08-10-2016, 03:27 PM
PERFORMANCE OF VAPOUR COMPRESSION REFRIGERATION SYSTEM WITH SUB COOLING BY USE OF R134a AND R600a REFRIGERANTS
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ABSTRACT
There are various obstacles faced in working of different refrigerants due to their environmental
impact (R 11, R12), toxicity (NH3), Flammability (HC) and high pressure (CO2); which makes them more hazardous than other working fluids according to safety and environmental issues. Performance analysis on a vapour compression refrigeration system with eco-friendly refrigerant
of HC600a were done and their results were compared with R134a as possible alternative replacement. In present work experimental setup is prepared for both existing and proposed system with R-134a and R-600a as refrigerants. In the proposed system suction line heat exchanger of 35cm length is used.
After conducting the experiment,
• In the proposed system with suction line heat exchanger the pull down period is found to be less than the pull down period of existing system. The percentage of decrease in pull down period using R-134a is 1.74%.
• In the proposed system with suction line heat exchanger the pull down period is found to be less than the pull down period of existing system. The percentage of decrease in pull down period using R-600a is 1.76%.
• In the proposed system the coefficient of performance is found to be greater than the coefficient of performance of existing system. The percentage of increase in COP using R-134a in no load condition is 3.27% and in loaded condition is 3.10%.
• In the proposed system the coefficient of performance is found to be greater than the coefficient of performance of existing system. The percentage of increase in COP using R-600a in no load condition is 4.09% and in loaded condition is 3.34%.
• From the above discussions, it can be concluded that the performance of vapour compression refrigeration system of domestic refrigerator can be increased by using 35cm length of a heat exchanger for different refrigerants.
The retrofitting of R600a in all conditions showed better performance than R134a. And also that if care is taken in flammable of R600a retrofitting gives better performance with R600a in domestic R134a systems.
Keywords: Compression refrigeration system, Refrigerant, COP, ODP, GWP
INTRODUCTION
Vapor compression Refrigeration system is an enhanced by air refrigeration system. The capacity of certain liquids to absorb giant quantities of heat as they vaporize is the basis of this system. Compared to melting solids (say ice) to find refrigeration effect, vaporizing liquid refrigerant has more advantages. To mention a few, the refrigerating effect can be started or stopped at will, the rate of cooling can be determined, the vaporizing temperatures can be governed by controlling the pressure at which the liquid vaporizes. Moreover, the vapor can be readily collected and condensed back into liquid state so that same liquid can be re-circulated over and over again to obtain refrigeration effect. Thus the vapor compression system employs a liquid refrigerant which evaporates and condenses readily. The system is a closed once the refrigerant never leaves the system. The coefficient of performance of a refrigeration system is the ratio of refrigerating effect to the compression work; therefore the coefficient of performance can be increased by increasing the refrigerating effect or by decreasing the compression work.
The vapor compression refrigeration system is now-a-days used for all purpose refrigeration. It is generally used for all industrial purposes from a small domestic refrigerator to a big air conditioning plant.
MATERIALS AND METHODS
Selection of Environment-Friendly Refrigerants: The range of possible alternative fluids is extensive; it includes Hydro-Fluorocarbons (HFCs), refrigerant mixtures, hydrocarbons, and natural fluids. Among these groups of alternatives, HFCs are the most useful. The possible environment-friendly refrigerants with zero ODP and lower Global Warming Potential (GWP) could be selected from derivatives of methane and ethane. In this work, full array of methane and ethane derivatives were considered and trade-off in flammability, toxicity and chemical stability concerning atmospheric lifetime with changes in molecular chlorine, fluorine and hydrogen content were carried out. Therefore, two promising alternative refrigerants (R134a and 600a) that contain no chlorine and that have short atmospheric lifetime were selected and investigated theoretically using sub-cooling coil.
Experimental set up
In order to know the performance characteristics of the vapor compression refrigeration system the temperature and pressure gauges are installed at each entry and exit of the components. Experiments are conducted on a domestic refrigerator of 165 liters capacity, with R-134a and R-600a as refrigerants and using liquid line-suction line heat exchanger of 35cm length.
Domestic refrigerator selected for the project has the following specifications
Refrigerant used: R-134a
Capacity of the refrigerator: 165 liters
Compressor capacity: 0.16 H.P
Condenser sizes
Length - 8.5m
Diameter - 6.4mm
Evaporator
Length - 7.62m
Diameter - 6.4mm
Capillary tube
Length - 2.428m
Diameter - 0.8mm
Among many possible variations of the basic refrigeration (vapor compression) cycle, the cycle with the liquid line/suction line heat exchanger (LLSL•HX) is probably used most often. As a result of employing this intra-cycle heat exchange, the high pressure refrigerant is sub cooled at the expense of superheating the vapor entering the compressor. Schematics of hardware arrangement for the basic cycle and cycle with the LLSL-HX are shown in Figure 1.
The use of liquid line/suction line heat exchangers is widespread in commercial refrigeration. The heat exchangers are often employed as a means for protecting system components, by helping to ensure single-phase liquid to the expansion device and single-phase vapor to the compressor. In residential refrigerators, a capillary tube/suction line heat exchanger is used to heat the suction line above the dew-point temperature of ambient air, thus preventing condensation of the water vapor on the outside of suction line.
Employing an intra-cycle heat exchanger alters refrigerant thermodynamic states in the cycle, which may have significant, positive or negative, performance implications. For any fluid and system, a LLSL-HX increases refrigerant temperature at the compressor inlet and outlet, this is shortcoming. The Coefficient of Performance (COP) and volumetric capacity may increase for some fluid/application combinations, while for others they may decrease.
EXPERIMENTAL PROCEDURE
The vapour compression system is initially cleaned and the evacuation of the system is carried out with the help of a vacuum pump for nearly 30 min and then the refrigerant is charged into the system.
Initially the system is charged with refrigerant R-134a and then the following tests were carried out.
1. Pull–down characteristics
2. No load performance
3. Performance with load
4. Frosting
1. For Pull-Down Characteristic
The pull-down period is the time required to reduce the air temperature inside the refrigerator from ambient condition to the desired cabin air temperature (i.e., +70C average cabinet temperature) after switching on the unit.
Procedure:
• Initially the refrigerator door is kept open until the evaporator cabin attains the environment temperature and then it is closed and system is switched ON to run.
• System is kept for running condition until required temperature is attained.
• After attaining required temperature the system is switched OFF.
• Time taken by the system to get to required temperature is noted.
• Energy meter readings also noted from starting to off conditions.
2. No Load Performance
In no load performance as the refrigerator is switched on and kept in running condition without placing any type of load inside the cabin until the steady state conditions are attained and then the readings are noted for calculating the COP of the system and system is switched off.
Procedure:
• Initially the system is switched ON.
• The system is kept in running condition continuously to obtain steady state conditions.
• After attaining steady state conditions the pressure gauge and temperature readings are noted.
• Energy meter readings also noted down as shown in table.8 and then system is switched OFF.
• COP calculations are made from the obtained values using p-h chart of the refrigerants.
3. Performance With Load
In load performance as shown in figure 4 the refrigerator is loaded by arranging a bulb having capacity of 60 watts inside the evaporator cabin and the system is switched on and kept in running condition until the steady state conditions are attained and then the readings are noted for calculating the COP of the system and system is switched off.
Procedure:
• Initially the system is switched ON.
• After some time a 60W capacity incandescent bulb is placed inside the evaporator cabin and switched on for loading the system.
• The system is kept in running condition continuously for obtaining steady state conditions.
• After attaining steady state conditions the pressure gauge and temperature readings are noted and bulb is switched off.
• Energy meter readings also noted down as shown in table.8 and then system is switched OFF.
• COP calculations are made from the obtained values using p-h chart of the refrigerants.
4. Frosting And Defrosting
For occurrence of frosting the system is switched on with a load of water in open trough is placed in the evaporator cabin and kept in continuous running condition for 72 hours and then the system is switched off. The system is allowed to defrost for certain period and the quantity of water is collected.
Procedure:
• Initially the load of 3litres of water is placed inside the cabin as shown in figure 5 in an open trough and then the system is switched ON.
• The system is kept in continuous running condition for 72 hours to allow for frosting of freezer as shown in figure 5.
• Then the system is switched OFF and allowed to defrost for some period.
• The defrosted water is collected.
• The quantity of water collected is noted down.