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Effect of operational parameters on heat and mass transfer in generator of
R134a-DMF absorption refrigeration system
Balamurugan PASUPATHY, Mani ANNAMALAI*
Refrigeration and Airconditioning Laboratory, Department of Mechanical Engineering,
Indian Institute of Technology Madras, Chennai, Tamilnadu, IndiaPhone: +91 44 22574666; Fax: +91 44 22570509; E-mail: [email protected]
* Corresponding Author
ABSTRACT
Vapour absorption refrigeration systems (VARS) has regained the attention due to their potential forrenewable/waste heat utilization. To improve the efficiency of these systems, it becomes obligatory to makecomponent level studies on processes. In this present study, investigations on the heat and mass transfer in compact
generator of the vapour absorption refrigeration system have been carried out using R134a-Dimethyl formamide
(DMF). An experimental facility of VARS has been fabricated using brazed plate heat exchangers as generator,condenser, absorber, evaporator and solution heat exchanger. Hot water source is used to supply hot water between
80 oC and 98oC to suit utilization of solar energy, waste heat, etc to the generator. Cooling water from cooling water
source is circulated through the absorber and condenser to remove the heat. Water from cooling load simulator is
circulated to the evaporator. Investigations have been carried out on VARS with a rated cooling capacity of 1kW byvarying the operating parameters viz, solution flow rate from 0.02 m3hr-1 to 0.05 m3hr-1, liquid refrigerant flow rate
from 0.002 m3hr
-1to 0.015 m
3hr
-1, hot water temperature from 85
oC to 97
oC. Generator pressure is varied from 620
kPa to 920 kPa, hot water flow rate from 0.12 m3hr
-1to 0.32 m
3hr
-1and solution initial concentration is varied from
0.59 kgkg-1 to 0.75 kgkg-1. The effect of solution flow rate, generator temperature and generator pressure on the
performance of generator and the absorption system has been investigated. Heat and mass transfer coefficients, heattransfer rate, mass desorption rate increase with generator temperature and solution flow rate but decrease with
increase in generator pressure.
1. INTRODUCTION
Many researches are carried out in absorption refrigeration technology, in the recent years, aiming to search for
environment friendly alternate working pairs and to improve the efficiencies of various major components of thevapour absorption refrigeration system. Even though traditionally used working fluids are NH3-H2O and H2O-LiBr,it is observed that the disadvantages associated with these working fluids have prompted researchers, to search for
alternate working fluids. Though R22-organic solvent based absorption refrigeration systems have been extensively
studied by Fatouh et al.(1994), Karthikeyan et al.(1995) and Sujatha et al.(1997), HCFCs along with CFCs, are
also covered by Montreal and other International Protocols and are being phased out. Alternatively, R134a based
VARS are being investigated. Nezu et al.(2002) and Yokozeki (2005) investigated R134a as a refrigerant in VARSwith various absorbent combinations and concluded that R134a-Dimethyl acetamide (DMA) and R134a-DMF are
promising for the absorption refrigeration system than other R134a-absorbent combinations. It is also found that thecirculation ratio is lower and COP is higher for R134a-DMF system compared to R134a-DMA system. Mani (2009)
carried out experimental studies on compact VARS system with plate heat exchangers and reported that this system
could be very competitive for applications ranging from -10C to 10C, with heat source temperature in the range of75C to 90C.
Generator is considered as one of the crucial components in the vapour absorption refrigeration system. Boiling
process in generator is characterized by simultaneous heat and mass transfer phenomena. Use of plate heatexchanger as the system components of VARS has increased due to its high heat transfer efficiency, high heat
transfer area to volume ratio, etc. Various authors (Kang et al.1998; Leeet al.2002 and Jesus Cerezo et al. 2009)have used plate heat exchanger as absorber and investigated its performance on VARS systems. Roriz et al. (2004)
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used the plate heat exchanger as generator and compared the performance of compact generator with that of a falling
film desorber with ammonia-water mixture and proved the feasibility of compact generator. Taboas et al. (2010)have measured heat transfer coefficient and pressure drop for ammonia-water combination under flow boiling
conditions in a vertical brazed plate heat exchanger, at different operating conditions. Yan and Lin (1999) carriedout experiments on evaporation, to measure heat transfer coefficient and pressure drop for R134a flowing in a plate
heat exchanger. Contrary to the mass flux effects, the heat flux did not show significant effects on the heat transferat high quality, but it showed some influence at low quality. Based on their experimental data, correlation equations
have been developed for heat transfer coefficient and friction factor. Experimental data of Taboas et al.(2010) on
flow boiling of ammonia-water in a plate heat exchanger are compared by Taboas et al. (2012) with the valuespredicted using the correlations available in the open literature for the boiling heat transfer coefficient and pressuredrop. A new correlation has also been proposed based on a separate model to obtain the boiling heat transfer
coefficient. Review of literature revealed that a comprehensive study on heat and mass transfer of compact generator
using R134a-DMF is yet to be investigated. The present investigation is focused on heat and mass transfer on plateheat exchanger used as generator in a 1 kW capacity VARS using R134a-DMF.
2. EXPERIMENTAL SETUP
Schematic diagram of the vapour absorption refrigeration system is shown in Fig.1. The setup consists of brazed
plate heat exchangers used as generator, condenser, evaporator, absorber and solution heat exchanger.
S3 A
R134a
Ball valve
Needle valve
Flow meter
Level gauge
Temp. gauge
Press. gaugeGas separator
Cold water pump
Solution Pump
Cooling water simulator
Cooling load simulator
Hot water pump
G
C
E
SHX
CHP
AT
T15
S4
T16T3 P1
BV14
BV13
BV15
DM
BV
16
PH
BV3
T2
BV2
T12
BV12
BV4
P2
T4
BV5
T5
T22
T6
BV6
RR
T21 NV1 T8
P4BV8
T7
P3
MS
L2
L1S1
BV7 BV9
S5T9
T17
T20
T14
T13 P6P7
T1
NV2
BV10
BV1
T11P5 L3
T10
T19S4
BV11
T18
Hot water
simulator
R134a/DMF
Water
S
BV
NV
L
T
P
Figure 1:Schematic diagram of R134a-DMF based VARS with plate heat exchangers
(Balamurugan and Mani, 2012a)
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Experimental facility also contains a solution pump, cooling water source, hot water source, cooling load simulator,
instrumentation and valves. Refrigerant loop starts from the gas separator where the R134a vapour generated in thecompact generator is separated and is allowed to get condensed in the condenser and stored in the receiver. Cooling
water source is used to remove heat of condensation. Liquid R134a collected in the receiver is expanded throughthrottle valve and evaporated in the evaporator. Chilled water from the simulator is used to give the cooling load.
R134a vapour from the evaporator is absorbed by the weak DMF solution in the absorber, which marks thebeginning of the solution loop. Heat of mixing is removed by water from cooling water source. A diaphragm type
reciprocating pump is used to pump the strong solution collected in the absorber tank through solution heat
exchanger and solution preheater to compact generator. Figure 2 shows the compact generator, used for the presentstudy, which is a plate heat exchanger with Chevron-H type plate channels. Hot water from the heat source issupplied to generator in counter flow direction. After desorption, the two phase mixture from the generator is sent to
gas separator. Weak solution remaining in the phase separator is sent to the absorber through solution heat
exchanger and pressure reducing valve.
2.5 0.3
1.5
42
77
207
A
A
Sectional view A-A
All dimensions in mm
172
Figure 2:Schematic diagram of compact generator (Balamurugan and Mani, 2012a)
Hot water source consists of an insulated hot water tank, electric heaters, pump, flow meter, PT100 sensor, PID
temperature controller, contactor, piping and valves. Cooling water source consists of R22 based vapourcompression refrigeration (VCR) circuit, insulated cooling water tank, electric heaters, pump, flow meter, PT100
sensor, PID temperature controller, contactor, piping and valves. Cooling load simulator features an insulated chilled
water tank, electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and
valves. It is used to supply water as cooling load to evaporator and maintains a constant desired value of chilled
water temperature at evaporator inlet.
Location of various temperature sensors, pressure sensors, flow meters, online density meter and valves areindicated in Fig. 1. All these measuring instruments have been calibrated. Copper-constantan thermocouples are
used as temperature sensors, to measure temperature at prominent locations, with a maximum measurement
uncertainty of 0.5 C. Piezo-electric type pressure transducers are used as pressure sensors with a measurementuncertainty up to 4.55 %. Metal tube rotameters are used to measure flow of liquid refrigerant, weak solution andhot water with a measurement uncertainty up to 5 %. Glass rotameters are used to measure flow of cooling waterand chilled water with a measurement uncertainty up to 2.5 %. An online density meter is used to measure densityof strong and weak solutions with a measurement uncertainty of 0.3 %.
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Figure 3:Experimental facility of VARS with compact generator (Balamurugan and Mani, 2012b)
Concentrations of strong and weak solutions are evaluated from the measured density values using HBT
(Hankinson-Brobst-Thomson) equation used by Reid et al.(1989) in their book. Data acquisition system connectedto a computer is used to monitor and record the readings from all these instruments and sensors. Photograph of the
experimental facility is shown in Fig. 3.
Gas separator
Compact Generator
Condenser
Liquid Receiver
AbsorberTank
Evaporator
Absorber
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3. EXPERIMENTAL PROCEDURE
Generally, when not in operation, refrigerant and solution loops are disconnected by closing the valve between i) gas
separator and condenser and ii) evaporator and absorber. Initially, hot water thermostat and cooling water thermostatare started. Hot water is circulated through the generator at a temperature higher than that to be maintained in thegenerator. Cooling water is circulated through the condenser and the absorber in series circuit. Cooling load
simulator is operated by circulating water through the evaporator. Water temperature in the chilled water tank is
maintained constant by switching on heaters equivalent to cooling capacity of system. Solution pump is then startedto circulate strong solution through generator. Level of weak solution collected in the gas separator, level of strong
solution in the absorber storage tank and pressure at salient locations of solution loop are monitored continuously.
When pressure in the gas separator becomes greater than that in condenser, the valve between them is opened to
allow refrigerant vapour to get condensed in condenser. The condensed liquid refrigerant is collected in the receiverand its level is monitored continuously. When sufficient amount of refrigerant is stored, the valve between
evaporator and absorber is opened and the liquid refrigerant is allowed though expansion devices to evaporator from
where the refrigerant vapour is sent to absorber. Flow rates of weak solution and liquid refrigerant are regulated to
maintain steady flow in the system. System is run continuously by monitoring pressure transducer, thermocouple,
flow meter and level gauge readings at various locations. When all these readings remain constant over a time period,it is presumed that system has attained steady state operating conditions and all these readings are recorded in the
computer. Water flow rates in the hot water thermostat, cooling water thermostat and cooling load simulator aremaintained constant at the design value. Experimental tests are repeated for different operating conditions. While
shutting down the system after experimentation, solution loop and refrigerant loop are isolated by closing the valve
between the evaporator and absorber and then closing the valve between the gas separator and condenser.
4. RESULTS AND DISCUSSION
Experimentation has been carried out on compact generator of VARS with a cooling capacity of 1kW by varying theoperating parameters viz., liquid refrigerant flow rate from 0.002 to 0.015 m3hr-1, solution flow rate from 0.02 to
0.05 m3hr
-1, hot water temperature from 85 to 97C, cooling water temperature from 15 to 30C. Parametric studies
were carried out from the following range of operating parameters: Generator temperature: 80-95 C, Generator
pressure: 600-1000 kPa, Solution initial concentration: 0.59-0.75 kgkg
-1
. Cooling water has been supplied toabsorber and condenser in series arrangement as shown in Fig. 1. In every run of the experiment, system is allowed
till it attains steady state condition, when the performance of compact generator is determined. Influence of solutionflow rate, generator temperature and pressure on desorption in compact generator is presented.
4.1 Effect of generator temperatureFigure 4 shows effect of flow rate of strong solution and generator temperature on heat transfer coefficient and mass
transfer coefficient during desorption process. As the refrigerant content in solution increases with flow rate, more
heat is utilized at higher desorption rates.
Figure 4:Effect of flow rate on heat and mass transfer Figure 5:Effect of flow rate on heat transfer and
coefficients at different generator desorption rates at different generatortemperatures temperatures
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0.1
0.21
0.32
0.43
75 80 85 90 95 100
Generator temperature,oC
Conc.
differenceacrossgenerato
r,
kgkg-1
Solution flow rate = 45 lph
Solution flow rate = 35 lph
Solution flow rate = 29 lph
Solution initial concentration = 0.68 kgkg-1
Solution inlet temperature = 65oC
Generator pressure = 745 kPa
Hot water flow rate = 200 lph
Condenser temperature = 20oC
Absorber temperature = 29oC
Figure 6:Effect of flow rate on exit quality and Figure 7: Effect of generator temperature on concentration
overall heat transfer coefficient at different difference across generator
generator temperatures
Heat transfer rate also increases with increase in generator temperature resulting in increased desorption of R134a
vapour. Thus, increase in heat transfer rate and mass transfer rate results in the increase in heat and mass transfer
coefficients with flow rate and generator temperature.
Variation of heat transfer rate and desorption rate with respect to solution flow rate at different generator
temperatures are shown in Fig. 5. Heat and mass transfer coefficients increases with increase in flow rates andgenerator temperatures, for the reasons explained above. Effect of solution flow rate on quality of refrigerant vapourat the exit of generator and overall heat transfer coefficient, at different generator temperatures, is shown in Fig. 6.
Exit quality and overall heat transfer coefficient increase with the increase in solution flow rate and generator
temperature. At higher generator and flow rates, heat transfer coefficient and mass transfer coefficient increase, for
the reasons explained above, resulting in increase in generation of R134a vapour. This increases the exit refrigerantvapour quality and overall heat transfer coefficient, with solution flow rate and generator temperature. As the
solution exists as two phase mixture, lower quality is measured at the exit of generator, as shown in the figure. Butthis quality is improved by using gas separator after generator and obtained between 97.49 and 99.84 % which iscontributed by refrigerant (R134a) vapour alone with very little traces of liquid absorbent.
2
4
6
8
10
80 85 90 95 100
Generator temperature,oC
Circulationratio
Solution flow rate = 19 lph
Solution flow rate = 29 lph
Solution flow rate = 45 lph
Generator pressure = 745 kPa
Hot water flow rate = 200 lph
Condenser temperature = 20oC
Absorber temperature = 29oC
Solution initial concentration = 0.68 kgkg-1
Solution inlet temperature = 65oC
Figure 8: Effect of generator temperature on circulation ratio
Effect of generator performance on the performance of the vapour absorption system with respect to generatortemperature is shown in Figs. 7 and 8. As the desorption rate increases with generator temperature, concentrationdifference between entry and exit of generator also increases. This requires the compact generator to use only less
amount of solution for desorption process to generate unit mass of refrigerant vapour, for the same operating
conditions. Hence, lesser circulation ratio is required at higher generator temperatures as shown in Fig. 8. Because of
lower circulation ratio, requirement of heat flux imposed on the generator will also be lower, resulting in higherCOP for the system, at higher generator temperatures.
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4.2 Effect of generator pressureEffect of generator pressure on heat transfer coefficient and mass transfer coefficient is depicted in Fig. 9. At a givengenerator temperature and solution flow rate, with decrease in generator pressure, there exists a high temperature
gradient between the hot water and the solution, due to the solution entering the generator at a lower equilibriumtemperature. Hence the heat transfer rate increases as the generator pressure decreases, resulting in enhanced massgeneration rate of R134a vapour. Hence increased heat and mass transfer rates result in increased heat transfer
coefficient and mass transfer coefficient respectively, with decrease in generator pressure. Variation of heat transfercoefficient and mass transfer coefficient with respect to solution flow rate is already explained earlier in Fig. 4
Figure 9:Effect of generator pressure on heat and Figure 10:Effect of generator pressure on heat transfer ratemass transfer coefficients and desorption rate
Figure 10 shows the variation of heat transfer rate and desorption rate with generator pressure. As the heat and masstransfer coefficients increase, heat transfer rate and desorption rate also increase at lower generator pressure and at
higher solution flow rate respectively. Figure 11 shows the role of generator pressure on the performance of vapour
absorption refrigeration system. With the increase in generator pressure, desorption rate decreases, for the reasons
explained above, resulting in decrease in the concentration difference of the solution. Figure 12, thus, reveals thathigher flow rate of strong solution is required by the generator to desorb unit mass of refrigerant vapour, resulting in
increase in the circulation ratio at higher generator pressures. Consequently, more amount of heat input is required to
be supplied to the generator for higher circulation ratio, resulting in lower COPs at higher generator pressures, asshown in Fig. 13.
0.15
0.25
0.35
0.45
0.55
550 650 750 850 950
Generator pressure, kPa
Conc.
differenceacrossgenerator,
kgkg-1
Solution flow rate = 45 lph
Solution flow rate = 35 lph
Solution flow rate = 19 lph
Hot water flow rate = 225 lph
Generator temperature = 91oC
0
5
10
15
20
550 650 750 850 950
Generator pressure, kPa
C
irculationratio
Solution flow rate = 19 lph
Solution flow rate = 35 lph
Solution flow rate = 45 lph
Hot water flow rate = 225 lph
Generator temperature = 91 oC
Figure 11:Effect of generator pressure on concentration Figure 12:Effect of generator pressure on circulation
difference across generator ratio
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0.4
0.45
0.5
0.55
0.6
0.65
700 800 900
Generator pressure, kPa
COP
Solution flow rate = 29 lph
Solution flow rate = 35 lph
Solution flow rate = 45 lph
Hot water flow rate = 225 lph
Generator temperature = 91 oC
Figure 13:Effect of generator pressure on COP
5. CONCLUSIONS
Experimental investigations have been carried out on a 1 kW capacity vapour absorption refrigeration system to
study heat and mass transfer during desorption process taking place in compact generator. R134a-DMF is used as
working fluid. Average heat transfer coefficient, volumetric mass transfer coefficient, heat transfer rate, desorption
rate, quality of refrigerant vapour, circulation ratio and COP of the system have been determined from theexperiments. Effect of important operational parameters viz., solution flow rate, generator temperature and generator
pressure on the performance of generator and VARS is studied. Results showed that heat and mass transfercoefficients, heat transfer rate and desorption rate increase with solution flow rate and generator temperature butdecrease with increase in generator pressure. Increase in generator temperature and decrease in generator pressure
requires less circulation ratio of solution and hence give better system COP.
NOMENCLATURE
A heat transfer area (m2) Subscripts
Cp specific heat (Jkg-1K-1) d desorption
CR circulation ratio (-) eq equilibrium
Hfg latent heat of vaporization (Jkg-1
) evap evaporator
h heat transfer coefficient (Wm-2
K-1
) gen generatork thermal conductivity (Wm-1K-1) hw hot water
LMCD log mean concentration difference (kgkg-1) i inlet
LMTD log mean temperature difference (oC) lat latent
m mass flow rate (kgs-1
) o outlet
M mass transfer coefficient (kgm-3
s-1
) ph preheaterQ heat transfer rate (W) s solution
T Temperature (C) sens sensiblet plate thickness (m)
U overall heat transfer coefficient (Wm-2K-1)V channel volume (m
-3)
x vapour quality (-)X liquid concentration (kgkg-1)
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REFERENCES
1. Balamurugan. P and Mani. A., 2012a, Heat and mass transfer studies on compact generator of R134a/DMFvapour absorption refrigeration system, International Journal of Refrigeration, 35 (3), 506-517
2. Balamurugan. P and Mani. A., 2012b, Experimental studies on heat and mass transfer in tubular generatorfor R134a-DMF absorption refrigeration system, International Journal of Thermal Sciences (Under review)
3. Cerezo, J., Bourouis, M., Valles, M., Coronas, A., Best, R., 2009, Experimental study of an ammonia/waterbubble absorber using a plate heat exchanger for absorption refrigeration machines, Appl. Therm. Eng., 29,
1005-1011.
4. Fatouh, M., 1994, Studies on HCFC based vapour absorption refrigeration systems suitable for lowpotential heat sources, PhD thesis, Indian Institute of Technology Madras, India.
5. Kang, Y.T., Christensen, R.N., Kashiwagi, T., 1998, Ammonia/water bubble absorber with a plate heatexchanger, ASHRAE Trans, 104, 1-11.
6. Karthikeyan, G., Mani, A., Srinivasa Murthy, S., 1995, Performance of different working fluids in transfer-tank operated vapour absorption refrigeration systems, Renew. Energy., 6(7), 835-842.
7. Lee, K.B. Chung, B.H., Lee, J.C., Lee, C.H., Kim, S.H., 2002, Experimental analysis bubble mode in aplate type absorber, Chemical Eng. Science, 57, 1923-1929.
8. Mani, A., 2009, Studies on compact bubble absorber of the vapour absorption refrigeration system, Areport to Department of Science and Technology, Government of India.
9. Nezu, Y., Hisada, N., Ishiyama, T., Watanabe, K., 2002, Thermodynamic properties of working-fluid pairswith R-134a for absorption refrigeration system. In: Natural Working-Fluids 2002, IIR Gustav Lorentzen
Conf. 5th., China, 446453.10. Reid, R.C., Prausnitz, J.M., Poling, B.E., 1989, The Properties of Gases and Liquids, Fourth ed. McGraw-
Hill Book Company, New York.
11. Roriz, L., Mortal, A., Mendes, L.F., 2004, Study of a plate heat exchanger desorber with a spray column fora small solar powered absorption machine. In: 3
rd International Conference on Heat Powered Cycles,
Cyprus.
12. Sujatha, K.S., Mani, A., Srinivasa Murthy, S., 1997, Analysis of a bubble absorber working with R22 andfive organic absorbents, Heat Mass Trans., 32, 255-259.
13.
Taboas, F., Valles, M., Bouruois, M., Coronas, A., 2010, Flow boiling heat transfer of ammonia/watermixture in a plate heat exchanger, Int. J. Refrig., 33, 695-705.
14. Taboas, F., M. Valles, M. Bouruois, and A. Coronas, 2012, Assessment of boiling heat transfer andpressure drop correlations of ammonia-water mixture in a plate heat exchanger, Int. J. Refrig., 35, 633-644.
15. Yan, Y.-Y., Lin, T.-F., 1999, Evaporation heat transfer and pressure drop of refrigerant R134a in a plateheat exchanger, J. heat transfer, 121, 118-127.
16. Yokozeki, A., 2005, Theoretical performances of various refrigerant-absorbent pairs in a vapour absorptionrefrigeration cycle by the use of equation of state, Appl.Energy, 80, 383-399.
APPENDIX
w hw hw,i hw,oQ m Cp (T T ) (1)w ,i s ,o hw ,o s ,i
hw ,i s ,o
hw ,o s ,i
(T T ) (T T )L M T D
T TlnT T
(2)
QU
(A) (LMTD) (3)
1
av g
hw w all
1 1 th
U h k
(4)
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dmM(V) (LMCD)
(5)eq,s,i s,i s,o eq,s,o
eq,s,i s,i
s,o eq,s,o
(X X ) (X X )LMCD
X Xln
X X
(6)
h ph ,sens ph ,latQ Q Q (7) h,lat s fg ph,oQ m H x (8)
h,o ph ph,sens gen,is fg
1x Q Q x
m H
(9)
gen
gen,o gen,i
s fg
Qx x
m H
(10)
s
d
mCRm
(11)evap
ge n
QCO PQ
(12)