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The Factors influencing the coefficient of performance of the refrigeration are:
- Subcooling of liquids;
- Superheating of vapor (includes three types of it – constant, increase or decrease);
- Change in suction pressure;
- Change in discharge pressure and superheating refrigerant;
- Effect of volumetric efficiency of the compressor.
Superheating (relating to superheating of the refrigerant vapor leaving evaporator) and subcooling (relating to subcooling of refrigerant liquid leaving the condenser) are two notable means in practical vapor-compression refrigerating systems and are implemented to provide better efficiency (COP) and avoid some technical problems. Studies had shown that the subcooling and superheating applications cause an increase of around 10% in the system performances. Plus, depending on the choice of refrigerants, an effective subcooling and superheating application causes a saving up of up to 10% in the energy consumption. What is subcooling and superheating system?
During the evaporation process, the refrigerant is vaporized partway through the evaporator. As the cool refrigerant vapor proceeds through the evaporator, extra heat is received to superheat the vapor. Under some conditions, such pressure loses affected by friction raise the amount of superheat. The enthalpy of the refrigerant is raised, if the superheating takes place in the evaporator, delivering extra heat and increasing the refrigeration impact of the evaporator. If it is provided in the compressor suction piping, no valuable cooling occurs.
Superheating in the evaporator which obtains the increase in the refrigeration effect is usually offset by a decrease in the refrigeration effect in the compressor. Because the volumetric flow rate of a compressor is continuous, the mass flow rate and the refrigeration effect are reduced by reductions in the refrigerant density caused by the superheating. In practice, it is well known that there is a loss of the refrigerating capacity of 1% for every 2.5C of superheating in the suction line. The lining of the suction lines is a solution to minimize undesirable heat accumulation.
The cooling process is when cooling proceeds, the refrigerant liquid below its condensing temperature at a given pressure. To enter the expansion device, subcooling provides 100% refrigerant liquid, preventing vapor bubbles from impeding the flow of refrigerant through the expansion valve. If the subcooling is caused by a heat-transfer method external to the refrigeration cycle, the refrigerant effect of the system is increased because the subcooled liquid has less enthalpy than the saturated liquid. Accomplished subcooling is by refrigerating the liquid line of the system, using a higher temperature system. Simply it can be stated that subcooling cools the refrigerant more and provides the following accordingly:
- Increases energy loading;
- Decreases electrical usage;
- Reduces pulldown time;
- More uniform refrigerating temperatures;
- Reduction in the initial cost.
Subcooling effectively improves the system performance. For example, subcooling of R-22 by 13C increases the refrigeration effect by about 11%. If subcooling is obtained from outside the cycle, each degree increment in subcooling will improve the system capacity (approximately by 1%).
The first step included the determination of the mass flow rate of the refrigeration cycle to get steady values of results. After that, the values of temperature were recorded for the different points corresponding to the PH chart. The temperature index is plotted in the PH chart to determine the values of enthalpy for the respective points. The ideal values of COP are compared to the actual values, noting the significance.
The implemented experiment obtained a true value of COP – 4.875. By the PH chart, the ideal value of COP designates 4.81. The work of the compressor is highly productive with superheating and is greater than the ideal cycle. Additionally, the superheating increases the effect of COP of the refrigeration cycle, as well as in the specific work compressor. The last thing to be mentioned, the more effort is needed to superheat a refrigerant by increasing the evaporator load, to remove its liquid, in greater extent the power input of the compressor would increase.
The obtained value of COP of the refrigeration cycle was 4.875. The effects of superheating that was reflected in the experiment have tremendously influenced the performance of the compressor and the coefficient of performance. The work of compressor and COP values tend to increase when the system incurs the superheating. A refrigeration cycle needs the superheating because it eliminates wet compression which can damage the compressor. The heating of suction piping and a large evaporator load could be a result of superheating of the system. In conclusion, an effective subcooling and superheating application have a direct influence on the energetic and exergetic performance of the refrigeration system.
. Cengel, Y.A., and Boles, 2011, Thermodynamics – An Engineering Approach, McGraw Hill, Chicago.
 Prof. T. Sundararajan and Prof. J.M. Mallikarjuna., Refrigeration Cycles, viewed 15 August 2013, http://nptel.iitm.ac.in/courses/IIT-MADRAS/Applied_Thermodynamics/Module_6/6_Simple_Vapor_Compression_RS.pdf.