Whether used for ventilation controls, door actuation, clutches, brakes, auxiliary panels, or any other function, pneumatic components in rail equipment may have to operate in temperatures as low as –40°.
Many manufacturers provide pneumatic cylinders and valves built to function in this demanding temperature range. However, unless an effective method is provided — and maintained — to dry the compressed air, equipment malfunctions and premature component failure will result. Even at moderate ambient temperatures, unless moisture is removed from the compressed air, it will condense as liquid water in the lines. If ambient temperature drops below freezing, ice can form in the lines and in the components. Even if it doesn't freeze, water in the compressed air stream can wash away lubricants in cylinders and valves.
Therefore, whether the application is rail, truck, bus, or any equipment that operates outdoors, if sub-freezing ambient temperatures may be a reality, then providing an effective means of drying air is essential. However, many different types of dryers are available — refrigerated, desiccant, deliquescent, and membrane types.
As it turns out, desiccant dryer systems are most suited for rail, truck, and bus applications because of their small size and simple operation.
Desiccant dryer systems
Desiccant (adsorption) dryers pass air over material that absorbs liquid from the compressed air system. Two types of desiccant dryer systems are commonly used in transportation applications – single tower and twin tower systems. Both remove water from the system, but some key differences make one style more advantageous over the other, depending on the application. The differences are size, cost, and the amount of service required to maintain the system.
Both single and twin tower desiccant dryers operate on the same basic principle of cyclical adsorption and desorption of water vapor from the compressed air stream. A twin tower dryer can provide a continuous stream of dry air to the application. This is possible because as the air to the application is dried in one tower, the desiccant in the opposite tower is regenerated. This occurs by taking a portion of the dry air at the dryer outlet, expanding it across a purge orifice, and using this purge air to regenerate the desiccant bed of the "offline" tower. This cycle switches every few minutes, and the regenerated desiccant bed begins drying the air stream, and the original tower begins regenerating.
In a single tower design, dry air to the application is stored in a storage tank (receiver) downstream of the dryer. The tank is sized with adequate capacity to maintain the air system, plus an additional volume to allow purge air to be stored. When the desiccant bed is ready for regeneration, the compressor is unloaded, and the desiccant bed is depressurized.
The purge air stored in the tank then is expanded across a purge orifice and counter-flowed through the desiccant bed to regenerate it. This cycle is fairly short, with purge occurring every 90 seconds or so. The purge air requirement for a twin tower dryer is typically 15% that of the rated inlet flow. The purge air for a single tower dryer can be 15% to 25% of rated flow, depending on the dryer and the application. All desiccant dryers require clean, high quality, oil-free air to operate reliably. High efficiency filtration, including a coalescing (oil removal) filter is a must, not an option.
Single tower dryers are better suited for buses and trucks because space is at such a premium. The smaller footprint is easier to accommodate in system designs. Along with the smaller footprint, and at a cost of nearly half the twin tower models, single tower units are considerably more economical when it comes to initial installation costs. However, the operating life expectancy of a single tower dryer is much shorter than its twin tower counterpart.
Maintenance requirements for single tower dryers are more demanding as well. The maintenance schedule for a single tower dryer calls for the desiccant to be replaced every 30 to 45 days. For this reason alone, the single tower system is not appropriate for rail applications. The industry standard for scheduled maintenance, set by the Federal Rail Administration, is 92 days. This means that all components must be designed and tested to function without failure for a minimum of 92 days.
The importance of dew point
Dew point suppression is the key measure of performance for air dryers. This rating is more important than the actual air temperature rating. For every 20° (F) increase in temperature, the air's ability to hold water vapor doubles. When choosing a dryer, establish the dew point for the application and then lower it at least 15° to 20° lower than the application's lowest ambient temperature.
The life of a desiccant dryer, whether a single or twin tower model, is greatly increased with the use of good filtration. Dryers are designed for the removal of water vapor rather than water in liquid form, thus require coalescing filters to be installed in front of them in the air system to ensure proper operation.
Coalescing filters are designed to remove aerosols, oil, and water, minimizing the chance of the desiccant becoming contaminated with hydrocarbon or oil. This, therefore, significantly extends the life of the desiccant in the dryer. A contaminated desiccant will lead to moisture forming in the system's lines and the resulting problems addressed above.
Beyond the rails
Dryer systems are also important to buses and coaches because pneumatics control doors, brakes, air bags, kneeling systems, and suspension systems. Moisture, along with oil and other contaminants, can cause valves and cylinders to stick. Sticking cylinders often result in doors failing to completely close, causing noise and vibration that is unpleasant to passengers. Doors failing to close completely are in violation of Department of Transportation (DOT) regulations.
Pneumatic system failures can also occur in air brake systems, resulting in a time delay between the signal and when air brakes are to engage. These delays can cause accidents, endanger passengers, and are in violation of DOT regulations.
Federal regulations require that all buses have kneeling systems by 2010. These systems are designed to lower a portion of the bus or coach to enable passengers to enter and leave the bus more easily. Parker's recently introduced kneeling module was developed in cooperation with a variety of bus and motor coach builders. A special molded rubber poppet makes the module perform well especially in extreme temperature environments.
Moisture damage and sticking valves can cause kneeling system failure or improper operation. This failure results in delays in raising and lowering the bus, creating time delays for bus routes.
Tire inflation control is connected to the brake system. Water crystals in the line cause valves that deliver air to the tires to stick, resulting in over/under inflation of the tire, reducing the life of the tire.
Paxton Augustine is with Parker Hannifin's Pneumatic Div, Richland Mich.; (269) 629-2427. Randy Greenwood is with Parker's Automation Group, Cleveland; (888) 242-7748. For more information, visit www.parker.com/pneumatics.