Introduction

Refrigerators are a household necessity and have been for some time, but how do they work?  The proceeding will delve into the how refrigerators use the 2nd law of thermodynamics, in addition to the explaining the most efficient way a refrigerator can run using thermodynamics and the Carnot engine. Engineering and design behind the principals of a refrigerator including the use of Freon will be discussed, along with the environmental impacts Freon can have on the environment if not properly disposed of. Finally, in closing there are some personal opinions form the authors.

Thermodynamic Concepts

How is Heat Removed? — The 2nd Law of Thermodynamics:

The two main ways to express the 2nd law of thermodynamics are through the Kelvin statement and the Clausius statement.

Kelvin stated that:  “It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects.”  Or, in other words heat cannot be 100% converted to work, as some heat will always be lost to the surroundings.

Clausius stated that:  “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.”  Or, more simply put, you cannot go from cold to hot without some amount of work being input into the system.

The Clausius statement is the most relevant way of looking at a refrigeration process.  That statement is illustrated below:

2nd Law of Thermodynamics - Clausius Statement

2nd Law of Thermodynamics – Clausius Statement

In the case of a refrigerator, heat is being moved from a cold reservoir (the inside of the refrigerator) to a warm reservoir (outside of the refrigerator).  By the 2nd Law, though, this can’t happen spontaneously.  So how does heat get removed from the inside of the refrigerator?  Although it can be described in practical terms by the heat transfer to the cold refrigerant flowing through the coils in the refrigerator, it can be described in thermodynamic terms by the work done by the compressor in the refrigeration system.  Ultimately, it is the work done by the compressor that controls how much heat can be moved from the inside of the refrigerator to the outside.

Why Doesn’t Opening a Fridge Cool the Room? — The 1st Law of Thermodynamics:

The 1st Law of Thermodynamics states that ΔU = Q – W.  In a cyclic process such as a refrigeration cycle, the final state is the same as the initial state, and so ΔU = 0.  The heat added to the system is equal to QH-QC, and so the 1st Law can be rewritten as:

QH = QC + W

QH is the heat added to the room, while QC is the heat removed from the inside of the refrigerator.  Since the work done by the compressor is always greater than 0, this means that the heat added to the room will always be greater than the heat removed from the refrigerator.  And so if you keep the refrigerator door open, more heat will be added to the room than is removed.

Entropy:

Although no real process is completely reversible, the refrigeration cycle can be estimated as a reversible process.  For a reversible process, the total change in entropy equals 0, which can be stated as:

ΔStotal = 0 = ΔSsys + ΔSsurroundings

The change in entropy for the system can be expressed as:

ΔSsys = dq/T

Because heat is being removed from the inside of the refrigerator, dq < 0, so ΔSsys < 0.  What this means is that while the entropy inside the refrigerator is decreasing, the entropy of the surroundings is increasing.

References:

Clausius, R. (July 1856). “On a Modified Form of the Second Fundamental Theorem in the Mechanical Theory of Heat”. London, Edinburgh and Dublin Philosophical Magazine and Journal of Science. 4th. 2: 86.

Thomson, W. (1851). “On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule’s equivalent of a Thermal Unit, and M. Regnault’s Observations on Steam”. Transactions of the Royal Society of Edinburgh. XX (part II): 265.

http://physics.csustan.edu/Ian/HowThingsWork/Topics/Temperature/ThermoLaws/Refrigerators.htm

Best Efficiency

The Reverse Carnot Cycle:

Reverse Carnot Cycle

The refrigeration cycle follows a Carnot cycle that moves in the opposite direction of that of a heat engine.  The process goes as follows:

b → c:  the refrigerant is compressed adiabatically in the compressor, raising its temperature

c → d:  in the condenser, the refrigerant is compressed at constant temperature while heat is transferred to the surrounding room

d → a:  the refrigerant expands adiabatically at the expansion valve, dropping to a lower temperature

a → b:  in the evaporator, the refrigerant expands at constant temperature while absorbing heat from inside the refrigerator

The efficiency of an idealized refrigeration process can be expressed as:

e = Tc/(Th-Tc)

As an example, if the refrigerator is cooling to 5° C, or 278 K, and the outside room is at room temperature (298 K), then efficiency would be:

e = 278 K/(298 K – 278 K) = 13.9

This is the best possible efficiency that can be achieved at these temperatures.  Because a real process is not ideal due to factors like friction within the components of the refrigeration system, an actual refrigerator will never reach this ideal efficiency.

References:

http://nptel.ac.in/courses/112106133/Module_6/2_Reversed_Carnot_Cycle.pdf

Engineering Details

Refrigerators take advantage of a fundamental behavior of gasses. Gasses heat up when they are compressed and, intuitively, cool when expanded. Refrigerators apply this principle through the use of a system involving five major components.

  1. A “gas” – typically Freon for home use.
  2. A compressor – to increase the heat of the gas and move Freon
  3. Condenser coils – to release head from the hot gas; comparable to a radiator
  4. Expansion valve – controls the amount of refrigerant flow, “metering device”
  5. Evaporator coils – inside cold portion of fridge.

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The Process:

The refrigerant, or coolant, is a liquid when it enters into the expansion valve. Immediately after the valve, the quick drop in pressure causes it to cool and expand.

Next, the coolant flows through a series of tubes inside of the cold portion of the fridge and thus absorbs heat from the air inside.

From there, the gas enters the compressor and the temperature and pressure are raised significantly.

The coolant then flows through the condenser coils which act like a car radiator. Here the gas has time to cool, and eventually turns into a liquid by the time it reaches the expansion valve.

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Sources:

https://books.google.com/books?id=eS_2UGJVA2EC&lpg=PR8&ots=bfW0x_ObE&pg=PA445 Refrigeration & Air Conditioning Technology

IMG https://www.fantasticfridges.com/YoungLearners/HowdoesaFridgeWork

Environmental Concerns

Refrigerators have provided an effective method to preserving food, allowing a meal or beverage to stay edible long past when it should have gone bad. The very use of the refrigerator helps to prevent waste of natural resources in our environment. Although in the history of the refrigerator there has been a bit of a dark past.

Domestic refrigerators did not go into production until the early 1900’s. Early refrigerators used several harmful reactants in the cooling process such as sulfur dioxide or methyl formate, which is toxic if ingested. Realizing that the continued use of this chemical was dangerous to customers, General Motors issued a team to find a replacement; the lead member of the team was Thomas Midgley Jr. In 1930, the product of the group’s hard work and dedication found its way into the domestic refrigerator, the product is commonly known as Freon (a chlorofluorocarbon, or CFC). This chemical was significantly less toxic to humans than the previous chemicals used and it proved to be an effective refrigerant. During the next several decades Freon was used in most refrigeration units.

CFC-12 is the commonly used refrigerant

Unfortunately, freon, unbeknownst to people during the mid-1900’s, was also effective at ozone depletion. CFC’s happen to be extraordinarily good at being bad. Greenhouse gases, when in the atmosphere, absorb radiation at the infrared range. This leads to the “warming” effect associated with global warming. Granted, many molecules are greenhouses gases, including water (H2O) and ozone (O3), and that these gases help to provide the stable warm temperature that the species of this planet currently enjoy, but the phrase “too much of a good thing” can be rather applicable in this case. The mass production of freon in refrigeration lead to an enormous impact on ozone depletion.

The graph depicts the minimum value of Total Column Ozone over Antarctica. Taken by the Total Ozone Mapping Spectrometer satellite system

Thankfully, the use of CFC’s was banned in 1987, and now many modern day refrigerators use vapor-compression for cooling, which does not leave a harmful effect on the environment. But the effects of freon are still felt and while it was banned almost thirty years ago, Earth’s ozone is still in a critical state.

 

References:

Sherwood, Frank S., and Mario J. Molina. “Stratospheric Ozone Depletion by Chlorofluorocarbons.” University of California – Irvine, 8 Dec. 1995, Stockholm, https://web.archive.org/web/20110909064451/www.eoearth.org/article/Stratospheric_Ozone_Depletion_by_Chlorofluorocarbons_(Nobel_Lecture)#gen10. Accessed 27 Nov. 2016.

Wofsy, Steven C., Michael B. McElroyp, and Nien D. Sze. “Freon Consumption: Implications for Atmospheric Ozone.” Science, vol. 187, no. 4176, 1975, pp. 535-36. Accessed 27 Nov. 2016.

 

Personal Opinions

The refrigerator has proved to be a highly useful tool in food preservation. Before refrigeration, the most common methods of food storage were through placing food with ice, which is difficult to obtain year-round, and with meat specifically to swathe it in salt. Both methods helped, but both were quite inefficient. Refrigeration is arguably one of the more important technological advancements made during the early 20th century. Thanks to the creativity and handwork of individuals who brought this tool to the domestic forefront, the common person is capable of efficiently keeping food without wasting. This tool has provided a significant amount of help to the human race.