Introduction

Refrigerators have been around for awhile and are a common necessity in households. They are primarily used to keep things cold. The technology behind the refrigerator has gone through many changes due to safety and environmental concerns. The following will explain the use of thermodynamics and efficiency of a fridge, as well as the engineering and environmental concerns.

Thermodynamics Principle

 

Zeroth law of thermodynamics

If two systems are in equilibrium with a third system, they are in equilibrium with each other. The concept of temperature. 

First law of thermodynamics

The equation for the first law of thermodynamics is ΔU = Q – W. This equation means that the change in internal energy is equal to the heat added to the system minus the work done by the system. In refrigeration the final state is equal to the initial state because it is a cyclic process.  This means that the change in internal energy is zero (ΔU = 0).  This equation can also be written as ΔU = Q + W, which is the same equation but instead of the work being done by the system it is done on the system. The heat added to the system is equal to QH-QC so this equation can be rewrote as QH = QC + W.  In this equation QH is the heat added to the room and QC is the heat removed. Since the compressor is always doing work on the system, W is positive. This means that the heat added to the room is always going to be greater than the heat removed from the refrigerator.  So if you were to leave your refrigerator door open the room would actually get warmer.

Second law of thermodynamics

The second law of thermodynamics deals with the Kelvin and Clausius, more specifically in the case of the refrigerator, the Clausius statement.  Clausius states that: “It is impossible to construct a device which operates on a cycle and produces no other effect than the transfer of heat from a cooler body to a hotter body.”  This means that you cannot have a transfer of heat from a cooler region to a warmer region without there being work done on the system.     

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With a refrigerator the heat is being removed from the inside of the fridge (cold reservoir) to the outside of the fridge (hot reservoir), this is all done in a cyclic process. According to the Clausius statement this cannot be done without work being done on the system so with a refrigerator a compressor is used to do work on the system.

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When talking about the change in entropy, for a reversible process, the change in entropy is 0. This can be expressed by ΔStotal = 0= ΔSsys + ΔSsurr. The change in entropy of the system can be expressed as Δsys=dq/T.  Since in refrigeration the heat added to the system is less than zero (dq < 0), this means that ΔSsys < 0.


http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html 
https://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/Chapter5.html 

Efficiency

The refrigeration process is a reverse Carnot cycle sometimes referred as a Carnot refrigerator.    

carnot

From the compressor the refrigerant vapor undergoes adiabatic compression (diagram, 3 to 4).The heated refrigerant vapor changes phase to a liquid as heat is exchanged with  the air outside the refrigerator while passing through the condenser coils and undergoing isothermal compression (diagram, 4 to 1). While moving through the expansion valve the liquid refrigerant undergoes adiabatic expansion and boils due to the decreased pressure changing phase to a vapor (diagram,1 to 2). The refrigerant gas exchanges heat with the air inside the refrigerator undergoing isothermal expansion while passing through the evaporator coils (diagram, 2 to 3).


The efficiency of the refrigeration cycle is given by the Coefficient of Performance (CoP). CoP=QL/(QH-QL)=TC/(TH-TC). QH = heat lost by the refrigerant to the air outside the refrigerator while in the condenser coils. QL = heat gained by the refrigerant from the air inside the refrigerator while in the evaporator coils. The efficiency of the refrigerator can be impeded by the formation of ice crystals on the evaporator coils inside the refrigerator as the temperature falls below 0℃. To remedy this, modern self  defrosting freezers will briefly heat the outside of the evaporator coils periodically to melt the ice and channel the liquid water to a runoff tray outside the refrigerator where it will naturally evaporate.

Engineering Details

Refrigerators work through the expansion and compression of a refrigerant. A refrigerant, usually a fluid, is commonly used in a heat pump and refrigeration cycle and can extract heat from another body.

  •   Liquid coolant arrives at expansion valve which expands the gas therefore cooling it.
  •  Coolant flows through evaporating coils cooling the system
  •  Gas enters the compressor which increases the pressure and temperature
  •  Coolant flows through condenser coils removing heat from system

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Components include the following.

  • Refrigerant – The refrigerant is a fluid that transitions between the gas and liquid phases due to pressure or temperature changes. An ideal refrigerant is one that is non corrosive, non toxic, non flammable, and non polluting. Additionally a refrigerant should have a boiling point below the target temperature. Most modern refrigerators today use Tetrafluoroethane,also known as norflurane, CH2FCF3 to fill this role.
  • Compressor- The compressor increases the pressure of the refrigerant vapor causing the vapor to become super-heated.

compresser

  • Condenser- The condenser is usually a long length of coils with an attached heat sink and possibly a fan. While in the condenser the heated vapor from the compressor exchanges heat through the heat sinks on the coil with the environment outside the refrigerator causing the vapor to cool and condense into a liquid.

condenser

  • Expansion valve- The expansion valve creates an area of low pressure by forcing the liquid refrigerant to flow through a narrow opening into a larger space causing it to evaporate into a gas and cooling it down. The valve also regulates the flow of refrigerant through the use of a diaphragm attached to a bulb filled with CO2. If the refrigerant temperature increases in the valve the CO2 expands in response and applies counter pressure to the diaphragm restricting the flow of refrigerant into the valve.

expansion valve

  •  Evaporator coil- The evaporator coil collects and exchanges the heat of the cooled refrigerant vapor with the inside of the refrigerator causing a temperature decrease inside of the refrigerator. The vapor is then taken by the compressor and the cycle restarts.

coil

 

Desired Refrigerant Properties:

A good refrigerant should have a small vapor density which allows the use of small compressors. To maximize the absorption of heat, it should have a high enthalpy of vaporization. A high thermal conductivity allows the heat to be transferred during condensation and evaporation at a faster rate. The dielectric strength is an important property for an airtight unit. A refrigerant should also have a critical temperature above its condensing temperature. The specific heat should be low so that the heat flow is fast and efficient. Refrigerants will leak on occasions and should be able to be identified from its color or odor. The cost and availability is also important to a good refrigerant, it should be inexpensive and have a high availability for maintenance purposes.

 

Environmental Impacts

Refrigerators have gone through many different phases of technology since they were introduced in the 1900’s. When initially created they used refrigerants such as, sulfur dioxide and methyl formate. These refrigerants were found to be toxic if ingested. After a few fatal incidents Frigidaire, General motors, and DuPont set out to find a better solution.

freonozone

The solution was a compound called Freon. Freon is an odorless, colorless, and nontoxic substance. It was a good at being a refrigerant, which draws heat from a system. Freon was used to in refrigerators up until the 1970’s when scientists were able to link Freon and other CFC’s to the depletion of the ozone layer. The ozone layer is an important part of the atmosphere. It keeps harmful radiation from entering the atmosphere and harming life on earth. The use of CFCs were banned in 1987. New solutions had to be found for cooling a refrigerator that were safe to people as well as the environment. Today most refrigerators use a form of vapor compression to cool.

The less harmful replacement for R-12 is R-134a (tetrafluoroethane)and has been in use since 1990. However, R-12 can still be found in older systems today. One gram of R-134a has the same global warming effect as 1.41 kilograms of CO2, while one gram of R-12 is similar to 8.5 kilograms of CO2. Most modern refrigerators have between 4-6 oz (113-170g) of R-134a in their respective system.

https://www.fridgefilters.com/fridgepedia-brief-history-of-refrigerators.html

http://www.theozonehole.com/

http://www.nytimes.com/2007/12/02/automobiles/02COOL.html?_r=0

Personal Opinions

Refrigerators come in a variety of forms. interior designs, and features; a fridge to fit anyone and everyone’s needs. We think refrigerators are a useful household item. The technology can be used for a variety of things such as keeping food cold to, air conditioning. Thanks to the fridge less food is being wasted, because now people can store leftover food for a longer period of time without it spoiling. Overall the refrigerator is a great piece of technology.