26-08-2014, 03:10 PM
Experimental analysis of a new refrigerant mixture as drop-in replacement project report
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ABSTRACT
An experimental performance study on a vapour compression refrigeration system with the new R290/R600a refrigerant mixture as drop-in replacement was conducted and compared with CFC 1 2 and HFC 1 34a. The vapour compression refrigeration system was initially designed to operate with R 1 2. Experimental results showed that the refrigerant R290/R600a had 19.9% to 50.1% higher refrigerating capacity than R12 and 28.6% to 87.2% than R134a.The refrigerant R134a showed slightly lower refrigerating capacity than R12. The mixture R290/R600a consumed 6.8% to 17.4% more energy than R12. The refrigerant R12 consumed slightly more energy than R134a at higher evaporating temperatures. The coefficient performance of R290/R600a mixture increases from 3.9% to 25.1% than R12 at lower evaporating temperatures and 11.8% to 17.6% at higher evaporating temperatures. The refrigerant R134a showed slightly lower coefficient of performance than R12. The discharge temperature and discharge pressure of the R290/R600a mixture was very close to R12. The R290/R600a (68/32 by wt%) mixture can be considered as a drop-in replacement refrigerant for CFC12 and HFC134a.The refrigeration efficiency of the system were also studied
INTRODUCTION
The refrigerants chlorofluorocarbon (CFCs) and hydro chlorofluorocarbon (HCFCs) both have high ozone depleting potential (ODP) and global warming potential (GWP) and contributes to ozone layer depletion and global warming. Therefore these two refrigerants are required to be replaced with environmentally friendly refrigerants to protect the environment. The hydro fluorocarbon (HFC) refrigerants with zero ozone depletion potential have been recommended as alternatives. R134a is the long-term replacement refrigerant for R12 because of having favourable characteristics such as zero ODP, non flammability, stability and similar vapour pressure as that of R12 [1–3]. The ODP of R134a is zero, but it has a relatively high global warming potential. Many studies are being carried out which are concentrating on the application of environmentally friendly refrigerants in refrigeration systems. The issues of ozone layer depletion and global warming have led to consideration of hydrocarbon refrigerants such as propane, isobutene-butane or hydrocarbon blends as working fluids in refrigeration and air-conditioning systems. Hydrocarbons are design noted as A3 (highly flammable) refrigerants by ASHRAE standard 34, the industry standard for refrigerant classification. The hydrocarbon (HC) as refrigerant has several positive characteristics such as zero ozone depletion potential, very low global warming, non-toxicity, high miscibility with mineral oil, good compatibility with the materials usually employed in refrigerating systems. The main disadvantage of using hydrocarbons as refrigerant is their flammability [4,5]. If safety measures are taken to prevent refrigerant leakage from the system then a flammable refrigerant could be as safe as other refrigerants. Fig. 1 shows the saturated vapour pressure versus temperature for R12, R134a and R290/R600a (68/32 by wt %) mixture
Experimental apparatus
An experimental setup of vapour compression refrigeration system was built up to investigate the performance R12, R134a, R290/R600a (68/32 by wt%) mixture refrigerants. Fig. 2 shows the schematic diagram of the experimental setup. It consisted of two loops; a main loop and a secondary loop. The main loop was composed of compressor, condenser, a filter-drier, refrigerant flow meter, sight glass, expansion valve and evaporator. The compressor was an open, reciprocating type. The rotating speed of the compressor was 855 rpm and its speed could be changed by a variable diameter belt pulley of the electrical motor. The condenser and evaporator are of both copper double tubes. In the double tube condenser, the refrigerant flows through the inner tube while the cooling water flows through the annular space between the inner and outer tubes. In the double tube evaporator the brine solution (calcium chloride/water solution) flows through the inner tube and the refrigerant flows through the annular space between them. For minimizing the heat loss, the outer tube was well insulated. Two sight glasses were incorporated into the system, one in the liquid line at the condenser outlet and another in the vapour line at the evaporator outlet in order to give a visual indication of the refrigerant circulation. The secondary loops were composed of a pump, a flow meter and an electrically heated unit within the insulated tank. One tank was filled with cooling water and circulated through the condenser tubes while the other tank was filled with brine solution and circulated through the evaporator tubes. The hot water coming out of the condenser tube was supplied to a cooling tower and gets cooled. This cooled water again pumped to the cooling water tank through a separate pump
Experimental procedure
The objective of the study was to compare the refrigeration performance of different refrigerants in terms of refrigerating capacity, compressor energy consumption and COP. Rota meters were used to measure the flow rates of the cooling water and brine solution with an accuracy of ±0.05 lpm. The refrigerant rot meter was used to measure the refrigerant flow rate with an accuracy of ±0.0125 kg/min. RTD type thermocouples were used to measure the temperatures with an accuracy of ±0.1 ◦C and pressures were measured using calibrated pressure gauges with an accuracy of ±1 psi. The thermocouples were located in the pockets on the surface of the tubes and each sensor was calibrated to reduce experimental uncertainties. The range and accuracy of equipment used in the experimental test setup are summarized in Table 1. The temperatures and pressures of the refrigerant and secondary fluid temperatures were measured at various locations in the experimental setup as shown in Fig.
Results and discussion
The experimental results obtained from the performance analysis of R12, R134a, R290/R600a (68/32 by wt%) are discussed with respect to the parameters such as refrigerating capacity, compressor energy consumption, COP and refrigerationRefrigerating capacity:- the variations of refrigerating capacity against evaporating temperature for condensing temperatures of 35 ◦C and 45 ◦C. It was observed that the refrigerant mixture R290/ R600a (68/32) had the highest refrigerating capacity than R12 and R134a.
Compressor energy consumption:- the energy consumed by the compressor increases as the evaporating and condensing temperature increases. Test results showed that the energy consumed by the system with R290/R600a (68/32) mixture was higher by 6.8%–17.4% than R12 and 8.9%–20% higher than R134a for all the operating conditions. The energy consumed by the system with R134a was slightly lower than R12 at higher evaporating temperatures above −10 ◦C. At lower evaporating temperatures below −10 ◦C both R12 and R134a consumed nearly the same energy.
Conclusions
A performance analysis on a vapour compression refrigeration system with the new refrigerant blend as substitute for CFC12 and HFC134a was made and the following conclusions were drawn.
• Refrigerating capacity of R290/R600a (68/32 by wt%) mixture was higher in the range 19.9-50.1% in the lower evaporating temperatures and 21.2–28.5% in the higher evaporating temperatures than R12.
• Refrigerating capacity of R290/R600a (68/32 by wt%) mixture was higher in the range 28.6–87.2% in the lower evaporating temperatures and 30.7–41.3% in the higher evaporating temperatures than R134a.
• Energy consumption of R290/R600a (68/32 by wt%) mixture was higher in the range 6.8–17.4% than R12 and 8.9– 20% than R134a.
• COP of R290/R600a (68/32 by wt%) mixture was higher in the range 3.9–25.1% in the lower evaporating temperatures and 11.8–17.6% higher in the higher evaporating temperatures than R12.
• The refrigeration efficiency of the system increases with the increase in condensing and evaporating temperature.
• The discharge temperature and discharge pressure of R290/ R600a (68/32 by wt%) mixture was nearly equal to those of R12 and R134a.