is pumped from the absorber to the main generator (G) through three preheaters (i.e. 10 Page 10 of 58

PH1, PH2 and PH3). The main generator is heated to boil out the refrigerant vapour from the solution. The vapour released from G, enters the condenser (C4) which condenses and releases heat that is utilized by the generator G3. The strong solution leaving the generator G at state 8, flows into the generator G3 through the preheater PH3, where it is cooled by the weak solution. In the generator G3 some more vapour is released that enters the condenser (C3), where from the heat of condensation is utilized by the generator G2. The strong solution, now leaving the generator G3 at state 8c, flows to the generator G2 through PH2 where it is further cooled by the weak solution coming from the preheater PH1. In the generator G2 still more refrigerant is released that enters the main condenser C from which heat is released to the sink. Thus, the total refrigerant entering the main condenser in the triple effect cycle will be sum of the refrigerants from all the generators. The liquidrefrigerant from the condenser (C) flow into the evaporator through a throttle valve, which vaporizes after taking heat from the space to be cooled. It then enters the absorber, and gets absorbed by the strongest solution coming from the generator G3 via the preheater PH1, thus, completing the cycle. Table 1 shows the different pressure levels of the single, double and triple effect cycles.

3.

MATHEMATICAL MODELLING

3.1 Absorption Cycle: The thermodynamic analysis of the system involves the principle of energy conservation. Energy conservation

  m h  i    m h  o +   Q i   Q o 

+W 0

(1)

Assuming each component of absorption cycles shown in the Figs. 1, 2 and 3 as control volume, energy balance lead to the following equations: Absorber:

Q a = m 7 h 7 + m 10 h 10  m 1h 1

(2)

Solution pump:

W p  m 2 h 2  m 1h 1

(3)

11 Page 11 of 58

Evaporator:

Q e  m 7h 7  m 6h 6

(4)

Main Generator:

Q g  m 4 h 4 + m 8h 8  m 3h 3

(5)

Main Condenser of the single effect cycle: Q c  m 4 h 4  m 5 h 5

(6)

Generator (G2), condenser (C3) and the main condenser (C) of the double effect cycle: Q g2  m 4c h 4c + m 8c h 8c  m 8b h 8b

(7)

Q c3  m 4 h 4  m 4a h 4a

(8)

Q c  m 4c h 4c + m 4b h 4b  m 5 h 5

(9)

Generators (G2 & G3), condensers (C3 & C4) and the main condenser (C) of the triple effect cycle: Q g3  m 4c h 4c + m 8c h 8c  m 8b h 8b

(10)

Q g2  m 8f h 8f + m 4f h 4f  m 8e h 8e

(11)

Q c3  m 4b h 4b + m 4c h 4c  m 4d h 4d

(12)

Q c4  m 4 h 4  m 4a h 4a

(13)

Q c = m 4e h 4e + m 4f h 4f  m 5 h 5

(14)

Effectiveness of the Preheaters:

Single effect cycle:

ε1 =

A ctual heat transfer M axim um posssible heat transfer

=

T8  T9 T8  T 2

Double effect cycle:

T  T9 ε 1 = 8c and ε 2 = T8c  T 2

T8  T8a T8  T 2a

Triple effect cycle:

T  T9 T  T8d T  T8a ε 1 = 8f , ε 2 = 8c and ε 3 = 8 T8f  T 2 T8c  T 2a T8  T 2b

(15)

(16)

(17)

12 Page 12 of 58

Coefficient of Performance of the cycle: COP 

Qe Q g  Wp

(18)

The crystallization values of LiBr salt in the cycle have been obtained using the following equation [32]: X c = 9.8459  10

-2

 T  273.15 

+ 59.7995

; In the range: 320 ≤ T ≤ 375 K

(19)

3.2 Irreversibility Analysis of Condenser-generator Assembly: The irreversibility generation associated with internal temperature difference between condenser and generator sets of double and triple effect cycles exchanging heat will be as follows: Entropy generation in the double and triple effect cycles when heat exchange is between C3 and G2 (refer Figs. 2 and 3):

S  gen

at P 3

=

Q g2



Q c3

Tg2

Irreversib ility

; kW K

1

(20)

Tc3

 I3 

= T s  S g en



at P 3

(21)

; kW

Similarly, the entropy generation in the triple effect cycle when C4 and G3 exchanging heat (refer Fig. 3):

S  gen

at P 4

=

Q g3



Q c4

Tg3

Irreversib ility

I4 

; kW K

1

(22)

Tc4 = T s  S g en



at P 4

(23)

; kW

3.3 Source of Energy: The absorption system can be driven by any source of heat energy such as solar, waste heat and gases [10]. Since the absorption machines are heat operated systems and can utilize any form of energy, the energy source can be selected depending on its availability and the cost incurred in procuring and converting it into the useful energy. Among many, solar energy and waste heat may prove to be cheapest provided they are available sufficiently at the location 13 Page 13 of 58

where the absorption machine is to be installed. The waste heat is generally available from the process or power industries which can be used to operate the absorption machine for air conditioning of places close to the plant because transporting the waste heat to a long distance may drop down its energy level. Here, large capacity air conditioner can be installed if sufficient energy is available in the waste heat at the point of its disposal. Similarly, the absorption system can also be operated by means of solar energy provided it is available in most of the days when cooling is required and there is sufficient open space for the solar collectors to be installed. In the present study liquefied petroleum gas (LPG) and compressed natural gas (CNG) have been selected as the sources of energy as they can very well operate the direct fired absorption machines. The direct fired absorption machines of varying capacity can be installed independently at any location where these gases are available at cheaper rate. Also both, LPG and CNG are less toxic greenhouse gases resulting in reduced pollution. The liquefied petroleum gas is generally a mixture of number of gases such as propane, butane and ethane. For a typical composition of LPG, say Propane (C3H8) = 6.88%, Isobutane (C4H10) =16.54% and n-butane (C4H10) = 76.58%, average molecular weight of LPG comes out to be M = 57.0368 kg kmol-1. The energy released during combustion of any gas can be calculated by applying steady flow energy equation per mole of fuel [33] as: Q rel +  n p  h f + h  h o o

 = n h r

p

o f

+h  h o



; kJ km ol

1

o f fu el

(24)

of fuel

(25)

r

This can further simplifies as: Q rel  Δ H s 

 n h  h  p

o

p

  n r  h  h o  r ; kJ km ol

1

Where, Δ H s = E nthalpy of com bust ion; kJ km ol -1 of fuel The combustion equation for the above composition of LPG, with 10% excess air, can be written as: 14 Page 14 of 58

[0.7658 C4H10 + 0.1654C4H10 + 0.0688 C3H8] + 7.036 O2 + 26.457 N2

3.9312 CO2 +

4.9312 H2O + 0.6392 O2 + 26.457 N2 (26) Based on the equation (26), the energy released as heat during combustion of 1 kmol of LPG at standard temperature of 25oC (298.15K), with the reactants assumed to be at 40oC (313.15K), can be written as:

 







 3.9312 h T  h C O 2  298.15   4.9312 h H 2 O  T p   h H 2 O  298.15    CO 2  p    QL   0.6392 h  T  h O 2  298.15   26.457 h N 2  T p   h N 2  298.15  O2  p   





  









 0.7658 h C H  313.15   h C H  298.15   0.1654 h C H  313.15   h C H  298.15    4 10 4 10 4 10 4 10    0.0688 h   H 313.15   h C 3 H 8  2 98.15   7.036 h O 2  313.15   h O 2  298.15   C3H 8  s    26.457 h  313.15   h  298.15   N2 N2  









k J km ol

-1

of LP G

(27) Substituting the specific enthalpy of each gases and enthalpy of combustion, taken from [33], for the known temperatures specified in the equation (27) we get, Q L = 4.9312h H 2 O (Tp )+3.9312h CO 2 (Tp )+0.6392h O 2 (T p )+26.4 5h N 2 (Tp )  2951411.875; kJ kmol of LPG -1

(28) Considering the respective values of enthalpy taken from [33] at different temperatures of the products of combustion (Tp), equation (28) gets correlated in the range of

273K ≤ Tp ≤

1800K as: 2

Q L  2944000.0  1008.6Tp  0. 144Tp ; kJ km ol

1

of LP G

(29)

Introducing the molecular weight of LPG (M = 57.0368 kg kmol-1) equation (29) simplifies as: 1

Q L = 51615.8  17.68Tp  0.00252T p ; kJ kg of LPG 2

(30)

15 Page 15 of 58

Similarly for the Compressed natural gas (CNG), with composition as: methane (CH4) = 91.9%, Ethane (C2H6)=3.7%, Propane (C3H8) =1.2%, Iso butane (C4H10)iso = 0 .4%, carbon dioxide (CO2) = 2%, Nitrogen (N2)=0.2%, n-butane (C4H10)n=0.1% and Heptane (C5H12) = 0.4%, the average molecular weight comes out to be 17.857 kg kmol-1. The combustion equation for the CNG, with 10% excess air is written as: [0.919CH4 + 0.037C2H6 +0.012C3H8+0.005C4H10+0.004C5H12]+0.02CO2 + 0.002N2+2.3232 1 .089CO2+2.046H2O+0.2112O2+8.735N2

(O2+3.76N2)

(31)

And, the energy released (QC) during combustion of CNG, in the similar manner as for the LPG, is correlated in the range of 273K ≤ Tp ≤ 1800K as: Q C = 53364.84  18.87 Tp – 2. 0 × 10

3

2

Tp ; kJ kg

1

(32)

of C N G

Thus, the mass flow rate and volume flow rate of the gases can be calculated as: .

(m ) LPG 

.

(m ) C N G 

and

Qg



QL

Qg QC



Qe COP  Q L

Qe COP  Q C

, kg s

-1

(33)

, kg s

-1

(34)

.

VL  (m ) LPG  S pecific volum e of LP G , m

3

s

s

-1

(35)

-1

.

VC  (m ) C N G  S pecific volum e of C N G , m

Where, specific volume of LPG;

Specific volume of CNG;

vc 

vl 

3

(36)

V o lu m e o f g as



M o lecu lar w eig h t

V olum e of gas M olecular w eight



25.68

2 5 .6 8

3

= 0 .4 5 0 2 , m k g

-1

5 7 .0 3 6

3

= 1.438; m kg

-1

17.857

3.3 Calculation Procedure: In order to simplify the simulation and analysis, several assumptions are made, including the following: _ The process in the cycle is under steady condition. 16 Page 16 of 58

_ The refrigerant (water) at the outlet of the condenser is saturated liquid. _ The refrigerant (water) at the outlet of the evaporator is saturated vapour. _ Temperature of absorber and condenser is considered as same (Ta=Tc). _ There is no heat loss to the surrounding. _ Expansion in throttling valve is isenthalpic. _ Friction and Pressure loss in the pipelines and all heat exchangers are negligible. _ No leakage of air in the system. Figures 4 to 6 show the flow charts for the calculation procedure of single, double and triple effect cycles. With fixed input data (Table 2) and thermo-physical property equations of the working fluids taken from references [33, 34], used in the form of subroutines, the temperature, pressure, density, specific heat and enthalpy of the solution and refrigerant at different state points are evaluated. Some fixed data for simulation of the absorption cycles are given in the Table 2.

For simplicity, the absorber temperature is taken as Ta=Tc as the main condenser and the absorber reject heat to the ambient using same coolant, which is generally water. With fixed values of Te and Ta, concentration of the LiBr-H2O solution Xa in the absorber gets fixed. Taking Xa as the base value the concentration of LiBr-H2O solution, Xg in the main generator is varied in steps. Similarly, concentration of LiBr-H2O solution in the subsequent generators at lower pressures are varied taking the base values of the concentrations that were in the preceding high pressure generators. Thus, temperatures in the generators vary as the concentration

of

the

solution

in

them

are

varied.

17 Page 17 of 58

3.4 Validation from the simulated results: The present results were at first validated with the simulation data of Arora and Kaushik [25] for the single and double effect cycles and those of Gomri [30] for the triple effect cycle. Table 3 gives the heat transfer rate at different components and COP of the cycles at same operating conditions from the corresponding authors. This is a close agreement of the present data analysis with the published works.

3.5 Comparison of simulated and experimental results: Most of the work reported so far in the open literature on the single, double and triple effect vapour absorption refrigeration systems are generally on simulation studies. Simulation modelling is an important tool to investigate the system performance in detail under varying operating conditions. Simulations are done using steady state or dynamic model. Most of the simulations carried so far on the absorption systems are based on steady state model with number of assumptions. The experiments, on the other hands, gives the real characteristics and involves factors like, efficiency, hysteresis and drop in pressure/temperature in the system-components. The experimental results, are expected to approach the theoretical values only if the system is properly designed, constructed and equipped with adequate measurement and control facilities. This is why it often becomes difficult to match the experimental results with the theoretical values. 3.5.1 Comparison for single effect cycle: Many Researchers [35-46] have carried experimental studies on the single effect vapour absorption refrigeration system using different sources of energy. A few of them have been 18 Page 18 of 58

considered to compare the present simulation results. Monne et al. [40] have performed experiments on the solar powered single effect absorption cooling cycle and presented average COP for the years 2007 and 2008. They [40] have validated their experimental result with a dynamic model of the solar cooling system using the simulation environment TRNSYS software. The experimental results of Monne et al. [40] and the present simulated result for the same operating temperatures have been presented in Table 4. Similarly, Izquierdo et al. [41] have made a prototype single-double effect air cooled absorption chiller. The machine operates in three modes: single effect (capacity 4.5 kW), double effect (7 kW) and simultaneous mode (11 kW) by adjusting the valve of the machine. This machine was evaluated experimentally in both years 2009 & 2010 at Madrid (Spain). The experimental data for five days were reported for single effect mode with average capacity of 3.4 kW yielding COP as 0.54, also given in the Table 4. Comparison of the experimental results of Monne et al. [40] and Izquierdo et al. [41] with the present work on a single effect cycle shows that the experimental COP is around 2630% less than the theoretical COP in the present work. Aphornratana and Sriveerakul [46] have carried a detailed experimental study on the single effect LiBr-water absorption system and compared with theoretical results evaluated for the same operating conditions. For comparison the present simulation work has also been carried for the similar conditions as adopted by Aphornratana and Sriveerakul [46]. The theoretical results have been obtained for fixed values of Xa,th=54.7% (salt concentration in the LiBr-water solution leaving the absorber) by varying Xg and Tg (salt concentration and temperature in the generator). All these results have been listed in Table 5. It is found that the results of the present simulation work match very well with the theoretical results of Aphornratana and Sriveerakul [46]. But, 19 Page 19 of 58

their [46] experimental results show significant increase in the solution circulation ratio (SCR) and decrease in the coefficient of performance (COP). The decrease in the experimental COP is around 17 to 24 % which seems to be due to low mass transfer performance of the absorber [46]. However, in depth analysis of the experimental results [46] is still required to identify the real parameters that have brought such a significant change. For this, the salt concentrations in the LiBr-water solution, leaving the absorber (Xa) and the generator (Xg), given in Table 5, are plotted in Fig. 7 against Tg. Through these plots it is inferred that the concentration Xa,th, which should basically be the equilibrium concentration (Xa,eqm) corresponding to the saturation temperatures Te and Ta in the evaporator and the absorber, remains constant because Te and Ta here are fixed. Whereas the concentration Xa,exp is seen to be increasing along with Xg as the generator temperature Tg is raised. Thus, causing a significant change in the value of (Xg –Xa,exp) from that of (Xg –Xa,eqm), resulting in deviation of the experimentally measured solution circulation rate (SCRexp) from those of the theoretically evaluated (SCRth).

SCR

SCR

th



exp

m sol



m ref



m sol m ref

m1



m4



m1 m4

Xg

(37)

X g  X a , eqm



Xg

(38)

X g  X a , exp

The theoretical and experimental values of SCR are also plotted and shown in the Fig. 7. Though they vary almost in a similar manner but have a significant change in their magnitude which is due to deviation of Xa,exp from the Xa,eqm. The absorption percentage in the absorber, as also pointed out by Aphornratana and Sriveerakul [46], can be determined from the following equation [47]:

20 Page 20 of 58

Ap 

X g  X a, exp X g  X a, eqm

 100.0

(39)

It is thus clear from the equation (40), that if the experimental salt concentration at the absorber exit (Xa,exp) remain close to the equilibrium value (Xa,eqm) then the absorption percentage of the absorber will be high and may even become 100% in an efficient absorber. But in the experimental study [46], the value of Xa,exp has increased with increase in the generator temperature, the absorption capacity of the absorber in their [46] setup has reduced. This can also be seen through the plot of Ap in the Fig. 7 which decreases drastically with the generator temperature. The theoretical and the experimental SCR can be related in terms of Ap as follows: SCR

th

 A p  SCR

(40)

exp

This shows that the theoretical SCR will be lower than the experimental SCR, if Ap is less than 100%. Thus, it is observed that the main parameter that has brought a significant change in the experimental values is the lithium bromide salt concentration of the solution leaving the absorber (Xa) at state point 1 (Fig. 1); leading to change in the properties of the lithium bromide salt solution in the circuit from state point 1 to 3 (Fig. 1) in the absorption cycle. The new property values evaluated for the system in Fig. 1 corresponding to the experimental Xa,exp show major change in the specific enthalpies h1 and h3 which can be seen from plots presented in Fig. 8. The experimental and theoretical COP which can be evaluated from the following equations indicate that the major quantities affecting the performance are SCR×(h8-h3) because the other properties remain almost unchanged.

21 Page 21 of 58

COP

th

Q  e Q  g

Q COP exp   e Q  g

 (h 7  h 6 ) th     th (h 4  h 8 ) th  SCR th (h 8  h 3 ) th

(41)

 (h 7  h 6 ) exp     exp (h 4  h 8 ) exp  SCR exp (h 8  h 3 ) exp

(42)

The theoretical and experimental values of SCR×(h8-h3) have been plotted in Fig. 9, which decrease with Tg following almost similar trend; the experimental values being more than the theoretical values. This in turn, changes the performance of the system as shown through the theoretical and experimental plots of COP in the same Fig. 9. The numerical and experimental COP also vary with Tg following almost similar trend with difference only in their magnitude. The experimental COP are lesser than the simulated values because of the experimental limitations discussed above.

3.5.2 Comparison for double and triple effect cycles: As for the double and triple effect systems, only a few researchers [41, 48 & 49] for a limited range of operating conditions have attempted to carry experimental studies. The experimental data on double and triple effect systems have been also compared with the simulated results of the present work, as given in Table 6. The simulated results of the double effect cycle is compared with the experiments of Izquierdo et al. [41] which show that the experimental COP deviates by 20% from the theoretical COP. Similarly the simulated results of the triple effect cycle are compared with the experiments of Alka Sonalki et al. [48]. Who have shown the effect of seasonal conditions (April, May, June and July) on the performance of the 22 Page 22 of 58

triple effect vapour absorption apparatus which is available at National Institute of Solar Energy, Gurgaon (Haryana) in India. The rated COP of their apparatus is 1.7 while the maximum COP achieved experimentally in the month of June was 1.56. For the purpose of comparison, the operating temperatures of the system were selected based on temperatures specified at various locations on the line diagram of their apparatus [48]. The experimental and present work simulation results are given in the Table 6, which shows that the COP of the actual (experimental) system deviates from the simulated values only by 7%. Because of limited experimental data available in the literature, the trend of the experimental results cannot be predicted exactly. However, from the results on the single effect system, which show almost similar variation in the experimental and simulated values of the COP, it is expected that the experimental COP of the double and triple effect systems should also follow the same trend of variation as that predicted theoretically because the deviation in their COP is relatively less. Thus, it is also expected that for the same operating conditions in the single, double and triple effect cycles, the optimum generator temperature for maximum COP in the actual experimental system should be close to those obtained theoretically. However, they may differ in their magnitude depending on the accuracy of the system design, construction and measurement techniques. RESULT AND DISCUSSION

4.1 LiBr Salt Concentrations: Figure 10 shows variation in concentration of the LiBr salt in the mixture of LiBr-H2O with main generator temperature of single, double and triple effect cycles at different values of the evaporator temperature and at Tc=30oC, Tc3=80oC and Tc4=130oC up to the actual state of system-operation, as shown in the algorithm of the respective cycles through Figs. 4 to 6. It is observed that concentration of the LiBr salt in the absorber (Xa) remain constant with increase in the generator temperature of all the three cycles. However, on increasing the evaporator temperature, the salt concentration in the absorber decreases. In the generator of the single effect 23 Page 23 of 58

cycle, the concentration increase linearly with increase in the generator temperature. It is interesting to see that, although the absorber concentration Xa changes with the evaporator temperature, the salt concentration in the generator Xg (though increasing with Tg), lie on a single line for all values of Te because Ta (=Tc=30oC) remain unchanged. Again in the double effect cycle, the absorber salt concentration is the same as it was in case of the single effect cycle, except that the system operates at high generator temperatures. Also, the concentration in the main generator follow almost similar trend as in the single effect cycle. However, the salt concentration (Xg2) in the generator G2, increase sharply; shifting towards the low generator temperatures with increase in the evaporator temperature. Similar trend is found in the triple effect cycle, which operates at higher temperatures with the absorber concentration remaining same as in the single and double effect cycles. The salt concentration in the main generator, lie on the same line shifting towards low-generator temperature on increasing the evaporator temperature. The concentration in generators G3 and G2 vary almost linearly, increasing with the main generator temperature and shifting towards the low-generator temperatures, as separate lines, at high values of the evaporator temperature. It is to be noted that the last values of Xg in the single effect cycle, Xg and Xg2 in the double effect cycle and Xg, Xg2 and Xg3 in the triple effect cycle for each temperature in the evaporator, are the actual operating points of the system. In the double and triple effect cycles, they correspond to the energy balance in the condenser and generator assembly exchanging heat. Another interesting feature observed here is that of the difference in the salt concentration, between the main generator and the absorber. The concentration-difference (Xg–Xa) in the double and triple effect cycles are less than (Xg–Xa) in the single effect cycle. This is because Xg, Xg2 and Xg3 in the respective cycles should lie between Xa and the crystallization line Xc due to 24 Page 24 of 58

which concentration Xg in the main generator becomes less in case of the double and triple effect cycles. This puts limit in increasing the temperature Tg in the main generator and hence the calculation terminates before the constant COP is attained. Figure 11 shows the variation of salt concentrations, Xg in the main generator G and Xg2 in the generator G2 at temperatures (Te=4oC and Te=10oC) in the evaporator. To clarify the variation of salt concentration in each generator, the concentrations shown in the Fig. 10 against the main generator temperature Tg are re-plotted in Fig. 11 for the double effect cycle and in Fig. 12 for the triple effect cycle. It is seen that the salt concentration varies almost linearly with its generator temperature; the temperatures in the respective condenser being fixed. In the Fig. 11 for the double effect cycle, it is also observed, that Xg2 varies from Tg2 = 60.95oC to 84.07oC at Te=4oC and from 53.72oC to 76.67oC at Te=10oC, while Xg varies from Tg =116.38oC to 126.11oC at Te=4oC and from 108.27oC to 119.99oC at Te=10oC. Similarly in the Fig. 12 for the triple effect cycle, Xg2 varies from Tg2=61.78oC to 78.24oC at Te=4oC and from 54.52oC to 79.48oC at Te=10oC, while Xg3 varies from Tg3=117.04oC to 129.59oC at Te=4oC and from 108.91oC to 126.73oC at Te=10oC and, Xg varies from Tg=172.99oC to 180.15oC at Te=4oC and from 163.97oC to 173.76oC

at

Te=10oC.

25 Page 25 of 58

4.2 Coefficient of performance (COP): Figure 13 shows variation in the coefficient of performance of single, double and triple effect cycles with the main generator temperature (to which heat is supplied) at different values of the evaporator temperature. Here the main condenser temperature in the single, double and triple effect cycles from which heat is rejected to the surrounding is Tc=30oC.

The

temperature of condenser C3 in the double and triple effect cycles is Tc3=80oC, while in the triple effect cycle the condenser C4 temperature is Tc4=130oC. From the plots in Fig. 13, it is seen that the coefficient of performance of the absorption cycle increase drastically from low values at the low generator temperatures, reach to a maximum value and then either remain constant with further increase in the generator temperature (as in the single effect cycle) or terminate (as in the double and triple effect cycles) due to the operation limit of the system. That is the salt concentration in the main generator (Xg) being lower in case of the double and triple effect cycles, result in low value of the generator temperature, as explained earlier. It is to be noted that in the double effect cycle, the highest values of the generator temperature and concentration at which the calculation terminates is when heat rejected from the condenser C3 balances the heat required by the generator G2. Similarly in the triple effect cycle, again the heat balance requirement between the condenser C4 and the generator G3 and, between the condenser C3 and generator G2, puts limit in the calculation terminating at relatively lower generator temperature before attaining the constant COP. It is seen that with increase in the evaporator temperature, COP of all the cycles increase and shift towards low generator temperatures. One can also notice significant enhancement in COP of the advance stage cycles, that is, COPtriple > COPdouble > COPsingle.

45

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4.3 Volume flow rate of the gases: Figure 14 shows variation in the volume flow rate of LPG and CNG that will be required to operate the single, double and triple effect cycles. It is observed that the volume flow rate of both gases in all the three cycles decrease with increase in the main generator temperature, reach to minimum values and remain almost constant as in the single effect cycle, or terminate as the heat balance occur in the condenser and generator sets exchanging heat in the double and triple effect cycles. It is seen that the volume flow rate of LPG and CNG gases decrease with increase in the evaporator temperature. The volume flow rate greatly decrease in case of the double effect cycle from that of the single effect cycle, with quite low values in the triple effect cycle. It is also seen that the volume flow rate of CNG (VC) is greater than the volume flow rate of LPG (VL) in the three cycles. This is because of difference in their specific volumes; the specific volume of CNG being higher than that of the LPG.

4.4 Optimization of Condenser and Generator Temperatures: 4.4.1 Single effect cycle: Since the single effect cycle has only one generator, the optimization here will be done in one step. Referring to the Fig.13, it is seen that the COP increases with increase in the main generator temperature, reaches to a maximum value and becomes constant thereafter. It shifts towards low generator temperature with increase in the evaporator temperature Te. The COP plots in the Fig. 13 are for Tc=Ta=30oC. Similar plots can be shown for other values of Tc=Ta. 46

Page 27 of 58

In Fig. 14, flow rates of LPG and CNG required in the system are plotted. Almost reverse effect is seen in the flow rate of the LPG and CNG. The minimum flow rate of the gases (Fig. 14) correspond to the maximum COP of the system (Fig. 13). Thus, the generator temperatures selected from the Fig. 13 corresponding to the maximum COP are termed as the optimum temperatures. The LiBr salt concentration Xg is then known corresponding to the selected value of Tg and fixed value of Tc. The optimum generator temperature, maximum COP and minimum flow rates, so obtained, for the single effect cycle at different values of Te and Tc=Ta are listed in Table 7. As also reported in the literature, there will be almost linear variation of the optimum Tg and maximum COP with Tc for each value of Te. Thus, knowing flow rate of the CNG or LPG, whichever is the source for operating the absorption system, one can estimate the yearly running cost similar to those carried by Saghiruddin and Siddiqui [10,13], based upon their availability and price at the locations of utilization.

4.4.2 Double effect cycle: In the Fig. 13 are shown variation of COP of the double effect cycle with Tg for Tc=Ta=30oC and Tc3=80oC. Similar plots can be shown for other values of Tc3 and Tc=Ta. Figure 15 show variation in COP of the double effect cycle for different values of Tc3 at Tc=Ta=30oC and Te=4oC. The COP for each Tc3 increases with Tg reaches to a maximum value and then terminates, either due to heat balance attained between the condenser C3 and the generator G2 or due to crystallization of the LiBr salt, as explained earlier through the Fig. 10. The double effect cycle having two generators will be optimized in two steps. In the first step, the maximum COP are selected for each value of Tc3 from the Fig. 15. The corresponding values of Tg are also noted down. Then these maximum COP are re-plotted against Tc3 in Fig. 16 (a) 47

Page 28 of 58

which increases from low values of Tc3, reaches to some maximum value and then decreases with further increase in Tc3. Similar variation of the maximum COP with Tc3 are seen for different values of Te; higher COP for high values of Te. The maximum COP for each Te can thus be identified that are shown by circles in the Fig. 16 (a). Thus the condenser temperature Tc3 corresponding to the maximum COP in the Fig. 16 (a) will be the optimum temperature in the condenser C3. The maximum values of COP and corresponding Tg (optimized values in the 1st step) obtained from the Fig. 15 are also re-plotted in the Fig. 16 (b). The values of COP in the Figs. 16 (a) and 16 (b) are same; Fig. 16 (b) gives the optimum generator temperatures in the second step of optimization corresponding to the maximum of the maximum COP, shown by the circles in the Fig 16 (b). Once the temperature in the condenser C3 are optimized, the temperature in the generator G2 also get optimized. Because the generator G2 operates by the heat of condensation from the condenser C3, hence their temperatures remain very close to each other, that is Tc3≈Tg2 along with the heat balance Qg2≈Qc3. The variation in the minimum flow rates of LPG and CNG, obtained for different values of Tc3 and Tg (optimized in the 1st step) are also re-plotted and shown in Figs. 17 (a) and 17 (b) respectively. They show minimum at certain values of Tc3 and Tg just corresponding to the maximum COP in the Figs. 16 (a) & 16 (b). Thus, the optimum values of Tc3 and Tg (optimized values in the 2nd step ) obtained corresponding to the maximum COP, also leading to the minimum flow rates of LPG and CNG, are listed in Table 8 for different values of Te and Tc=Ta. The corresponding values of Tg2, Xg2 and Xg are also given as the optimum parameters in the double effect cycle.

48

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4.4.3. Triple effect cycle: The triple effect cycle having three generators, will be optimized in three steps. The plots of COP with main generator temperature, shown in the Fig. 13 for Te=4 to 10oC, Tc=Ta=30oC, Tc3=80oC and Tc4=130oC are again plotted for different values of Tc4 in the Fig. 18. They all show increase in COP, reach to the maximum value and terminate due to attainment of heat balance or crystallization as explained earlier with the help of the Fig. 10. Similar plots can also be shown for other values of Tc3 and also for Te and Tc=Ta. Thus, in the 1st step of optimization, maximum COP for each values of Tc4 and the corresponding temperatures in the main generator Tg are selected. The maximum values of COP, obtained from the Fig. 18 are plotted against Tc4 and Tg (optimized values in the 1st step) in the Figs. 19 (a) and 19 (b), respectively for Te=4oC, Tc=Ta=30oC and Tc3 varying from 75 to 95oC. All the COP curves, for each value of Tc3 give maximum values as shown by the circles in the same figure. This is the second step of optimizing Tc4 and Tg. The maximum COP (indicated by circles) are again plotted against Tc3 and Tg (optimized values in the 2nd step) for different values of Te and shown in Figs. 20 (a) and 20 (b), respectively. The COP for each Te increase from low values, reach to a maxima and decrease. Again the maximum values of COP in each case are marked with circles. For the same conditions of Te and Tc=Ta, as in the Fig. 20 (b), the volume flow rates of LPG and CNG are plotted against the main generator temperature Tg (the optimized values in the 2nd step) and shown in Figs. 21 (a) and 22 (b), respectively. They show exactly the reverse trend as the COP shown in the Fig. 20 (b). The minimum flow rates of LPG and CNG shown by the circles in the Figs. 21 (a) and 21 (b) are corresponding to the maximum COP in the Fig. 20 (b). 49

Page 30 of 58

Thus, for maximum COP shown by circles in the Figs. 20 (a) and 20 (b), the corresponding values of Tc3 and Tg (the optimized values in the 3rd step) are selected for different values of Te and Tc=Ta and listed in the Table 9. Now, we get the optimum values of Tc4 from the Fig. 19 (a), Tc3 from the Fig. 20 (a) and Tg from the Fig. 20 (b) for maximum COP of the triple effect cycle. It should be remembered that the optimum generator (G) temperature Tg has been obtained after three steps of optimization. Now, knowing the optimum values of Tc4, the optimum generator (G3) temperatures Tg3 will be known because Tg3≈Tc4 due to heat exchange between the condenser C4 and generator G3. Also, the optimum generator (G2) temperature Tg2 will be known because the optimum Tc3 are known for the given values of Te and Tc=Ta. All such optimum values; the condenser temperatures and the generator temperatures with salt concentrations along with maximum values of the COP and minimum flow rate of both gases for different values of Te and Tc=Ta are given in the Table 9.

4.5 Irreversibility generation due to temperature difference of the Condenser-generator assembly: As discussed above, the double effect cycle consists of two generators while the triple effect cycle has three generators. Referring to the Figs. 2 and 3, the generator G3 in the triple effect cycle requiring heat Qg3 at Tg3 operates with the heat of condensation (Qc4) at Tc4 from the condenser C4. While, the generator G2 in the double and triple effect cycles with heat requirement as Qg2 at Tg2, operate with the heat of condensation (Qc3) from the condenser C3 at Tc3. In the exchange of heat between the generator and condenser at the intermediate pressure levels of the respective cycles, there may be irreversibility generation if there is large 50

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difference in their temperature in case there is no complete heat balance between them. To check this, the irreversibility generated in each set of the generator and condenser exchanging heat has been estimated using the equations (20) to (23). The irreversibility, thus obtained has been exhibited graphically in Figs. 22 (a) and 22 (b) for the double and triple effect cycles against their respective generator temperatures, for fixed temperatures in the respective condenser which is giving the heat. In Fig. 22 (a) for the double effect cycle, it is seen that with increase in the temperature Tg2 (with Tc3 = 80oC) the irreversibility increases from a low value, reaches to a maxima and then decreases sharply becoming very low as (Tc3 – Tg2) approaches zero. Referring to the values of Tg2 and Tc3 in the Table 8 that correspond to the optimum operating conditions, since (Tc3 – Tg2) is very small. The irreversibility in the double effect cycle will be quite low, which can also be seen from the Fig. 22 (a). Similarly for the triple effect cycle, in Fig. 22 (b) as the temperature Tg2 in the generator G2 increases, the irreversibility decreases sharply from high to very low values. Again, from the Table 9 the values of Tc3 and Tg2 that correspond to the optimum operating conditions in the triple effect cycles, show that Tc3-Tg2 is quite small. Therefore, the irreversibility in G2 and C3 will be very low that can also be seen from the Fig. 22 (b). Again, the irreversibility in G3 and C4 of the triple effect cycle, also shown in the Fig. 22 (b), initially increases with increase in Tg3 reaches to some maximum value and then decreases sharply becoming low as the difference (Tc4 – Tg3) approaches zero. The tabulated values of Tg3 and Tc4 in the Table 9 again confirm that | Tg3 – Tc4 | is very small, hence the irreversibility will also be low. 5. CONCLUSIONS In the present work the single, double and triple effect absorption cycles have been simulated and optimum operating temperatures obtained for maximum COP. LPG and CNG are 51

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selected as the source of energy and minimum flow rate required to operate the absorption cycles is estimated. The present simulation results have been compared with the simulation models and experimental data available in the literature. The following main conclusions are drawn: i.

Concentration of LiBr-salt in the generator and the crystallization line, both increase with an increase in the generator temperature, whereas, the concentration in the absorber remain constant due to fixed values of Te and Ta as considered in the analysis.

ii.

In all the three absorption cycles (single, double and triple effect) COP initially increases with generator temperature and then becomes almost constant or terminates due to limitation in the operation.

iii.

In the single effect cycle, because of one generator, optimization of the generator temperature is carried in one step because other temperatures Te, Tc and Ta are assumed as constant. However, the optimum generator temperature varies linearly with Tc=Ta for each value of Te.

iv.

In the double effect cycle, optimization of the main generator temperature is done in two steps. The optimization process gives optimum temperatures for the high pressure condenser as well as for the intermediate generator.

v.

The triple effect cycle gives optimum temperature of the main generator in the third step of optimization. This optimization process also gives optimum temperatures for the two intermediate condensers and generators simultaneously.

vi.

Increase in COPs of the double effect and triple effect cycles from those of the single effect cycle are in the range of 62 to 77% and 90 to 132% respectively.

vii.

Volume flow rate of the gases, being inversely proportional to the COP, result in minimum values. Also the volume flow rate of CNG appears to be more than that of LPG. This is because specific volume of CNG is higher than that of LPG. Both the gases are a better alternative to operate the absorption systems, because these can provide high temperatures and are easily available at nominal costs.

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viii.

In the condenser-generator set(s) of the double and triple effect cycles which exchange heat, the irreversibility induced reduces to very low values as the generator temperature approaches the condenser temperature in each set.

Thus, through the present simulation it has been established that the triple effect cycle yields higher COP and requires low volume flow rate of the gases that is, it is more efficient and economical in operation. The results of the present study are useful for the optimal design of the absorption systems. The work can be further extended for thermo-economic analysis.

Acknowledgement The authors wish to acknowledge the Department of Science and Technology, New Delhi for providing financial support under DST-PURSE-II programme for carrying the Computational Work and also thanks to University Grant Commission, New Delhi for providing scholarship.

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doi:10.1016/0140-7007(91)90019-D 20. Cai D, He G, Tian Q, Tang W. Thermodynamic analysis of a novel air-cooled nonadiabatic absorption refrigeration cycle driven by low grade energy. Energy Convers Manag 2014;86:537–47. doi:10.1016/j.enconman.2014.06.008. 21. Cai D, Jiang J, He G, Li K, Niu L, Xiao R. Experimental evaluation on thermal performance of an air-cooled absorption refrigeration cycle with NH3–LiNO3 and NH3– NaSCN

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solutions.

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2016;120:32–43.

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water-lithium

bromide absorption

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2009;32:1247–58. doi:10.1016/j.ijrefrig.2009.01.017. 25. Arora A, Kaushik SC. Theoretical analysis of LiBr/H2O absorption refrigeration systems. Int. J Energy Res 2009;33:1321–40. doi:10.1002/er.1542. 26. Marcos JD, Izquierdo M, Palacios E. New method for COP optimization in water- and air-cooled single and double effect LiBr-water absorption machines. Int. J. Refrig., 2011. doi:10.1016/j.ijrefrig.2011.03.017. 27. Gomri R, Hakimi R. Second law analysis of double effect vapour absorption cooler system. Energy Convers Manag 2008;49:3343–8. doi:10.1016/j.enconman.2007.09.033. 28. Ratlamwala T. A. H., Dincer I, and Gadalla M. A. Performance analysis and evaluation of a triple-effect ammonia-water absorption-refrigeration system. Int. J Energy Res. 2013; 37:475–483. doi: 10.1002/er.2998. 29. Khaliq A, Kumar R, Dincer I and Khalid F. Energy and Exergy Analyses of a New Triple-Staged Refrigeration Cycle Using Solar Heat Sources. J Sol Energy Eng. 2014; 136;011004-1-11.doi: 10.1115/1.4024126. 30. Gomri R. Thermodynamic evaluation of triple effect absorption chiller. 2008 Second Int Conf Therm Issues Emerg Technol 2008. doi:10.1109/THETA.2008.5188778. 56

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39. Z.Y. Xu, R.Z. Wang, H.B. Wang “Experimental evaluation of a variable effect LiBr– water absorption chiller designed for high-efficient solar cooling system”, International Journal of Refrigeration 59 ( 2 0 1 5 ) 135–143. doi: 10.1016/j.ijrefrig.2015.07.019 40. C. Monné, S. Alonso, F. Palacin and L. Serra, "Monitoring and simulation of an existing solar powered absorption cooling system in Zaragoza (Spain)," Applied Thermal Engineering, vol. 31, pp. 28-35, 2011. doi:10.1016/j.applthermaleng.2010.08.002 41. M. Izquierdo, A. Gonzalez-Gil and E. Palacios, "Solar powered single-and double-effect directly air-cooled LiBr–H2O absorption prototype built as a single unit,"Applied Energy, vol. 130, pp. 7-19, 2014.doi:10.1016/j.apenergy.2014.05.028 42. Abdullah Kececiler, H. Ibrahim Acar, Ayla Dogan, “Thermodynamic analysis of the absorption refrigeration system with geothermal energy: an experimental study”, Energy Conversion & Management 41 (2000) 37-48. 43. Syed A, Izquierdo M, Rodriguez P, Maidment G, Missenden J, Lecuona A, et al. A novel experimental investigation of a solar cooling system in Madrid. International Journal of refrigeration 2005; 28: 859-71. doi:10.1016/j.ijrefrig.2005.01.007 44. Agyenim F, Knight I, Rhodes M. Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store. Solar Energy 2010; 84: 735-44. 45. Lizarte R, Izquierdo M, Marcos J, Palacios E. An innovative solar-driven directly aircooled LiBr–H2O absorption chiller prototype for residential use. Energy and Buildings 2012; 47: 1-11.

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46. Aphornratana S, Sriveerakul T. Experimental studies of a single-effect absorption refrigerator using aqueous lithium-bromide: effect of operating condition to system performance. Exp Therm Fluid Sci ;32:658–69, 2007. 47. Florides GA, Kalogirou SA, Tassou SA, Wrobel LC. Design and construction of a LiBr– water

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http://dx.doi.org/10.1016/S0196-8904(03)00006-2. 48. Alka Solanki, S .K Singh and Yash Pal, “ Performance Evaluation of Triple Effect Vapour Absorption Cooling System,” International Journal of Emerging Technology and Advanced Engineering, vol. 5, special Issue 4, pp-53-59, March-2015. 49. Matsushima H., Fujii T., Komatsu T., Nishiguchi A., “Dynamic simulation program with object-oriented formulation for absorption chillers (modeling, verification and application to triple-effect absorption chiller)”, International Journal of Refrigeration 33, 2010, 259-268.

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Qg Main

Qc

Main Generator (G)

4

Condenser (C)

Tg

Tc 5

3

8

TV1

PH

9 6

TV2 2

Evaporator (E)

10

PH=PREHEATER,TV=THROTTLE VALVE Q=HEAT TRANSFER RATE WP =PUMP WORK

Te

Absorber (A)

Ta

7

Qe

1

Wp

Qa

Fig. 1. Single Effect Absorption Refrigeration System

Qg 4a

4

Condenser (C3)

Main Generator (G)

Tc3

Tg

TV1 Qg2

4b Main Condenser (C)

Qc

Generator (G2)

4c

Tc

3

8

Tg2

8b

TV3

8a

PH2

5

2a

TV2

PH1

8c 9

2

TV4

6

10 Evaporator (E)

Absorber (A)

Te

Ta

1

7

Wp

Qa

Qe

Fig. 2. Double Effect Absorption Refrigeration System

60

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Qg 4a

Qc

Condenser (C3)

4e

Generator (G3)

Tc

Tg3

4c Qg2

Main Condenser (C)

Tg 8

Qg3

Tc3

TV2

Main Generator (G)

Tc4

TV1 4b 4d

4

Condenser (C4)

3

TV4 8b

PH3

8a

2b

8c

Generator (G2)

4f

8e

Tg2

TV5

PH2

8d

2a 5

8f PH1

TV3 9

TV6

6

2

10 Evaporator (E)

Te

Absorber (A)

Ta

7

1

Wp

Qe

Qa

Fig. 3. Triple Effect Absorption Refrigeration System

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Fig. 4 Flow chart for the Single effect cycle Fig. 5 Flow chart for the Double effect cycle

Fig. 6 Flow chart for the Triple effect cycle

45

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70

65

60

60 50

Xa,eqm

55

40

50 45

SCRexp

Ap

30

40 20 35

SCRth

30

Soltion circulation ratio (SCR)

LiBr salt concentration (X), Absorption %age (Ap)

70

10

25

0 60

65

70 75 80 Generator Temperature, Tg in oC

85

90

Fig. 7 variation of salt concentration, absorption percentage and SCR with generator temperature 180 Theory Experiment

Enthalpy, kJ/kg

160

h3

140 h3 120 h1 100 80

h1

60 60

65

70 75 80 85 Generator Temperature, Tg in oC

90

Fig. 8 variation of enthalpy at state points 1 and 3 (Fig.1) with generator temperature 45

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0.8

4000 COP,Theory COP,Exp SCR,Theory SCR,Exp

3500

0.7

3000

0.65

2500

0.6

2000

0.55

1500

0.5

1000

0.45

500

0.4

SCR×(h8-h3), kJ/kg

COP

0.75

0 60

65

70

75

80

85

90

Generator Temperature, Tg in oC

Fig. 9 variation of COP and SCR×(h8-h3) with generator temperature

45

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66

Te=4 deg C

Te=6 deg C

Te=8 deg C

Xc

Te=10 deg C

Xc

Xc

Xg2

Xg3

Concentration of LiBr salt, X (%age)

63

60 Single Effect Ta=Tc=30OC

Double Effect Ta=Tc=30OC Tc3= 80OC

Xg

Triple Effect Ta=Tc=30OC Tc3=80OC Tc4=130OC

57

Xg2

Xg Xg 54

Xa

Xa

Xa

51

48 45

60

75

90

105

120

135

Main Generator Temperature, Tg in

150

165

180

195

OC

Fig. 10 Variation in concentration of the LiBr salt in the LiBr-water solution with main generator temperature at different temperatures in the evaporator and the condense 45

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Concentration of LiBr salt, X (%age)

65 Te=4 deg C

Te=10 deg C

Tc=30OC (P2)

60

Tc3=80oC (P3)

55

50

45 50

60

70

80

90

100

110

120

130

Tg Tg2 Temperatures in the generators at pressure levels P2 and P3

Fig. 11 Variation in concentration of the LiBr salt in the LiBr-water solution for the double effect cycle against the respective generator temperatures at different pressure levels

Concentration of LiBr salt, X (%age)

65 Te=4 deg C

Tc=30oC (P2)

Te=10 deg C

Tc3=80oC (P3)

60

Tc4=130OC (P4)

55

50

45 50

70

Tg2

90

110

130

Tg3

150

170

190

Tg

Temperatures in the generators at different pressure levels (P2, P3 and P4)

Fig. 12 Variation in concentration of the LiBr salt in the LiBr-water solution for the triple effect cycle against the respective generator temperatures at different pressure levels

45

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2

Coefficient of Performance

Te=4 deg C

Te=6 deg C

Te=8 deg C

Te=10 deg C

Double Effect Tc=Ta=30oC Tc3=80oC

1.7

1.4

1.1

Single Effect Tc=Ta=30oC Triple Effect Tc=Ta=30oC Tc3=80oC Tc4=130oC

0.8

0.5 50

65

80

95

110

125

140

155

170

185

Main Generator Temperature, Tg in oC

Fig. 13. Variation in COP of single, double and triple effect cycles with main generator temperature at different temperature in the evaporator and the condensers

46

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Tc=Ta=300C Tc3=800C Tc4=1300C

Volume Flow Rate of gases (in m3 s-1)

0.015

Te=4 deg C Te=8 deg C

Te=6 deg C Te=10 deg C

Double Effect

0.013

Triple Effect 0.011

Single Effect

0.009 0.007

CNG

0.005

LPG 0.003 0.001

50

70

90

110

130

Main Generator Temperature, Tg in

150

170

190

oC

Fig. 14. Variation in volume flow rates of LPG and CNG with main generator temperature at different temperatures in the evaporator and the condensers

47

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Coefficient of performance

1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6

Tc3=65 deg C TC3=80 deg C Tc3=95 deg C

Te=4oC & Tc=Ta=30oC

0.5 95

105

115

125

Tc3=70 deg C Tc3=85 deg C Tc3=100 deg C

135

Main Generator Temperature, Tg in oC

Tc3=75 deg C Tc3=90 deg C Tc3=105 deg C

145

155

Fig. 15. Variation in COP of double effect cycle with main generator temperature at different temperature in condenser C3

Fig. 16. Variation in maximum of the maximum COP of double effect cycle with temperature Tc3 and Tg at different temperature in the evaporator

48

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Fig. 17. Variation in volume flow rate of double effect cycle with main generator temperature at different temperature in the evaporator and fixed value of main condenser

Coefficient of Performance

1.8 1.7

Te=4oC, Tc=30oC &Tc3=80oC

1.6 1.5 1.4 1.3 1.2 1.1 Tc4=120 deg C Tc4=130 deg C Tc4=140 deg C Tc4=150 deg C Tc4=160 deg C

1 0.9 0.8 155

165

175

185

195

Tc4=125 deg C Tc4=135 deg C Tc4=145 deg C Tc4=155 deg C

205

215

Main Generator Temperature, Tg in oC

Fig. 18. Variation in COP of triple effect cycle with main generator temperature at different temperature in condenser C4

49

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Fig. 19. Variation in maximum COP of triple effect cycle with temperature Tc4 and Tg at different temperature in the evaporator

Fig. 20. Variation in maximum of the maximum COP of triple effect cycle with temperature Tc3 and Tg at different temperature in the evaporator

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Fig. 21. Variation in volume flow rate of triple effect cycle with main generator temperature at different temperature in the evaporator and fixed value of main condenser

Fig. 22. Irreversibility generation in the generator and condenser sets against their respective generator temperatures for the selected values of Te, Tc, Tc3 and Tc4

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Table 1. Pressure levels of single, double and triple effect cycles Pressure levels I. II. III. IV

Single effect

Double effect

Triple effect

Te = f (Ta, Xa) P1 = Pe = Pa Tc = f (Tg, Xg) P2 = Pc = Pg ------------

Te = f (Ta, Xa) P1 = Pe = Pa Tc = f (Tg2, Xg2) P2 = Pc = Pg2 Tc3 = f (Tg, Xg) P3 = Pc3 = Pg

------------

------------

Te = f (Ta, Xa) P1 = Pe = Pa Tc = f (Tg2, Xg2) P2 = Pc = Pg2 Tc3 = f (Tg3, Xg3) P3 = Pc3 = Pg3 Tc4 = f (Tg, Xg) P4 = Pc4 = Pg

Table 2. Fixed data used in the simulation of the single, double and triple effect cycles Evaporator cooling capacity Evaporator temperature, Te Absorber temperature, Ta Main Condenser (C) temperature, Tc Condenser (C3) temperature, Tc3 Condenser (C4) temperature, Tc4 Heat exchanger efficiency Pump efficiency

300 kW 4oC, 6oC, 8oC, 10oC 30oC, 33oC, 36oC, 39oC 30oC , 33oC, 36oC, 39oC 60oC to 110oC in steps of 1oC 100oC to160oC in steps of 1oC 70% 85%

Table 3. Comparison of the present results with Arora and Kaushik [2009] and Gomri [2008] Heat Transfer Rate [in kW] and COP

Absorber Generator Condenser Evaporator Pump work COP

Single Effect Operating conditions Tg=87.8oC, Te=7.2oC, ɛ=0.7 Tc=Ta=37.8 oC, mass flow rate of refrigerant = 1 kg s-1 Ref [25] Present work

2945.269 3095.698 2505.910 2355.450 0.03143 0.7609

2941.292 3091.727 2506.465 2355.995 0.03409 0.762

Double Effect Operating conditions Tg=140.6oC, Te=7.2oC, ɛ=0.7 Tc=Ta=37.8 oC, mass flow rate of refrigerant = 1 kg s-1 Ref [25] Present work

2942.175 1868.711 1282.052 2355.450 0.3598 1.26

2942.319 1869.100 1283.171 2355.995 0.3960 1.260

Triple Effect Operating conditions Qe=300 kW, Tg=190oC Te=4oC, Tc=Ta=33oC ɛ=0.85, ɳp=0.95 Ref [30]

Present work

357.67 169.68 112.23 300.0 0.220 1.766

357.265 168.152 111.228 300.0 0.3424 1.784

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Table 4. Comparison of the present results with Monné et al [2011] and Izquierdo et al [2014] for single effect cycle

Parameter

Operating conditions: Te,out=11.5oC (Te=9.0oC),Tg,in= 91.0oC and Qe=5.7 kW Ref [40]

Present work

%age deviation

0.57

0.809

30%

Thermal COP

Operating conditions: Te,out=16.0oC (Te=13.0oC),Tamb=36.0oC and Qe=3.4 kW, Ref [41] Tg=80-95oC Average = 0.54 %age deviation

Present work Tg=80 C Tg=95oC 0.648 0.765 26% 30% o

Table 5. Comparison of the present work with Aphornratana and Sriveerakul [2007] for single effect cycle Operating conditions: Te=5oC, Tc=29.8oC, Absorber inlet temperature=50oC o Tg=65 C Tg=75oC Tg=85oC Parameters

Mref Msol Xa Xg SCR COP %age deviation in COP Absorber percentage

Experiment Ref [46] 0.016 1.05 55.7 56.6 65.0 0.467

Theory Ref [46] 0.016 1.05 54.7 56.6 30.0 0.610 24% 47.36%

Present work

Experiment Ref [46]

0.016 1.05 54.7 56.6 29.78 0.621

0.027 0.95 58.4 60.1 35.8 0.598

Theory Ref [46] 0.027 0.95 54.7 60.1 11.2 0.745 19.6% 31.48%

Present work

Experiment Ref [46]

0.027 0.95 54.7 60.1 11.129 0.747

0.034 0.82 61.7 64.3 24.9 0.644

Theory Ref [46] 0.034 0.82 54.7 64.3 6.7 0.773 16.4% 27.08%

Present work 0.034 0.82 54.7 64.3 6.697 0.771

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Table 6. Comparison of the present results with Izquierdo et al [2014] and Alka Sonalki et al [2015] Double Effect cycle Triple Effect cycle Operating conditions: Rated values of the experimental setup: Qe=100 kW, Te,out=12.0oC(Te=9.0oC),Tamb=35.5oC, High generator: Tg=180oC, source temperature in the evaporator= 7/12oC Tc= Tamb+11oC, Ta=41.5oC, Qe=4.5kW (fixed Te= 5oC) Low generator: Tg,low=85oC and Xg=65% (calculate Parameter o Tg=175 C, εhigh=0.9, εlow=0.6 Tc=30oC) Present %age Rated COP of the Ref [41] Ref [48] Present work %age deviation work deviation Apparatus Ref [48] Thermal COP 1.0 1.246 20% 1.56 1.7 1.66 7%

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Table 7. Optimum generator temperature corresponding to maximum COP for single effect cycle At Pressure P1 Te (oC)

Xa (%)

4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10

53.06 51.74 50.38 48.96 54.74 53.48 52.18 50.85 56.35 55.13 53.89 52.61 57.92 56.72 55.52 54.29

At Pressure P2

Ta (oC)

Tc (oC)

30

30

33

33

36

36

39

39

Tg (oC)

Xg (%)

72.51 69.70 66.86 63.22 79.75 76.93 74.11 70.86 87.04 84.18 81.34 78.52 94.43 91.51 88.16 85.31

59.56 58.24 56.88 55.06 61.24 59.98 58.68 57.15 62.85 61.63 60.39 59.11 64.42 63.22 61.82 60.59

Maximum COP 0.795 0.811 0.828 0.845 0.772 0.787 0.803 0.820 0.751 0.765 0.780 0.796 0.731 0.745 0.758 0.773

Minimum VL (m3s-1) 0.00352 0.00344 0.00337 0.00330 0.00363 0.00356 0.00348 0.00341 0.00374 0.00367 0.00360 0.00352 0.00385 0.00378 0.00371 0.00364

Minimum VC (m3s-1) 0.01094 0.01071 0.01048 0.01025 0.01130 0.01106 0.01083 0.01060 0.01165 0.01142 0.01119 0.01095 0.01200 0.01177 0.01155 0.01132

Table 8. Optimum temperatures corresponding to maximum COP for double effect cycle At Pressure P1 Te(oC)

Xa (%)

4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10

53.06 51.74 50.38 48.96 54.74 53.48 52.18 50.85 56.35 55.13 53.89 52.61 57.92 56.72 55.52 54.29

Ta(oC)

At Pressure P2 Tc(oC)

30

30

33

33

36

36

39

39

At Pressure P3

Tg2 (oC)

Xg2 (%)

Tc3 (oC)

Tg (oC)

Xg (%)

78.61 73.99 69.41 68.32 84.04 83.1 78.41 75.59 90.40 89.46 88.54 83.79 94.90 94.92 94.98 91.10

62.30 60.24 58.11 57.59 63.10 62.70 60.64 59.37 64.25 63.87 63.48 61.46 64.61 64.61 64.64 63.06

79 75 70 69 85 84 79 76 91 90 89 85 96 96 96 92

124.96 116.45 106.96 103.72 134.99 131.72 122.01 115.62 145.63 142.31 139.00 130.34 154.13 152.49 150.84 142.48

57.66 55.94 54.18 53.16 58.94 58.08 56.38 55.05 60.35 59.53 58.69 57.01 61.32 60.72 60.12 58.69

Maximum COP 1.359 1.399 1.444 1.491 1.299 1.339 1.380 1.424 1.244 1.283 1.322 1.359 1.181 1.226 1.266 1.302

Minimum VL (m3s-1) 0.00214 0.00208 0.00202 0.00195 0.00224 0.00217 0.00211 0.00204 0.00234 0.00227 0.00220 0.00214 0.00246 0.00237 0.0023 0.00224

Minimum VC (m3s-1) 0.00669 0.00650 0.00630 0.00610 0.00700 0.00679 0.00659 0.00639 0.00731 0.00709 0.00688 0.00669 0.00770 0.00742 0.00718 0.00699

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Table 9. Optimum temperatures corresponding to maximum COP for triple effect cycle At Pressure P1 Te Xa Ta (oC) (%) (oC) 4 53.06 6 51.74 30 8 50.38 10 48.96 4 54.74 6 53.48 33 8 52.18 10 50.85 4 56.35 6 55.13 36 8 53.89 10 52.61 4 57.92 6 56.72 39 8 55.52 10 54.29

At Pressure P2 Tc Tg2 Xg3 (oC) (oC) (%) 78.27 62.15 74.37 60.41 30 71.63 59.15 68.89 57.86 85.41 63.68 82.69 62.52 33 77.49 60.23 74.82 59.01 91.42 64.67 89.86 64.03 36 88.51 63.47 83.12 61.17 92.36 63.57 94.57 64.47 39 93.06 63.86 91.55 63.24

At Pressure P3 Tg3 Xg2 Tc3 (oC) (%) (oC) 128.47 59.12 79 119.94 57.49 75 113.52 56.17 72 108.25 54.80 70 140.75 60.72 86 135.26 59.50 84 123.49 57.52 78 117.11 56.24 75 151.12 61.92 92 147.61 61.09 91 143.10 60.28 89 132.27 58.30 84 153.87 61.65 95 154.64 61.91 95 151.17 61.10 94 146.60 60.28 92

At Pressure P4 Tc4 Tg Xg (oC) (oC) (%) 129 179.01 56.26 121 166.20 54.74 114 155.14 53.38 109 146.40 51.96 141 197.19 57.94 136 188.09 56.68 124 170.19 54.98 118 160.27 53.65 152 213.96 59.35 148 206.37 58.33 145 200.0 57.29 133 181.97 55.61 155 219.16 59.92 155 217.72 59.52 152 211.52 58.52 147 202.88 57.49

Maximum COP

Minimum VL (m3s-1)

Minimum VC (m3s-1)

1.728 1.797 1.878 1.955 1.632 1.692 1.762 1.839 1.539 1.60 1.663 1.728 1.392 1.508 1.568 1.635

0.00168 0.00162 0.00155 0.00149 0.00178 0.00172 0.00165 0.00158 0.00189 0.00182 0.00175 0.00168 0.00209 0.00193 0.00186 0.00178

0.00526 0.00506 0.00484 0.00465 0.00557 0.00537 0.00516 0.00495 0.00591 0.00568 0.00547 0.00526 0.00653 0.00603 0.00580 0.00556

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