07_chapter2.pdf

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CHAPTER 2 LITERATURE REVIEW

2.1

INTRODUCTION The suitable and alternate working fluids for refrigeration systems,

heat pumps and air conditioners have gained considerable attention due to growing environmental awareness. The unsafe fluids are banned in response to the environmental issues discussed in Chapter 1 and new environment friendly alternatives are being developed. New approaches are being developed in design, maintenance and servicing of refrigeration equipments. As per Montreal Protocol, HCFC22, the generally accepted and most suitable refrigerant for air conditioners must be phased out by 2030 by developed countries and by 2040 by developing countries because of its Ozone Depleting Potential (UNEP 2000). Even if the control measures of the Montreal Protocol are implemented by all nations, the atmospheric abundance of chlorine will at least double during the next few decades (Kurylo 2003). To return the ozone layer to its natural state will require very strong measures, including a complete phase out of all fully halogenated CFCs, halons, carbon tetrachloride and methyl chloroform, as well as careful use of HCFC substitutes. There are concerns that continuous use of HCFC22 will lead to increased levels of stratospheric chlorine and to more significant ozone depletion (Cavallini 1995). Due to contribution of HCFC22 to the ozone depletion, substitutes for HCFC22 have been developed (Hughes 1994). The phasing out of ozone depleting refrigerants has led to the quest for

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eco-friendly alternative refrigerants. Refrigerant mixtures have been introduced to achieve acceptable properties reasonably well matched to those of the fluids to be replaced. The demand for air conditioners increases day by day as the technological revolution based on information technology and globalization impact upon the societies and people (Paul Spoonley 2001). In the modern world, refrigerators and air conditioners installed in homes, business and industry are the leading users of electric energy. As the fossil fuel reserves are getting depleted at a fast rate, it is an urgent need to improve the energy efficiency of vapour compression system as it is most widely used in majority of modern cooling equipments. Preliminary survey revealed that one of the methods to improve the efficiency of these systems is to provide internal heat exchanger for the full use of evaporator, higher cooling capacity and better dehumidification (Meyer and Wood 2001). With the above objectives in mind a detailed literature survey has been carried out to assess the ongoing research in the area of alternate refrigerants and the possibility of adopting internal heat exchanger in small capacity window air conditioners. 2.2

REVIEWS ON ALTERNATE REFRIGERNTS The environmental problems by CFCs and HCFCs have been

encouraging engineers and researchers to develop new alternate refrigerants with highly efficient machines (Kurylo 1993).

Earlier

investigators

like

Hwang et al (1997) have reported that there are no pure alternative refrigerant for HCFC22. Tashtough et al (2002), Jansen and Engels (1995) and Joseph Sekhar et al (2003) have studied the aspects of new alternate refrigerants to domestic refrigeration systems. Similar studies have been reported on R22 replacement by Green (1995) and Hwang et al (1995) in the earlier stage.

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During the process of selecting potential alternate refrigerant mixtures to replace R22, certain screening approaches have been followed. In such an attempt Vineyard et al (1989), applied criteria like toxicity, instability, ozone depletion potential, flammability, boiling point and commercial availability to more than 200 pure compounds that would make up the components of zeotropic mixtures. Studies have been reported in the literature on alternatives to R22. Various alternatives have been proposed and tested in an effort to comply with the Montreal Protocol. Among a number of refrigerants assessed as potential replacements for HCFC22, the most promising alternative refrigerants that emerged were R410A and R407C. Among these two, R410A is a near azeotropic mixture with a gliding temperature difference of less than 0.2 oC. Its vapour pressure is roughly 50% higher than that of HCFC22 and hence the capacity increases significantly. The better vapour compression system performance of R410A is attributed to high evaporation heat transfer coefficients and compressor efficiency and low system pressure drop (Devotta et al 2001). Due to high pressure, compressors need to be redesigned completely and also the heat exchangers need to be optimized to accommodate lower volumetric flow rates associated with the use of R410A (Dongsoo et al 2000). Studies conducted by Chin and Spatz (1999) also reveal that R410A seems to be the leading long term candidate for the new residential and light commercial equipment. They also noted that the performance of R410A is higher than R22 when the ambient temperature is lower than 35 oC. Due to the compressor efficiency degradation, the performance of R410A is inferior to that of R22 at higher ambient temperatures. On the other hand, R407C is a non azeotropic refrigerant mixture whose gliding temperature difference is roughly 6 oC (Dongsoo et al 2000). Its

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vapour pressure is similar to that of R22 and hence it is expected that R407C may be used in existing equipment without having major changes. At present it seems that R410A can be adopted in new systems while R407C are used in the existing systems. HFC407C is a close match to HCFC22 in existing equipment with respect to energy efficiency and other performance parameters such as compressor discharge temperature and pressure. Studies by Aprea et al (2004) show that the performance of HFC407C is superior to blends like HFC507 and HFC417A. Heat transfer behavior of R407C has been described by Bivens et al (1994). The performances of some new refrigerant mixtures like R32/125/152a, R125/290, R32/125/290 have been theoretically and experimentally investigated by Yang Zhao et al (1999) under varying working conditions and they can be suitable replacements for R22. The evaporator temperature range considered is -35 to 10oC and condenser temperature range from 30° to 60 oC. The performances of the R32/125/152a mixtures are close to that of R22 under all range of operating conditions. Devotta et al 2001 assessed the suitability of some selected fluids as alternatives

to

R22

for

air conditioners,

using NIST CYCLE_D

thermodynamic analysis. The refrigerants studied are R134a, HC290, R407C, R410A and three blends of R32, R134a and R125. Evaporator and condenser temperatures were fixed as 7.2 oC and 55 oC respectively, neglecting the pressure drops. Pressure ratio of R407C is higher than R22 for the range of evaporator temperatures due to higher discharge pressure. They also have reported that discharge temperature of R407C is considerably lower than R22. Specific compressor displacement of R407C is same as that of R22. Hence it may be possible to retrofit R22 compressors with R407C. Table 2.1 shows the summary of derived thermodynamic data of the refrigerants studied by

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Devotta et al 2001 relative to R22 and the variations in COP of the refrigerants are shown in Figure 2.1. 3.5 3.0

COP

2.5 2.0 R410

1.5

R407C

1.0

HCFC22

0.5

HFC134a

0.0 -5

0

5

10

15

o

Evaporating Temperature C

Figure 2.1

Variation

of COP

with

evaporator temperature

at

o

condenser temperature of 55 C (Devotta et al 2001)

Table 2.1 Summary of Derived Thermodynamic Data (Devotta et al 2001) % Relative to HCFC-22 Refrigerant

COP

Cooling Pressure Compressor Discharge power Capacity ratio Pressure

R407-C

- 1.76

1.72

6.60

1.75

7.67

R-410A

-8.90

41.21

-2.29

9.81

55.60

HFC-134a

4.40

-33.00

13.75

-4.27

-31.45

HC-290

1.00

-14.13

-6.87

-1.00

-12.42

HFC-32/HFC-34a

1.00

-1.00

8.80

-1.00

1.30

-8.32

45.40

2.00

8.30

57.72

0.34

2.93

7.44

0.35

6.71

(30/70 by wt%) HFC-32/HFC-125 (60/40 by wt%) HFC-32/HFC-125/HFC134a (30/10/60 by wt%)

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The performance of nine R22 alternatives have been studied by Pannock and Didion (1991) and Domanski and Didion (1993), using Cycle-11. Sagia (2001) developed an algorithm on the basis of heat and thermodynamics theory to define the blend with the most favorable composition, as an environmentally acceptable solution for R22 replacement. Judge et al 1997 have developed a heat exchanger simulation for transient and steady state mixtures and pure components. Simulation is focused on air to refrigerant condensers and evaporators found in residential heat pumps to quantify the effects of using a zeotropic mixture R407C, with cross, parallel and counterflow heat exchangers. According to this study whenR407C is used with a pure counterflow heat exchanger, the capacity can be improved by 4.4% for typical heat pump operating conditions. Experimental comparison between R22 and R407C has been done by Aprea et al 2003, to evaluate the compressor performance in a purpose built rig using semi-hermetic compressor. COP as a function of outlet water temperature at the condenser shows that the overall energetic performance of R22 is always better than that of R407C. The difference is in the range 8% 14%. This study also shows that R407C has smaller actual volumetric efficiency due to smaller mass flow rate and lower isentropic compressor efficiency than that pertaining to R22. Experimental analysis has been carried out by Devotta et al 2005 on 1.5 TR window air conditioner retrofitted with R407C shows a lower COP compared to R22 (Figure 2.2) under various operating conditions. Lower COP is due to the reduced cooling capacity and increased power consumption with R407C. They have also found that the pressure drop reduces both in evaporator and condenser when the working fluid is R407C (Figure 2.3). The decrease in pressure drop may be attributed to the lower R407C mass flow rate (Navarro et al 2005).

12 3.0 R407C

2.5

R22

2.0 1.5 1.0 0.5 0.0 DT

ETA

ETB

Operating conditions

Figure 2.2

Comparison of COP of R22 and R407C at various operating conditions (Devotta et al 2005)

Pressure drop (kPa)

140 HCFC-22 Evaporator

120

R407C Evaporator

100

HCFC 22 Condenser

80

R407C Condenser

60 40 20 0 DT

ETA

ETB

Operating conditions

Figure 2.3

Comparison of evaporator and condenser pressure drops of R22 and R407C at various operating conditions (Devotta et al 2005)

It is perhaps not surprising that the search for alternatives has concentrated on chemically similar compounds as CFCs and HCFCs which could be used as substitutes. An alternative solution, which

has until been

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largely overlooked, may be provided by hydrocarbons, which offers the possibility of a cheap, readily available and environmentally acceptable alternatives to CFCs and CFC substitutes. The absence of chlorine results in zero ozone depletion potential. The global warming potentials are also very low. Major hydrocarbons under consideration are propane, isobutene, nbutane, perfluorocyclobutane, cyclopropane and propylene. Among these refrigerants, R290 is considered as a replacement for R22 and R502. These natural refrigerants are environmentally friendly, non toxic, chemically stable, compatible with many materials and miscible with mineral oils. Besides this the zeotropic refrigerant mixtures of hydrocarbon refrigerants have potentials to enhance the performance and efficiency of a system due to their temperature gliding effect (Chang et al 2000). The only major problem is the flammability of hydrocarbon refrigerants. Experimental study

on the performance of hydrocarbon

refrigerants, namely propane and LPG mix as a suitable replacement for the widely used refrigerant R22 in refrigeration and heat pump applications conducted by Purkayastha et al 1998

revealed that the hydrocarbon

refrigerants performed better than R22 but with a small loss of condenser capacity. The mass flow rate and compressor discharge temperature have been found to be significantly lower than R22. Chang et al 2000 also have experimentally investigated the performance of a heat pump system using hydro carbon refrigerants. Propane, isobutane, butane and propylene, the single component hydro carbons and Propane / isobutane and propane / butane are the binary mixtures. The cooling and heating capacities of R 290 are found slightly smaller than those of R22 with slightly higher COP than that of R22. The capacity and COP of R1270 are slightly greater than R22, which is a good indication for R1270 to be the possible alternative for conditioning and heat pump applications. When

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zeotropic mixtures of R290 / R600a and R290 / R600 are used, the cooling and heating capacities increase almost linearly with respect to mass fraction of R290. COPs of the mixtures are found to be higher than linearly interpolated values based on those of pure components. COP of hydro carbon mixtures for the cooling condition is higher than that of R22 for a wide range of composition of mixtures. Propane and mixtures as alternate refrigerant has also been reported by various investigators like Treadwell (1994), Halozen et al (1994), Richardson and Butterworth (1995), Kim et al (1994) and Richardson and Ritter (1996). Lee et al 2002, Alok and Agarwal (1998), Ritter (1996), Lee and Su (2002) and Horst and Florian (1997) also have reported studies on HC. The major set back reported with HCs is the flammability. James and Missenden (1992) claimed that in case of the household refrigerators, the possibility of explosion by flammability can be negligible because half the amount of HCs can be charged compared to general CFC. Also, some simple safety devices such as a ventilation system and a leak detector can be installed to overcome the flammability problem in large sized air conditioning systems. Extensive studies on flammability of hydrocarbons have been conducted by Richard and Shanland (1992). Studies on the use of hydrocarbon in refrigeration system have also been done by various investigators including Chen et al (1994), Colbourne (2000) and Jung (1996). 2.3

REVIEWS ON INTERNAL HEAT EXCHANGER As vapour compression cycle is widely used in modern cooling

equipment, the energy efficiency improvement of this cycle draws more attention. Most residential and mobile air conditioning and refrigeration systems are direct expansion units and have protection against liquid slugging in the compressor by utilizing about 60%-90% of the evaporator capacity for cooling and the remaining for superheating the refrigerant. This superheating

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portion of the evaporator provides only little contribution to the total cooling capability. This practice also results in excessive evaporator volume. To provide higher cooling capacity, liquid overfeeding operation (LOF) used in larger systems, has been suggested to small air conditioning systems in recent years. This is achieved by using a heat exchanger referred as internal heat exchanger. Internal heat exchanger patents date back to the 1970s. Since then many forms and variants have been investigated (Meyer et al 2001) Internal heat exchangers may have positive or negative influences on the plant overall efficiency, depending on the working fluids and the operating conditions. According to Navarro et al 2005, the main benefits of liquid line/suction line heat exchanger (internal heat exchanger) are: increasing refrigerating effect at the evaporator, sub cooling liquid refrigerant ensuring liquid phase entrance to the expansion device and minimizing the risk of liquid refrigerant presence at the compressor inlet. The possible disadvantages are: increasing suction specific volume at the compressor entrance, decreasing the refrigerant mass flow rate which is delivered by the compressor, increasing compressor discharge temperature, increasing pressure drops at the suction and liquid lines and becoming a possible lubricant trap. Boewe et al (1999) and Boewe et al (2001) have analyzed the influence of IHE when using R744 as working fluid in automotive air conditioning systems, where the presence of the IHE has a great influence on the energy efficiency. Mei et al (1994) designed and tested a new liquid overfeeding mobile air conditioning system that can use the evaporator 100% effectively. An accumulator heat exchanger (IHE) has been included in the study and the much subcooled liquid enters the expansion device. In the evaporator, the

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liquid is not fully evaporated and remaining liquid is evaporated in the accumulator heat exchanger by heat transferred from the condenser liquid. The compressor suction line will always have saturated or nearly saturated vapour and therefore a high mass flow rate. The compressor discharge temperature will decrease and thus the compressor power consumption per unit mass flow rate will also decrease. In the mean time, the whole evaporator is used and the system’s cooling capacity is increased. It is estimated that this concept can improve the overall efficiency of mobile air conditioning system by 15% to 20%. At 2020 rpm, the mass flow rate of LOF is about 29% higher than for operation with no LOF feature. The refrigerant side heat transfer coefficient is a function of the mass flow rate and hence a higher mass flow rate means a higher heat transfer coefficient. At 880 rpm LOF has shown 7.6% improvement in cooling capacity and at 2020 rpm 35%. Power consumption has been lowest with LOF (the lower compressor discharge temperature of the LOF reduces the power consumption). Discharge pressure is at the highest for LOF. However the compressor high – low pressure ratios for the LOF are the lowest. Because of the LOF system’s higher cooling capacity, coupled with lower compressor power consumption, COPs of LOF operation are higher. A two TR(EER 10) off the shelf window air conditioner was modified and tested by Mei et al,1996, with and without liquid overfeeding (LOF) feature. A recuperative accumulator heat exchanger has been added and the system charged with an additional 15% of R22 for additional piping. Original components have not been replaced. LOF operation is reported to improve the system cooling capacity and COP. At 28oF, LOF improves the cooling capacity by 14% and COP by 10%. As ambient temperature increases, the improvement decreases. At 43oF, the COP of the LOF becomes equal to, or less than, that of the baseline unit. LOF has a lower compressor pressure ratio, lower compressor discharge temperature, higher refrigerant

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mass flow rate, slightly higher power consumption and slightly higher suction pressure. Bivens et al 1997 has tested heat pumps and water chillers with accumulators (IHE). Commercial split type heat pump of capacity 8.8 kW with scroll compressor, fin and tube heat exchangers, suction line accumulator and thermostatic expansion valve have been used and the capacity of R407C is found to be increased by 0.98% - 1.09% and COP by 0.94% - 0.97%, compared to R22. Further, a water chiller of capacity 528 kW with screw compressor, water inside tube shell and tube condenser and evaporator was tested. The weight percentages of components (R23/R125/R134a) have been fixed as 30/10/60. During this test, the capacity with the mixture (R407C) is found to be reduced by 36% compared to operation with R22 due to a reduced mixture heat transfer coefficient in the evaporator. Power required was increased by 14% due to reduced mixture heat transfer coefficient in both the evaporator and condenser (causing lower refrigerant evaporator temperature and pressure and higher condenser pressure). The combined effect was a reduction in COP of 44%. The effects of suction/liquid heat exchanger COP of cycle have been discussed by Aprea et al 1999, from a thermodynamic point of view. A simple criterion for evaluating the suitability of using a suction /liquid heat exchanger is presented and discussed. The criterion has been positively checked for several working fluids such as CFCs, HCFCs and other substitutes. When a 15 oC heating of the refrigerant in the suction / liquid heat exchanger takes place, it was found that there is rise of 2% in COP for R502 and 3% decrease for R32. The application of the criterion to R502 and R22 together with their most credited substitutes has been presented. Graphical approach made by them shows that the thermodynamic advantage of adopting a suction line heat exchanger is more for R22 than R407C.

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Dongsoo et al 2000 have tested 14 refrigerant mixtures composed of R32, R125, R152a, R290 and R1270 in a breadboard heat pump in an attempt to find a substitute for R22 used in residential air conditioners. The capacity of heat pump was 3.5kW and heat transfer fluid has been water both in evaporator and condenser. Suction line heat exchanger (E Stick) of 530mm long, 12.8mm inner diameter and 22.8mm outer diameter have been in this study. It has been reported that R407C has 0.8% more COP than R22 without suction line heat exchanger (Figure 2.4). Also the capacity of R407C is 8% more than that of R22 without suction line heat exchanger (SLHX). Table 2.2 shows the reference numbers of refrigerants for Figure 2.4. For most of the refrigerants tested, COP increases with the addition of suction line heat exchanger (Figure 2.5), showing a maximum of 3.9% for the mixture containing large quantities of R125. This is due to the large specific heat of R125. For R22, the COP and capacity have been found to increase by 2% and 0.8% respectively. For R407C, COP increases by1.5% whereas the capacity decreases by 1.2%. In the application of alternate refrigerants, the lifetime and reliability of the system as well as the stability of the refrigerant and lubricant should be considered. These characteristics can be examined indirectly by measuring the dome and discharge temperatures. The dome temperatures measured during this study for R22 and R407C are found to be 50 oC and 45oC respectively, without suction line heat exchanger. The compressor discharge temperatures with R22 and R407C have been found to be 116 oC and 91oC respectively. With the addition of SLHX, compressor discharge and dome temperatures increase by a range of 4.5 oC – 8.6oC. Still these temperatures are lower than those with R22 without the use of SLHX. Therefore it can be said that even though SLHX is added, there seems to be no problem in the system with the mixtures tested in the study.

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Table 2.2 Refrigerants tested in the study by Dongsoo et al 2000 Ref. Number

Temperature glide (oC)

Refrigerants

1

R22

0.00

2

23% R23/ 25% R125/ 52% R134a (R407C, Du Pont’s AC9000)

6.64

3

26% R32/ 14% R125/ 60% R134a

6.87

4

26% R32/ 20% R125/ 54% R134a

6.74

5

50% R125/ 30% R134a/ 20% R152a

4.64

6

70% R125/ 10% R134a/ 20% R152a

4.85

7

75% R125/ 5% R134a/ 20% R152a

4.77

8

25% R32/ 71.5% R134a/ 3.5% R152a

10.91

9

26%R32/ 60% R134a/ 14% R152a

6.19

10

30% R125/ 60% R134a/ 10% R1270

6.08

11

33% R125/ 55% R134a/ 12% R1270

5.82

12

43.6% R125/ 45.9% R134a/ 10.5% R1270

5.40

13

30% R32/ 70% R134a

6.93

14

26% R32/ 74% R134a

6.77

15

45% R290/ 55% R134a

0.00

Change in COP (%)

10 8 6 4 2 0 -2

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

-4 -6 Refrigerants

Figure 2.4

Effect of using various refrigerants when compared to R22 reported by Dongsoo et al 2000 for systems without SLHX

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Change in COP (%)

5 4 3 2 1 0 -1

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

-2 Refrigerants Figure 2.5

Effect of using SLHX on COP for various refrigerants (Dongsoo et al 2000)

A new dimensionless group has been identified by Klein et al (2000) to correlate the performance impacts attributable to liquid-suction heat exchangers. Also it extends the previous analyses to include new refrigerants. The analysis includes the study of impact of pressure drops through the liquid – suction heat exchanger on the system performance. Refrigerants R507A, R404A, R600, R290, R134a, R407C, R410A, R12, R22, R32 and R717 are the refrigerants investigated. This study specifically considers the effects of pressure drops and it presents general relations for estimating the effect of liquid – suction heat exchangers for any refrigerant. The beneficial effects of a liquid suction heat exchanger are offset by the refrigerant pressure drops that occur in the heat exchanger. A potential increase in the capacity is possible by sub cooling the liquid refrigerant before expansion. Neglecting the reduction in refrigerant mass flow rate, liquid suction heat exchangers lead to performance improvement for any refrigerant. They have reported that liquid suction heat exchangers increase the temperature and reduce the pressure of the refrigerant entering the compressor causing a decrease in the refrigerant density and compressor volumetric efficiency. The potential performance

21

advantage of a liquid suction heat exchanger is reduced due to pressure losses in the heat exchanger. They also report that the cooling of the condensate that occurs on the high pressure side serves to increase the refrigeration capacity and reduce the likelihood of liquid refrigerant flashing prior to reaching the expansion device. On the low pressure side, the liquid –suction heat exchanger increases the temperature of the vapour entering the compressor and reduces the refrigerant pressure, both of which increases the specific volume of the refrigerant and thereby decreases the mass flow rate and capacity. Despite these developments and achievements, there had been no documented mathematical process, model or design procedure that can describe the sizing of accumulators (internal heat exchangers) with respect to their relevant operating systems. Also there had been no evidence of any equations that accurately and sufficiently describe the heat exchange process taking place within the heat exchanger accumulator. Heat exchanger accumulator design seemed to be an experimental trial and error procedure with each design improving with experience gained. In these circumstances, the heat exchange process that takes place within the heat exchanger accumulator (internal heat exchanger) has been studied by Meyer and Wood 2001 and they have developed a mathematical model of a heat exchanger accumulator. This model has been used to develop a universal design procedure to size the heat exchanger according to the operating system into which it has to be installed. The model predicted that for a 10oC increase in evaporator temperature and condenser temperature will cause a 3% (23 mm) increase in the required coil length while 10oC decrease in each caused a 2% (17 mm) decrease in the required coil length. The experimental facility established by them comprised a compressor with a cooling capacity of 3780W and 0.83kg of R22 charge. Experiments were

22

conducted at an evaporator temperature of 7oC and condenser temperature of 50 oC. The experimental results show that the LOF operation increases 0.4% of the condensing pressure. On the other hand, increase in exit temperature has been noted to be very small. 2.1% increase in evaporator pressure indicates a reduction in the work required. It reduces the pressure ratio also by 1.7% resulting in less work and longer compressor life. Better compressor isentropic efficiency is due to the reduced pressure ratio. An increase of 4% in the mass flow rate is attributed to the higher evaporator temperature, lower pressure ratio and increase in compressor isentropic efficiency. As a result, there is a decrement in power consumption by 1%, and increment in cooling capacity by 6.5% (related to the fact that refrigerant is sub cooled in the heat exchanger). COP has also increased by 7.5% (Figure 2.6).

8

Percentage

6 4 2 0 -2

Pc

Pe Pc/Pe

ηi

m

P

Qe

Qc COP

-4 Figure 2.6

Effect of using Internal Heat Exchanger in an R22 system by Meyer and Wood (2001)

Influence of adopting an internal heat exchanger on a single stage vapour compression plant energy performance at different operating conditions has been studied by Navarro et al (2005). The relative variation in refrigerating capacity has been observed to be more for R407C when compared to R22 due to the adoption of IHE (Figure 2.7). The relative COP

23

variation due to adoption of IHE is also more for R407C in lower compression ratios and it is reversed when compression ratio increases (Figure 2.8). It has also been reported by these authors that the effectiveness of IHE remains approximately constant during the experimental tests carried out. 5.0 Relative capacity variation

4.5 4.0 3.5 3.0 2.5 2.0 1.5 R134a

1.0

R22

0.5

R407C

0.0 3.5

4.0

4.5

5.0

5.5

6.0

6.5

Compression ratio with IHE

Relative variation of COP

Figure 2.7

Relative variation in refrigerating capacity by using IHE (Navarro et al 2005)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0

R134a R22 R407C

3.5

4

4.5

5

5.5

6

6.5

Compression ratio with IHE

Figure 2.8 Relative Variation in COP by using IHE (Navarro et al 2005)

24

In general, in small capacity air conditioners and heat pumps, a condition almost similar to liquid overfeeding operation in large system can be achieved by including an IHE. This has been addressed by various authors. Some of them like Meyer and Wood 2001 used the name as accumulator heat exchanger. This can also be achieved by using capillary tube suction line heat exchanger as noted by few authors like Bansal and Rupasinghe (1998), Pate and Tree (1984), Peixoto and Bullard (1994), Meyer and Dunn (1998), Bittle et al (1995) and Bansal et al (1996). Capillary tube suction line heat exchanger is nothing but a capillary tube soldered to the outer surface of the suction line. The capillary tube and the suction line form a counter flow heat exchanger. Dirik et al (1994) and Wolf and Pate (2002) also have studied the aspect of using non-adiabatic capillary tubes in vapour compression systems. Unlike simple liquid suction line heat exchange, the problem in a capillary tube suction line heat exchanger is more complex. Refrigerant may enter the capillary tube in either a two phase or sub cooled state, flash due to friction in the adiabatic inlet section or later in the diabetic segment, and recondense in the diabetic section and flash again in the adiabatic outlet segment. It normally exits into the evaporator in a choked condition, at a vapour quality of approximately 10% as quoted by Yang and Bansal (2005). 2.4

CONCLUSIONS FROM THE LITERATURE SURVEY AND OBJECTIVES OF THE PRESENT RESEARCH WORK From the literature review presented so far, the following are the

major conclusions. (a) Based on studies related to alternate refrigerants: (i)

Refrigeration

and

air

conditioning

indispensable in the modern world.

systems

are

25

(ii) Among various options, vapour compression system occupies over 80% of refrigeration and air conditioning applications. (iii) Man-made CFCs were extensively used in all vapour compression systems upto 1990s. Ozone layer depletion due to the use of CFCs is found to be of major concern. Hence replacements for CFCs were attempted. At present HCFCs are used which also needs replacement due to environmental concerns. (iv) While attempting to find replacement for CFCs and HCFCs, two broad approaches have been tried, viz., alternate refrigerants with totally new refrigeration system design, and retrofitting the existing machines with alternate refrigerants. (v) Considerable studies have been reported for CFC and HCFC replacements with totally new system design and limited studies have been reported on the retrofitting approach. (vi) At present R22 has been widely used in air conditioning systems. The promising alternatives reported out of the various research studies are R410A, R407C and hydrocarbons. (vii) From the studies reported, it appears that R407C is suitable for retrofitting in existing systems and R410A for adopting in new system designs. Flammability of hydrocarbons is said to be of major concern in practical use.

26

(viii) Almost in all studies, R407C is found to be a close match for R22. However, the COPs of retrofitted R407C systems are about 7-15% lower than R22 ones. (b) Based on studies related to the use of IHE (i)

Use of IHE is found to have either positive or negative effect on the performance of VCR system depending upon the refrigerant used, system design and operating conditions.

(ii) Fully flooded

evaporator (evaporator with liquid

overfeeding system) with IHE is found to have better performance in large capacity systems. (iii) In case of small capacity systems, the efficiency improvements are offset by the cost factors. Hence least attempts have been made earlier. In the recent times, few studies have been reported due to increased energy costs and increased awareness on reducing CO2 emissions. (iv) While adding IHE with the existing systems, increased suction vapour superheat is found to be a major negative factor. However, the liquid sub cooling has a positive effect. Net result depends on the particular situation. In considerable number of occasions, 5-10% improvement in COP are reported, Limited studies have been reported in the literature regarding the use of IHE in small capacity systems.

27

2.4.1

Background and Objectives of the Present Work From the above said conclusions, it is felt that R407C can be a

suitable replacement for R22 systems for retrofit considerations. The drop (about 5-15%) in COP reported by replacing R22 with R407C can be offset by judicially using IHE, which has the potential to improve the COP by about 5-10%. If such an approach works, retrofitting of existing R22 systems with R407C and by the addition of IHE, almost similar performance can be obtained. Including IHE with existing system, is found to superheat the suction vapour to a large extent. It is claimed to be a major detrimental factor. Hence it is decided to isolate a portion of evaporator while using IHE. Keeping the above aspects in mind it has been decided to carry out the following in the present research work. (i)

Creating an experimental facility to study the performance of one TR of air conditioning system.

(ii)

With

the

experimental

facility

created,

conducting

performance studies with R22 system without IHE. (iii)

Conducting experiments by retrofitting the same system with R407C (without IHE).

(iv)

Conducting experiments to find out the fraction of evaporated area to be isolated while adding IHE.

(v)

Conducting experiments with R22 and IHE by isolating a predetermined portion of evaporator.

(vi)

Conducting experiments with R407C and IHE by isolating a predetermined portion of evaporator.

28

(vii)

Performing simulation studies with IMST-ART v 3.20 software.

(viii) Presenting and discussing the results to arrive at certain conclusions regarding the background which stimulated this research work. The following chapters present these details.