The choice of compressed air dryers is driven by the air quality demands of your plant. If ultra dry compressed air is required with very low dewpoints in the minus 40°C range, then the choice is really about which type of desiccant air dryer you will install. Pressure swing heatless air dryers can handle volumes under 50 3. These can be equipped with dew point controls to reduce expensive purge air wastage drastically. Above these volumes, heat regenerative dryers could be considered. They are more expensive than the heatless varieties, but offer operational cost savings in reduced air purge wastage. The air feed ideally should be from oil-free compressors.
If the flow requirements for low dewpoint compressed air are relatively small compared with the total airflow, a decentralised desiccant air dryer can be installed at the point of use to reduce capital cost and compressed air purge air wastage.
For dew points above 3°C the obvious choice is a refrigeration air dryer as the capital cost is well below its desiccant cousin. Reliable and cost-effective, refrigeration dryers are easy to install and maintain. Always remember to re-rate the dryer’s capacity to your local ambient air temperature. Failure to do so will lead to a multitude of problems caused by excess heat loads affecting the refrigeration circuit within the dryer. Lubricated compressed air is handled by these dryers with no real problems, unless the oil carry-over is excessive.
Inlet pressures will also affect the sizing of a dryer, so don’t forget to consult the potential supplier before you purchase the unit. If re-rate factures for ambient inlet air temperature and air pressure are not shown in the sales brochure, look for an alternative supply chain.
Desiccant air dryers are susceptible to compressor oil contamination. Oils are detrimental to the adsorption chemical’s ability to remove water vapour from the compressed air stream and will lead to poor dew points. Hence compressor oils need to almost be eliminated from the incoming air stream.
The use of pad separators and inline coalescing filters to remove compressor oils is essential. After filtration it is also necessary to ‘polish’ the air downstream of the dryer before delivering the compressed air to your system. High inlet air temperatures and low inlet pressures also detrimentally affect filtration media. Lower temperatures below 40°C are preferred. It is possible to get oil vapour contamination down to 0.001 mg/m3 or even lower if activated carbon filters are installed. Filtration is never cheap and to give the elements the best chance of a reasonable life span between changeouts, it is prudent to oversize the casings relative to the inlet air flow rate. This will also assist with reducing pressure drops across the filter bank.
The drainage of condensate from air filters and refrigeration dryers is vital. Reliable auto drains are critical. Elements ‘swimming’ in condensate in the base of the casing with no function may well collapse the unit. The choices are from low-cost, simple timer solenoid drains to sophisticated, level-controlled units that will send an alarm out if they fail to eject condensates.
While considering the choice of air filters and dryers, do not neglect the oily waste condensate treatment system that ensures that waste oils are separated from the water condensates, retained and properly disposed of when the adsorption cartridge is saturated. The clear clean water condensate – the largest component – is allowed to run to waste, while the oils are retained for proper waste disposal.
A litre of waste compressor oil can contaminate a square kilometre of wetland area. Contamination of water in stormwater drains is a serious offence. Take steps to avoid the fines that will be levied on the managing director.
Having bought the equipment, take a long hard look at where and how the equipment will be installed. This is another discussion topic in its own right, and too broad for this article. Please call or email us to have the notes on “Compressor House Design” sent to you – it’s a provocative worthwhile read.
Compressed air quality testing
Artic Driers can monitor particle, dewpoint and residual oil levels from one mobile or static auditing data logger, and is also able to monitor a variety of gases. Dew point monitoring provides an early warning of a dryer treatment system hiccup. It is a better solution than relying on an operator or infecting your factory’s air line.
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| www: | www.articdriers.co.za |
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