Vapor Compression Refrigeration Cycle Engineering Essay
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Published: Mon, 5 Dec 2016
A compressor is a mechanical device that used increases the pressure of a compressible fluid. The inlet pressure level can be any value from a deep vacuum to a high positive pressure. The discharge pressure can range from sub-atmospheric levels to high values in tens of thousands of pounds per square inch. The inlet and outlet pressure are related, corresponding with the type of compressor ant its configuration. Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. The fluid can be any compressible fluid, either gas or vapor, and can have a wide molecular weight range, that are from 2 for hydrogen to 352 for uranium hexafluoride. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, so the main action of a pump is to pressurize and transport liquids. Applications of compressed gas vary from consumer products, such as the home refrigerator, to large complex petrochemical plant installations.
A vapor compression refrigeration system uses a refrigerant sealed in an airtight and leak proof mechanism. The refrigerant is circulated through the system and it undergoes a no of changes in its state while passing through various components of the system. Each such change in the state of vapor is called a process. The process of repetition of a similar order of operation is called a cycle. The compression cycle is given this name because it is the compression of the refrigerant by the compressor which permits transfer of heat energy. The refrigerant absorbs that from one place and releases it to another place. In other words the compressor is used to put the heat laden refrigerant vapor in such a condition that it may dispute the heat it absorbed at low pressure from the refrigerated space, to an easily available cooling medium. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.
2.1 Description of Vapor Compression System
Most of the modern refrigerators work on this cycle. In its simplest form there are four fundamental operations require to complete one cycle.
Compressor – The low pressure vapor in dry state is drawn from the evaporator during the suction stroke of the compressor. During compression stroke the pressure and temperature increase until vapor temperature is greater than the temperature of condenser cooling medium.
Condenser – When the high pressure refrigerant vapor enters the condenser heat flows from condenser to cooling medium thus allowing the vaporized refrigerant to return to liquid state.
Expansion Valve – After condenser the liquid refrigerant is stored in the liquid receiver until needed. From the receiver it passes through an expansion valve where the pressure is reduced sufficiently to allow the vaporization of liquid a low temperature of about -10C.
Evaporator – The low pressure refrigerant vapor after expansion in the expansion valve enters the evaporator or refrigerated space where a considerable amount of heat is absorbed by it and refrigeration is furnished.
The schematic diagram of the arrangement is as shown in Figure 2.1 below. The low temperature, low pressure vapor at state B is compressed by a compressor to high temperature and pressure vapor at state C. This vapor is condensed into high pressure vapor at state D in the condenser and then passes through the expansion valve. Here, the vapor is throttled down to a low pressure liquid and passed on to an evaporator, where it absorbs heat from the surroundings from the circulating fluid (being refrigerated) and vaporizes into low pressure vapor at state B. The cycle then repeats.
Figure 2.1: Simple Vapor Compression System.
The exchange of energy is as follows:
Compressor requires work, w. The work is supplied to the system from the surroundings
During condensation, heat Q1 the equivalent of latent heat of condensation etc, is lost from the refrigerator.
During evaporation, heat Q2 equivalent to latent heat of vaporization is absorbed by the refrigerant.
There is no exchange of heat during throttling process through the expansion valve as this process occurs at constant enthalpy.
2.2 Simple Vapor Compression Cycle
Figure 2.2 below shows the simple vapor compression cycle:
Figure 2.2: Simple Vapor Compression Cycle
Process 1-2: The refrigerant as a mixture of liquid and vapour corresponding to state point 1 enters the compressor where isentropic compression takes place. The compression process increases the temperature of refrigerant from lower limit T2 to the upper limit Tl. Work is supplied to the system and after compression, the vapour is wet or saturated but not superheated.
Process 2-3: The refrigerant in the form of vapour enters the condenser at state 2 and heat is rejected at constant pressure and temperature. At exit from the condenser, the refrigerant becomes saturated liquid at state point 3.
Process 3-4: The refrigerant at state point 3 enters the expansion cylinder expands isentropic ally and its temperature drops to lower temperature T2 at the end of the expansion process. Work is obtained during the expansion process.
Process 4-1: The liquid refrigerant at point 4 enters the evaporator and extracts heat at constant pressure and temperature from the space or substance being cooled and thus produces refrigerating effect.
Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called a heat pump. “Freon” is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced, such as sulfur dioxide.
Newer refrigerants that have reduced ozone depletion effect include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine. However, CFCs, HCFCs, and HFCs all have large global warming potential.
Newer refrigerants are currently the subject of research, such as supercritical carbon dioxide, known as R-744. These have similar efficiencies compared to existing CFC and HFC based compounds, and have many orders of magnitude lower global warming potential.
3.0 TYPES OF COMPRESSORS
3.1 Centrifugal Compressor
Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants.
Their application can be from 100Â horsepower (75Â kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000Â psi (69Â MPa). Many large snowmaking operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines.
3.2 Axial Flow Compressor
Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like airfoils to progressively compress the working fluid. They are used where there is a requirement for a high flow rate or a compact design. The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary.
The rotating airfoils, also known as blades or rotors, accelerate the fluid. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation.Axial compressors can have high efficiencies; around 90% polytropic at their design conditions. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials. Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants.
4.0 APPLICATION OF COMPRESSORS
Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed:
Pipeline transport of purified natural gas – To move the gas from the production site to the consumer, or the transportation of goods through a pipe. Most commonly, liquid and gases are sent, but pneumatic tubes that transport solid capsules using compressed air have also been used. Often, the compressor in this application is driven by a gas turbine which is fueled by gas bled from the pipeline. Thus, no external power source is necessary.
Plants and refineries – Petroleum refineries, natural gas processing plants, petrochemical and chemical plants, and similar large industrial plants for compressing intermediate and end product gases.
Refrigeration and air conditioner equipment – To move heat from one place to another in refrigerant cycles: see Vapor-compression refrigeration.
Gas turbine systems – To compress the intake combustion air
Commercial industry – Storing purified or manufactured gases in a small volume, high pressure cylinders for medical, welding and other uses.
Cylinder – SCUBA diving, hyperbaric oxygen therapy and other life support devices to store breathing gas in a small volume such as in diving cylinders.
Many various industrial, manufacturing and building processes to power all types of pneumatic tools.
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