The Refrigeration Cycle Kaitlin Keegan English 202C 10/24/2012
Audience and ScopeThe purpose of this description is to inform someone who has not had a classicalThermodynamics course on how the refrigeration cycle works. This descriptionassumes no prior knowledge, and aims to be understandable on face-value. Thisinformation is important because refrigeration is something that Americans deal withon a daily basis, whether they are grabbing a glass of milk in the morning, cranking upthe AC in the summer, or working at an industrial plant. However, the process bywhich all these common appliances work is rarely appreciated and examined, exceptperhaps by a struggling Engineering student in a Thermodynamics class. Therefrigeration cycle revolutionized how modern society operates; without refrigeration,our current standard of living would be unattainable. This type of description could befound in a science magazine, or as an intro to an entry level Thermodynamics course.IntroductionThe refrigeration cycle is the process by which heat is removed from a system, and a cooltemperature in the surroundings is achieved. This is done by a series of phase changes of aspecial fluid, and the heat required to power them. Refrigeration has been exploredsince the 1750’s, starting with a scientist in Scotland named William Cullen. Over thecenturies, the process has been extensively developed, and used in producing theappliances that provide us with modern comforts such as refrigerators, freezers, and airconditioners.The refrigeration cycle can be described by the science of Thermodynamics.Thermodynamics is the study of the relationships between heat, energy, and work. Therefrigeration cycle utilizes these principles in a four step process that continuously coolsthe surroundings inside the refrigerator.The four steps are as follows: throttling, evaporation, compression and condensation. Thesesteps are repeated in a continuous loop through a refrigerating unit, which requires aspecial type of coolant to operate. To fully understand and appreciate the refrigerationcycle, we will examine each step of the process, the coolants involved, and therelationship between the heat loss and energy of the system.
Figure 1: Diagram of a refrigeratorAbove is a diagram of the mechanical components of a refrigerator. Each component: the compressor,condenser coils, evaporator coils, and capillary tube all contribute to a step in the cycle. QL representsthe heat being lost from the freezer compartment when the heat is used as energy to power theevaporation. QH represents the heat being dispelled by the condenser. These concepts will beexplored in more detail in the following steps.Refrigerants The first step requirement for cooling a system is to start with a coolant, which is acompressed liquid. A compressed liquid is a substance that is naturally a vapor at aspecific temperature of the system, but enough pressure has been applied that thesubstance is now in the liquid phase. Common coolants used in industry are shownbelow in Table 1. These coolants are used because they work well as compressedliquids at temperatures and pressures achievable in industry. Coolants, or refrigerants,are organized into different classes, based on how a certain thermodynamic property isused. For this discussion on the refrigeration cycle, we will be concerned with coolantsthat operate as vapors at ambient temperature.
Chemical Name Normal Boiling Point (⁰C) UnitIsobutane -12 RefrigeratorR-22 (a type of Freon) -41.44 Air ConditionerEthylene Glycol 135-139 Car Engine Table 1: Displays common coolants and where they are used. Figure 2: A picture of a pressurized tank of Freon-22Stage One: ThrottlingThe process can be examined starting with the compressed liquid about to enter athrottle. A throttle can be viewed as an “expansion valve”; it is a valve that controls thearea of the pipe line. Entering the expansion valve, the coolant is a compressed liquidwith a high pressure, and a relatively low temperature. Once the refrigerant travelsthrough the throttle, the cross sectional area is increased, and the pressure will drop.This drop in pressure allows the refrigerant to leave the compressed liquid state to astate of mixed liquid and vapor. This mixture is at the saturation temperature and pressurefor the substance. Saturation is when a substance is at its boiling point. Saturationpressure is the pressure that corresponds to this temperature. This vapor/liquidmixture then travels to the evaporator.
Stage Two: EvaporationThe second stage in the refrigeration cycle is the evaporation of the coolant. This is themost critical stage and the part of the process where the cooling effect is produced. Therest of the cycle is just a recycle of the coolant to allow it to re-reach this stage of thecycle. What happens is that when the vapor/liquid mixture reaches the evaporator, themixture wants to complete the change of state to the pure vapor phase. The coolantwants to reach the vapor phase, because as a fluid, it desires to totally fill the volume ofthe coil that it flows through. When the volume is increased to the size of theevaporator, the refrigerant tries to reach the vapor phase to fully fill the coil. Thisrequires work, or the use of energy. To drive this process, or to do the work necessary,the refrigerant removes heat from the surroundings to power this phase change fromvapor/liquid to pure vapor. This loss of heat is felt by the surroundings as coldness, ora drop in temperature. This evaporation process occurs at constant pressure. An Every Day Example This loss of heat in evaporation can easily be seen on a small scale, to truly understand why this works. If you have ever used nail polish remover, you may have noticed that your fingers become cold. This is because the active ingredient in nail polish remover is acetone, and acetone likes to evaporate at room temperature. This evaporation requires energy to drive it, so the evaporation removes heat from your fingers to power the process. This is why your fingers become cold. This process happens also when you use hand sanitizer. The active ingredient is alcohol, and alcohol also likes to evaporate at room temperature. This evaporation process works the same way, by removing heat from your fingers to power the process.
Step Three: CompressionThe third step of the refrigeration cycle is the compression stage. This is to begin re-prepping the coolant for the evaporation. When the refrigerant left the evaporator, itexited as cold vapor. The compressor then takes in this cold vapor and pressurizes it, sothat it will reach the superheated vapor phase. As a consequence, the temperature ofthe vapor rises past the boiling point. A superheated vapor is a vapor that iscontinuously heated to a temperature higher than its boiling point. This superheatedvapor has a temperature and pressure higher than the saturated pressure andtemperature of the coolant. To compress this vapor, the compressor does work on thesystem. The energy to power this work is supplied by an electrical connection.Step Four: CondensationThe final stage of the refrigeration cycle is the condensation step. This is where the fluidis returned to its initial compressed liquid state. The refrigerant enters a condenser afterit exits the compressor. The purpose of the condenser is to dump the excess heatgenerated through the compression to the surroundings. This loss of heat allows thesuperheated vapor to condense back to the compressed liquid phase. The refrigerantcondenses to a compressed liquid because it maintains the high pressure generatedfrom the compression, but loses the high temperature, allowing the substance to reachthe compressed liquid region. The refrigerant is now ready to repeat the cycle.
Figure3: image of a piston powered compressorGraphical Representation of ProcessBelow is a diagram displaying all four stages and components required for therefrigeration cycle. It is a cyclical process, so there is no true beginning or end. One canstart as we did, with the expansion vale. This diagram shows that as the refrigerantexits the expansion valve, it is a mixture of liquid and vapor. As it passes through theevaporator, cold air is felt as the coolant removes heat from its surroundings so that itmay change phase to pure vapor. This pure vapor then travels through the compressor,where while the diagram does not show, it exits as a hotter, higher pressurized vapor.This pressurized, or superheated vapor then travels through the condenser whichremoves the excess heat, and allowing the refrigerant to undergo another phase change,back into a compressed liquid. Figure 4: Circular diagram of refrigeration cycle
Conclusion:In summary, the refrigeration cycle can be broken down into four basic stages: 1. Throttle : pressure is decreased, allowing the coolant to expand into a liquid/vapor mixture 2. Evaporation : cross sectional area is increased so that the coolant expands into a vapor, removing heat from the surroundings and producing a cooling effect on the desired area 3. Compression : the refrigerant is compressed into a superheated vapor, which results in an increase in pressure and temperature 4. Condensation : the condenser dumps the excess heat generated from compression, allowing the temperature to drop, and for the refrigerant to return back to the compressed liquid phase, where it is under high pressure, but a low temperatureSpecial chemicals called refrigerants are used as the substance that enables the processto function. The cycle involves the changes of states, and the energy consumption andheat loss associated with these phase changes is the key to the process. This cycle hasrevolutionized how modern society operates. Without this process, modern grocerystores would be unable to sell perishable goods such as meat, frozen vegetables, anddairy products. Our homes would not be cooled with air conditioning, and buying iceat the local convenience store would be impossible. Even many large scale productionprocesses of goods and services would be impossible without an understanding of thiscycle. The refrigeration cycle has quietly influenced and shaped modern society, andwithout it, our progress would be halted.
Glossary of TermsAmbient Temperature: Temperature of surroundingsCompression: The change of volume by applied pressure, greater compression, smallervolume, higher pressureCompressor: A mechanical unit that applies compression, powered by electricityCompressed Liquid: A liquid that is under high pressure, but kept at low temperature, higherthan saturation pressureCondensation: The phase change by which a vapor is changed back into a liquidSystem: Refers to what is undergoing the thermodynamic change; example: the fluid travelingthrough the pipe is a “system”Surroundings: The universe outside of a systemEnergy: The potential to do work, the “fuel” that powers a processEvaporator: A unit that allows evaporation, usually a coil of pipe with a higher cross sectionalarea than the piping that was upstream from the expansion valveEvaporation: The phase change by which a liquid is changed to a vaporExpansion Valve: A valve that separates a section of piping with different cross sectional areas,creates a pressure drop from the high pressure upstream, to the low pressure downstream ofthe valveHeat: The thermal equivalent of energy, related to temperaturePhase Changes: A change that a fluid undergoes, from either a liquid to a vapor or from avapor to a liquidSaturation Temperature and Pressure: The boiling temperature of a chemical, and thecorresponding pressure. When a fluid is in a vapor/liquid mixture, it is at saturationSuperheated Vapor: Vapor heated past its boiling point, high temperature, high pressureThrottling: The way a pressure drop is achieved, by a change in cross sectional area of pipingWork: The transfer of energy required for a process to be completed
Works CitedGeneral Information: 1. Brain, Marshall, and Sara Elliott. "How Refrigerators Work" 29 November 2006. HowStuffWorks.com. <http://www.howstuffworks.com/refrigerator.htm> 24 October 2012. 2. Matsoukas, Themis. "Chapter 6 Balances in Open Systems." Fundamentals of Chemical Engineering Thermodynamics. 1st ed. Vol. 1. Upper Saddle River: Pearson Education, 2013. 287-309. Print.Figures: 1. Figure 1: http://www.google.com/imgres?q=refrigeration+cycle&start 2. Figure 2: http://www.google.com/imgres?q=freon+r22&num 3. Figure 3: http://www.central-air-conditioner-and- refrigeration.com/Air_Conditioner_Compressors.htm 4. Figure 4: http://www.google.com/imgres?q=refrigeration+cycle+diagram&um=1&hl=en &sa=X&biw= 5. Cover Picture http://www.google.com/imgres?q=refrigeration&start=89&hl=en&biw=1304& bih=664&Empirical Data: 1. http://www.sigmaaldrich.com/catalog/product/sial/324558?lang=en®ion=US 2. http://www.sigmaaldrich.com/catalog/product/aldrich/295450?lang=en®ion= US 3. http://www.engineeringtoolbox.com/r22-properties-d_365.html