ES3B5 valve. Next, the refrigerant enters the filter/dryer

ES3B5 Engines and Heat PumpsSummaryThis vapour compression lab had flaws causing one of the results to give a negative efficiency. Two of the remaining tests had a COP higher than the Carnot COP for the system, an impossibility.

These problems were caused by the control of the solenoid valves in the system allowing the system to fluctuate around the set temperature rather than converging on the set temperature.IntroductionThis lab was undertaken as part of the third-year module ES3B5 Engines and Heat Pumps. It was designed to provide hands-on experience of the thermodynamic machines on which the course is based and to see the theory work in practice.Description of the systemThe lab was undertaken using an air conditioning demonstration unit (ET915.06 from GUNT). Starting at the compressor, the gaseous Tetrafluoroethane (R134a) is compressed from P1 to a higher-pressure P2, raising the temperature of the fluid from T1 to T2. The high pressure, high temperature fluid then passes through a condenser fan, rejecting some of the heat. The refrigerant cools from T2 to T3 and condenses into high-pressure liquid.

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The collector/receiver then captures any liquid refrigerant that didn’t boil off and ensures that only liquid refrigerant enters the expansion valve. Next, the refrigerant enters the filter/dryer which filters out any system contaminants such as water or dirt that would damage system components. The refrigerant then passes through a thermostatic expansion valve/throttle which reduces the pressure of the liquid flowing towards the evaporator in a constant enthalpy process.

The reduction in pressure causes some of the refrigerant to boil and turn to vapour, reducing the temperature of the mixture from T4 to T5. The refrigerant finally passes through an evaporating chamber with a high air flow passing through it due to a fan at the entrance. The refrigerant absorbs heat from the air, lowering the temperature of the air in the chamber from T7 at the entrance to T8 at the exit whilst raising the temperature of the refrigerant from T5 to T6. The refrigerant then evaporates back to a gaseous form and flows back to the compressor.Experimental procedureThe machine was running for a period of time before arriving at the lab in an attempt to reach the lowest temperature set point (6°C) within the labs allotted time frame.

The first set temperature value was therefore chosen to be the value at which the apparatus had levelled off to when the lab was due to start. At each set temperature, the temperatures, enthalpies, three pressure values and two phi values were read off of the schematics and diagrams produced and screenshot from LabView. The screenshots for the results were to be taken when the temperature lines on the temperature vs time graph stopped oscillating around the set point value and reached a more stable constant value close to the set point.This did not work well for the higher values of ‘T8 Set’ as the machine continued to oscillate for a very long period of time without any sign of eventual convergence towards the set point value. For this reason, the screenshots had to be taken very quickly before the machine started ramping up or down away from the desired value. The difference in time between each of the screenshots produced some rather sizable errors in the data. ResultsTable 1 shows the four primary test results found in the lab, followed by three data points given on the module webpage. Analysis The values for h2s were worked out graphically as shown in figure 3 for given data point 3.

COP=Q_in/W=(h_6-h_5)/(h_2-h_1 )  ?COP?_carnot=T_ev/(T_con-T_ev )=T_5/(T_3-T_5 ) ?_isentropic=(h_2s-h_1)/(h_2-h_1 )The first test (T8 = 6°C), gives a good set of results, comparable to those given on the module webpage. The coefficient of performance (COP), Carnot COP and ? values are all of the same size as those calculated from the given values. In the second test (T8 = 12°C), the value of h1 ended up being very close to the value of h2 giving an impossibly large isentropic efficiency which should range between 0 and 1. The COP of 611 is also far too high for a real system. The results for test 2 were taken when the system was still unstable as shown by the fluctuations in figure 2 overleaf. The results for test two will not feature in the remaining analysis.In the third test (T8 = 14°C), the COP is lower but still too high for a real system. The isentropic efficiency of the machine is above 1 due to h2s being a lot bigger than h2.

This is likely due to the result being recorded as the machine was part way through a fluctuation. It may have been close to the ‘Set’ temperature initially but was not stable. For the fourth test (T8 = 16°C), the system did not reach a T8 value of 16°C. It was therefore decided to take the results at the highest value it reached in its fluctuation, which was 15.7°C. As shown in figure 3, the recorded value of h2 is less than the value of h1 causing a negative work value giving a negative COP and ?.

At this point in time, the refrigeration unit was effectively acting as a heat pump. This was due to the temperature gradient between the system and the ambient temperature of the room not being great enough for the system to reach its ‘Set’ temperature.Figure 4 shows that h7 is always higher than h8 due to the reduction in temperature from T7 to T8 with enthalpy being a function of temperature; h=u+RT for a perfect gas. The absolute humidity decreases as the set temperature increases as more of the vapour in the gas evaporates out of the fluid. Figure 5 shows a negative relationship between the COP of the system and the chamber exit temperature, T8, whilst the heat into the system (cooling the air) increases with the input temperature. The system must do more work to cool warmer input air than to cool cooler input air to the same set temperature.

For hotter air, the refrigerant must absorb more heat per given mass of air as it passes through the chamber to reach the set value. The heat out of the system decreases with increasing air supply temperature as it takes less energy to condense higher temperature refrigerant. The COP : COP Carnot ratio shown in figure 6 is around 54%. The actual coefficient of performance for the system is therefore roughly half of the ideal Carnot refrigeration coefficient. This is due to the losses incurred in the real system. In reality, friction acts upon the refrigerant in the system causing pressure and temperature drops in the lines and components.

This results in wasted (non-useful) energy from the system. There is a lot of unwanted heat transfer from the refrigerant to the components and surroundings of the system. This reduces the ability of the refrigerant to absorb as much heat from the air in the cooler, reducing Qin and therefore the COP. The compression in the real system was ?90% isentropic as opposed to the Carnot cycle of fully isentropic compression. There is therefore a 10% loss in efficiency in the compression process. The expansion process also differs from the Carnot model due to the equipment having an isenthalpic expansion valve rather than an isentropic valve. The expansion therefore does not occur at 100% isentropic efficiency.Known IssuesThe solenoid valves in the demonstration unit are poorly controlled by the software, allowing the temperatures to fluctuate around the temperature set by the user.

This caused the issues with tests 2, 3 and 4 where results were taken whilst the machine was not in a stable state. For these tests, the screenshots started being taken when the temperature reached the set point value but, due to the amount of time used to take and save the separate shots, it can be assumed that the values in subsequent screen shots did not stay steady. The values in the screenshots will have changed by the time they were taken creating an error in the results.The high thermal inertia of the system meant the unit took a very long time to raise its exit temperature to the temperature set by the user. Heat losses through all components in the system also hindered the process of raising to the required temperature.

ConclusionThe refrigeration unit had flaws that rendered test 4 to have a negative efficiency which is not possible whilst tests 2 and 3 had a COP higher than the Carnot COP, another impossibility. These problems were caused by the control of the solenoid valves in the system, allowing the system to fluctuate around a temperature rather than stabilise at the ‘Set’ temperature making it impossible to capture all the values needed at one time instant. If the experiment was to be run again, the results which gave impossible figures should be repeated with the reading only taken when the system is sufficiently stable.

The COP of the system is smaller than the Carnot COP due to irreversibilities in the real system discussed above. These irreversibilities cause the heat absorption to reduce and the work input to increase, reducing the system COP.


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