In the vapour phase, the oil is present as a molecule in the compressed air and is thus not separated out by mechanical filters. The quantity of oil vapour, see Diagram 4.8.3.1, depends on the temperature and, under unfavourable circumstances, can amount to a larger quantity than the oil droplets or oil mist concentration.
Diagram 4.8.3.1 - Vapour content of mineral oil in compressed air
The separating efficiency of a filter deteriorates with rising temperature due to an increase in oil vapour content. Thus, at a temperature of 20°C, up to ten times more oil passes through a filter than at 10°C. For this reason, care has to be taken, particularly with depth and high performance filters, to install at locations of low temperature. Oil vapour is condensed by cooling. Condensed oil can be filtered without problem from compressed air by means of depth filters. The oil vapour remaining in compressed air is reduced by the use of activated charcoal adsorption filters mounted downstream. By means of such filters, residual oil contents down to 0.003 mg/m3 can be achieved. Compressed air treated to this extent can be graded as technically oil free. Activated charcoal filter elements are available in various constructions such as:
- Ground activated charcoals embedded on a deep filter bed manufactured from borosilicate - glass paper (Fig. 4.8.3.1A)
- Granular activated charcoal contained within a pleated element
- Moulded tubes of activated charcoal in the form of a hollow cylinder.
- Granular bed (Fig 4.8.3 1B)
Figure 4.8.3.1A
Figure 4.8.3.1B
Activated charcoal is preferentially used if hihg quality of oil free compressed air is required for a particular application. The claimed residual oil content is, however, achieved only if the operating conditions are adhered to. Optimum separation requires:-
- A low inlet temperature to the adsorption filter
- Relative humidity of the compressed air of about 60% maximum
- Pre-filtration with a depth filter
The activated charcoal filter elements are fitted into standard filter housings to form a complete filter. Activated charcoal adsorbs the oil vapour from compressed air until it becomes saturated. As the quantity of activated carbon in charcoal filters is relatively small, the service life of activated charcoal elements is limited, with regard to oil vapour adsorption. Depending on the geometry of the elements, service lives of 300 - 1000 operating hours are realistic, given optimum separation under normal conditions of use.
Figure 4.8.3.2
Photo 4.8.3.2
As an alternative to activated charcoal filters, activated charcoal adsorbers are used. These adsorbers consist of a container filled with activated charcoal. Flow diffusion screens at the inlet and outlet of the vessel contain hold the activated charcoal. Using an adsorber, a residual oil content down to 0.003mg/m3 is achieved. For this, the dwell time in the activated charcoal bed, itself determined by the adsorber’s geometry, is decisive. The specified quality is achieved from a relatively long contact time. An activated charcoal adsorber is always used when the highest quality compressed air requirements are demanded and cannot be achieved by filters alone.
Figure 4.8.3.3
The service life of the activated charcoal filling inside the adsorber reaches 8000 - 10000 hours of operation under normal loading conditions.
