Keeping compressed air dry

Some applications and environmental conditions impose particularly stringent requirements on the quality and production of compressed air. An innovative full-stream rotation dryer combined with a dry-running rotary screw compressor delivers both exceptional drying quality and maximum efficiency — even under the most challenging conditions.

Ambient air contains water vapour. When the air is compressed, numerous key parameters increase, such as the air temperature, proportion of water vapour per volumetric unit of air and consequently, the air’s dewpoint temperature — or pressure dewpoint. Measured in °C, it indicates the lowest temperature at which water will not condense out of compressed air — in other words, condensation will result if the air is chilled below the pressure dewpoint. The lower the pressure dewpoint, the drier the compressed air. If the compressed air is not dried following the compression process, water can condense when the compressed air cools. It can then accumulate in the downstream compressed distribution air network or even within the realms of the production process itself — which can have even more serious consequences, as water can then damage not only the compressed air system, but also downstream equipment that uses compressed air and the products being produced. So it’s extremely important to exercise due care in selecting the appropriate degree of compressed air drying for the specific process in mind.

Ultimately, it’s the degree of compressed air drying that determines the drying method, and the cost of drying the compressed air. Refrigeration drying generally provides the most efficient and cost-effective method for most applications, usually ensuring a pressure dewpoint of +3°C. If a lower pressure dewpoint is required due to the nature of the processes in question, more complex desiccant dryers or combination dryers can be used. However, these types of dryers involve higher costs as a result of the additionally required materials and increased energy consumption. In these types of dryers, the compressed air is treated using desiccants, such as silica gel or activated aluminium. During the drying phase, water vapour contained in the compressed air binds to the desiccants. Once the adsorption capacity of the desiccant is exhausted, it must itself be dried out — either continuously or at intervals, depending on its saturation. This process is called regeneration and is responsible for the greater part of desiccant dryer operating costs.

In technical terms, regeneration processes are differentiated into chamber and drum processes. In chamber regeneration, the desiccant is usually in granulate form and is contained in two separate pressure receivers. Desiccant regeneration takes place non-continuously and, depending on the type of unit, may employ cold compressed air that has already been dried. In the case of dryers that have been specially adapted to the compressor, hot compressed air supplied directly from the second compressor stage is used for regeneration.

Compact and energy saving

One type of drying process involves dryers with significantly more compact dimensions capable of superior adaption to the compressor; these are known as ‘heat of compression’ (HOC) dryers. In this design, the desiccant is contained in a drum through which the compressed air flows in an axial direction. Desiccant regeneration and compressed air drying take place continuously, within a single pressure receiver. The drying and regeneration sectors are separated, however, both structurally and in terms of process. Slight pressurisation of the drying sector ensures that once dried, the compressed air does not reabsorb moisture from the regeneration air flowing by in the adjacent sector.

The dryers are integrated in dry-running compressors, which — in contrast to oil-injected compressors — feature two compression stages and generate significantly higher temperatures during the compression process.

Drying without additional energy consumption

In these integrated rotation dryers, desiccant regeneration takes place continuously, using the heat that already exists in the hot compressed air. Following regeneration, this heat is not lost, but rather the hot air is cooled down by the second compressor stage’s cooler and the heat is fed into the drying sector by a radial blower. This means that the heat arising as a result of compression of the air is also used for desiccant regeneration. Consequently, this heat is freely available without cost, since no additional energy is required for the drying process. This translates into maximum efficiency and outstanding drying reliability. This perfect interplay between the compressor and dryer also avoids additional energy costs, which are unavoidably incurred in the case of conventional desiccant dryers that use additional, external energy for desiccant regeneration. Furthermore, the energy cost savings from integrated rotation dryers continue to apply even with variable free air deliveries.

Reliable pressure dewpoint

Modern integrated rotation dryers such as i.HOC units also guarantee reliable and stable maintenance of low pressure dewpoints to -20°C and, under special conditions, even to -40°C — regardless of the operational conditions or free air deliveries needed.

The unassuming exterior hides the exceptional versatility that lies beneath: an i.HOC rotation dryer integrated with a dry-running rotary screw compressor.

In integrated rotation dryers, the attainable pressure dewpoint is determined by the compressed air inlet temperature in the drying sector and the available regeneration potential. This, in turn, depends on the mass flow of regeneration air and its temperature.

The i.HOC integrated rotation dryer therefore uses the entire mass flow of hot compressed air available at the end of the second compression stage for regeneration purposes. This is why it is referred to as a ‘full stream’ rotation dryer.

Conversely, partial-stream rotation dryers use only part of the hot compressed air for desiccant regeneration. All other conditions being equal, they therefore have lower potential available for removal of moisture from the desiccant.

The higher regeneration potential of full-stream rotation dryers is especially advantageous when it comes to high cooling medium temperatures at the regeneration air cooler, low compression ratios in the compressor and — in partial-load operation — more consistent and significantly lower pressure dewpoints.

The second important factor affecting the pressure dewpoint is the inlet temperature in the drying sector. A general rule of thumb dictates that the lower the inlet temperature, the better the drying performance, all other conditions being equal. In practice, this means that the lower the temperature of the available cooling medium (air or water), the better the drying results.

Thanks to full-stream regeneration technology, the i.HOC rotation dryer is capable of exploiting much more heat than conventional dryers — an efficient feature that translates into lower energy costs.

Intelligent regeneration management

The attainable pressure dewpoint therefore fluctuates with the temperature of the ambient air (insofar as it acts as a cooling medium) — and this effect is especially pronounced in air-cooled compressors with integrated rotation dryers. For instance, if temperatures temporarily peak around 40 °C in the inlet area during summer months, it may be necessary to enhance the rotation dryer’s regeneration potential during this time in order to avoid exceeding a required pressure dewpoint of -20°C.

Far from posing an obstacle, in the i.HOC full-stream rotation dryer, the discharge temperature following the second stage (the regeneration air temperature) can be increased by a controlled bypass around the first compression stage cooler. The regeneration air temperature (and consequently, the regeneration potential) increase in order to ensure maintenance of the target pressure dewpoint. From an energy perspective, it makes sense to use the bypass to increase the regeneration air temperature to meet process requirements — especially since the conventional technology available on the market for electrically heating the regeneration air consumes significantly more energy. Pressure dewpoint management can also be beneficial for compressors with a low discharge pressure (less than 5 bar) in which the discharge temperature — and consequently, the regeneration potential — is limited due to the nature of the compression process. Intelligent dewpoint management is easily put in place when the compressor and dryer each share a common controller capable of perfectly harmonising the performance of both components.

Double heat exploitation

The extensive benefits of integrated rotation dryers don’t end there. Aside from drying compressed air, they can also be used for heat recovery purposes. This means that the heat generated during the compression process is then used for other purposes; in the case of water-cooled compressors, it can be used for heating process water or heating adjacent rooms — delivering major cost savings in other areas.

This is possible because the i.HOC integrated rotation dryer does not need an additional regeneration air cooler; rather, it simply uses the compressed air cooler of the second compressor stage — which is already optimised for heat recovery purposes. All in all, an efficient way to avoid unnecessary heat wastage.

Because of their relatively compact design, compressors with integrated rotation dryers require significantly less space than two separate units — while also cutting installation and maintenance costs accordingly.

A dry-running compressor with an integrated rotation dryer is therefore an excellent choice for users with a certain usage profile: demanding requirements in terms of compressed air quality and pressure dewpoint consistency; relatively little installation space; and a desire to take advantage of heat recovery, not to mention those facing challenging environmental conditions. For all the reasons outlined, in such cases these machines often deliver the most reliable and efficient supply of compressed air while also cutting energy costs across the board.

Top image: When combined with a dry-running rotary screw compressor, the i.HOC system is easy to integrate into a compressed air station thanks to its compact design.