Clean Compressed Air Brochure - Norgren - #7

Dew Points are normally measured at atmospheric pressure and can be related to Pressure Dew Points through appropriate charts.

In order to remove water vapour from a compressed air system Air Dryers must be employed. The efficiency of these devices is much increased by ensuring that they are not contaminated by liquid water or oil (or combinations - emulsions) and are supplied with air at the lowest possible temperature. So they are additions to the system and not alternatives to filters and after coolers.

There are 3 principle types of Air Dryer; Refrigerant,

Regenerative Adsorbent Desiccant and Deliquescent Absorbent Dryers (The general comparative abilities

and comparative costs are tabled in the

Reference Data on page 23)

In order to keep the costs of air

drying to a minimum consider the

following:-

a) Does the particular process require air drying or will efficient after coolers, receivers and filters suffice?

b) Do not specify extremely low Dew Points if the process does not warrant them.

c) Limit the volume of air being dried to that actually needed for the particular process with an adequate margin for future expansion. This may indicate only one area of a process plant need employ a dryer.

d) The major requirement for air dryers in general industrial applications is where high ambient temperatures exist.

Figure 12. PARTICLE SIZES

Particle Diameter, Microns 0,01 0,1 1,0 10,0 100

AerosolsHIII l l l l MJJJJ_

Spray

t : a..' Smoke Human Hair

V ' ..Se'

Ba ;ter a Silt

H ne Sand Car: '¹ Black C a Dust Pollens

Figure 13. GENERAL PURPOSE FILTER

Like water, solid particles exist in any compressed air system regardless of the type of compressor. These can arise from four principle sources:-

a) Atmospheric dirt inhaled at the compressor inlet port.

b) Corrosion products due to the action of water and weak acids, formed by the interaction of water and gases such as sulphur dioxide inhaled by the compressor.

c) Carbon products formed by the action of the heat of compression on the lubricating oil or the normal wear of the carbon piston rings used in some types of oil free compres­sors.

d) Particles originating from the mechanical fixing of the metal pipe work and components into the air distribution system.

The size of dirt particles covers a very wide range from several hundred to below one micron (see figure 12) and the level of filtration depends upon the degree of cleanliness needed for the particular process involved. Generally it is inadvisable to provide finer filtration than is absolutely necessary because the finer the filtration, the greater the quantity of dirt trapped by the filter element and the more rapidly it will become blocked.

Particles can be broken broadly into two groups, coarse (40 microns and above) or fine. Most normal air line filters will satisfac­torily remove particles down to 40 microns.

Fi ne fi l trati on in the region 10 -25 urn is normally required for high speed pneumatic tools or process control instru­mentation. Filtration of 10 um and below is essential for air bearings and miniature pneumatic motors. Norgren general purpose filters are available with different grades of element to offer these various filtration levels. Some applications may need filtration better than this and indeed for paint spraying, breathing air and food related applications particle removal below 1 um is also essen­tial. Standard air line filters cannot be used

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