Thermodynamics Principle

Entropy
The total entropy of the universe is never decreasing, rather, the total entropy of the universe is either increasing or remaning constant depending on the thermodynamic process that is applied, reversible or irreversible. The change in entropy of a thermodynamic process is equal to the heat transfer of a system divided by temperature.

equation

For reversible thermodynamic processes, the total change in entropy of the system and the surroundings remains constant such that ΔStotal = 0.

For irreversible thermodynamic processes, the total change in entropy of the system must always increases towards more “disorder” such that ΔStotal > 0.

Second Law of Thermodynamics
1. The second law of thermodynamics states that not all heat can be converted to work such that some of it will always be lost to the surroundings (Kelvin statement).
2. The second law also states that heat cannot be transferred from a cold reservoir to hot reservoir without the addition of extra work (Clausis statement).

HEATENGINE
Figure 1: The Kelvin and Clausis statement of the second law of thermodynamics.HEATengine2
Figure 2: The Kelvin and Clausis statement of the second law of thermodynamics.

The thermodynamics principle involved in the process of refrigeration is the “reverse” process of a heat engine, where extra work is needed for heat to flow from a cold reservoir to a warm reservoir.

The process of refrigeration directly follows the Clausis statement of the second law of thermodynamics where heat is taken in from a cold reservoir, work is applied, and heat is expelled to a hot reservoir- a cyclic heat engine process known as the “reverse Carnot Cycle.”

A refrigerator’s net effect is to remove heat from a cold reservoir, making the reservoir colder, and transferring the removed heat to a hot reservoir, making the hot reservoir even “hotter.” This process in consistent with the discussion of entropy, such that the total entropy of the universe is never decreasing.

fridge
Figure 3: Refrigerator- work applied to system to remove heat from cold reservoir and transfer it to hot reservoir.

Sources:
1. Engel, Thomas, and Philip Reid. “Entropy and the Second and Third Laws of Thermodynamics.” Thermodynamics, Statistical Thermodynamics, & Kinetics. 2nd ed. Vol. 1. Pearson Prentice Hall, 2010. 80-94. Print.

2. Brown, Robert. “Second Law of Thermodynamics Summary.” Second Law of Thermodynamics Summary. 12 Apr. 2012. Web. 2 Dec. 2015.

3. Hall, Nancy. “Second Law of Thermodynamics.” Second Law of Thermodynamics. NASA, 5 May 2015. Web. 2 Dec. 2015. <https://www.grc.nasa.gov/www/k-12/airplane/thermo2.html>.

4. “But Wait!” Dave’s Physics Shack. Morningside College. Web. 2 Dec. 2015. <http://webs.morningside.edu/slaven/Physics/entropy/entropy6.html>.

Efficiency

Image

There are two ways that the efficiency can be calculated.

  1. Using the ratio: Which is the amount of heat that is removed from the system over the amount of work done to remove the heat from the system.
    Example: 50J are removed from the inside of the refrigerator by doing 13J or work, then the efficiency is (50J/13J)=3.846
  2. The Carnot Cycle:The refrigeration cycle is the opposite of the Carnot cycle for a heat engine.

Refrigeration Process

1 -> 2: adiabatic expansion – gas expands and temperature drops with no loss or gain of heat.

2 -> 3: Isothermal Expansion – Gas is compressed at a constant temperature of Th and heat is absorbed.

3 -> 4: adiabatic compression – no heat is lost or gained. Gas is compressed and the temperature raises.

4 -> 1: Isothermal Compression – Gas is compressed at a constant temperature TC, removing heat from the system.

Under Ideal conditions (perfect seal and no friction, which is not acheivable), the maximum efficiency for a heat engine is:Capturewhile the maximum efficiency for the refrigeration cycle is reversed, leading to an equation of:Capture2

The efficiency will be higher than 1 when the temperature of the cold reservoir is warmer than half of that of the hot reservoir” (Schroeder)

Example:Surrounding temperature is 298 K while the temperature of the refrigerator is 276K. The efficiency will be 276/(298-276)=12.55.

 

Piccard, Dick. Carnot Engines, Heat Pumps, and Refrigerators. January 2013. <http://www.ohio.edu/people/piccard/phys202/carnot/carnot.html>.

Schroeder, Daniel. Refrigerators and the Second Law of Thermodynamics. n.d. <http://physics.csustan.edu/Ian/HowThingsWork/Topics/Temperature/ThermoLaws/Refrigerators.htm>.

 

Engineering

When a substance changes its state, heat is either absorbed or liberated.  Think of ether, or alcohol evaporating from your hand, it feels cool when it evaporates.  This is essentially how a refrigeration system operates.  In a refrigerator, to save the vapor, it must be used in a closed system such as a pipe system.  According to the First Law of Thermodynamics all energy must be conserved.  Energy is conserved in a refrigeration system by having both an evaporator and a condenser.  A simplified design of a refrigerator is shown here:

Refrigeration Diagram

Substances used as an ideal refrigerant must follow this criteria:

  1. The piping system is usually made of a metal, so the refrigerant must be non-corrosive.
  2. Non-toxic, so if/when it leaks it does not damage the user.
  3. Not a fire hazard under any conditions.
  4. Non-explosive.
  5. Boiling point must be at a level where it can be liquefied and vaporized without great pressure or a large vacuum.
  6. Large heat of vaporization
  7. Does not decompose
  8. Low cost

Brief System Walkthough

The liquid used will be called the refrigerant.

Refrigerant contained in a liquid tank

Simplified design of an Evaporator System

Evaporator Diagram

  • Refrigerant moves in through liquid inlet from the liquid tank
  • Refrigerant moves through circulating pump to evaporator
  • Travels through a pipe to an expansion valve
  • Evaporator uses reduced pressure to decrease boiling point of refrigerant
  • Refrigerant evaporates into (ammonia) receiver
  • Evaporated refrigerant is suction to the compressor
  • Vapor moves into the compressor at its low temperature, work is done on the vapor and it is compressed.
    •  The vapor greatly increases in temperature.
  • Vapor moves to another coil where it is cooled to RT and sent to the condenser

Simplified design of Condenser

condensor

  • Low pressure gas enters in through gas inlet
  • Gas condenses as it flows down condenser
    • Cold water flows through small tubes within the large tubes to cool the large tube.
  • Liquid flows out liquid outlet to liquid receiver
  • Goes back to liquid reservoir to be used again.

Sources:

Evans, Ward V. J. Chem. Educ. 1942, 539-544.

Macintire, H. J.; Hutchinson, F. W. Refrigeration Engineering. John Wiley & Sons: United States, 1950.

Environmental Effects

The earliest refrigerants used in commercial applications were the gases sulfur dioxide, methyl chloride, and ammonia. However, these were poisonous to humans and a safer alternative needed to be found. CFCs, or chlorofluorocarbons, were developed in response, and became popular after World War II. CFCs are composed of carbon, fluorine, hydrogen, and other halogens–often chlorine–and are odorless, noncorrosive, nonflammable, and generally stable molecules.

Dichlorodifluoromethane (Freon-12), a common CFC. Destroys ozone because of its chlorine atoms.

This stability, however, meant that they could only be broken in environments of high energy. In the 1970s and 1980s, scientists found that CFCs were having a major effect on the ozone layer that absorbed most of the sun’s harmful ultraviolet radiation. They would slowly accumulate in the stratosphere, and when exposed to ultraviolet radiation, they would break down into chlorine atoms and react with the stratospheric ozone, catalyzing their breakdown into diatomic oxygen that absorbed no ultraviolet radiation. This was very noticeable over Antarctica, where, because of its unique atmospheric conditions, provided an environment for concentrated ozone destruction.

Chlorine Reaction Equations with Ozone

In response to these studies, the Montreal Protocol on Substances that Deplete the Ozone Layer was passed. It is an international agreement to control the damage of ozone-damaging substances by phasing out their production and use. Developed countries have completely stopped production of CFCs by 1995 as a result.

1,1,1,2-tetrafluoroethane, an HFC. Does not destroy ozone, but contributes to the greenhouse effect.

Additionally, manufacturers began to produce alternatives to CFCs called HCFCs and HFCs, or hydrochlorofluorocarbons and hydrofluorocarbons, respectively. Because of the added hydrogen atom to these molecules, they photodegrade much earlier than CFCs in less-energetic sunlight and, as such, have less of a chance to destroy the dwindling stratospheric ozone. However, these molecules contribute to greenhouse effect and HCFCs in particular are being phased out because of their ozone-destroying ability, unlike HFCs.

In 2007, countries under the Montreal Protocol agreed to follow a schedule that aims to phase out the use of HCFCs completely by 2030 in steps, and in 2015, a proposal was submitted that aims to also reduce the production and use of HFCs because of its contribution to the greenhouse effect. Meanwhile, alternatives for refrigeration are currently being researched that have even less negative environmental effects.

 

Sources & Additional Reading:

Cole-Misch, S. Hydrochlorofluorocarbons. The Gale Encyclopedia of Science, http://www.encyclopedia.com/topic/Hydrochlorofluorocarbons.aspx (accessed 2015).

HCFC Phaseout Schedule. United States Environmental Protection Agency, http://www3.epa.gov/ozone/title6/phaseout/hcfc.html (accessed 2015).

The Montreal Protocol on Substances that Deplete the Ozone Layer. United States Department of State, http://www.state.gov/e/oes/eqt/chemicalpollution/83007.htm (accessed 2015).

Tran, C.; Chong, D.; Kieth, A.; Shively, J. Depletion of the Ozone Layer. The Ozone Hole, http://www3.epa.gov/ozone/title6/phaseout/hcfc.html (accessed 2015).

Welch, C. Chlorofluorocarbons. The Ozone Hole, http://www3.epa.gov/ozone/title6/phaseout/hcfc.html (accessed 2015).

Personal View Points

Sarah’s Views - Like almost everything else that makes our lives easier, refrigeration does have some negative effects, including the possibility for environmental damage. But, refrigeration is a staple in our daily lives. If it were not for refrigeration, foods would still have to be preserved using salts and being smoked or dried, and buildings would not be very comfortable in hot climates. Even though the refrigeration process is not perfectly efficient, there have been great improvements made, with more room to make even more improvements in the future.

Emmielito’s Views – Refrigeration is remarkable process when you get into the specifics of it. Who knew if that you reversed a Carnot engine cycle that you can make spaces cold? When I was a child living in the Philippines, either my bedroom was close enough or the refrigerator was loud enough that I could hear that hum and it would put me to sleep, not knowing this was the compressor that takes in and heats up the refrigerant being cycled endlessly. It’s a shame, however, that refrigeration has caused much environmental damage, seeing that major classes of refrigerants have been banned for doing so. I wish to see progress in this matter and that scientists discover the best alternatives possible.

Jennifer’s Views - Before taking this course I did not consider the application and principles behind the process of refrigeration. I’ve had a refrigerator in my house since the day I was born so I never had a second thought about refrigerators other than the fact that they keep our food cold. Obviously work is needed to run a refrigerator, however learning about the Carnot cycle and applying the concept of entropy to refrigerators is eye opening to prove that simple machines really do impact our lives in a more detailed manner than I previously thought. Image how we would have to jump through obstacles to keep our food cold if refrigerators were not developed.

Noel’s Views –  The engineering of the refrigerator has a crucial impact on all of our lives.  I had no idea that the design of the refrigerator was so simple.  I thought that there would be many more components necessary for the design of an appliance that is so vital to food preservation.  However, it is a relief at the same time that the appliance that keeps our food from rotting and becoming ridden with disease is a relatively easy design.  If it was a complex design it may be too hard to recreate or fix when people are in need.  Hopefully in the future, with such an easy design we can make a more efficient appliance, and at a cheaper price, so everyone can use refrigeration to conserve their food.