Dec 032018

1.    Abstract

This report is about giving a true understanding of the refrigerator cycle to its audience. Refrigeration cycle (RC) is a very important cycle used in cooling down a fluid, which has a wide range of applications. In this report, what consists of refrigeration cycle is explained in depth through various figures. The difference between a Carnot RC and real RC is discussed. This is followed by analyzing the different components of Whirlpool 3’ EEV-101484 refrigerator. It’s coefficient of performance is compared with the Carnot and possible reasons are explained why this might be so. The discussion is concluded with how the performance of Whirlpool 3’ can be improved. Some of the solutions include changing the refrigerant, ensuring cleanliness in the condenser and evaporators amongst many others.

2.    Refrigeration cycle (RC)

The refrigeration cycle consists of four individual parts that come together to lower the temperature of the coolant being used. The refrigeration cycle pulls heat from the low temperature sink and dumps it in the high temperature as a result, removing the heat from the system.[i] This is done through a fluid 4-way cycle that will be described in each of the section as shown in figure 1.[ii]

Figure 1: Refrigeration cycle and its different components

The objective of the refrigeration cycle is to remove as much heat as possible from a cold environment for a given amount of work. The cycle above can be represented on a Pv cycle as shown in figure 2.[iii]

Figure 2: Pv diagram of a refrigeration cycle

Most common fluid used in a refrigeration cycle is R134a, therefore, we will use that to analyse the whole system.

a.      Compressor (1-2)

The role of the compressor is crucial in RC. The power input () from the electrical source is majorly used by the compressor. As the name describes, compressor quickly increases the pressure the R134a, which is fed in as a saturation vapor demonstrated at point 1 on Figures 1 and 2. The pressure increase significantly increases the temperature (TH) of the coolant and acts as a source for the energy to be lost. As evaporation cools the surface down, RC uses this evaporation to its advantage that will be discussed in the next sections. Most of the compressors are found at bottom of the back side of the refrigerator. They are generally a pot kind of structure which helps in increasing the pressure and therefore, temperature.

b.      Condenser (2-3)

As the name suggests here also, the condenser helps to reduce the temperature and the state of the coolant changes to saturated liquid. At this stage, the pressure continues to remain the same. In this condensation process, most of the heat is lost ( or ) to the surroundings. This is the second law of thermodynamics. So, if a refrigerator is left open in a room, it cannot act as an air-conditioning system because the heat from the condenser is being dumped in the room also. Condensers are long tube-like structures that can be found at the back side of the fridge connected to compressors. When the coolant leaves the condenser at 3, it is saturated liquid and at high pressure.

c.      Throttling Valve (3-4)

Throttling valves are essentially valves that can reduce the pressure at varied rates. When the saturated liquid at high pressure goes through this stage, throttling valves significantly reduce the pressure of R134a. Due to a reduction of pressure, some of the liquid vaporizes and the resulting coolant at stage 4 is a saturated mixture as shown in figure 2. Throttling valves play a crucial role in changing the pressure which helps in continuing the cycle forward.

d.      Evaporator (4-1)

The last stage of the RC is the evaporator stage. The coolant is at a lower pressure and temperature (TL), which helps in cooling the surroundings by taking heat away ( or ). In the example of a refrigerator, heat from the various food items kept inside is taken away by the evaporator. Evaporator section again looks like a tube present at the top-back side of the refrigerator, generally near the freezer section. The low temperature air is transferred from the freezer to the normal fridge through a small window that can be manually or automatically controlled.

3.    Real Life Scenario

a.      Carnot Refrigeration Cycle

Carnot refrigeration cycle is the most ideal case considering the best efficiency and performance of the refrigerators. This coefficient of performance is given by

Considering that the refrigerator is able to cool to 5°C on average and the heat generator in the condenser makes the temperature be 60°C, this would give the Carnot COP of:

However, this performance is an ideal case and would be almost impossible for any refrigerator to reach due to the inefficiencies present in the different devices mentioned in the previous section.

b.      Real Refrigeration Cycle

As mentioned in the previous section, reaching the ideal case of the refrigeration cycle is almost impossible to reach due to numerous inefficiencies present in the different components of RC. This section will focus on these inefficiencies. In order to visualize these, we need to consider a T-s diagram to truly understand the different additional mechanisms taking place. This has been placed before as figure 4.[iv]

Figure 3: Displaying a real T-s diagram of a RC

In the figure 3, green cycle represents the ideal cycle discussed in section 3.a. However, with the numerous inefficiencies, the green region reduces to the yellow area, which represents the ideal case. As it can be seen, the area between the two closed loop of the cycle represents the COP and it reduces in the real cycle.

In the first stage (1-2), the compressor is supposed to be completely isentropic. However, it is very hard to reach true isentropic stages and there is a very small increase in entropy leading to 1-2 curve having a slight slope. In the second stage (2-3), the pressure is supposed to remain constant in the condenser. However, pressure slightly changes. The total entropy change is also more. In the third stage (3-4) at the throttling valve, pressure is supposed to reduce but it changes at an uneven rate with uncertainties to come back to stage 4. In the final stage (4-1), just like the condenser, the pressure slightly drops even though it is supposed to remain constant. The entropy change is also more than before.

Therefore, there are numerous uncertainties that are introduced at various stages of the RC because of which temperature expected is actually much higher while the lower temperature remains the same. This is why the real COP is much lower than 4.74 calculated from equation 1.

4.    Whirlpool 3’ EEV-101484

Whirlpool 3’ EEV-101484 is a mini fridge that I own in my room. It can be found in the figure 4 below.

Figure 4: A picture of Whirlpool 3’ EEV-101484

This refrigerator draws 74W () from the power source which requires a 115V outlet at 60Hz. The maximum pressure, as shown in figure 2, it reaches is 1.6MPa (88psig) and the lowest is 0.6 MPa (88psig). All this information is drawn from the table on the back of the refrigerator which is shown in figure 5.

Figure 5: Whirlpool Refrigerator Specifications

As described in the first section, this refrigerator also consists of the four parts of the RC. All of these are not visible without taking the cover off the refrigerator. In most of the fridges, compressors are always visible because that is the area where the hot air leaves the fridge. This is generally at the bottom of the back side of the fridge. In this Whirlpool EEV-101484, compressor is clearly visible shown in figure 6 below.

Figure 6: Compressor of EEV-101484

This compressor works at 115 V and accepts 60 Hz of frequency. On the right side of this compressor, there is a bronze tube which transfers the R134a to condenser section. These condenser is not entirely visible, however, a small part is visible and is shown in figure 7 below.

Figure 7: Tube connecting to condenser

There are some additional parts in the refrigerator that are not discussed in the section 2 that also crucial to improve the performance.

a.        Two Fans

In real refrigerators, at least two small fans are always present. Since RC is a dynamic system, heat can never accumulate at any one area. Therefore, too cold chambers cannot continue to remain cold and too hot areas cannot continue to remain hot. This is why fans are installed. These fans are shown in a small schematic in the following figure 8, which are highlighted in yellow.

Figure 8: Displaying the two fans in the RC

When I touched the condenser from the figure 6 while the fridge was running, it was hot. When the fridges keep running all day, compressor chambers get really hot. In order to remove the heat from that area, compressor section is left open so that heat can be dissipated to the room. This fan plays a crucial role in keeping the condenser from burning out.

Another place where fans are used in the evaporator chamber as shown in figure 8. Since the temperatures becomes really low at the evaporator stage, this cold air is delivered to the refrigerator to cool the items present. It is only because of this fan that the refrigerator remains cold and preserves the food present inside.

Another problem that happens is at evaporator tubes. Because the temperatures here are so low, the evaporators generally freeze and frost bites.

b.        Heating evaporator tubes

From very low temperatures at the evaporator, tubes generally freeze and get frost bites as shown in figure 9 below.

Figure 9: Frost bites at the evaporator tube in refrigerators

Therefore, a small heating unit is added here to stop this from happening. Older refrigerators did not used to have this, and it has only been started adding in the recent years. Up to three times a day, this heating unit will be active to remove the frost bites.

c.      Measuring power output

In order to measure the power input of the refrigerator, I connected a multimeter shown in figure 10 below to the voltage and current to measure the true watts.

Figure 10: Multimeter used for the experiment

The multimeter read 119V and 0.49A for voltage and current respectively. Since power is given by the multiplication of these variables, the power input for the refrigerator is

As it can be seen that this value is actually about 188W less than the ideal power input mentioned in the figure 5. This value is . In order to find the COP, we have to estimate the . This is the heat gained by the RC. This heat comes from the food that we keep inside the refrigerator. We will conduct an experiment to measure how much is the heat dissipated.

For this experiment, I placed a bottle of 1L water inside that was initially at 50°C. After cooling down in the refrigerator, the temperature became 5°C. The experiment was stopped at 5°C because at temperatures lower than this, anomalous expansion of water takes place and getting data for that stage becomes complicated. The total duration of the experiment was approximately 85 minutes. Using these, we can find the heat loss in the RC.

The average density between these two temperatures is

Using the table A6 in the Introduction to thermal and fluids engineering[v] and have been placed in the equation 5.

In a similar form, we can find the cp of the water also.

The cp values can again be found in the A6 of reference.v

Finally, we can find the Qin as:

We can, therefore, find the value of the COP of the refrigerator as:i

Therefore, it can be seen that the real COP (0.756 – equation 10) from the Carnot (4.95 – equation 2) is much different. As explained previously, this happens due to numerous discrepancies that take place in the different chambers of RC as shown on figure 3.

5.    Design improvements to performance of the refrigerators

Numerous improvements can be added to the refrigerators that go through the RC to improve the coefficient of performance. These have been listed below in the form of bullet points. They have been further developed to support how it reduces the COP value also. For other points, performance has been

  1. Since refrigerator parts often need to be repaired, various parts can easily be taken out with simple screwdrivers. Use the screwdrivers, one should always check that the condenser tubes are clean. These tubes often catch dust and other particles, which reduces the ability of the fluid to lose heat. This increases the temperature of these tubes and can be fatal to the performance. Increase in temperature (TH) also decreases the COP value.
  2. In reference to the point above, the evaporator tubes must also remain clean. If the heating chamber, which was discussed in section 4.b stops working, the temperatures (TL) can lead to lower COP value.
  3. Placing the refrigerator away from hot areas is an important aspect of making sure that the temperature that builds up around the compressor and condenser is not too much. This would affect the rate of the heat dissipated from these stages and further increase TH decreasing the COP value.
  4. The efficiency of the compressor could be improved lubricating it frequently and checking for leaks.[vi]

Other miscellaneous methods:

  1. R134a, which seems to be one of the most common coolant used is quite dangerous to use. It depletes the Ozone air and if the fridge is not opened for a very long time, R134a’s concentration in the fridge can increase and it can lead to massive explosions. Some of the other refrigerants being utilized these days are R-600a and HFC-134a. Replacing the cooling agent frequently makes sure that the refrigerant remains pure.
  2. Keeping the fridge closed at all times would mean that the cold air would not leave the fridge and cold the items inside the fridge faster. This is why the rubber of the fridge should also be frequently checked to make sure that it is not causing any leakage.[vii]

6. References

[i] Jensen, Michael. “Chapter 7.” INTRODUCTION TO THERMAL AND FLUIDS ENGINEERING, by DEBORAH A. KAMINSKI, JOHN WILEY, 2017, pp. 262–280.

[ii] HighamT. “Ideal Refrigeration Cycle.” History Refrigeration Cycle, 1 Jan. 1970, refrigerationbest.blogspot.com/2014/09/schematic-refrigeration-cycle.html.

[iii] “Chapter 6 Engines, Refrigerators and the Second Law of Thermodynamics.” NTNU Widgets, Norwegian University of Science and Technology, phy.ntnu.edu.tw/~chiact/thermo_ch6.htm.

[iv] HighamT. “Refrigeration T-s diagram.” History Refrigeration Cycle, 1 Jan. 1970, refrigerationbest.blogspot.com/2014/09/schematic-refrigeration-cycle.html.

[v] Jensen, Michael. “Appendix 6.” INTRODUCTION TO THERMAL AND FLUIDS ENGINEERING, by DEBORAH A. KAMINSKI, JOHN WILEY, 2017, pp. 262–280.

[vi] Nampel, Corey. “5 Steps to Achieve Maximum Air Compressor Efficiency.” Rolair, 23 Feb. 2017, www.rolair.com/blog/5-ways-to-improve-air-compressor-efficiency/.

[vii] “Homeowner left with serious cuts and bruises after fridge-freezer exploded in his kitchen says he only survived because he was kneeling down”. Archived from the original on 6 February 2016. Retrieved 14 June 2017. Daily Mail February 2016

Pranav Mohan

Change and progress are two words that define my character and my ultimate goals. I have a vision to bring a global change by targeting the psychology, because that is the easiest to change. My aim is to incur a self-progressive routine for myself and then help the people around me to progress themselves. In my perspective, walking towards a defined target should be everyone’s goal while keeping in mind that things don’t go as planned but still the target should remain unchanged.

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