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REMOVING CONTAMINANTS

take-off lines. See figure 1 for a typical good distribution main arrangement.As stated earlier most efficient waterremoval will take place at high pressure, so anything which will produce a pressure dropwithin the distribution system should beavoided. This will also be a loss of energy to the system and increase the the cost of compressed air generation. Areas to avoid here are complex flow paths with undue bends and inadequately sized piping. See page 23 Reference Data for friction losses in pipe and for recommended pipe flows.The action of water removal can beachieved by drip leg drains, automatic drain valves and as discussed later, filters. These devices should be located in positions where liquid water is present in amounts large enough to be removed. (See figure 11). Because of the possibility of cooling occur- ring during the passage of the air through distribution mains and branch lines it is preferable to install smaller individual filters as near to the actual point of air usage as possible, rather than rely on one large filter adjacent to the air receiver. A point to remember is that since most water will be present at higher pressures, always locate filters upstream of any pressure reducing valves.Filters which have the ability toremove water are designed for efficient water removal and low pressure drop in accor-dance with the recommended pipe flows (seepage 23) and Norgren filters will have high efficiencies up to 200% of this recommended figure.

WATER VAPOUR

The air produced by a compressor is hot, wetand dirty. The first step in good air prepara- tion is to filter out these contaminants. This section considers the removal of liquid water, water vapour, solid particles and finally oil. A properly designed air line filter of the correct size, in the correct location will effectively and efficiently remove liquid water, but will not reduce the water vapour content of the air. Further air cooling may result inmore water condensing out. If completefreedom from water contamination is essen- tial then the water vapour content of the air must be lowered such that the ‘Dew Point’ of the air is lower than any temperature that the air can be exposed to in the system.Once all liquid water is removedfrom compressed air, then normally the airwill be completely saturated with watervapour. The particular temperature and pressure at which the compressed air exists at that moment is known as the ‘Pressure Dew Point’.

LIQUID WATER

In compressed air systems water vapour exists as a contaminant originating at the compressor outlet in vapour form, but as the air cools, it will exist as both liquid and vapour.The amount of water vapour that canexist in any given volume of compressed air is directly proportional to the air temperature and inversely proportional to the pressure.Most liquid water will be presentwhen the temperature is lowest and the pressure is highest and removal at this point will achieve the highest efficiency.In order to achieve this an essentialelement of any system following thecompressor is an efficient after cooler of sufficient capacity to reduce the temperature of the outgoing air to within 8°C of the temperature of the water entering the after cooler.The outgoing air should then bepiped to a receiver of adequate capacity located in the coolest location available, definitely not within the compressor house itself. This will permit further cooling of theair to occur and therefore more condensa-tion.Generally the capacity of the receiveris about 30 times greater than the rated free air delivery of the compressor when oper- ating in the 7 bar g region, typical of most industrial air supplies. See figure 10 for a typical compressor installation.Further cooling may occur in thedistribution mains themselves. These shouldbe laid out with a pitch in the direction of airflow so that gravity and air flow will carry water to drain legs located at appropriate sites. Down loops in distribution mains should be avoided, if not locate a drain leg at the down loop. With the exception of drain legs all air take-off points from the distribu- tion mains should be taken from the top of the main to prevent water from entering the
Figure 10.TYPICAL COMPRESSOR INSTALLATION Figure 11. DRIP LEG DRAIN 6