Refrigeration System for Inter Modal Containers - Essay Example

Energy Conversion and Management: Refrigeration System for inter modal containers Transportation of temperature sensitive cargo requires the use of refrigerated containers. These refrigerated containers are ideal for inter modal freight transport and is termed as reefer. These reefers have an internal refrigeration unit and they actually operate on external power provided through the fuel based power generators. Refrigeration systems that are well suited for containers are basically liquid suction heat exchangers. The old reefers were equipped with a water cooling system but the new generation reefers rely on ventilation to remove heat. But these technologies are very much a mechanical refrigeration system with higher cost of maintenance (NIST). The liquid carbon dioxide based system is also very much popular. Based on Cryogenic concepts, this type of cooling system has the capability of freezing things at the level of 30 to 50 K and is very much environment friendly. But the latest technology in field of refrigeration systems is the Closed-Cycle Air Refrigeration Technology or CCAR (NIST). CCAR is based on the thermodynamic process termed as reverse Baryton Cycle which had been almost more than a century before and the refrigeration system working in jet planes were based on the same only. As an open cycle refrigeration system, this air based refrigeration technique is very much inefficient but the same air being used as a refrigerant for CCAR while using the advance technologies and much improved compressor, expander and seal, can be an excellent for any refrigeration plant. It can be used in food processing units, food storage plants, pharmaceutical industry and above all for the freight transport of intermodal containers as well as natural gases or transport gases (NIST). Different refrigeration technologies and refrigeration of containers Mechanical refrigeration system has been the first refrigeration system which has been used for any commercial purpose. It is based on the principle that the absorption of heat by a fluid while it is changing from liquid to gas makes a downward turn in the temperature of objects around it. The fluid used is termed as refrigerants. Post 1996 the hydrochlorofluorocarbons (HCFCs) based refrigerants are the most widely used. HFC-134a, HCFC-123 and HCFC-124 are the some of the refrigerants which are very much suitable for mechanical refrigeration. Mechanical refrigeration cycle (WOU) The mechanical refrigeration systems are based on the principle that absorption of heat by a fluid (refrigerant) as it changes from a liquid to a gas lowers the temperature of the objects around it. Compression system based refrigeration is employed in electric home refrigerators and commercial installations and involves a compressor which is being controlled by a thermostat and forcing the vaporized refrigerants to pass through a condenser by exerting pressure on a vaporized refrigerant (engel & Boles). At the condenser the vaporized gas loses heat and liquefies and is again moved through the coils of the refrigeration compartment where it again vaporizes there by drawing heat from whatever object are present in the compartment (Cavilini). The refrigerant then passes back to the compressor, and again the same cycle is repeated. Since the refrigerants have to go for quick vaporization and again getting cooled into liquid form. The temperature and pressure range of HCFCs and HFCs make it suitable for thermodynamically suitable for any mechanical refrigeration with much lower ozone depletion rate than CFCs ("Refrigeration"). Temperature entropy diagram of a compression vapour-compression refrigerating cycle (engel &. Boles) The effectiveness of the system can be measured by using the formula given below with all measurement of temperatures being done. The condenser design is being optimized according to the choice of refrigerant. The thermal performance of the different refrigerants condensing at the sides of the tubes is being compared. The optimal design value of the condensation heat transfer coefficient with pipe diameter and saturation temperature can make way to calculate refrigerant mass flux index which is unique for each refrigerant and then the optimum length of circuit can be calculated (Klein, Reindel & Brownell). The Cryogenic refrigeration system is also a solution to the container refrigeration for freight transport. This refrigeration on cryogenic concept is actually liquid carbon dioxide based and an environment friendly technology. The most interesting factor with this heat exchanger based technology is that it doesn't require any power supply or fuel and the container can be transported to any other location. The cryogenic approach of refrigeration requires very low temperature gases mainly liquid nitrogen or solid carbon dioxide. So the refrigerants required here is basically liquid nitrogen or dry ice (Majumdar & Steadman). The refrigeration requires chilling down of fluid transfer lines. This chilldown of fluid transfer lines is a very important part of the whole cryogenic system. The chilldown process is a complex combination of a number of phenomena involving both thermal and fluid transients. A cryogenic fluid like nitrogen and liquid carbon dioxide flows through the transfer line that is actually at the higher temperature than the cryogen. The liquids get vaporized due to transient heat transfer processes between the liquid and the transfer line. This change of phase from liquid to gaseous may cause transient pressures and flow surge. But with the cooling of the transfer line, these effects get diminished until the cryogenic fluid reaches a steady flow condition in the chilled transfer line (Majumdar & Steadman). A cryogenic system based food processing unit (NIST) The cryogens pass through throttling valves while entering spiral and tunnel freezers so as to expand to atmospheric pressure and then vaporize (Venkatrathnam, Reddy & Murthy). The liquid nitrogen which vaporizes into cold nitrogen gas is sprayed into freezers to freeze objects via direct contact ((Venkatrathnam). The extremely cold nitrogen gas which is initially at -320F moves through warmer zones and is again re-circulated throughout the freezer before being vented. When carbon dioxide based system is operated at temperature which is quite lower than the one through liquid nitrogen, the temperature range is -150F to -200F (NIST). Carbon dioxide is passed through the throttling valves and is finally sprayed on the objects as a mixture of dry ice i.e., the solid form of carbon dioxide and its in vapor form. The sublimation of the dry ice or the solid carbon dioxide into the vapor form derives heat from the object and then keeps the whole in refrigerated condition. The vapors formed of the refrigeration effect enter the chamber directly (engel & Boles). A cryogenic refrigeration system (engel & Boles) The temperature entropy diagram of a cryogenic refrigeration system (engel & Boles) The work output of the turbine of the system can be used to reduce the work input requirements to the compressor. Thus, the COP of a this refrigeration cycle is Now moving on to CCAR; Closed-cycle air refrigeration (CCAR) is a new refrigeration technology which has been developed through the combination of components from mechanical and cryogenic refrigeration technologies thereby enhancing the capabilities of the component and then working out the integration phase to meet the necessary performance conditions. The complete system depends on dry, high-pressure air as the fluid being configured in a closed system. The closed system ensures that the makeup air has totally been removed of moisture (NIST). This refrigeration system has been developed while considering air as a working fluid. This environmentally friendly and safe fluid is actually unlimited in volume and quantity. So, the most suitable refrigerant in this case is the air and the refrigeration technique for the purpose is based on the reverse Brayton Cycle. This air-based refrigeration system units working according to this cycle and has been used in commercial aircraft for their air based refrigeration units (NIST). Brayton cycle has normally three components: a gas compressor, a burner or the combustion chamber and an expansion turbine. Initially these air based systems were configured in an open cycle with compressed cold air being clown into a cooling chamber and hence lost for further use and the makeup air has to be dehumidified and is then compressed so that the loss of cold air could be compensated (Lane). The process for the purpose of removing moisture from the air and then its compression leads to low efficiencies. So to have any recognizable improvement in the efficiency level of any air based system, it has to be operated at higher pressure and in closed cycle (Lane). Brayton Cycle and engine (MIT& NASA) Closed Cycle Air Refrigeration (NIST) The CCAR technology has its own standard on factors like efficiency and operating reliability. The improvements in these areas have been brought through many elements. The system is operating at a very high pressure while maintaining the temperature at around -150F and compander shaft speed being 30.000 rpm (NIST). The compressor and the expander designed are all single wheeled. The compressor output to expander output is normally maintained at 1.6 to 1. So the compression ratio is maintained at a very low level. Any possible escaping of high pressure air at the compressor shaft is being negated with the use of ultra low leakage seal. The technological advancement in CCAR systems has its proof when it was tested at a Kodak facility in Rochester, New York (NIST). While being operated for almost 6000 hours, the CCAR system exceeded all of its design specifications. While being specified to work at 50 tons of refrigeration, the plant operated at 60 tons i.e., it exceeded the design point by 20 percent. System reliability was supposed to be 95 percent but the plant actually operated at 98 percent (NIST). A CCAR refrigeration based storage unit (NIST) The refrigeration temperatures remained within a close range of -73C. The coefficient of performance or COP level at -56.7F was 0.75 while the same at -73C remained at a very competitive value of 0.66 design points. The actual difference between the conventional refrigeration system and the CCAR and its strength gets visible when there is load reduction. With 40 percent turndown, the CCAR unit continued performing at the efficiency level which is just 3 percent less than the best that can be achieved by it. But a conventional mechanical refrigeration unit shows a sharp fall in efficiency that is around 37 percent when the load reduction is at around 40 percent (NIST). The CCAR technology with it success brought many revolutionary products into the market. The new shaft seals used for CCAR implementation can easily be incorporated in other industries. The high-efficiency aluminum plate core heat exchanger can have its use in the petrochemical and natural gas industries. The casting technology used in CCAR are actually three-dimensional rapid prototyping technology with quick cast honeycombed advanced materials. This approach has caused significant reduction in time and cost requirements of building prototypes (NIST). Now the condition over here is to preserve the food items at a temperature of around -70C while the temperature of the surrounding is at 40C, so the required COP value can be calculated using this formula: COP = TC / (TH - TC) Now TC in the given scenario is -70C while TH is 40C, so COP required is 0.64. Since, the COP of CCAR ranges from 0.75 to 0.66, hence this technology is very much ideal for the refrigeration of cargo at -70C. References National Institute of Standards and Technology. Closed-Cycle Air Refrigeration Technology for Cross-Cutting Applications in Food Processing, Volatile Organic Compound Recovery, and Liquid Natural Gas Industries. December 2001 < http://www.atp.nist.gov/eao/gcr_819.pdf> engel, Y. A. & Boles, M. A. Refrigeration Cycles:Thermodynamics: An Engineering Approach, June 3, 2005. 5th edition McGraw-Hill Science/Engineering/Math < highered.mcgraw-hill.com/sites/dl/free/0072884959/240292/Chapter11.ppt> Venkatarathnam, G., Reddy, V. R. & Murthy S. S. Performance of a very low temperature refrigeration system operating with natural fluids, December 2003 World Climate & Energy Event < www.rio3.com/proceedings/RIO3_275_G_Venkatarathnam.pdf> "Refrigeration", IPCC/TEAP Special Report: Safeguarding the Ozone Layer and the Global Climate System, May 19, 2005 Venkatarathnam, G. Liquefaction of nitrogen using mixed refrigerant processes < http://www.aiigma.org/G.Venkatarathnam.pdf> Majumdar, A & Steadman, T. Numerical Modeling of Thermofluid Transients During Chilldown of Cryogenic Transfer Lines, January 2003. < http://www.gfssp.msfc.nasa.gov/pdf/AKMAJUMD.pd> Cavalini, A. Heat Transfer and Energy efficiency of Working fluids in Mechanical Refrigeration 2002 < www.iifiir.org/en/doc/1046.pdf> Klein, S. A., Reindel D. T. & Brownell K. Refrigeration System Performance using Liquid-Suction Heat Exchangers, 2000 International Journal of Refrigeration, Vol. 23, Part 8, pp. 588-596. Lane D., Brayton Cycle: The Ideal Cycle for Gas-Turbine Engines In Relation to Power Plants , 2001 < web.me.unr.edu/me372/Spring2001/Brayton%20Cycle.pdf> "Mechanical refrigeration cycle" "Temperature entropy diagram of a compression vapour-compression refrigerating cycle" < highered.mcgraw-hill.com/sites/dl/free/0072884959/240292/Chapter11.ppt> "A cryogenic system based food processing unit" < http://www.atp.nist.gov/eao/gcr_819.pdf> "A CCAR refrigeration based storage unit" < http://www.atp.nist.gov/eao/gcr_819.pdf> "Closed Cycle Air Refrigeration" < http://www.atp.nist.gov/eao/gcr_819.pdf> "Brayton Cycle and engine" "A cryogenic refrigeration system" < highered.mcgraw-hill.com/sites/dl/free/0072884959/240292/Chapter11.ppt> "The temperature entropy diagram of a cryogenic refrigeration system" < highered.mcgraw-hill.com/sites/dl/free/0072884959/240292/Chapter11.ppt> Read More