Why air dryer?
Compressed air is used on diesel and electric locomotives for brake system and operation of other auxiliary devices. The compression cycle of ambient air includes compression in locomotive compressor and then cooling in after cooler. During compression, pressure & temperature of ambient air increases hut its relative humidity decreases. However, absolute amount of the moisture present in the air remains same. After compression when the air is cooled in after cooler, it becomes saturated and condensation takes place. This condensed moisture is drained out from main reservoirs. A major portion of moisture is removed in this process, but water still remains present in the form of vapour in the compressed air. This water vapour condenses in air brake equipment/valves, air pipes and auxiliaries during train operation.
It is important to 'manage' this moisture contained in the air (humidity) in order to minimize its effect on air brake equipment. Excessive moisture in the compressed air system can lead to several undesirable conditions such as the condensation which takes place in air valves and air pipes leads to following problems:
- Malfunctioning of air valves due to rusting.
- Sluggish operation of moving parts of the valves.
- Poor performance of the valves due to washing away of the lubricant.
- High flow resistance and pre-mature blocking of filters.
- Rusting of air pipes.
- Poor performance and low life of rubber components of the valves.
- Sticky and greasy accumulation on the components of the air valve.
- Freezing of the water droplets when ambient temperature drops below freezing point. Although this is not a major problem in India.
- Choking of small chokes and strainer in the valves.
Moisture present in compressed air result in malfunctioning poor reliability and increased maintenance of brake system of locomotive and trailing stock. The problem of moisture in compressed air is overcome by using air dryer on locomotive.. Twin tower heatless regenerative air dryer reduces moisture content of compressed air to such an extent that condensation of moisture does not takes place during expansion of air in brake valve even at very low ambient temperature.
How it works?
Condensation of water vapour in brake circuit can be overcome if dew point temperature of air is reduced. This can be achieved by reducing moisture content of compressed air by adsorbing the water vapour in air dryers. On locomotives heat less regenerative twin tower compressed air dryer is used. This type of air dryers use desiccant to adsorb the moisture. Desiccant used is normally in the form of granules. However, after some time, desiccant is completely saturated and no longer capable to adsorb moisture. Therefore, water adsorbed is to be given aeration of saturated desiccant is done by drying it by dry air (regenerative air). For this a small quantity (10 to 15 %) of the air dried by the desiccant is used. Rest of the dried air is used for locomotive operation.
Function of Air Dryer
As mentioned moisture removed by cooling is limited by ambient temperature. Therefore air dryers are used as drying system to reduce moisture content of compressed air. Air dyer works on refrigeration or adsorption principle. Refrigerated cooling of compressed air to remove moisture is not practical on locomotive, therefore, on locomotives, adsorption type air dryers are used to remove moisture to reduce dew point depression below ambient temperature. Air dryer removes moisture of compressed air by adsorbing water vapour. Drying agent in air dryer is desiccant which is in spherical or granular form. Desiccant varies in composition and size depending upon the type of dryer. Generally all desiccants have a high surface activity and can be considered as a vast honeycomb of very small pores. They provide a large internal surface area for drying. Most commonly used drying agents are silica gel, activated aluminum oxide and molecular sieves consisting of sodium, magnesium and aluminum silicates.
Specific choice of desiccant is made on the basis of various parameters like air inlet temperature/pressure, flow rate and quantity of contaminants. Silica gel performs best when relative humidity of air is 40-80%. Its pore size is 4-20 Angstrom; it is a low cost desiccant. Molecular sieve is a high grade desiccant and perform well when relative humidity of air is below 40%. Its pore size is extremely small and ranges between 3-5 Angstrom. Activated alumina is suitable for relative humidity of 80-100%. Its pore size is of the order of 4-20 Angstroms.
With time desiccant bed adsorbing water vapour looses its capacity to adsorb further moisture; therefore adsorbed moisture is to be given away from its pores to reactivate it for moisture adsorption. This process is called regeneration. Regeneration is done by two processes.
- By heating the desiccant.
- By dry compressed air.
Since heating is not practical on locomotive, heatless regeneration by dry compressed air is employed on locomotive. Locomotive air dryers consist of two desiccant beds in two separate towers. One tower adsorbed moisture of incoming air whereas other tower at that time is in regenerative mode. A part of dry air of first tower is utilized to regenerate desiccant of second tower. Dry air utilize for regeneration is normally 10-15% of tot inlet air supply.
Operation of Air Dryer
Fig.3 shows operation of a typical twin tower, heatless regenerative air dryer used on locomotive. Compressed air- after main reservoir (1) of diesel' locomotive (MR-2 in case of electric loco) passes through a coalescing filter (2). This filter removes significant amount of debris, moisture and oil from the air. After coalescing filter air enters into the actuator (3) which decides the entry of air to either of two desiccant towers (4a and 4b). Position of actuator is controlled by timer unit (5). For example in a particular cycle wet air enters from the bottom of the tower (4a) and passes through desiccant bed where moisture is adsorbed by the desiccant. Dry air with dew point less than ambient temperature comes out from the top and finally goes out of dryer to final filter (6). Final filter removes desiccant powder if any from the dry air and send it to main reservoir-2 (7) of the locomotive A small part of the dry air (10-15 % of inlet air capacity) is drawn from the dry air outlet of tower (4a) and is passed through regenerative orifice of non-return valve (8) and on to the desiccant bed of the other tower (4b). It removes the moisture collected in to second lower and thus reactivates the desiccant. The moisture laden air is vented through exhaust valve which is also called purge valve (9). Thus in this cycle tower (4a) works as dehydrating tower and tower (4b) works as regenerative tower.
After completion of this cycle the working of two towers reverse and now the tower (4a) works as regenerative tower and tower (4b) works as dehydrating tower. In these way two towers alternatively works as dehydrating/regenerative tower. This reversal takes place after a predetermined time period. This is called cycle time of the air dryer. Cycle time of air dryer is normally 1 to 2 minutes.
Performance Parameters of Air dryer
- Dew point depression
- Purge Loss
- Pressure Loss + Desiccant life
- Actual performance on locomotive
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