Mechanical Seminar Topics
Mechanical Seminar Topics Abstract
This Automobile mechanical seminar topics paper presentation presents new trends in the area of automobile air conditioning, which is fast becoming standard equipment. Attention is focused on the refrigerant and ventilation circuit of the air conditioning equipment, and on the control system. An analysis is presented here of a commonly used contemporary refrigeration system working with the refrigerant R134a and a promising trans critical refrigeration system using CO2 as a refrigerant. The article also deals with the configuration of some components of a trans critical refrigeration system. Additionally, attention is paid to physiologically controlled air conditioning and multi zone air conditioning systems which make it possible for every individual in the automobile cabin to have an optimum micro climate.
Key words: air conditioning, automobile, refrigerant circuit, ventilation circuit, control system.
Automobile air conditioning has become more and more commonplace. The air conditioning system ensures a thermal comfort for passengers, but also contributes to the defogging of windows and thus increases the active safety factor. Automobile air conditioning equipment is markedly different from the air conditioning facilities found in buildings since, in contrast to the stable conditions of a building, conditions inside an automobile vary markedly in both time and space. Among the most significant new trends in automobile air conditioning is the use of alternative refrigerants to replace the more usual R134a, the use of physiologically controlled air conditioning, and multi zone air conditioning systems. Because of its contribution to the greenhouse effect, the long-term prospects for R134a, which has been used to the present time, are not very good. The best candidate to replace R134a as a refrigerant in automobile air conditioning is CO2. The refrigeration cycle functioning under CO2 is trans critical and, due to the working pressures in the refrigeration system, some components require special configuration. At the present time, the majority of fully automatic air conditioning equipment used in automobiles regulates themselves based upon temperature measurements in the area near the driver. Evolution is toward so-called physiologically controlled air conditioning, in which an optimum thermal microclimate is reached by means of several sets of sensors which measure the thermal state in the automobile cabin. This form of regulation also includes a sensor to measure pollution in the outside environment. Also among these new trends are systems 2 which make it possible to climatise the cabin using several zones, thus creating an optimal microclimate for every passenger, at both the head and foot level. The automobile air conditioning system consists of the refrigerant and ventilation circuit, and a system of control.
Analysis of Air Conditioning Systems Operating with CO2 and R134a
Automobile air conditioning systems presently use the refrigerant R134a. Because of its contribution to the greenhouse effect, which is 1300 times that of CO2 , its long-term prospects are not good. Intensive research and development is underway into refrigeration systems using alternative refrigerants (Joudi et al 2003). CO2 is among the alternative refrigerants used for automobile air conditioning. CO2 refrigeration system functions in air conditioning equipments right at the critical pressure of 7.38 MPa, or sometimes above it (Kim et al 2004). Heat transfer therefore takes place in many cases at supercritical temperatures and the cycle is trans critical, i.e., it has a subcritical low-pressure side and a supercritical high-pressure side.
At supercritical pressures, saturation conditions do not exist and pressure is independent of temperature. For a given evaporation temperature and minimum heat rejection temperature on exit from the cooler, a trans critical cycle has greater thermodynamic loss than a condensation cycle (Fig. 4).
As a consequence of the higher median temperature of heat rejection in the CO2 cooler and the greater heat loss due to throttling, the theoretical cycle work for CO2 increases compared to a conventional refrigerant like R134a. The use of two-stage compression and a cycle with sub cooling liquid refrigerant in the internal heat exchanger leads to improved capacity and coefficient of performance (COP) and a reduced size for the refrigeration equipment components, and also prevents wet vapor from entering the throttling valve. The refrigerant is sub cooled by vapors exiting the evaporator which are thus overheated, preventing the compressor from taking on wet vapor.
Draft for a CO2 component system
In CO2 systems, the compressor works under high median effective pressure and the pressure ratio (discharge pressure to suction pressure) is relatively low. The pressure ratio to deliver identical refrigerating capacity is 3.1 using CO2 and 5 using R134a. The compressor becomes more efficient as the pressure ratio tends lower. For the trans critical cycle, the compressor requires thicker walls, but it is still smaller than a compressor which delivers the same capacity using R134a as a refrigerant. Piston and rotary compressors (rotary vane, rolling pistons, scroll) both single- and two-stage, are being developed for CO2 refrigeration systems. The use of two-stage compressors improves COP by up to 20%. A CO2 gas cooler has better heat transfer as a result of greater convection heat transfer in the vicinity of the critical point and as a result of the high pressure, which allows for higher velocities of refrigerant flow. Because of the high pressures, a cooler with flat micro channel tubes is used, fitted with folded louvered fins as we can see in Fig. 5 (Pettersen et al 1998). The internal heat exchanger, in which the refrigerant exiting the cooler is sub cooled by vapors exiting the evaporator, can increase the efficiency of the cooling cycle working under CO2 by up to 25%. An example of micro channel configurations for the internal heat exchanger is given in Fig. 6 (Kim et al 2004). These configurations, in comparison with the conventional concentric tube designs, reduce demands on the material by 50% and increasing effectiveness by 10%.
The analysis offered here of automobile air conditioning systems working with the refrigerant R134a and more promising trans critical systems using CO2 demonstrates that CO2 technology brings with it many advantages. As an example, when refrigeration systems work with CO2 and R134a and with heat exchangers of identical proportions, system using CO2 will have greater refrigerating capacity and allow lower cabin temperatures to be achieved in the automobile at the same time it cuts fuel demands by 25 to 30%. Some components of refrigeration systems working under CO2, however, require fresh configuration. Modern physiologically controlled air conditioning makes possible the automatic control not only of temperature in the automobile cabin, but also humidity and velocity of air, and permits automatic switching between external and internal air circulation. With multi zone automatic air conditioning systems, it becomes possible to create an optimal microclimate for each individual in the passenger compartment of the automobile.
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