Rail vehicles must have a reliable braking system. They generally use compressed air, which is generated by screw compressors. Photo: Fotolia / VRD Rail vehicles without compressed air? Practically inconceivable! Screw compressors make reliable braking systems possible Christian Rau and Ekkehard Cramer Rail vehicles irrespective of whether they are locomotives, carriages, regional trains, trams, or underground trains need a reliable braking system. Such systems generally use compressed air, which is generated by screw compressors, or in rare cases, by piston compressors. These compressed air generators have to be developed specifically for the very special applications and installation conditions in these vehicles. Compressed air in rail vehicles Compressed air is a simple medium used to supply various consumers primarily the brake actuators in rail vehicles the world over. The compressed air is generated with a compressor which is normally installed in the locomotive or the multiple unit. The compressed air is conveyed from the primary receiver to the consumers via the primary receiver line with hose connections. Compartment doors, external doors, lavatories, pantographs and suspension systems use compressed air. However, the braking system is the consumer which determines how much air a rail vehicle needs. The braking systems on main-line European railways are primarily controlled with the help of an electropneumatic system. Screw compressors for compressed air generation The compressed air required is generated with a piston or an oil-injected screw compressor. Screw compressors are preferred because they run noiselessly and reliably with a higher output and are designed to be particularly space-saving and compact. Screw compressors also generate the compressed air required to shunt locomotives, as they often have to move and decelerate very long trains. On the other hand, piston compressors are mainly used where the require-
78 Innovations & trends Figure 1: Compressed air generator in a framework for installing under the floor: Minimising the weight and using the space available are crucial decision factors. ment for compressed air is not high, in regional trains, for example. The compressor supplies the main air receiver(s) in the locomotive with a maximum overpressure of 8 to 10 bar, while the actual braking system operates at less than 5 bar. The compressed air is filtered and dried before being supplied to the vehicle s network. Illustrations: ALMiG Kompressoren The high quality of the air is achieved with an intelligent arrangement of various coordinated oil/water filters, air filters in a range of classes and and effective air-drying system. Requirements of the compressed air Both air consumers and compressed air generators and treatment systems must cope with the demands of rail vehicle operation. The primary focus is on their safety and availability. High availability is particularly relevant to safety, not only to ensure smooth operation under normal circumstances but especially in emergencies. These demands are met with a concept that coordinates monitoring and control mechanisms. A high level of safety is realised particularly by the choice of high-quality components which undergo testing in real conditions. One of the special conditions posed by rail vehicle operation is the wide range of ambient temperatures, normally from -25 to +50 C, in which an air supply system must work; even extremes of -40 to +60 C are not uncommon. Extensive testing in a climate chamber provides the necessary evidence of the suitability of individual components and even of whole systems. Air is usually required to comply with quality class [2.2.2] in accordance with DIN ISO 8573-1, in order to effectively prevent condensation from forming anywhere in the braking system and therefore to protect all the applications from freezing in sub-zero temperatures in winter, and to ensure that all the pneumatic components can function as reliably as possible. The system is also required to be resistant to the vibration and shocks that come from jolting over the rails and points, for example. The components are tested on test benches in accordance with specific rail standards and the durability of the products is simulated in long-term experiments. A rail vehicle and its major components are designed to last for 30-40 years; a general overhaul of the major components every 6 to 8 years is usual and in some cases required by the authorities. Another important safety aspect is to minimise the risk of fire which can be posed by a component. It is important that the operation of the system is monitored and crucial that the materials used are chosen to minimise the general risk of fire. Appropriate evidence regarding the materials in the form of test certificates complying with special rail-related standards are therefore a significant part of the licensing process. Figure 2: Condensate receiver made of high-quality, flexible plastic also suitable for frozen condensate. System solution for compressed air generation Modular oil-injected screw compressors offer a system solution for compressed air generation and treatment in harsh environments on
COMPRESSORS, COMPRESSED AIR & VACUUM TECHNOLOGY 79 trams, underground and railway trains, buses and HGVs. They can be tailored to specific customer requirements and thanks to their compact design they are perfect for installation in tight spaces. They can be supplied as both a free-standing unit or in a framework construction made from steel or aluminium, designed for underfloor and roof installation. As an option, the systems can be equipped with all the components required to prepare compressed air (filter and drying systems, for example) and to control the system. Depending on the model, the customer can opt for a direct drive with an AC motor, which is predominately used, or a gear, universal shaft or V-belt drive, hydro drive or speed-controlled drive. Other options are modules with inputs from 4 to 40 kw, volume flows from 500 to 3,500 l/min pressure levels from 5 to 13 bar start-up with a star-delta control system or a frequency converter. Typically, what customers want from compressed air generation is a long service life and high reliability plus low maintenance costs, long service intervals, low weight, low-vibration and quiet operation. As the compression stages used by ALMiG in its modular series can be equipped for any drive technology required by the customer, these screw compressors always provide tailored solutions. The number of interfaces on the modules is kept low, so as to minimise installation input for the customers and to eliminate as many sources of error as possible during installation in the vehicle. This is why ALMiG fits all the necessary components, including the control cabinet and compressed air dryer, on the compressor module in advance, where at all possible. The overall concept is furthermore designed to be very maintenance-friendly. The vehicle s control device provides information about all the important units on board. This system also records the screw compressor s operating hours. All the
80 Innovations & trends the start of a journey. This is an application that requires a considerable amount of compressed air. It was essential to comply with the following performance parameters: Performance: Operating pressure: Voltage supply: 2,400 l/min 10 bar 400 V /50 Hz Ambient temperatures: -25 to +50 C Compressed air dried in a two-chamber adsorption dryer Compact design, simple installation, low noise level, already tested in rail vehicles and certified. Figure 3: Installation in a diesel locomotive: High-end compressed air generator in a tiny space. control and monitoring functions (switch-on and switch-off procedures, loaded/idle mode control, over-run times, temperatures etc.) have been coordinated with each other. Faults are registered in the locomotive s control unit and reported to the driver. Modular oil-injected screw compressors offer a system solution for compressed air generation and treatment in harsh environments on trams, underground and railway trains, buses and HGVs. A sample application: Electric locomotives for shunting and freight trains A leading European manufacturer of rail vehicles was looking for a compressed air generation solution for a newly developed range of electric shunting and freight locomotives. The performance of the compressed air supply, 2400 l/min, was deliberately specified at a rate normally reserved for large locomotives, so that the compressed air system of a longer train could be filled within the shortest time possible. The individual carriages are also equipped with separate auxiliary air receivers with a volume of e.g. 200 l. When a train has been parked for a long period of time, the entire line and receiver system has to be filled as quickly as possible before Solutions and applications A compact compressor module was developed using standard components. An initial sample system was first tested by ALMiG at -20 C in a climate chamber, and then tested to ensure that it complied with the overall concept and installation dimensions specified. It was then approved. A very short and compact, electrically driven, directly coupled screw compressor with oil-injection cooling formed the basis of this module. The compressor system is intermittently moved by the pressure measured in the reserve receivers depending on its level (switch-on pressure 8 bar, switch-off pressure 10 bar). The fresh air drawn into the locomotive is first cleaned of leaves and large particles of dirt in a pre-filter. As it is installed in a permanently clean and dry location, the compressor can be equipped with a standard intake filter. The oil used in the screw compressor for lubrication, internal sealing of the compression stage and transmitting the heat generated during compression releases the heat into the atmosphere via an air-cooled cooler. Any condensate produced is collected in a receiver (with a volume of 70 l in this case), which, thanks to the elasticity of the plastic used, is capable of withstanding water expanding when frozen in winter. A level probe sends a signal to the control centre when the receiver is 80% full, so that the condensate containing residual oil can be drained at the depot and disposed of in accordance with regulations. At an ambient summer temperature of +30 C, ambient pressure of 1 bar, relative humidity of 70% and pressure of 10 bar, a screw compressor in continuous operation produces just 0.04 kg/h
COMPRESSORS, COMPRESSED AIR & VACUUM TECHNOLOGY 81 (around 40 cm³/h) of condensate, despite the extreme conditions assumed here. After 24 hours of continuous operation, just 1.008 l of condensate would therefore be produced. But because the screw compressor is used intermittently and not in 24-hour continuous operation, the amount of condensate produced is actually much lower. The condensate receiver therefore only needs emptying every two months at the most, even under such extreme conditions and in 24-hour continuous operation. As a result of low air temperatures in winter and the therefore greatly reduced humidity of the compressed air, the amount of condensate produced might even be just 10% of the figure given above. It is therefore theoretically possible that during the winter months, the receiver will not need emptying, even when operation is continuous. Dry compressed air The mixture of oil and air generated during the compression stage is mechanically separated into the oil and air components in the oil separator receiver. An additional wash plate in the Figure 4: TrackAir module installed below the vehicle. oil receiver prevents movement in the oil from building up when the locomotive is underway and guarantees a constant oil level. Vaporous oil components are held back in a downstream fine separator. A two-part oil air cooler (combi cooler) cools both the compressed air and the oil separated in the oil separator receiver. This cooler lies horizontally below the compressor unit. The ventilator mounted on the cooler intakes
82 Innovations & trends the clean cool air from the compressor chamber and pushes it through the cooler and the floor of the locomotive box into the atmosphere from above. The cooler continually blows the cooling air outside and therefore effectively prevents dirt particles from entering the cooler from the track bed below. After passing through another cyclone separator and a two-stage pre-filter, the compressed air, which has a residual oil content of just 0.1 to 0.4 mg/m³, is dried in a cold regenerative two-chamber adsorption dryer. Unlike standard industrial practices, this dryer does not operate at a constant pressure dew point, but a variable one, which results in a so-called dew point reduction of 20 to 25 C below the corresponding ambient temperature. Within the entire compressed air network, this system prevents free humidity from escaping inside the locomotive, carriages and the entire pipe system. The variable dew point also permits particularly small sizing compared with an adsorption dryer with a pressure dew point that is constant all year round. The adsorption dryer is regenerated fully automatically over a pre-defined period using a low partial flow of compressed air that has already been dried. The drying agent has a service life of around 3 to 4 years. Unlike a vertical adsorption dryer used in industry, in which the regenerative air flows from the bottom up, the dryer was installed horizontally in the solution implemented. A supporting screen prevents the drying agent from being destroyed as a result of internal friction caused by motion and vibrations when the locomotive is moving. The Figure 6: The lengthy IRIS certification process: Shake and vibration testing. small number of drying agent particles that do however get through the screen are effectively removed from the compressed air by an afterfilter. The construction tolerances and the materials used in the overall design mean that the compressor can run smoothly at supply air temperatures down to -30 C. Additional heating systems make for faultless operation of the compressor, even when temperatures are very low outside. A fast-acting electric auxiliary heater in the oil receiver guarantees an oil temperature of -10 to -5 C and therefore that the oil is sufficiently viscous before the compressor is started after the locomotive has been stationary for a long time, even during extreme winter temperatures. A safety circuit prevents the compressor from starting prematurely. A sample application: Systems for regional multiple units and urban rail networks Another application of compressors in the railway environment are systems for regional multiple units and urban rail networks. At 600 to 1500 l/min, less compressed air is required in these vehicles than for locomotives which have to supply whole trains with compressed air. In a large modern city, a metro train is often in use for more than 20 hours every day. It is therefore particularly important that it operates without breakdowns and that the air supply systems are highly available. Figure 5: Faultless compressed air generation even at -55 C: Testing in the climate chamber. These trains operate both above and below ground and travel constantly into and out of
COMPRESSORS, COMPRESSED AIR & VACUUM TECHNOLOGY 83 tunnels. This makes great demands on the resistance across the system to extremes of temperature and particularly on the effectiveness of the air drying systems. Modern regional and metro vehicles usually have a lightweight carriage structure. The weight of the modules installed must therefore be kept as low as possible. With all its components and the sound-insulated framework, a metro air supply system weighs less than 300 kg. Systems that run as quietly as possible, with little noise or structure-borne sound, are preferred so that the passengers have a pleasant travelling experience. The air supply systems are normally installed under the floors of the vehicles. The framework modules have to be designed especially for the installation, with the air supply and treatment components integrated in them. With installation constrained by these conditions, it is particularly important to protect the components from stones, dirt and corrosion. Furthermore, the design had to take into account easy access to the components requiring maintenance, such as filters and separators, to make it as simple as possible for the vehicle operator. Modern, innovative screw compressor systems will continue to have to meet this wide range of demands posed by different vehicle types. Authors: Dipl. Wirt.-Ing. Christian Rau Dr.-Ing. Ekkehard Cramer ALMiG Kompressoren GmbH