Last month, we wrote about Speaking the Same Industrial Language, with the intent of clearing up misconceptions surrounding different terminology. This month we’re looking at terms associated with various forms of dryers used with compressed air systems.
Dry air is a critical part of a compressed air system. Water, when it goes where it isn’t supposed to go, can cause tremendous problems. It’s messy. And expensive. That’s why removing water from air during the compressed air process is important for keeping system components dry downstream. Dryers make that happen. Here’s a breakdown of the various ways water can be removed from compressed air:
Membrane drying – This technique uses the process of selective permeation of the gas components in the air to separate and remove water vapor. Thousands of tiny hollow polymer fibers with an inner coating line the dryer’s cylinder, thus creating a membrane. The water vapor in the air is collected between the fibers while the dry air passes through the fibers and onto its intended process.
Cooling plus separation – Cooling plus separation happens exactly how it sounds. The hot compressed air is cooled, which allows a large amount of water to condense. Once the water is condensed, it can then be separated from the air through an after-cooler (or a refrigerant dryer). Almost all stationary compressor installations use an after-cooler and most modern compressors have after-coolers integrated in the machine.
Over-compression – Over-compression is similar to the cooling plus separation technique: the air is compressed and cooled, and the resulting water is separated from the air. However, the air is compressed to a higher pressure beyond what is needed for the intended application. After separation, the air is allowed to expand to the working pressure so it can be used in the intended process. Though this method is the easiest for drying compressed air, it is only suitable for very small flow rates because it consumes a large amount of energy.
Absorption drying – Absorption drying is a chemical process in which water vapor is bound to an absorption material such as sodium chloride or sulfuric acid. This technique is not often used because of the possibility of corrosion and the high consumption of absorption materials.
Adsorption drying – No, you’re not seeing double. With adsorption drying, moist air flows over a hygroscopic material or desiccant that removes water. Typical materials used are silica gel, molecular sieve and activated alumina. The desiccant is eventually saturated with adsorbed water and needs to be regenerated regularly to maintain its drying capacity. Because adsorption dryers can achieve a pressure dew point of -40°F, this technique is used in many critical applications.
Dry air is good, helpful and productive air. Make sure your air is working for you – not against you – and doesn’t have any water in it.
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