Annamalai-2012-Heat and Mass Transfer Generator

8/12/2019 Annamalai-2012-Heat and Mass Transfer Generator

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

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 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




Generator pressure = 745 kPa


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





Generator temperature = 91oC

0

5

10

15

20

550 650 750 850 950


C

irculationratio





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


COP





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)

Documents

Annamalai-2012-Heat and Mass Transfer Generator