More and more often, industrial applications expect mobility and flexibility from compressed-air processing. For example for the employment at different sites or outdoors. A solution to that is the installation of complete compressed-air stations in transportable sea containers. These mobile performance centres make special demands on the technology applied, in particular as far as compressed-air drying is concerned.

A company which recognised the growing need for mobile compressed-air stations particularly early is Karl Uhl GmbH in Bochum, Germany. For 80 years now already, the experts from the Ruhr area have operated as wholesalers and service providers of compressed-air technology.

During the past years, this closeness to the market made more and more clear that, besides the "traditional" installation of industrial compressed-air stations in existing buildings, mobile concepts are increasingly in demand, be it for application on large-scale building sites, for the temporary support of stationary plants or as flexible, independent "isolated applications" remote from stationary operating facilities, such as corporate buildings or control centres.

Due to their decades-long experience, it was relatively easy for Karl Uhl GmbH to meet to the new demand with coherent concepts and convenient solutions. In this respect, projecting and implementation of a very special type of compressed-air generation was the focus of attention: mobile compressed-air stations in sea and storage containers.

Sven Stampa, the person responsible for the engineering / plant construction division at Uhl, is standing in the recently-constructed additional production hall of the expanding company, in front of one of the newest units. "A real gem of its kind", smiles the engineer contentedly and undoes the bolt, making the double door of the mobile performance centre swing wide open.

It consists of two firmly interconnected sea containers, the sides of which are open up to ceiling-height. They form a room with a length of six metres, a width of five metres and a height of two and a half metres. "Plenty of space for compressed-air technology at its finest" explains Sven Stampa, highlighting the advantages of the system.

Neatly arranged, the interior - which is also insulated against noise and temperature - accommodates three oil-injected, air-cooled screw compressors with an operating energy input of 22 kW each. Downstream of these, there is a 3,000 l compressed-air tank as a buffer tank, and three high-capacity adsorption dryers with the corresponding compressed-air filters, condensate drains and a dew point control. All installed resources are connected via signaling lines with an equipment cabinet which is also mounted in the container.

"A platform which, except for the supply of electric power, is a virtually completely independently functioning platform for the generation of qualitatively high-grade compressed air", explains the Uhl engineer Stampa.

All alone in the meadow

This is exactly what is expected of the mobile compressed-air station because it is employed several hundred kilometres away from the "home" control centre, at a German energy supplier. Its work site is in the pump station of a natural gas pipeline near the coast between the gas field in the open sea and the refinery in the upcountry.  A site which is not only all alone in the meadow, but which is also subjected to extreme climatic conditions as a result of the proximity of the sea.

"The plant is to supply compressed air to the pneumatics of the pump station in the outdoor area“, explains Sven Stampa. "Therefore, special importance is attached to compressed-air drying. Nothing must go wrong or corrode, nothing must freeze up. Everything must run failure- and maintenance-free over weeks and months in all seasons. It is impossible, after all, to send out a servicing team every few days."

In view of the required process reliability and of the special operating conditions of this mobile station, Uhl decided to employ cold-regenerated adsorption dryers.

The method of adsorption drying is based upon a purely physical process, where the air humidity, through adhesive forces, is absorbed to a desiccant (e.g. alumina or silica gel) in the dryer container. When the adhesive forces of the desiccant are neutralised by hydration the desiccant must be regenerated, meaning that the moisture needs to be removed. To allow this, continuously operating adsorption dryers have two separate pressure vessels. While one of the containers actively dries compressed air, the second one has got the time to regenerate the previously moistened desiccant.

Why cold-regenerated adsorption dryers?

In cold-regenerated adsorption dryers, the treatment of the desiccant takes place without the additional application of heat, e.g. via an energy-consuming heating system. In lieu thereof, a part of the dried volume flow is diverted. This partial flow, also referred to as purge air, expands to a pressure slightly above the atmospheric pressure and is, therefore, extremely dry. The purge air is led through the compressed-air container to be regenerated and absorbs the moisture accumulated in the desiccant. Via an outlet valve, the purge air finally emerges into the environment.  

Further advantages of cold-regenerated adsorption dryers are simple set-up, applicability even at high ambient temperatures, fast drying and regeneration times and regeneration without the supply of extra energy.

However, a disadvantage of conventional standard adsorption dryers is their system-inherent compressed-air pressure  drop, resulting among other things from the divergence of purge air described above, and the function of the pre and after-filters. This deficiency ultimately needs to be compensated by increased compressor performance, involving, of course, a higher energy input.

A problem that Uhl does not have to face however. Because this company relies on the special dryer technology of Beko Technologies GmbH. For years already, Uhl has preferably integrated the numerous products of this internationally-operating compressed-air system provider into the company's mobile stations.

In this particular case of application, the company decided in favour of three Drypoint AC adsorption dryers. With these, Beko has developed devices which are able to limit the pressure drop (including pre-and after-filter) to only 0.25 bar on average. This corresponds to an improvement of over 50 per cent compared with conventional constructions. For the users, the acquisition costs of these dryers will therefore pay off after a period of only three years on average, and this via energy-cost savings alone.

The energy efficiency of the Drypoint AC adsorption dryers is achieved through a whole range of specific measures. Beko uses, for example, inclined-seat valves with a large orifice cross section to minimise the flow resistance. Direct mounting of the valves at the pressure vessels, as well as rectilinear piping without redundant changes in direction, have the same effect.

Moreover, flow-optimised Clearpoint compressed-air filters including a Bekomat 20 FM condensate drain with a filter management are employed as standard.

The large desiccant bed also contributes significantly to the low pressure drop. This results in additional capacity to maintain the full dryer performance, for example at a sudden high volume flow or a low operating pressure.

And finally an adjustable regeneration-air nozzle allows changeovers to other operating conditions and provides additional operational reliability regarding the dosage of the correct and most profitable amount of regeneration air.

The intelligent control with an accurately defined program run, which is maintained even after an interruption or a power outage, is particularly interesting. It is capable of distinguishing between a planned interruption of the operating sequence and a breakdown of the power supply.

Top priority during the mobile employment: process reliability

The operational reliability of the dryers is of vital importance for the employment in mobile Uhl compressed-air stations.

During an external interruption of the power supply, conventional cold-regenerated adsorption dryers are overloaded and lose their function. This results in water introduction into the compressed-air system. In contrast, the concept of Drypoint AC prevents undesired loading of the dryer during a power interruption and ensures full operational capability - the compressed-air system remains dry.

In cold-regenerated adsorption dryers, both desiccant containers are under alternating pressure. The peculiarity of the Beko design is the fact that they are, including all fittings, constructed to be durable throughout alternating pressure operation. Their suitability for unlimited load alterations prolongs their service life and reliably excludes any risk for the operator and for the entire system.

And finally: the devices are also suitable for discontinuous operation. Reduction of the regeneration air according to the needs of the customer takes place through coupling with the compressor, meaning according to the principle of the compressor synchronisation control. A load independent control can be retrofitted upon request. With this control, the saving potentials of a dryer are used particularly efficiently and the operating costs are reduced even further. The advantage of the Beko Drypoint AC dryers is the retrofitability of the load-dependent control through a simple add-in.  In contrast, other manufacturers devices normally require a costly replacement of the complete control unit.

For the specific application at Uhl, the stable pressure dew point of minus 40 °C of the Drypoint AC dryers employed there is also important. This pressure dew point ensures trouble-free operation also during the winter months and protects the pneumatics of the gas pumping station in the outer area against freezing.

Decisive factors:
Ease of installation and maintenance

"Ease of installation and maintenance are two particular factors which should be in the focus of attention for mobile compressed-air stations are concerned", explains Sven Stampa from Uhl. "Because first of all, it must be possible to arrange everything optimally here in a crowded space. And secondly, the future service personnel should be able to act without problems, quickly and safely in the plant."

The Drypoint AC adsorption dryers allow for these points in many respects. For example, all components of the devices are accessible from the front. Therefore, no complicated demounting and mounting of the pipe bridge is required when replacing the desiccant. In addition, each desiccant container is equipped with an easily accessible overhead service sleeve.

When delivered by Beko, dryers up to a volume flow size of 800 m³/hour are, as a matter of course, already mounted by the manufacturer and are ready to operate. Even problem-free transport was taken into consideration and the devices were integrated as standard into a lift-truck-suitable supporting frame.

"The sum of all these properties makes the Beko adsorption dryers ideally suited for use in our mobile compressed-air stations. By the way, the same goes for all other Beko products ", concludes the Uhl engineer Stampa. "With the meanwhile approximately 20 plants that we mount annually for worldwide employment for mostly extreme operating conditions, we need to know precisely which technology we make use of. Because this business does not forgive mistakes."