In some applications, compressed air lines may be exposed to temperature below freezing levels. In these applications, the pressure dew point must be reduced to below the levels that a refrigerant air dryer can achieve, in order to prevent ice from forming in the compressed air lines. There are many applications where compressed air may come into contact with chemicals, food products or other products in which water vapor in the compressed air may interact negatively in the process. These applications are referred to as “process” applications, and it is often necessary to remove almost all of the water vapor from the compressed air. There are also applications where compressed air is used for instrumentation purposes, and water vapor will negatively affect the instruments.
These are all applications where desiccant air dryers are required. There are international standards for purity of compressed air for particulate and oil content, and dew point levels. These levels are readily available under the ISO8573-1 standard. For most of the desiccant dryer applications, Class 2 is the most common level, which calls for -40ºF (or ºC) pressure dew point in compressed air. Class 1 is the maximum level which calls for dew point levels to -94ºF (-70ºC).
Understanding Desiccant Air Dryers
Twintower regenerative, desiccant air dryers are the industry standard for achieving these dew point levels. These dryers are most commonly available in either a “heatless” or “heated purge” types. Desiccant used in these dryers is most commonly activated alumina, but other desiccants used may include molecular sieves, silica gels, or combinations of these products. All of these adsorptive products are hygroscopic porous materials which have an affinity to water vapor. The pores are of a specific size (measured in angstroms) that water vapor particles are actually attracted to, and condensed into. As a change from vapor phase to liquid phase requires the transfer of latent heat, heat is therefore released during the drying process, raising the sensible temperature of the desiccant bed as compressed air is dried. With twintower dryers, after a period of drying on one tower, the desiccant in the chamber must be regenerated to remove the moisture which was condensed into the desiccant bed during the drying phase. In heatless dryers, this is accomplished using the “pressure swing” principle, in which a portion of the dried air from the outlet of the on-line tower is expanded and channeled back through the regenerating tower, counter-flow to the drying phase. Key to the success of full and efficient regeneration, is that the volume of air used for purging is equivalent to the volume previously dried, but at a much lower pressure and dew point.
Factors assisting Moisture Removal during Regeneration
For 100 PSIG applications, when the purge air is expanded to close to atmospheric conditions, its dew point level will fall approximately 30ºF lower than the outlet dew point from the dryer. Therefore, if -40ºF pressure dew point purge air is expanded to near atmospheric level, the dew point will fall to approximately -70ºF. This super dry air, passing through the desiccant bed which has retained heat from the previous on-line drying period, will cause the condensate in the desiccant pores to return to vapor phase, and then be carried in the purge air stream to be vented to atmosphere in vapor form. Other factors that assist in removing moisture during regeneration, are the heat of adsorption which retains heat in the desiccant bed when the purging starts, and the high velocity of depressurization as the tower blows down.
Design Considerations for Heatless Desiccant Air Dryers
As mentioned earlier, the volume of purge air should be equivalent to the volume of air previously dried. In designing the dryer, consideration must be given to desiccant bed size, pipe and fitting size and valve sizing, all of which will create resistance to air flow. Normally, flow resistance through the desiccant bed, piping and exhaust mufflers should be limited to approximately 2-3 PSID. Therefore, if the operating pressure is 100 PSIG, and the purge air is reduced to approximately 2.5 PSIG after consideration to flow restriction pressure drop, the expanded volume will represent approximately 15% of the compressed air volume. If operating pressures are higher, this percentage will be lower, and if the operating pressure is lower, the percentage of purge air consumption will become higher.