Bryan Chambers – 16 April 2009 Vapor Compression Refrigeration Cycle An Introduction for GE Appliance Salesmen The purpose of this publication is to educate GE Appliance Salesmen on the Vapor Compression Refrigeration Cycle – the newly discovered process that makes the GE Refrigerator possible. Salesmen should be able to explain the cycle to a customer and identify the parts involved. The Vapor Compression Refrigeration Cycle is a process that cools an enclosed space to a temperature lower than the surroundings. To accomplish this, heat must be removed from the enclosed space and dissipated into the surroundings. However, heat tends to flow from an area of high temperature to that of a lower temperature. How can we make the reverse happen? This is where the Vapor Compression Refrigeration Cycle steps in. During the cycle, a substance called the refrigerant circulates continuously through four stages. The first stage is called Evaporation and it is here that the refrigerant cools the enclosed space by absorbing heat. Next, during the Compression stage, the pressure of the refrigerant is increased, which raises the temperature above that of the surroundings. As this hot refrigerant moves through the next stage, Condensation, the natural direction of heat flow allows the release of energy into the surrounding air. Finally, during the Expansion phase, the refrigerant temperature is lowered by what is called the auto- refrigeration effect. This cold refrigerant then begins the Evaporation stage again, removing more heat from the enclosed space. Each of the four stages will now be revisited in detail, explaining the physical changes that occur in the refrigerant and the devices used to accomplish these changes. A visual representation of the cycle is displayed below. With the explanation of each stage, a picture of the part responsible is included to aid in identification. ©1911 General Electric Company* 1/3 Bryan Chambers – 16 April 2009 Evaporation During this stage, the refrigerant travels through a device called an evaporator that has a large surface area and typically consists of a coiled tube surrounded by aluminum fins. The cold fluid is a mixture of liquid and vapor refrigerant as it begins this stage. While flowing through the evaporator, all the liquid evaporates and absorbs heat from the enclosed space. The energy absorbed is used to change the state of the refrigerant from liquid to vapor. This lowers the temperature of the space, along with whatever food or beverages are stored in it. The refrigerant exits this stage as a saturated vapor. Compression The heat that was absorbed in the Evaporation stage must be released into the surroundings, but this will not happen unless the temperature of the refrigerant is higher than the outside air. This is the purpose of the Compression stage. A device, predictably called a compressor, raises the pressure of the refrigerant vapor. Due to basic thermodynamic principles, this causes the temperature of the refrigerant to rise, leaving the stage as a superheated vapor. Energy is needed to power the compressor, which is why electricity is required to operate a refrigerator. Condensation Now that we have increased the temperature of the refrigerant above that of the surroundings, we can dissipate the heat necessary to continue the process. This is accomplished with a device very similar to the evaporator. It also uses a coiled tube with aluminum fins, but may have different dimensions than the evaporator to accommodate the different state of the refrigerant. As the hot vapor flows through the condenser, the outside air removes energy and the refrigerant becomes a saturated liquid. At this point the slightest drop in pressure will initiate evaporation, which is the basis for the final stage of the process. Expansion To begin a new cycle, all that must happen is a lowering of the refrigeration temperature to below that of the enclosure. This is the key to the entire cycle, because this was the problem that we started with. However, in this situation we can utilize what is called the auto-refrigeration effect. When a saturated liquid experiences a sudden drop in pressure, a small amount of liquid is instantly vaporized and the temperature of the mixture is drastically reduced. This cold liquid-vapor mixture can now begin a new cycle. The pressure drop is accomplished by the simplest, yet most important, part of the system – a simple flow restriction. This part is commonly called a throttle or expansion valve. ©1911 General Electric Company* 2/3 Bryan Chambers – 16 April 2009 Summary The Vapor Compression Refrigeration cycle is a simple four stage process that cools a small enclosed area to a temperature lower than the surroundings. During the cycle, a refrigerant removes heat from the enclosed space and dissipates it to the surrounding air. The Evaporator draws heat from the space as the cold liquid-vapor refrigerant evaporates. The Compressor increases the pressure of the vapor, which increases the temperature above that of the surrounding air. The Condenser then releases heat to the surroundings as the hot vapor refrigerant condenses. This saturated liquid passes through the Expansion Valve, abruptly lowering the pressure and initiating the auto-refrigeration effect. This drops the temperature of the refrigerant and begins a new cycle. After reading this document, you should be able to answer most questions asked by customers. If you are interested in learning more, contact the GE Appliance Engineering Department. We Bring Good Ideas To Life Glossary Evaporation – The process of changing the physical state of a substance from a liquid to a vapor. The rate of evaporation can be increased by increasing temperature, decreasing pressure, or a combination of both. Energy is absorbed from the surroundings during evaporation. The steam rising from a hot cup of coffee is an example of evaporation that you can see. Condensation – The process of changing the physical state of a substance from a vapor to a liquid. This can be accomplished by decreasing temperature, increasing pressure, or a combination of the two. During condensation, energy is released into the surroundings. A common example is the moisture that collects on the outside of a glass containing a cold beverage. Saturated Vapor – A substance is considered a saturated vapor if it’s in the vapor stage, but just barely. This means that the slightest increase in pressure or removal of heat will begin condensation Saturated Liquid – Similar to the saturated vapor phase, a saturated liquid is in the liquid stage, but barely. This means that the slightest decrease in pressure or addition of heat will begin evaporation. Superheated Vapor – A finite amount of energy must be removed from a superheated vapor before condensation begins. A saturated vapor that is heated becomes a superheated vapor. Auto-Refrigeration Effect – When a saturated liquid experiences a sudden drop in pressure, a small portion of that liquid instantly vaporizes. A vapor contains more energy than its liquid counterpart at the same temperature, but energy is not added to the mixture. Therefore, the temperature of the mixture decreases. If you have ever used an aerosol keyboard cleaner and were baffled by how cold the can got, you were witnessing the auto-refrigeration effect. *This is not an official copyright. Neither this document nor its author have any affiliation with GE; logos and slogans are used only to simulate a hypothetical GE publication for the purpose of a course assignment.
Pages to are hidden for
"Vapour Comp ref2"Please download to view full document