Добірка наукової літератури з теми "Refrigeration and refrigerating machinery Environmental aspects"

This paper proposes a critical review of the different aspects concerning magnetic refrigeration systems, and performs a detailed analysis of thermodynamic cycles, using mathematical models found in the literature. Langevin’s statistical mechanical theory faithfully describes the physical operation of a refrigeration machine working according to a magnetic Ericsson cycle. Results of mathematical and real experimental models are compared to deduce which best describes the Ericsson cycle. The theoretical data are not perfectly consistent with the experimental data; there is a maximum deviation of about 30%. Numerical and experimental data confirm that very high Coefficient of Performance (COP) values of more than 20 can be achieved. The analysis of the Brayton cycle consisted of finding the mathematical model that considers the irreversibility of these machines. Starting from the thermodynamic properties of magnetocaloric materials based on statistical mechanics, the efficiency of an irreversible Brayton regenerative magnetic refrigeration cycle is studied. Considering the irreversibility in adiabatic transformations, the lower limit of the optimal ratio of two magnetic fields is determined, obtaining a valid optimization criterion for these machines operating according to a Brayton cycle. The results show that the Ericsson cycle achieves a higher Coefficient of Performance than the Brayton cycle, which has a higher cooling capacity as it operates with a larger temperature difference between the magnetocaloric material and source.

Thesis (Master Diploma (Electrical Engineering))--Cape Technikon, Cape Town,1996
In order to design a coolerbox, utilising Thermo-electric technology as a heat pump, it was necessary to determine if this type of technology would be suitable for a coolerbox heat pump application. A detailed TEC (Thermo Electric Cooler) sizing estimation was done, using formulas supplied by the manufacturer, and using thermo-electric themy formulas to calculate the absolute theoretical performance parameters for a suitable TEe The correct heat pumping capability is important since.it is possible to obtain TEes with different input currents, voltages and heat pumping capabilities. Using the TEC as a heat pump, it was decided to use water cooling due to the extremely high cost of a suitable air cooled heatsink. It should be noted that to cool the hot side of theTEe a vel}' efficient heat exchanger should be used. A simulator was constructed to simulate, under variable operating temperatures and input powers, the cooling capacity of the TEC heat exchanger. The cooling characteristics were then used to determine if the theol}' and manufacturers claims correspond with the cooling characteristics of the coolerbox. It might be possible that the performance of the TEC would be drastically influenced, since installation con~itions may not be ideal when installed; and, that the manufacturers performance claims are done when the TEC is operated under ideal conditions. This would ensure optimum results because, should an under sized TEC be used, the result would be poor cooling characteristics; or, if the TEC is over sized, the coolerbox would consume too much power, resulting in an inefficient system.

This work was undertaken to investigate the perceived problem of Thermal Discomfort in Malawi. One observable effect of thermal discomfort was the amount of foreign exchange that was spent to import air conditioning devices. The purpose of the work was to find out, and quantify the problem of thermal discomfort and outline its effects to the people and country. In order to investigate the problem of thermal discomfort in depth in a place where the necessary data hardly existed a lot of work had to be done. The work has been outlined in four stages of research, analysis and documentation and these are as follows 1 Literature Review The subject of Thermal Comfort appears to be location specific, but the general principles are universal. In that context it was necessary to read widely on both historical and contemporary current work. The problem of thermal comfort in general was being discussed as early as 1758 and still remains a big area of research and discussion today. A considerable number of literature that specifically relate to the problem of thermal comfort in the tropics has been reviewed. The problem of scales for thermal comfort measurement has been discussed in detail. It is still not possible to quote a scale that is satisfactory. However, the recent approach of Adaptive Thermal Comfort Model seems to be closer to the answer than the others 2 Analysing Existing Relevant Information And Data In Malawi In the course of this work it was found out that quite a large amount of useful data existed in Malawi. However, this data was not standardised. Most of this data had to be cleaned and updated. Some of the old formulae are quoted in their original formats in order not to confuse the referencing. The data that exists in Malawi has been recorded on three types of instruments; namely the Gunn Bellum Spherical Pyranometer, the Camp Bell Stoke Sunshine Recorder and the Eppley Pyranometer. Most of the data was recorded using the Camp Bell Stokes Sunshine Recorder. The data recorded on the Gunn Bellum Spherical Pyranometer had to be related to that from the Camp Bell Stokes Sunshine Recorder. The former gave data that was more accurate as was found out when a comparison was made with data recorded on an Eppley Pyranometer. A paper on this subject was accepted for publication in the Renewable Energy Journal of WREN. Wind speeds, air temperatures, and humidity have been analysed to investigate the severity of thermal discomfort relative to locations in Malawi. This has resulted in the identification of three climatic zones. A tool for testing Thermal Discomfort severity of a location by calculating number Degree Days (D. d) if the altitude (AL) has been developed; as D. d = -575.994 In AL + 4226.6 3 Field Measurements In order to investigate some of the issues that came out of this work, it was felt simpler to conduct field measurements. For example it would have been possible to build typical experimental houses, and extract performance data on Thermal Comfort from these buildings. However, this approach would have been very expensive. On the other hand it was felt that it was possible to find in the field that were representative of typical buildings and could be prepared and tested to extract performance data for use in the work. The latter approach was adopted and has proved to be more realistic than the former. 4 Field Surveys There were certain areas where the only way to find information was not to conduct experiments but to conduct field conduct surveys. This was done once to find the Preferred Bath Water Temperature (PBWT) and deduce the Neutral Temperature Range for Malawi. This yielded very useful results. The first published paper on this work was in this area (copy of this publication is attached). The second area of field survey was to survey traditional buildings in seven selected districts stretching from latitude 9°S to 17°S; covering a terrestrial distance of over 1000 km; over altitudes from 52 to over 1600 metres above mean sea level (m. a. m. s. l). This again yielded very useful environmental data that explained why traditional buildings have certain structural elements as functions of the environment and the need to achieve Thermal Comfort. A number of useful equations have been developed. From that sub routine of this research of PBWT survey an equation was developed that related the bath temperature (h) to the air temperature (tab) as; tb =0.3772 tab + 36.4401. Part of this work was also published separately in 2001. From this equation the Thermal Comfort Temperature Range for Malawi was deduced as 22-27°C. From the survey of the traditional buildings, a number of structural elements were that are functions of Thermal Comfort were identified as derivatives of the desire to have Thermal Comfort in the buildings. A regression equation that can give values of irradiation of the locality in MJm 1 Day' was developed. Lastly the results have been extracted as recommendations directed at policy makers, and both Architects and Engineers to use this data and the results in their design work. It is also further recommended that the national buildings regulations could be updated and revised to incorporate some of the findings. It is strongly believed that some of the findings will be incorporated to update the two main Laws that regulate Public Health in Malawi. These are the Public Health Act; Cap. 34.01, and the Health and Safety at Work Act, 1977; of the Malawi Laws. All data that has been cleaned up or measured specifically for this work has been organised and tabulated into ready-to-use tables and are included.