CHAPTER 4 EXPERIMENTAL STUDIES - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9877/9/09_chapter 4.pdf · CHAPTER 4 EXPERIMENTAL STUDIES 4.1 INTRODUCTION Based on the results

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CHAPTER 4

EXPERIMENTAL STUDIES

4.1 INTRODUCTION

Based on the results of the theoretical studies on the air-cooled GAX

based vapour absorption refrigeration system that were carried out, the design

and fabrication of the system have been done. The results of the pinch point

analysis helps to identify the streams responsible for heat recovery and

thereby heat load capacity for exchange between the streams. This helps to

design the internal heat recovery components such as high pressure GAX, low

pressure GAX, solution cooler, solution heat exchanger 2, condensate pre-

cooler. The heat load obtained for these components are used in the

experimental test-rig design. An experimental investigation of the

performance of the air-cooled GAX based vapour absorption refrigeration

system designed for 10.5 kW cooling capacity is presented. The experimental

plan and procedure, measurement of parameters and data reduction are

discussed in detail in this chapter.

4.2 COMPONENTS OF THE SYSTEM

The components of the air-cooled GAX based vapour absorption

refrigeration system, are the absorber, condenser, evaporator, generator, high

pressure GAX, low pressure GAX, solution heat exchanger 1, solution heat

exchanger 2, solution cooler and condensate pre-cooler. The components of

the air-cooled GAX based vapour absorption refrigeration system are

designed for the heat loads obtained from the thermodynamic analysis,

114

corresponding to a cooling capacity of 10.5 kW. At the designed conditions of

the generator, sink and evaporator temperatures of 150°C, 45°C and -10°C,

respectively and at a split factor of 0.8, based on the pinch point analysis, the

amount of heat recovered in the various internal heat exchanging components

are given in Table 4.1.

Table 4.1 Heat load of the internal heat exchanging components

ComponentHeat Load

(kW)

High Pressure GAX 3.6

Low Pressure GAX 4.6

Solution Heat Exchanger 1 12.2

Solution Heat Exchanger 2 3.6

Solution Cooler 5.3

Condensate Pre-Cooler 1.3

The components are designed using standard procedure with relevant heat and

mass transfer equations / coefficients. The specifications of the designed

components are given in Table 4.1. The component drawings are shown from

Figures 4.1 to 4.11.

4.3 EXPERIMENTAL SET UP

All the components are fabricated as per design specifications, and

then assembled after testing for leaks. The schematic of the experimental

setup of the air-cooled GAX based vapour absorption refrigeration system is

shown in Figure 4.12.

115

Table 4.2 Specifications of the major components of the system

Component Design Conditions Specifications

Absorber

(Figure 4.1)

Weak solution temperature: 45°C

Strong solution temperature: 67 °C

Inlet air temperature: 28°C

Outlet air temperature: 38°C

Mass flow rate of weak solution: 0.058 kg/s

Mass flow rate of strong solution: 0.050 kg/s

Mass flow rate of refrigerant: 0.008 kg/s

Type: Finned

Material: Mild steel

Tube Diameter (ID): 15 mm

Thickness: 4 mm

Tube Length: 750 mm

Number of Tubes: 20

Heat Transfer Duty: 21.85

kW

Heat Transfer Area: 0.89 m2


Condensor

(Figure 4.1)

Refrigerant vapour temperature: 48°C

Refrigerant liquid temperature: 46°C

Inlet air temperature: 28°C

Outlet air temperature: 38°C


Type: Finned


Tube Diameter (ID): 15 mm

Thickness: 4 mm

Tube Length: 750mm

Number of Tubes: 16

Heat Transfer Duty: 11.96

kW

Heat Transfer Area: :0.72 m2

116

Table 4.2 (Contd….)


Evaporator

(Figure 4.2)

Refrigerant liquid temperature: -12°C

Refrigerant vapour temperature: -10°C


Type: Shell and Coil


Shell Diameter (OD): 600 mm

Shell Length: 1150 mm

Coil Diameter (OD): 25.4 mm

Thickness: 7.4 mm

Length: 1360 mm

Number of Coils: 28

Heat Transfer Duty: 10.5 kW



Generator

(Figure 4.3)

Weak solution temperature: 119°C

Strong solution temperature: 150°C



Type: Direct Fired


Diameter (ID): 488.6 mm

Diameter (OD): 514 mm

Length: 1552 mm


Heat Transfer Area: 1 m2

117



High Pressure

GAX

(Figure 4.4)

Weak solution inlet temperature: 45°C

Weak solution outlet temperature: 59°C

Refrigerant inlet temperature: 112°C

Refrigerant outlet temperature: 48°C



Type: Shell and Tube


Shell Diameter (ID): 103.5 mm

Thickness: 10.8 mm


Tube Diameter (ID): 3.6 mm

Thickness: 2.4 mm

Tube Length: 1200 mm

Number of Tubes: 132




Low Pressure

GAX

(Figure 4.5)



Strong solution inlet temperature: 76°C

Strong solution outlet temperature: 67 °C

Refrigerant temperature: 38°C







Thickness: 20.6 mm



Thickness: 2.4 mm

Tube Length: 375 mm

Number of Tubes: 76



118



Condensate

Pre-Cooler

(Figure 4.6)

Refrigerant liquid inlet temperature: 46°C

Refrigerant liquid outlet temperature: 20°C

Refrigerant vapour inlet temperature: -10°C

Refrigerant vapour outlet temperature: 38°C





Thickness: 14.2 mm



Thickness: 3.3 mm


Number of Tubes: 76




Solution

Heat

Exchanger 1

(Figure 4.7)







Type: Tube-in-Tube


Outer Tube Diameter (OD): 40.9 mm

Thickness: 7.4 mm

Length: 1456 mm

Inner Tube Diameter (ID): 3.6 mm

Thickness: 2.4 mm

Length: 1456 mm

Number of Tubes: 24



119



Solution Heat

Exchanger 2

(Figure 4.8)







Type: Tube-in-Tube


Inner Tube diameter (ID): 62.7 mm

Thickness: 10.3 mm


Outer Tube diameter (ID): 203 mm

Thickness: 16 mm





Solution

Cooler

(Figure 4.9)








Material: Mild steel (Shell)

Mild Steel (Tube)


Thickness: 14.2 mm



Thickness: 15.2 mm

Tube Length: 750 mm

Number of Tubes: 4



120

Table 4.2 (Concluded)

Component Specifications

Weak Solution

Reservoir

(Figure 4.10)

Type: Vertical Cylinder


Diameter (ID): 307.1 mm; Thickness: 16.8 mm

Length: 385 mm

Component Specifications

Refrigerant Reservoir

(Figure 4.11)

Type: Vertical Cylinder


Diameter (ID): 128.2 mm; Thickness: 13.1 mm

Length: 385 mm

121

LOW PRESSURE

REFRIGERANT

VAPOUR IN

HIGH PRESSURE

REFRIGERANT

VAPOUR IN

HIGH PRESSURE

REFRIGERANT

LIQUID OUT

WEAK

SOLUTION OUT

STRONG

SOLUTION IN

AIR IN

AIR OUT

LOW PRESSURE

REFRIGERANT

VAPOUR IN

HIGH PRESSURE

REFRIGERANT

VAPOUR IN

HIGH PRESSURE

REFRIGERANT

LIQUID OUT

WEAK

SOLUTION OUT

STRONG

SOLUTION IN

AIR IN

AIR OUT

Figure 4.1 Schematic of the absorber and the condensor

122

CHILLED

WATER OUT

REFRIGERANT IN

CHILLED WATER IN

REFRIGERANT OUT

CHILLED

WATER OUT

REFRIGERANT IN

CHILLED WATER IN

REFRIGERANT OUT

Figure 4.2 Schematic of the evaporator

123

STRONG

SOLUTION OUT

WEAK

SOLUTION IN

REFRIGERANT

VAPOUR OUT

STRONG

SOLUTION OUT

WEAK

SOLUTION IN

REFRIGERANT

VAPOUR OUT

Figure 4.3 Schematic of the generator

124

WEAK

SOLUTION IN

WEAK

SOLUTION OUT

REFRIGERANT VAPOUR IN

REFRIGERANT

VAPOUR OUT

WEAK

SOLUTION IN

WEAK

SOLUTION OUT


REFRIGERANT

VAPOUR OUT

Figure 4.4 Schematic of the high pressure GAX

125

REFRIGERANT

VAPOUR IN

STRONG SOLUTION IN

STRONG

SOLUTION OUT

WEAK SOLUTION OUT

WEAK

SOLUTION IN

REFRIGERANT

VAPOUR IN

STRONG SOLUTION IN

STRONG

SOLUTION OUT

WEAK SOLUTION OUT

WEAK

SOLUTION IN

Figure 4.5 Schematic of the low pressure GAX

126

REFRIGERANT

LIQUID OUT

REFRIGERANT VAPOUR OUT

REFRIGERANT

LIQUID IN


REFRIGERANT

LIQUID OUT

REFRIGERANT VAPOUR OUT

REFRIGERANT

LIQUID IN


Figure 4.6 Schematic of the condensate pre-cooler

127

STRONG SOLUTION IN

WEAK

SOLUTION IN

WEAK

SOLUTION OUT

STRONG SOLUTION OUT

STRONG SOLUTION IN

WEAK

SOLUTION IN

WEAK

SOLUTION OUT

STRONG SOLUTION OUT

Figure 4.7 Schematic of the solution heat exchanger 1

128

WEAK

SOLUTION IN

WEAK

SOLUTION OUT

STRONG SOLUTION OUT

STRONG SOLUTION IN

FLUE GAS IN

FLUE GAS OUT

WEAK

SOLUTION IN

WEAK

SOLUTION OUT

STRONG SOLUTION OUT

STRONG SOLUTION IN

FLUE GAS IN

FLUE GAS OUT

Figure 4.8 Schematic of the solution heat exchanger 2

129

WEAK SOLUTION IN

WEAK SOLUTION OUT

STRONG SOLUTION OUT

STRONG SOLUTION IN

WEAK SOLUTION IN

WEAK SOLUTION OUT

STRONG SOLUTION OUT

STRONG SOLUTION IN

Figure 4.9 Schematic of the solution cooler

130

WEAK SOLUTION OUTWEAK SOLUTION OUT

Figure 4.10 Schematic of the weak solution reservoir

REFRIGERANT OUTREFRIGERANT OUT

Figure 4.11 Schematic of the refrigerant reservoir

131

SP

SC

SX1

G

SX2

LPGAX

HPGAXE

RRSR

CPC

FGO

FGI

A

C

B

PRV

EV

F

RFMSFM

PT

PT

2

1

3

4

5

6 7

8

9

10

11

12 13

14 15

16

1718

19

20

21

2223

24

WEAK SOLUTION

STRONG SOLUTION

REFRIGERANT

SP

SC

SX1

G

SX2

LPGAX

HPGAXE

RRSR

CPC

FGO

FGI

A

C

B

PRV

EV

F

RFMSFM

PT

PT

2

1

3

4

5

6 7

8

9

10

11

12 13

14 15

16

1718

19

20

21

2223

24

WEAK SOLUTION

STRONG SOLUTION

REFRIGERANT

LEGEND

A – Absorber FGO – Flue Gas Outlet SC- Solution Cooler E – Evaporator

B – Burner G - Generator SFM – Solution Flow Meter EV – Expansion Valve

C – Condensor HPGAX – High Pressure GAX SP – Solution Pump F – Fan

CPC – Condensate Pre-Cooler LPGAX – Low Pressure GAX SR – Solution Receiver FGI – Flue Gas Inlet

PRV – Pressure Reducing Valve SX1 – Solution Heat Exchanger 1 RFM – Refrigerant Flow Meter

PT – Pressure Transducer SX2 – Solution Heat Exchanger 2 RR – Refrigerant Reservoir

Figure 4.12 Schematic of the experimental set up

132

Figure 4.13 Pictorial view of the components of the experimental set up before insulation

133

Figure 4.14 Pictorial view of the experimental set up before insulation

134

Figure 4.15 Pictorial view of the components of the experimental set up after insulation

135

1 Absorber

2 Burner

3 Condenser

4 Condensate Pre-Cooler

5 Evaporator

6 Generator

7 High Pressure GAX

8 Low pressure GAX

9 Pressure Transducer

10 Refrigerant Flow Meter

11 Solution Cooler

12 Solution Heat Exchanger 1

13 Solution Heat Exchanger 2

14 Solution Pump

15 Weak Solution Reservoir

13

54

10

13

2

4

7

6

5

89

10

11

12

13

14

15

16

13

54

10

13

2

4

7

6

5

89

10

11

12

13

14

15

16

Figure 4.16 Pictorial front view of the experimental set up after insulation

136

Figure 4.17 Pictorial rear view of the experimental set up after insulation

137

Power

Analyzer Data LoggerPower

Analyzer Data Logger

Figure 4.18 Pictorial view of the experimental set up with measuring instruments

138

The pictorial views of the components, and the experimental set up

before and after insulation are shown in Figures 4.13 to 4.18. The system has

two pressure levels. The absorber is at evaporator pressure and the generator

at condenser pressure. Weak solution (ws) is pumped by a 1.5 kW solution

pump (SP) of diaphragm type, from the absorber (A) to the high pressure

GAX (HPGAX). The HPGAX is an indirect counter current gas liquid heat

exchanger, which cools the refrigerant vapor (state point 16) coming from the

generator through the solution cooler (SC), by the incoming weak solution

entering the HPGAX at state point 2. The heat of rectification is thus

transferred to preheat the incoming weak solution. The weak solution enters

the low pressure GAX (LPGAX). The LPGAX absorbs a partial amount of

the refrigerant vapor (state point 24) from the condensate pre-cooler (CPC).

The simultaneous absorption of the refrigerant vapour takes place in both the

LPGAX and the absorber, due to the splitting of the refrigerant vapour at a

split factor of 0.8 to the absorber and the LPGAX. The pipe diameter from the

outlet of the condensate pre-cooler to the low pressure GAX, is reduced in

such a way that the refrigerant flow rate is at a split factor of 0.8 for a

diameter ratio of 1:2. Thus the split factor is maintained constant during

experiments and it is not measured. The heat of absorption, and the cooling of

strong solution from state point 12 to 13 are utilized to heat the weak solution

from state point 3 to 4, thus reducing the heat input. Then, the weak solution

enters the solution heat exchanger 1 (SX1). SX1 is the heat exchanger similar

to the one used in the conventional single effect system, where it exchanges

heat with the strong solution returning from the generator through the solution

heat exchanger 2 (SX2). It does not differ from SHX. The weak solution then

enters the solution heat exchanger 2 (SX2). In the SX2, the weak solution

(state point 5) is heated by both the strong solution (state point 9) and the

waste flue gases from the burner leaving the SX2. After extracting the energy

from the flue gas and the strong solution in the SX2, the weak solution finally

enters the generator through the Solution Cooler (SC), where it is further

139

heated by the strong solution leaving the generator. In the generator, the

external heat input from a diesel fired burner is supplied to generate the

refrigerant vapour. The purified vapor leaves the HPGAX and enters the

condenser at state point 17. Both the absorber and the condenser are air-

cooled. The air circulation is accomplished by a 0.75 kW fan.

The liquid refrigerant from the condenser enters the condensate pre-

cooler (CPC). The CPC is the economizer that is used in the system, which

heats the refrigerant vapour from the evaporator by sub-cooling the liquid

refrigerant from the condenser, before being throttled. The refrigerant vapour

from the CPC enters the absorber and the LPGAX. . Table 4.2 shows the type,

heat transfer area and the designed heat duty of the various components of the

air-cooled vapour absorption refrigeration system.

Table 4.3 Component details

S.No Component Type

Heat

Transfer

Area

(m2)

Heat Duty

(kW)

1 Absorber Finned 0.89 21.85

2 Condenser Finned 0.72 11.96

3 Evaporator Shell and Coil 5.77 10.5

4 Generator Direct Fired 1 23.27

5 High Pressure GAX Shell and Tube 2.98 3.65

6 Low Pressure GAX Shell and Tube 0.54 4.57

7 Condensate Pre-Cooler Shell and Tube 4.55 1.33

8 Solution Heat Exchanger 1 Tube-in-Tube 0.68 12.23

9 Solution Heat Exchanger 2 Tube-in-Tube 0.10 3.57

10 Solution Cooler Shell and Tube 0.24 5.26

140

4.4 CHARGING PROCEDURE

The individual components of the system are fabricated as per the

design specifications and subjected to hydraulic pressure testing upto 20 bar

to check for any leakage. Due to limited facility in the laboratory, the pressure

testing of the individual components was restricted upto 20 bar only The

pressure is maintained for a period of 2 to 3 days. The components are then

assembled and instrumented with pressure, temperature, and flow rate

measurements at the required location, as shown in Figure 4.12. The entire

system was again tested for leakage following the above procedure. The

system is then evacuated to an extent of 30 mm of Hg and the vacuum is

maintained for 3 days. To remove the non-condensable gases that may be

present, the system after evacuation is first charged with the refrigerant

vapour. The system is charged with calculated quantities of the refrigerant and

absorbent. The calculation procedure to determine the required quantity of

refrigerant and absorbent are given in Appendix 2 and the specifications of

the equipments such as the solution pump, absorber and condenser fan, burner

and power analyzer are given in Appendix 3.

4.5 MEASUREMENT OF PARAMETERS

The details of the instrumentation are presented in this section. The

parameters that are to be measured are the pressure, temperature, density and

mass flow rate of the solution, the air velocity, density and mass flow rate of

the refrigerant, and mass flow rate of the fuel. The uncertainty analysis of the

measured / calculated parameters is presented in Appendix 4.

4.5.1 Pressure

Pressure measurements are made by calibrated Pressure

Transducers. The low and high pressures in the system are measured with the

instrument ranging from 0 to 10 bar and 0 to 25 bar respectively, with an

uncertainty of ± 0.20%.

141

4.5.2 Temperature

Temperatures are measured with T type copper-constantan

thermocouples. The ends of all the thermocouples are connected to a data

acquisition system (Make: Agilent 34970A). The thermocouples were

calibrated at ice point of water and ambient temperature. Good agreement is

observed between the thermometer readings and measured readings of the

temperature using T-type thermocouples. The thermocouples are fixed at

various locations on the experimental set up. The measurements are made

with an uncertainty of ± 0.5oC. These data are stored in a data acquisition

system.

4.5.3 Flow Rate

The mass flow rate of the weak solution and the refrigerant are

measured by coriolis mass flow meters with an uncertainty of ± 0.2% and ±

0.15% respectively.

4.5.4 Concentration

The density and the temperature of both the weak solution and the

refrigerant are measured by coriolis mass flow meters. Using the correlation

which gives the relation between the specific volume, the temperature and the

concentration of the saturated ammonia-water solution, the concentration of

the weak solution and the refrigerant are determined as given below

3 3

0 0

( , ) j i

ij

i j

v T X a X T (4.1)

The coefficient of Equation (4.1) is taken from the ASHRAE

Handbook Fundamental (1993).

142

4.5.5 Fuel Consumption

The fuel flow rate to the generator is measured by a digital

weighing machine with an uncertainty of ± 0.05%.

4.5.6 Heat Loss

The heat loss from the surface of the generator to the surrounding is

calculated, based on the measurement of the temperatures at the generator

surface, insulation surface and the thermal conductivity of the insulation

material. The amount of heat loss from the generator to the surroundings is

estimated to be 0.175 kW, which is about 1.5% of the actual generator heat

input. The calculation procedure is mentioned in Appendix 5.

4.6 EXPERIMENTAL PLAN AND PROCEDURE

The following ranges of operating conditions are fixed for testing

the performance of the fabricated experimental setup.

(a) The mass flow rate of the weak solution is varied between

0.0248 kg/s and 0.0807 kg/s.

(b) The fuel flow rate to the generator is varied between 1kg/h

and 1.5 kg/h, by using different nozzles in the burner.

First, the absorber condenser fan is switched on, followed by the diesel

fired burner to supply heat input to the generator. The weak solution is then

circulated between the absorber and the generator, by switching on the

solution pump for the generation of the refrigerant vapour in the generator.

The condensing and evaporating temperatures varies with the cooling medium

and water temperatures respectively. The evaporating and condensing

temperatures were maintained by regulating the flow of strong solution and

143

the refrigerant using the pressure reducing valve and refrigerant expansion

valve.

The flow rates of the refrigerant, the pressure and temperature of

the components, air velocity, and fuel flow rate are noted periodically. After

the refrigerant reservoir is filled about 50%, the pressure reducing valve

between the condensate pre-cooler and the evaporator is adjusted in such a

way that a constant flow of liquid refrigerant is established. The level of the

weak solution in the weak solution reservoir in maintained constant for the

steady state operation, regulating the flow of the strong solution from the

generator to the absorber through the expansion valve. The refrigerant is also

maintained at a constant level in the refrigerant reservoir. When two to three

successive readings are almost the same, it can be concluded that the system

has attained the steady state.

As a safety precaution, since water has good affinity towards ammonia,

sufficient quantity of water has been kept as it can easily dilute the

concentration of the refrigerant. Safety face masks and gloves were also used

while operating the system. Any traces of ammonia leaked into the

atmosphere, can be easily detected before it reaches the toxic level due to its

pungent odour.

4.7 DATA PROCESSING

The circulation ratio (CR) is determined by the measured flow rate

of the weak solution and the refrigerant.

. .

ws rCR m / m (4.2)

144

The heat recovered by the internal heat recovery components is

estimated from the mass flow rate of the weak solution and the fluid

enthalpies.

The heat recovered by the high pressure GAX is

.

HPGAX ws 3 2Q m (h h ) (4.3)

The heat recovered by the low pressure GAX is

.

LPGAX ws 4 3Q m (h h ) (4.4)

The heat recovered by the solution heat exchanger 1 is

.

SX1 ws 5 4Q m (h h ) (4.5)

The heat recovered by the solution heat exchanger 2 is

.

SX2 ws 6 5Q m (h h ) (4.6)

The heat recovered by the solution cooler is

.

SC 7 7 6Q m (h h ) (4.7)

The heat recovered by the condensate pre cooler is

.

CPC 23 23 22Q m (h h ) (4.8)

In Equations (4.3) – (4.8), the temperatures were measured, using

calibrated copper-constantan thermocouples (T-type). To calculate the

enthalpy, the measured temperature and the solution concentration were used

in the correlation proposed by Sun (1997).

The heat rejected by the absorber and condensor is

.

AC a a ao aiQ m Cp (t t ) (4.9)

The measured temperatures were recorded in a data acquisition

system. The air velocity was measured by an anemometer at the inlet to the

145

absorber and condenser. The air flow rate is then calculated using the

following relation

.m a = (density of air) x (area) x (air velocity) (4.10)

The heat supplied to the generator is

.

fuel fuel fuelQ m (CV) (4.11)

CVfuel = 44,000 kJ/kg

Air-Fuel ratio = 14:1 (Ganesan 2002)

The cooling capacity is estimated by calculating the rate of heat

removal from a constant quantity of water.

Q m Cp dt / dw wE (4.12)

The volume of the evaporator shell is calculated and the required

quantity of water is filled in. The water temperature in the evaporator shell

was measured at three different locations (top, middle and bottom) using

copper-constantan thermocouples. The measurements were recorded and

stored periodically every 15 minutes. A graph between time and temperature

is plotted and the slope is taken for the calculation. The variation of ambient

air temperature during the course of experiment varies in the range of ± 3°C

only. This variation does not contribute significantly on the performance of

the system. The fuel COP is then determined using the relation

fu el E fu e lC O P Q Q (4.13)

The total COP is calculated considering the auxiliary power

required to operate the solution pump, fan and the burner.

total E fuel SP f bC O P Q (Q W W W ) (4.14)

146

The auxiliary powers were measured by using a calibrated digital

power analyzer.

4.8 RESULTS AND DISCUSSION

The performance of the system was evaluated by varying the mass

flow rate of the weak solution and the fuel flow rate to the generator.

4.8.1 Variation of the system pressures

Figure 4.19 shows the variation of the system pressures with time

for a constant weak solution and fuel flow rate and split factor. The split

factor is defined as the ratio of the mass flow rate of the refrigerant to the

absorber, to the total mass flow rate of the refrigerant in the cycle. The low

0

5

10

15

20

25

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

Condensor

Evaporator

Time (h)

Pres

sure

(ba

r)

Z = 0.8.

0.0578 /wsm kg s.

1.05 /fuelm kg h

0

5

10

15

20

25

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

Condensor

Evaporator

Time (h)

Pres

sure

(ba

r)

Z = 0.8.

0.0578 /wsm kg s.

1.05 /fuelm kg h

Figure 4.19 Variation of the system pressures with time

147

and high pressures stabilize at about 4 bar and 18 bar respectively, and the

system reaches the steady state in about 120 minutes of operation.

4.8.2 Variation of the component’s temperatures

Figure 4.20 shows the variation of the component’s temperature

and the ambient temperature with time. The ambient temperature was initially

at 28°C, but reached upto 35°C during the operation of the system. It is

observed from Figure 4.20, that the generator temperature increases steadily

and after nearly three hours, it stabilizes at a temperature of about 125°C. The

variation of the absorber and condenser temperatures with respect to time also

follows a pattern similar to that of the generator. The absorber temperature is

higher than the condensor temperature due to the large amount of heat of

mixing during the absorption process. The evaporator temperature decreases

with time, initially. The decrease in the evaporator temperature during the

initial three hours of operation, is due to the heat capacities of the different

components of the system. After 2 to 3 hours, it became stable, thus providing

the cooling capacity at a constant rate.

Figure 4.20 Variation of the component’s temperature with time

-5

20

45

70

95

120

145

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

Absorber Condensor

Evaporator Generator

Air inlet Air outlet

Time (h)

Tem

per

atu

re (

°C)

Z = 0.8.

0.0578 /wsm kg s

.1.05 /fuelm kg h

-5

20

45

70

95

120

145

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

Absorber Condensor

Evaporator Generator

Air inlet Air outlet

Time (h)

Tem

per

atu

re (

°C)

Z = 0.8.

0.0578 /wsm kg s

.1.05 /fuelm kg h

148

4.8.3 Temperature history of the internal heat recovery components

The temperature history of the internal heat recovery components

are shown from Figures 4.21 to 4.23. The temperatures were measured at all

the inlets and outlets of the components with respect to the solution /

refrigerant streams. The mass flow rate of the weak solution and the fuel flow

rate are 0.0578 kg/s and 1.05 kg/h respectively. The internal heat recovery

components comprise of the CPC, HPGAX, LPGAX, SX1, SX2 and SC.

Figure 4.21 Variation of the temperatures of the CPC and SX1 with time

-25

0

25

50

75

100

125

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

rli CPC rlo CPC rvi CPC rvo CPCwsi SX1 wso SX1 ssi SX1 sso SX1

Time (h)

Tem

per

atu

re (

°C)

Z = 0.8

.0.0578 /wsm kg s

.1.05 /fuelm kg h

-25

0

25

50

75

100

125

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

rli CPC rlo CPC rvi CPC rvo CPCwsi SX1 wso SX1 ssi SX1 sso SX1

Time (h)

Tem

per

atu

re (

°C)

Z = 0.8

.0.0578 /wsm kg s

.1.05 /fuelm kg h

149

Figure 4.22 Variation of the temperatures of the LPGAX and SX2 with

time

0

25

50

75

100

125

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

wsi HPGAX wso HPGAX rvi HPGAXrvo HPGAX wsi SC wso SCssi SC sso SC

Time (h)

Tem

pera

ture

(°C

) Z = 0.8

.0.0578 /wsm kg s

.1.05 /fuelm kg h

0

25

50

75

100

125

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

wsi HPGAX wso HPGAX rvi HPGAXrvo HPGAX wsi SC wso SCssi SC sso SC

Time (h)

Tem

pera

ture

(°C

) Z = 0.8

.0.0578 /wsm kg s

.1.05 /fuelm kg h

Figure 4.23 Variation of the temperatures of the HPGAX and SC with

time

Tem

pera

ture

(°C

)

-25

0

25

50

75

100

125

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

wsi LPGAX wso LPGAX ssi LPGAX sso LPGAX rvi LPGAX

wsi SX2 wso SX2 ssi SX2 sso SX2

Time (h)

Tem

pera

ture

(°C

)

-25

0

25

50

75

100

125

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

wsi LPGAX wso LPGAX ssi LPGAX sso LPGAX rvi LPGAX

wsi SX2 wso SX2 ssi SX2 sso SX2

Time (h)

150

The rise in the temperature of the weak solution is about 5 to 10 °C

in the GAX component, namely, the HPGAX and LPGAX, and also in the

SC, whereas it is about 5°C in the SX2. The SX1 accounts for nearly 25 to 30

°C rise in the amount of heat gained by the weak solution. The effectiveness

of the SX1 is determined to be 80 to 85%.

4.8.4 Circulation ratio and weight fraction

The variation of the circulation ratio and the weight fraction, with

respect to time for a typical operating condition is shown in Figure 4.24. The

circulation ratio has a greater impact on the system performance. The

circulation ratio was initially high, because of the less amount of refrigerant

circulation, and it became steady after 2 hours from the starting time of the

system operation. The weight fraction of the weak solution is high initially,

mainly due to the refrigerant that was ‘emptied’ out of the refrigerant circuit

and ‘stored’ in the solution after the previous run. The weight fraction of the

pure refrigerant remains almost constant from the starting time of the

refrigerant circulation. The refrigerant vapour leaving the HPGAX is

condensed in the condensor and is stored in the refrigerant reservoir.

Time (h)

CR

Wei

gh

t F

racti

on

(%

)

0

3

6

9

12

15

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

0

20

40

60

80

100

Xr

Xws Xss

Z = 0.8

CR

.0.0578 /wsm kg s

.1.05 /fuelm kg h

Time (h)

CR

Wei

gh

t F

racti

on

(%

)

0

3

6

9

12

15

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

0

20

40

60

80

100

Xr

Xws Xss

Z = 0.8

CR

.0.0578 /wsm kg s

.1.05 /fuelm kg h

Figure 4.24 Variation of the circulation ratio and weight fraction with

time

151

The concentration of the liquid refrigerant before it enters the evaporator

through the condensate pre-cooler, is estimated by using the correlation which

gives the relation between the specific volume, the temperature and the

concentration. The density and the temperature of the refrigerant is measured

by a coriolis mass flow meter. The HPGAX is very effective as the estimated

concentration of the refrigerant, was found to be 0.99, as inferred from Figure

4.24. The weight fraction of the strong solution also follows a trend similar to

that of the weak solution.

4.8.5 Pinch point diagram for a typical operating condition

The pinch point diagram for a typical operating condition is shown

in Figure 4.25. The heating process is represented by state points 2 to G. It

represents the amount of heat recovered by the weak solution in the various

internal heat recovery components. Processes 8 to A represent the cooling

process. The difference in the heat load between state points 2 and A, is the

absorber heat load. The difference in the heat load between state points 2 and

8 represents the total amount of heat recovered by the heat recovery

components. The amount of the external heat input that is supplied to the

generator is represented by the region between the state points 8 and G. The

pinch point is the closest approach temperature between the heating and cooling

curves. It can be inferred from the above figure that the pinch point occurs

between state points 4 and 11 on the heating and cooling curves respectively. A

pinch point of 7°C is obtained and a maximum cooling capacity of 9.5 kW is

attained. About 16 kW of internal heat has been recovered.The experimental

pinch point diagram which shows a pinch point of 7°C, has been plotted for a

particular operating condition of generator, absorber, condenser and

152

evaporator temperatures of 120°C, 52°C, 46°C and 2°C respectively, whereas

the pinch point at the other operating conditions is greater than10°C.

0

25

50

75

100

125

150

0 10 20 30 40 50 60

weak solution

strong solution

2 34

5

6

7

8

910

1113

G

A

QA

QG

QIHR

Pinch point

Z = 0.8

Heat Load (kW)

Tem

per

atu

re (

°C)

.0.0578 /wsm kg s

.1.05 /fuelm kg h

0

25

50

75

100

125

150

0 10 20 30 40 50 60

weak solution

strong solution

2 34

5

6

7

8

910

1113

G

A

QA

QG

QIHR

Pinch point

Z = 0.8

Heat Load (kW)

Tem

per

atu

re (

°C)

.0.0578 /wsm kg s

.1.05 /fuelm kg h

Figure 4.25 Pinch point diagram for a typical operating condition

4.8.6 Heat load

Figure 4.26 shows the variation of the heat supplied, the heat recovered

and the heat rejected by the system with respect to time for the generator, absorber,

condenser and evaporator temperatures of 120°C, 52°C, 46°C and 2°C

respectively. The heat supplied is estimated by adding the heat load of the

evaporator and that of the generator. Due to the increase in the refrigerant

circulation the evaporator heat load increases, and this results in an increase in the

amount of heat supplied to the system, until the steady state is reached. The heat

recovered is estimated, by adding the heat loads of all the internal heat recovery

components such as the CPC, HPGAX, LPGAX, SX1, SX2 and SC. The amount

of heat recovered by the heat recovery components increases gradually with time,

due to the rise in the temperature of the weak solution at the outlet of the

corresponding component. The SX1 contributes to about 60 to 70% of the total

153

amount of heat recovered internally. The internal heat recovered in the GAX

component and the SC is approximately 1.5 kW each.

Time (h)

Hea

t L

oad

(k

W)

10

15

20

25

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

Heat Supplied Heat Recovered Heat Rejected

tG = 120°C, tA = 52°C, tC = 46°C, tE = 2°C, Z = 0.8.

0.0578 /wsm kg s

.1.05 /fuelm kg h

Time (h)

Hea

t L

oad

(k

W)

10

15

20

25

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

Heat Supplied Heat Recovered Heat Rejected

tG = 120°C, tA = 52°C, tC = 46°C, tE = 2°C, Z = 0.8.

0.0578 /wsm kg s

.1.05 /fuelm kg h

Figure 4.26 Variation of the heat load with time

Overall, the heat recovery components provide nearly 16 kW of

heat recovered to the generator. The amount of heat rejected by the system is

the total amount of the heat rejected by both the absorber and the condenser.

The variation in the heat rejection load is not uniform, due to the variation in

the absorber and condenser load in different ambient conditions.

4.8.7 Cooling capacity and COP

The performance of the system in terms of the cooling capacity,

fuel and total COP, with respect to time is shown in Figure 4.27. The fuel

COP, calculated based on the fuel consumption rate and the rate of heat

removal from a fixed quantity of water, reached a maximum of 0.61 for the

generator, absorber, condenser and evaporator temperatures of 120°C, 52°C,

46°C and 2°C respectively.

154

CO

P

Cooli

ng C

ap

aci

ty (

kW

)

Time (h)

0.00

0.20

0.40

0.60

0.80

1.00

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

5

7

9

11

13

15

tE = 2°C tE = -2°C

COPfuel COPtotal Cooling Capacity

tG = 120°C, tA = 52°C, tC = 46°C.

0.0578 /wsm kg s.

1.05 /fuelm kg h

Z = 0.8

CO

P

Cooli

ng C

ap

aci

ty (

kW

)

Time (h)

0.00

0.20

0.40

0.60

0.80

1.00

7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

5

7

9

11

13

15

tE = 2°C tE = -2°C

COPfuel COPtotal Cooling Capacity

tG = 120°C, tA = 52°C, tC = 46°C.

0.0578 /wsm kg s.

1.05 /fuelm kg h

Z = 0.8

Figure 4.27 Variation of the cooling capacity and the COP with time

The total COP, calculated by considering the parasitic power

consumption, reached a maximum of 0.58 for the same operating conditions,

which is about 8 % less than the fuel COP. The cooling capacity of the system

is about 7.4 kW and it remains almost constant. When the evaporator

temperature decreases from 2°C to -2°C while the other operating parameters

remain constant, the cooling capacity and the performance of the system are

reduced. This is because, when the evaporator temperature decreases, the

amount of the refrigerant mass flow rate decreases, and hence, the cooling

capacity and the COP of the system also decrease. The decrease in the actual

performance of the system, when the evaporator temperature varies from 2°C

to -2°C, is nearly 10 to 15%.

155

4.8.8 Experimental runs

Figure 4.28 depicts the performance of the system with respect to

the experimental runs. The mass flow rate of the weak solution is kept

constant at 0.0578 kg/s, and the fuel flow rate to the generator at 1.05 kg/h.

The system reached the maximum COP after 3 hours of operation during each

Figure 4.28 Variation of the COP with experimental runs

experimental run. The fuel and the total COP of the system vary between 0.60

and 0.63 and 0.56 and 0.59 respectively, for different experimental runs, for

the same operating conditions. The deviation between the thermodynamic and

fuel COP is estimated to be 20 to 25% due to heat losses and the internal

irreversibility in the system. For a particular set of experimental observations,

the performance parameters are given in Appendix 5. The efficiency of the

diesel fired burner is 95 to 98%. For calculation purposes, the efficiency is

taken as 100%.

CO

P

Experiment Runs

0.00

0.25

0.50

0.75

1.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Fuel

Total

Thermodynamic

tG = 120°C, tA = 52°C, tC = 46°C, tE = 2°C, Z = 0.8

.0.0578 /ws sm kg

.1.05 /fuelm kg h

CO

P

Experiment Runs

0.00

0.25

0.50

0.75

1.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Fuel

Total

Thermodynamic

tG = 120°C, tA = 52°C, tC = 46°C, tE = 2°C, Z = 0.8

.0.0578 /ws sm kg

.1.05 /fuelm kg h

156

4.8.9 Effect of fuel consumption

Figure 4.29 depicts the effect of the fuel flow rate in kg/h on the

cooling capacity and the fuel COP of the system, for an evaporator

temperature of 2°C. The mass flow rate of the weak solution is kept constant

at 0.0578 kg/s and the split factor at 0.8. It is inferred from the Figure that the

cooling capacity increases with an increase in the fuel flow rate. The increase

CO

P

Coo

lin

g C

ap

aci

ty (

kW

)

Fuel Consumption (kg/h)

0.40

0.60

0.80

1.00

1.00 1.10 1.20 1.30 1.40 1.50

5

6

7

8

9

10

Fuel COP

Cooling Capacity

tE = 2°C

Z = 0.8

.0.0578 /wsm kg s

CO

P

Coo

lin

g C

ap

aci

ty (

kW

)

Fuel Consumption (kg/h)

0.40

0.60

0.80

1.00

1.00 1.10 1.20 1.30 1.40 1.50

5

6

7

8

9

10

Fuel COP

Cooling Capacity

tE = 2°C

Z = 0.8

.0.0578 /wsm kg s

Figure 4.29 Variation of the cooling capacity and COP with the fuel

consumption

in the fuel flow rate increases the heat input to the generator, which leads to

the generation of more amount of refrigerant vapour, for a constant weak

solution flow rate. As a result of the increased refrigerant vapour generation,

the cooling capacity increases. However, the fuel COP of the system does not

increase because, firstly, the heat of generation increases as the generator

temperature increases. Secondly, the generator efficiency decreases with an

increasing fuel consumption rate. It is found that the fuel COP of 0.61 is

attained under the conditions analyzed for the fabricated system.

157

4.8.10 Influence of the sink temperatures on the circulation ratio

Figure 4.30 shows the variation of the circulation ratio with respect

to the sink temperature for the generator and evaporator temperatures of

120°C and 2°C respectively. For a constant weak solution flow rate, as the

Sink Temperature (°C)

CR

tG = 120°C, tE = 2°C

3.0

6.0

9.0

12.0

15.0

25 28 31 34 37 40

tG = 120°C, tE = 2°C.

0.0578 /wsm kg s

.1.05 /fuelm kg h

Z = 0.8

.1.40 /fuelm kg h

.1.25 /fuelm kg h


CR

tG = 120°C, tE = 2°C

3.0

6.0

9.0

12.0

15.0

25 28 31 34 37 40

tG = 120°C, tE = 2°C.

0.0578 /wsm kg s

.1.05 /fuelm kg h

Z = 0.8

.1.40 /fuelm kg h

.1.25 /fuelm kg h

Figure 4.30 Variation of the circulation ratio with the sink temperatures

sink temperature increases, the circulation ratio increases. This is because, as

the sink temperature increases the weak solution concentration decreases,

which results in a lower degassing width. The lower degassing width

increases the circulation ratio. For a constant sink temperature, the increase in

the fuel flow rate lowers the circulation ratio. This is due to the increase in the

refrigerant flow rate, for a constant weak solution rate.

4.8.11 Influence of the sink temperatures on the heat load

The variation of the heat supplied, heat recovered and heat rejected

with the sink temperatures is shown in Figure 4.31. The heat supplied is the

summation of the evaporator and generator load and the heat rejected is the

158

summation of the absorber and condenser heat load. The heat recovered by

the system is the total heat load of all the internal heat recovery components,

such as the high pressure GAX, the low pressure GAX, solution heat

exchangers 1 and 2, solution cooler and the condensate pre-cooler. As the sink

temperature increases, the total heat supplied reduces, due to the decrease in

the evaporator load at higher sink temperatures. The heat rejected from the

system also decreases with the sink temperature. However, the heat recovered

by the system increases due to the increase in the temperature difference

across the internal heat recovery components. The reason for large difference

between the heat supplied to the system and the heat rejected by the system as

inferred from the Figures 4.26 and 4.31 is because, the heat loss from the

other components and the connecting pipelines were not measured.


Hea

t L

oa

d (

kW

)

5

10

15

20

25

25 28 31 34 37 40

Heat Supplied

Heat Recovered

Heat Rejected

tG = 120°C, tE = 2°C.

0.0578 /wsm kg s.

1.05 /fuelm kg h


Hea

t L

oa

d (

kW

)

5

10

15

20

25

25 28 31 34 37 40

Heat Supplied

Heat Recovered

Heat Rejected

tG = 120°C, tE = 2°C.

0.0578 /wsm kg s.

1.05 /fuelm kg h

Figure 4.31 Variation of the heat load with the sink temperatures

159

4.8.12 Influence of the sink temperatures on the cooling capacity and

COP

The variation of the cooling capacity and the COP of the system

with respect to the sink temperatures is shown in Figure 4.32, for a constant

weak solution flow rate of 0.0578 kg/s and fuel flow rate of 1.05 kg/h. When

the sink temperature increases, the decrease in the degassing width increases

the circulation ratio. For a constant weak solution flow rate, the increase in

the circulation ratio reduces the refrigerant mass flow rate. Hence the cooling

capacity decreases. The decrease in the cooling capacity decreases the COP of

the system.


CO

P

Co

oli

ng

Ca

pa

city

(k

W)

0.4

0.5

0.6

0.7

0.8

25 28 31 34 37 40

0

2

4

6

8

10

Fuel COP Total COP Cooling Capacity

.0.0578 /wsm kg s

.1.05 /fuelm kg h tG = 120°C, tE = 2°C


CO

P

Co

oli

ng

Ca

pa

city

(k

W)

0.4

0.5

0.6

0.7

0.8

25 28 31 34 37 40

0

2

4

6

8

10

Fuel COP Total COP Cooling Capacity

.0.0578 /wsm kg s

.1.05 /fuelm kg h tG = 120°C, tE = 2°C

Figure 4.32 Variation of the cooling capacity and the COP with the sink

temperatures

160

4.8.13 Second law efficiency

Figure 4.33 shows the variation of the second law efficiency with

respect to the sink temperatures, for a constant weak solution flow rate and

constant generator and evaporator temperatures. With increase in the sink

temperature, the fuel and the carnot COP decreases. Since the rate at which

the carnot COP decreases is higher that that of fuel COP, the second law

efficiency increases.

0.20

0.25

0.30

0.35

0.40

0.45

25 28 31 34 37 40


Sec

on

d l

aw

eff

icie

ncy

tG = 120°C, tE = 2°C.

0.0578 /wsm kg s.

1.05 /fuelm kg h

0.20

0.25

0.30

0.35

0.40

0.45

25 28 31 34 37 40


Sec

on

d l

aw

eff

icie

ncy

tG = 120°C, tE = 2°C.

0.0578 /wsm kg s.

1.05 /fuelm kg h

Figure 4.33 Variation of the second law efficiency with the sink

temperatures

4.9 ECONOMIC ANALYSIS

The incorporation of high pressure GAX, low pressure GAX,

solution cooler and the solution heat exchanger 2 increases the first cost of the

proposed system. Hence, it may not be economically viable when compared

to that of a simple system with only a solution heat exchanger and condensate

pre-cooler. However, for small and medium capacities up to 35 kW, air-

161

cooled systems are more economically viable compared to water-cooled

systems, due to large size of the cooling tower, installation problems etc.

Since the water cooled systems need a cooling tower, cooling

water pump etc, the installation cost will be higher than that of an air-cooled

one, by 20 to 25%, for the same capacity. For a conventional ammonia-water

system, the COP is about 0.5 and in the GAX system it is about 0.61, at a

generator, absorber, condenser, evaporator temperatures of 120°C, 52°C,

46°C, 2°C respectively. The amount of internal heat recovery is about 30 to

40 % more than in the conventional one, which would reduce the operating

costs by about 20%.

4.10 CONCLUSION

In this chapter, the design of the components of the air-cooled GAX

based vapour absorption refrigeration system, the working fluid charging

procedure, the measurement of parameters, and the experimental plan and

procedure have been presented. The results of the experimental investigations

on the air-cooled GAX based vapour absorption refrigeration system are

discussed. The influence of the various parameters such as the sink

temperature and the fuel flow rate on the performance of the system is also

discussed. The performance parameters studied are the circulation ratio,

internal heat recovered and the coefficient of performance. The conclusions

drawn from the experimental investigations are presented in the next chapter.

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CHAPTER 4 EXPERIMENTAL STUDIES - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9877/9/09_chapter 4.pdf · CHAPTER 4 EXPERIMENTAL STUDIES 4.1 INTRODUCTION Based on the results