Why Do We Need to Dry Compressed Air?

When air cools down, as happens following a compressors compression process, water vapour precipitates as condensate. (this is from the water that is naturally contained in the atmosphere in the air we breath. The humidity of the air reflects the ability of the air to hold moisture the higher the humidity to more water that is contained in the air.)

Under average conditions a 30kW compressor with a free air delivery of 5 m3/min operating at 7.5 bar will generate around 20 litres of condensate over a 8 hour period.

This water / condensate needs to be removed from the compressed air in order to prevent damage and avoid costly system downtime.

Compressed air drying or Dry Compressed Air is therefore and important part of the air treatment process. The favoured method of removing the moisture from the compressed air is via refrigeration or desiccant dryers.

Refrigerated dryers work on the same principle as a home refrigerator where a circuit is used to chill the compressed air down, normally to a temperature of +3 degC at which point the moisture drops out. The compressed air is then reheated back up to approximately +20 degC. After this process you would have to chill the air down to less than 3 degC to see any of the remaining moisture. If we now expanded the dried compressed air back to atmospheric pressure you would have reduced the water content by a factor of more than ten.

Desiccant dryers on the other hand are known as adsorption dryers. As opposed to cooling the compressed air we pass it over a substance (desiccant that is normally activated alumina) that adsorbs the moisture. Once the moisture has been adsorbed the material has to be regenerated and for this reason twin tower desiccant dryers are the norm. (one tower dries out whilst the other is removing the moisture from the air.) The main benefit of desiccant dryers is the dewpoint achievable can be as low as -70 degC where extremely dry air is required, normally classed as instrument quality air (this would also require some additional particulate filtration).

Whilst desiccant dryers can achieve much lower dewpoints they cost more to buy and cost a lot more to run. On average a desiccant dryer uses between 10 and 25 percent more energy to run in comparison to a refrigerated dryer.

Regular maintenance on both refrigerant dryers and desiccant dryers is essential to ensure maximum reliability and ensure the lowest possible energy costs.

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