Introduction
A refrigerator is an electrical device used in temperature maintenance below ambient temperatures. To achieve this function, the refrigerator should be able to create a heat sink at cold room temperatures a temperature that should be lower than the ambient temperature while at the same time also rejecting heats having higher temperatures than the ambient temperatures. To achieve this refrigerant is forced to undergo evaporation at temperatures lower than the ambient temperature(Griffiths, 1937). When the refrigerant is compressed at high pressure and condensing temperatures above the ambient temperature results in the loss of heat and condensation back to the liquid in the process the vaporized refrigerant is reclaimed, thus maintaining a continuous cycle.
Objectives
The objectives of this laboratory experiment include plotting of actual refrigeration cycle on a pressure specific enthalpy chat. Further the experiment also aimed at determining the chart, system and overall coefficient of performance of the refrigeration. Lastly to determine the isentropic and volumetric efficiency of the compressor under study (Griffiths, 1937).
Procedure
The experiment was conducted using Hilton R715 having twin cylinder compressors made up of R134a refrigerant as the working fluid. The application of this refrigerant was aimed to replaceR12 (CFC) that had been in use in domestic and commercial refrigerators and air conditioning systems but was prohibited due to its effects on the ozone layer.
There is heat loss from the refrigerant to the water that is directed to pass through the condenser. Heat is supplied by an electrical heating element that is usually wrapped in the evaporator tube and insulated; the vaporizing liquid absorbs the supplied heat in the evaporator tube. A thermostatic expansion valve is used to equip the unit. Measurement of temperatures and pressure flows are done at appropriate points as can be seen from the fig 1. Sufficient time was given to stabilize all the readings; the readings were therefore taken using the log sheet that was provided.
Figure.1: The Ideal Vapor Compression Refrigeration Cycle
For simplification of the calculations the following assumptions are made; steady operation conditions. Minimal or no change in the kinetic and potential energies in the whole system. No frictional pressure that may cause drops in the velocity of the moving fluids. A steady and constant flow of the fluids across the compressor and the refrigerator. Lastly the flow should be adiabatic in order to fulfill the theoretical conditions desired.
The post ceding calculations were done using the ideal vapor-compression refrigeration cycle with the following considerations made. The heat transfer that occurs between the condenser and the surrounding air was estimated from Newton’s law of cooling as;
Qcondensor = 0.8(Tair – Tcondensor)
The direction of the heat flow is solely dicated by the temperature difference between the condenser and the surrounding air. Estimation of the heat transfer that occurs between the surrounding and the evaporator was also done from Newton’s law of cooling.
Qevaporator = 0.8(Tair – Tevaporator)
The direction of heat flow was dictated by the temperature difference that exists between the evaporator and the surrounding air. The coefficient of performance can thus be expressed as;
COP=BenfitCost
As a refrigerator the system results into heat gain while as a heat pump there is release of heat from the condenser. The cost can thus be calculated as;
Power = Voltage* Current*power factor – losses
A first law energy balance is done first before the heat gained by the evaporator is calculated. The concept applied in the calculation works on the principle that the heat acquired by the refrigerant is the heat lost by the water flowing through the evaporator. Further, it should also be noted that the heat gained by the water that flows through the condenser is the heat lost by the refrigerant. A detailed calculation of the COP’s for different mass flows is done here below, and the results were shown.
Mass flow calculation of the refrigerant occurring through the cycle, requires only the state properties at three different stages are necessary these are state 1, state 3 and state 4. The state properties at state two can, however, be determined through two different approaches, for example, in the first case, the enthalpy at state two can be calculated from the mass flow rate of the refrigerant to achieve energy balance between the condenser and the evaporator. The use of energy balance on the condenser prompts the use of the equation below.
h2=QbMr+h3
Determination of the temperature can be done from the pressure-enthalpy diagram after the determination of the enthalpy for the refrigerant. The other sure way is by determination of the enthalpy at state two by the use of mass flow rate of the refrigerant; from the energy balance it is possible to write the following equation:
h2=WCompressorMr+h3
After the determination of the enthalpy, there is determination of the temperature from the pressure-enthalpy diagram for the refrigerant. It should however be noted that this method is not quite accurate because of the varying mass flow rates and generalized efficiencies.
Results and calculations
Two different methods were used in the calculation of properties of state 2. The results were then tabulated as shown in the tables below;
The following data was measured from the experimental set-ups; each cylinder bore was measured to be 38 mm, while the stroke of each cylinder was measured to be 19 mm. Each torque arm was measured to be 165 mm and the speed to motor to compressor ratio calculated to be 1:98:1.
Discussion
Some variables were noticed in the refrigeration cycle observed in the lab; the variables resulted into deviations when the COP was being calculated from the experimental data. The variation observed was attributed to so many factors key among them are the errors due to the measuring equipment, the equipment’s set-up and human errors in taking the different gauges (Griffiths, 1937). The calculations were done using the experimental results; the experimental results were compared to the theoretical and ideal results.
Different types of equipment were used in measuring various aspects in the laboratory, the equipment’s used could have resulted in the errors noticed. Below are some of these equipment’s that were used. Temperature measurements were taken with a thermometer, the pressure of the R141b was taken through a Bourdon gauges (Griffiths, 1937). The current supplied to the motor was measured using the ammeter while the voltage across the motor was measured using a voltmeter. Lastly water flow in the evaporator and the condenser were monitored by the use of a flow meter.
The use of a thermometer in measuring of temperature realized some problems according to the set-up. The position at which the thermometer was placed in the refrigerant was above the refrigerant; it, therefore, means that the measured temperature is the average temperature of the refrigerant and the surrounding air. The allowance left for pulling and inserting the thermometer may act as a source of heat escape thus resulting into unprecedented cooling or heating thus interfering with the recorded measurements (Vasta, Freni, Sapienza, Costa, & Restuccia, 2012). This problem can, however, be mitigated against through the application of a non-removable thermometer that will not allow for such cooling allowances as the reading can be taken directly by not removing the thermometer.
The calculated COP is directly influenced by the value of the varying temperature measured. In calculating the refrigerant values for states 1 and three, the table values are read based on the pressure but not the temperature.
Conclusion
The main objectives outlined in the introduction of this report were achieved. The pressure, enthalpy, and temperature were determined and the experimental values compared to the theoretical values at the four different states of the refrigeration cycle (Wasternack, 2014). The COP was then determined from the power utilized by the compressors and the state point properties that were also determined from above. The work input to the compressor can be used in determining the COP.
The experimental results differed from the theoretical results. The difference in these results was attributed to the assumptions and the errors that have been discussed above (Zhang et al., 2015). For example, the isentropic assumption of a compressor (Vasta, Freni, Sapienza, Costa, & Restuccia, 2012).
Most errors were as a result of the measuring equipment’s accuracy and the difference in the methods used in obtaining the measured values (Wasternack, 2014). An ideal refrigeration cycle assumes a constant pressure.
References
Vasta, S., Freni, A., Sapienza, A., Costa, F., & Restuccia, G. (2012). Development and lab-test of a mobile adsorption air-conditioner. International Journal Of Refrigeration, 35(3), 701-708. http://dx.doi.org/10.1016/j.ijrefrig.2011.03.013
Zhang, Y., Zhang, L., Zheng, Q., Zheng, X., Li, M., Du, J., & Yan, A. (2015). Enhanced magnetic refrigeration properties in Mn-rich Ni-Mn-Sn ribbons by optimal annealing. Sci. Rep., 5, 11010. http://dx.doi.org/10.1038/srep11010
Wasternack, C. (2014). Perception, signaling and cross-talk of jasmonates and the seminal contributions of the Daoxin Xie’s lab and the Chuanyou Li’s lab. Plant Cell Rep, 33(5), 707-718. http://dx.doi.org/10.1007/s00299-014-1608-5
Griffiths, E. (1937). Refrigeration. Rep. Prog. Phys., 4(1), 124-133. http://dx.doi.org/10.1088/0034-4885/4/1/308
