Increasing awareness of the value of compressed air treatment
in rail vehicles is starting to demand a higher quality of compressed
air in terms of contamination and condensate than ever before. The
introduction of new and sensitive pneumatic control systems coupled
with economic considerations related to maintenance costs are making
the effective treatment of compressed air with drying systems a
must. To achieve these improvements, class BR 111 electrically driven
locomotives operated by „Deutsche Bahn Regio“ are being
upgraded with new and innovative equipment.
1. Introduction
Along with electricity, compressed air is the most important source
of energy on board a rail vehicle. The flexibility this powerful
medium offers is displayed in it´s universal use in many applications.
A fact which stems from the general use of compressed air throughout
industry as well as in rail vehicles and even commercial road vehicles.
Compressed air can be produced by many types of compressor and distributed
to the consumer simply and easily via a central system. The main
uses of compressed air for rail vehicles are as follows;-
• Braking systems
• Control and monitoring systems
• Pantograph raising and lowering
• Door operation
• Air suspension
All of these applications place different demands on this alleged
uncomplicated medium. Not only is reliability a major concern but
the entire safety of the engine, the train and therefore the entire
rail transport network is heavily dependent on a reliable source
of compressed air.
Unfortunately the quality requirements placed on this medium are
very often underestimated, especially where older locomotives are
concerned, where the question of compressed air quality has often
been neglected. The physical processes which take place when air
is compressed, the type of compressor installed, together with it´s
condition, and finally the arrangement of the compressed air components
in the whole system, can lead to disruption, mainly in the form
of contamination from water, dirt and oil. This contamination causes
corrosion and wear to componentry and is the major constituent of
condensate resulting from the compression process, which itself
is often the cause of breakdown where it freezes in winter.
The task of effective compressed air treatment is to remove these
damaging contaminants in order to guarantee the reliable operation
of all of components but also to reduce the level of maintenance
required. The results definitely justify the additional measures
and the cost.
2. Compressed air treatment
In order to clearly understand the necessity for compressed air
treatment, let´s examine some of the important facts, taking
water as the main constituent of the condensate produced.
At 100% relative humidity, the water content of the air being
pulled in by the compressor is 17 g/m³. At 70% this is still
12 g/m³. During compression with a piston compressor the compressed
air is heated to approx. 100°C or more. In the after-cooler
this temperature is then reduced to about 35°c. As soon as we
fall short of the 100% saturation point condensate is formed. At
35°C and 7 bar pressure, compressed air can only hold 5 g/m³,
the remainder, 7 g/m³ forms condensate. Taking a compressor
with a performance of 2,4 m³/h, this means 1 kg/hour of condensate
over the compressor´s running time. Taking average compressor
running hours to be 5 hours per day, the result is 5 kg. This means
approximately 5 liters of aggressive oily liquid, which should not
enter the compressed air system, but should be collected and disposed
of as toxic waste.
The fact that compressed air cools down even further in pipework
and vessels means that without additional treatment condensate continues
to form to a lesser or greater extent depending upon the ambient
temperature. Taking this fact into account there is potential for
further disruption caused by freezing in winter. This situation
can only be avoided where continuous draining of the condensate
from the entire compressed air system is carried out.
Generally new locomotives are already fitted with appropriate
treatment. The situation is completely different when we take a
look at older rolling stock which has been in service a number of
years, but is still heavily relied upon to perform. In order to
operate these locomotives reliably and economically, many are currently
undergoing step-by-step re-fit with the newest technology. In the
class BR 111 electrically driven locomotives operated by DB Regio
in Germany, these measures also include improving the quality of
the compressed air. The reasons for this are;-
• Breakdown avoidance caused by frost and dirt
• Safe operation of the doors
• Less maintenance
• Avoidance of corrosion in the compressed air receivers and
other componentry
• Protection of sensitive pneumatic controls
• Collection of condensate
3. Innovative Air treatment
3.1 Requirement
Compressed air quality is defined in ISO 8573.1 in terms of three
characteristics, each being split into 5 or 6 classifications. (
Table 1 ) Air treatment systems for rail vehicles should meet the
quality requirements of classification 1.2.1 ( highlighted in the
table ). In addition to this an air drying system should be capable
of reducing the dewpoint of the compressed air to such an extent
that the physical appearance of renewed condensate, resulting from
a reduction in temperature cannot take place after treatment. In
order to achieve these results, the following solution describes
a combination of filtration and adsorption utilising high-performance
compressed air filters and purpose-built modular adsorption dryers.
4. Water separation and filtration
The fist step in upgrading the BR 111 involved the removal of the
existing oil separator and replacing this with a water separator
( Photo 2 ). This product exhibits extremely efficient separation
of up to 99%, whilst maintaining low differential pressure. It is
located outside at the lowest point in the compressed air system.
The condensate is removed from the separator housing via a timed
solenoid valve. Following this, the particles and oil are removed
to meet the levels already described by the pre-filtration stage,
which consists of either a one-stage filtration or a two-stage,
depending upon compressor type.
4.1 Air drying
For reasons already mentioned, drying this pre-filtered compressed
air is an important step in the treatment process.
All classic air dryer systems consist of one or two steel vessels,
filled with adsorption material, in which the process of humidity
adsorption and regeneration takes place in alternating fashion.
Traditional air dryers often suffer from channeling throughout the
adsorption bed ( Photo 4 ). This results in ineffective drying.
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| Photo
4 – Channeling in adsorption bed |
The innovative air dryer installed in the BR 111 locomotive works
along similar lines, however based upon the principle of heatless
regeneration of the saturated adsorption material. The changeover
process is fixed and controlled by an electronic timer, which has
been specially developed for rail applications and can be operated
at various voltages. This system not only controls and monitors
the solenoid valves on the dryer but also those located on the upstream
equipment of water separator and filter, a fault feature is also
provided.
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| Photo
5 – Cut-away air dryer. ( Length across flanges
990 mm ) |
The adsorption dryer ( photo 5 ) consists of two chambers filled
with adsorption material. Whilst the compressed air is flowing through
the adsorption material in the active chamber and being dried, regeneration
of the other chamber takes place simultaneously. This necessitates
a small amount of dry air, which when expanded from operating pressure
to atmospheric pressure aids the removal of the humidity adsorbed
by the adsorption material.
The adsorption dryer was sized for the water loading expected
at the volumetric flow, the operating temperature and the operating
pressure of the existing compressor. In the development of this
product considerable attention was given to the following;-
• Optimisation of a vibration-free adsorption – bed.
• Reliable controls and up-to-date valve technology, sized
for the special demands of rail vehicle technology.
Above all, consideration was given to the „foot-print“
and weight of the air dryer, which must meet the requirements of
the rail industry, especially when upgrading into existing system
configurations. Most of the time the installation is subject to
severe space limitations.
The solution to this problem is to use aluminium extruded modules
instead of traditional steel vessels, which are far more space-saving
and considerably lighter in weight. These modules are treated internally
and externally with a special anti-corrosion method, which guarantees
long service-life.
The dryer modules are filled with adsorption material utilising
the „snow-storm“ filling method, which was specially
developed for filling dryers of this type, achieving absolute packing
density. In order to maintain this, the contents of the dryer are
held firmly in place by springs located at the top of the adsorption
bed. The bed is thus able to cope with extreme vibration without
suffering from fluidisation. The air is not able to take the path
of least resistance through the bed, forming channels, instead it
is forced to flow through the entire bed in an even manner. All
of these features guarantee the operational performance of the unit.
4.2 Interplay
In addition to the direct cleansing effect of Compressed air filters
on the medium compressed air itself, filters also contribute considerably
to an extension of the life-time of the adsorption material in the
downstream dryer. Insufficient filtration increases the danger of
premature contamination and leads to far poorer drying performance
resulting from oil aerosols coating the surface area of the adsorption
material, preventing the adsorption of humidity.
An after-filter, with the same dimensions as the pre-filter, prevents
any dust from the adsorption material reaching system componentry
and the compressed air user.
Regular maintenance by replacing filter elements secures the trouble-free
operation of the entire air treatment system. In addition to this
it also ensures the safe, reliable operation of the pneumatic controls
and monitoring systems installed in the locomotive´s air system.
4.3 Handling condensate
A requirement of the updating procedure was the recovery and removal
of the condensate drained from the water separator and the pre-filter.
Utilising timed solenoid drains, the condensate is regularly evacuated
from vessels and fed by way of the system pressure to a collecting
tank within the locomotive. Within the timescales between regular
maintenance periods this liquid is safely collected and then disposed
of. With this system the regular, time-consuming drainage of the
system in former times by railway personnel is no longer necessary.
The timing of the solenoid drain valves is governed by the control
unit on the dryer and adapted to cope with the amounts of condensate
being experienced.
4.4 Construction
By combining the tried and trusted advantages of air-drying systems
with modern equipment, a compact, reliable system has been developed.
This system provides a high proportion of installation flexibility
either inside or underneath the locomotive, installed vertically
or horizontally.
In updating the BR111 class of locomotive, we had to match our
product to the particular circumstances we found on board this type
of vehicle. As well as requiring a favourable location for the dryer,
the engineering works wanted a product which could be run and maintained
with a minimum of time and effort. The solution provided is supplied
practically ready to install, with all of the components such as
the dryer, pre-and after filtration and control box, piped up and
mounted to an easy accessible frame. ( photo 6 ). This not only
enables ease of installation into the locomotive, but also helps
the integration of the unit into existing documentation.
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| Photo 6
- Air treatment system in the engine room (side access) of
a class BR 111 locomotive. |
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Photo 7 -
Air dryer installed in class BR 294/295 diesel locomotive
operated by DB Cargo, Germany.
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5. Final comments
Following long-term trials with the new air-treatment systems in
BR 111 class locomotives, during which all parameters were checked,
we were capable of demonstrating the extremely high operating reliability
of the unit. Based upon these trials a standard re-fit programme
was embarked upon in 2001 covering over 200 BR 111 class locomotives
in Dessau, Germany.
The decision to take this system was furthermore supported by
experience already gained with a dryer concept installed in BR 294/295
diesel locomotives operated by Deutsche Bahn Cargo, Germany (Photo
7).
The modular construction of the dryer and the flexible installation
this provides means that the unit is not only an interesting proposition
for upgrading rolling stock but also for new rail vehicles under
construction.
Engineer Eckert Machein ( 43 ). Engineering degree at the Technical
University of Chemnitz, Germany.
Adress:- domnick hunter gmbh, Karl-Arnold-Strasse 13, D-47877 Willich.
Tel:- +49 (0) 2154- 4810-0. Fax:- -10; E-mail;- info@domnickhunter.com
Table 1;- Quality Classification for compressed air in accordance
with ISO 88573.1
Highlighted values are those recommended for air drying systems
on rail vehicles.
Class |
Dirt particles
Size µ
µm |
Water |
Oil including
oil vapour
mg/m³ |
Pressure dewpoint
at 8 bar absolute °C |
Humidity mg/m³ |
| 1 |
0.1 |
-70 |
0,2 |
0.01 |
| 2 |
1 |
-40 |
11.7 |
0.1 |
| 3 |
5 |
-20 |
94.5 |
1.0 |
| 4 |
15 |
+3 |
695 |
5 |
| 5 |
40 |
+7 |
919 |
25 |
| 6 |
- |
+10 |
1126 |
- |
• Normal conditions
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