Compressed air might have moisture content in them and if reached to the processing environment or processing equipment, can adversely affect the quality of product by causing undesirable changes in them. Hence it is necessary to have all water or moisture to be removed from air before coming in contact to food products. In this scenario, air dryers come into picture, which removes content of water vapor from air. Air dryers are commonly found in a wide range of industrial and commercial facilities. The process of air compression concentrates atmospheric contaminants, including water vapor. Some cases of rusting in tanks or tools, chances of being emulsified with grease, washing out lubrication oils etc., are some of the disadvantages of water vapor in air.
Application of Air Dryer
Compressed air contains more water vapor content than the air in atmospheric condition. There are chances they can condense in the pipelines or can come in contact with critical equipment or product causing corrosion or rusting of equipment, affecting the effectiveness of equipment and reducing the shelf life of product by initiating microbial growth like fungus and molds. Air dyer removes water from compressed air by employing different technologies that are discussed further. Water content in compressed air are measured by pressure dew point i.e., PDP, where low value of PDP indicates less amount of water vapor in the compressed air.
Types of Air Dryer
Performance characteristics of air dryer are typically defined by the dew point and depending upon there working they can vary. There are mainly four types of air dryers that are being discussed here. Air dryers are widely categorised into primary and secondary types and the major four categories comes in this.
- Regenerative desiccant dryer: The compressed air is passed through a pressure vessel with two towers filled with a media such as activated alumina, silica gel, molecular sieve or other desiccant material. This desiccant material attracts the water from the compressed air via absorption. As the water clings to the desiccant, the desiccant bed becomes saturated. The dryer is timed to switch towers based on a standard NEMA cycle, once this cycle completes some compressed air from the system is used to purge the saturated desiccant bed by simply blowing the water that has adhered to the desiccant off. The tower is drying tower and regenerative tower.
- Refrigerated dryer: Refrigeration dryers employ two heat exchangers, one for air-to-air and one for air-to-refrigeration. However, there is also a single heat exchanger that combines both functions. The compressors used in this type of dryer are usually of the hermetic type and the most common gas used is R-134a and R-410a for smaller air dryers up to 100 CFM. Older and larger dryers still use R-22 and R-404a refrigerants. A refrigerated dryer delivers a dew point not lower than approximately 35 °F (2 °C).
- Deliquescent dryer: A deliquescent dryer typically consists of a pressure vessel filled with a hygroscopic medium that absorbs water vapor. The medium gradually dissolves or deliquesces to form a solution at the base of the pressure vessel. The liquid must be regularly drained from the vessel and new medium must be added. A deliquescent dryer delivers a dew point suppression that fluctuates with air temperature. Typically, this suppression is 20 °F (11 °C) below the compressed air temperature.
- Membrane dryer: In a Membrane dryer the compressed air is first filtered with a high-quality coalescing filter. This filter removes liquid water, oil and particulate from the compressed air. In order to desorb the water Vapors, the partial flow of dried air expanded to atmospheric and passed through outside of the hollow fiber bundle in counter flow direction which will flush out the moisture.
It is not always required to have compressed air operating at minimum dew point as it can be economically not feasible, as reducing pressure dew point can be really expensive. It is advised to supply the degree of dryness needed for each application along with considering all other operating parameters in mind while selecting the correct air dryer.
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