Download PDF - Voith Turbo

More documents

Recommendations

Info

3. Operation of cooling system fans Fan design depends on the following factors: - the type of vehicle and thus the type and arrangement of the cooling system in the vehicle, - the nature of the components to be cooled and the type of coolant, - the type of drive and control of the fan and cooler group, - the operating mode and design of heat exchangers, and - the total flow loss of the system. It has become apparent over time that certain arrangements, cooling system types, and fan drives should be preferred depending on vehicle type. 6 Fig. 2: Axial flow fan For locomotives, the cooler group can usually be arranged next to the component to be cooled. For railcars and trainsets with underfloor powerpacks a greater distance may be necessary or the cooler group may have to be arranged under the roof of the vehicle. Sometimes, the axial-flow fans (fig. 2) mostly used in cooling systems are replaced by radial-flow fans today if the installation conditions and ratings are favourable and noise is to be minimized. In the majority of cases, however, axialflow fans are adequate for the total pressure loss mostly found today. Moreover, axial-flow fans can be easily adapted to the required cooling air flows. With radial-flow fans, in contrast, the possible pressure head cannot be utilized. Cooling air flow is limited due to the limited intake velocity of air in the ∆ptot in Pa 600 500 400 300 200 100 Fan characteristic η = 0,58 η = 0,75 intake funnel so that efficiency levels remain unsatisfactory, as can be seen from the location of the working point on the performance curves (fig. 3). The working points where efficiency is higher are further to the left or right on the performance curve of the fan and cannot be reached in practice. Cross-flow fans are rarely used in rail vehicles. Their advantages regarding the arrangement of radiator and fan are offset by an extremely low pressure head and low efficiency, among other things. η = 0,74 0 0 0,3 0,6 0,9 1,2 1,5 1,8 V L in m 3 /s Fig 3: Performance curve of a radial-flow fan without spiral case in an underfloor-mounted cooling unit. B = Operating point B η = 0,53 System characteristic
3.1 Drive and control Today, the drive and control of the fan cannot be separated any longer from coolant temperature control. Both are performed using advanced electronic systems. The requirement is to keep the operating temperature of the components to be cooled as constant as possible and to maintain close temperature limits for certain operating conditions, irrespective of variables such as drive speed, loading condition, or cooling air temperature. For all liquid-cooled components, temperature control of the system is realized via speed control of the fan and carefully matched to the thermal behaviour of the system, taking into account the overall concept of the cooling circuit. This presupposes the availability of information and knowledge about the different characteristic operating conditions of the components to be cooled. With modern diesel engines, the thermal characteristics of the start-up phase and the effects of prolonged idling are of particular importance. Additionally, the partially conflicting requirements of cylinder cooling and charge-air cooling in the case of single-circuit systems and the interaction of the circuits in the case of two-circuit systems must be considered. For engines with charge-air-air cooling, in particular, care must be taken to prevent critical operating conditions such as excessive cooling or overheating. Further development of diesel engines to improve efficiency and more stringent exhaust gas regulations in conjunction with the use of exhaust gas turbochargers have resulted in an increase in operating temperatures. The higher coolant temperatures resulting from this require higher safety in respect of coolant circulation. Moreover, the reduced temperature margin up to the failure limit caused by the higher operating temperatures makes it necessary to define the control temperature range even more accurately than it was the case with older types of diesel engine. With diesel-hydraulic vehicles, waste heat from the hydrodynamic transmission and from the hydrodynamic brake, if any, must be dissipated in addition. The situation with regard to transformer cooling is less complex and easier to understand. Temperatures in the cooling circuit are usually lower than in the case of engine cooling, and they change less quickly due to the high transformer mass. The control is matched to the operating conditions of the transformer, preventing both excessive temperatures and excess cooling when ambient temperature is low. Coolant circulation problems can occur when the system is started at a very low temperature and coolant flow through the radiator element is impaired or even inhibited by a highly viscous coolant. This situation cannot be handled by the temperature control alone but requires selection of a suitable coolant. Converter cooling requires a comparatively high cooling air flow rate due to the small difference between operating/ coolant temperature and ambient temperature. In countries with a hot climate the coolant temperature is only marginally higher than the ambient temperature. The high efficiency of present day converters is due to the use of temperature-sensitive components with a low mass. Adherence to close temperature limits and avoidance of excess temperatures are essential for a long service life and high efficiency of these converter elements. This presupposes reliable temperature control and quick response of the fan drive control. When the converter and transformer are both cooled by the same fan, unfavourable or extreme conditions may require concessions to be made for one of the two cooling circuits. This also applies when further electrical components in the cooler group are to be cooled directly by the cooling air flow. There are three different approaches towards meeting the above demands regarding cooling system control and fan speed control for the fan drive. 3.1.1 Mechanical / hydrodynamic drive The simplest way of driving a fan - with the cooler group arranged directly next to the power pack - would be a purely mechanical, rigid drive without control options. Fan speed would then be coupled to the speed of the prime mover, usually a diesel engine. This is an inadequate solution because of the fluctuations in operating temperature that result in changes in the required cooling capacity due to different load stages and ambient temperatures. Even the addition of system components like coolant temperature controllers or mixers would not make this type of drive efficient. The shortcomings of cooling systems with rigidly coupled fan and prime mover led to the development of mechanical/hydrodynamic and hydrostatic drive systems for diesel-powered vehicles that offer different control options and high flexibility with regard to fan speed. 7
Page 1 and 2: Fans in rail vehicle cooling system
Page 3 and 4: Summary Fans are a key component in
Page 5: specified working point (L in fig.
Page 9 and 10: Capacity of hydrostatic pump 1,0 0,
Page 11 and 12: temperature overlaps between coolan
Page 13 and 14: 3.4 Arrangement of the fan The vehi
Page 15 and 16: 3.6 Noise emission and sound insula
Page 17 and 18: 5. Economical considerations Develo
Page 20: 20 Voith Turbo GmbH & Co. KG Divisi

Download PDF - Voith Turbo

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?