The specific water vapour content of air or, in general, of any gas or mixture of gases, expressed in terms of vapour weight per unit of mixture volume, is limited by the temperature to which it is subjected (dew point). At a certain temperature, air may contain a maximum of a given amount of water vapour; that amount will fall as the temperature of the air is reduced. It is possible to reduce the water vapour content of compressed air by lowering its temperature. The excess water vapour condenses and, in its liquid state, can be easily separated from the flow of the gas stream. That is the physical phenomenon that takes place in end coolers, which use ambient air or water for cooling purposes and are installed at air compressor outlets. However, the term 'refrigeration dryer' is only used for equipment capable of cooling air to temperatures far below the ambient temperature. Such units have cooling systems that are able to reduce the temperature of compressed air (generally around +3ºC). It should not be cooled to below 0ºC, as ice would form on the heat exchange surfaces, restricting the passage of air and/or reducing the transmission of heat. Technical specifications refrigeration dryers water-driven condensation Technical specifications refrigeration dryers RD range Tabla de datos técnicos secadores frigoríficos gama RD        When a fluid (a gas or a liquid) comes into contact with solid matter, some of the molecules of the fluid become strongly attached to the surface of the solid, due to the action of forces of mutual attraction, and only become detached if they acquire the energy necessary for that to happen. The phenomenon in question is called physical adsorption. Virtually all solids are adsorbent to some degree, but only in the case of a small number of them is that property particularly notable. Referred to as adsorbents, such bodies have the common characteristic of containing a large quantity of micropores, the surface of which can be up to a hundred thousand times greater than the external particle area. Existing adsorbents do not have the same affinity with all fluids. On the contrary, each adsorbent has an exclusive, noteworthy affinity with certain fluids. Such selectivity makes it possible to use them specifically in numerous fluid purification and drying processes. Activated alumina, silica gel and molecular sieves are the most appropriate adsorbents for the purpose of drying gases, for which reason they are known as desiccants. Almost all adsorption dryers are of the fixed bed variety. The drying process carried out by such dryers takes place in a vertical recipient called a drying tower, which contains a given amount of desiccant. Air passes through the desiccant, as a result of which the water vapour contained in the former comes into contact with the latter and is adsorbed thereby. At the outlet of the drying tower, the air still contains a small amount of water vapour that has not been adsorbed. That residual vapour can be gauged using an appropriate measuring device, called a dew point meter, which makes it possible to determine how much water vapour remains in the air, in degrees of temperature. The dew point is the lowest temperature, at constant pressure, to which the air may be cooled without any risk of the remaining vapour producing condensation. Once the dew point of the air has been ascertained, it is possible to establish the quantity of residual water vapour in terms of grams per cubic metre of air. The continuous adsorption of water vapour leads to the saturation of the desiccant. After use for a certain period of time, called a half cycle, it is necessary to halt the process taking place in the drying tower, as the residual water vapour in the air at the outlet could henceforth begin to exceed the required amount. Carrying out the regeneration process takes a certain amount of time, meaning that only intermittent service is possible with a single drying tower (a "simplex" dryer), as the drying and regeneration processes have to be alternated cyclically. Twin drying towers (a "duplex" dryer) are used for continuous service, with no interruptions to the supply of dry gas. The two towers are combined in such a way that the drying process takes place in one of them while the regeneration process is being carried out in the other. The regeneration process consists of releasing the water vapour retained by the desiccant. It is only possible to do so by ensuring a very low moisture level inside the drying tower. The difference in vapour tensions between the surface of the desiccant and the surrounding atmosphere overcomes the forces of adsorption and the vapour molecules are released. Maintaining such conditions for a sufficient amount of time regenerates the desiccant. The released vapour can be removed from the tower in the following ways: By means of a vacuum pump (a method that is rarely used). By passing a stream of purge gas through the inside of the tower. As the vapour is released, it is incorporated into the purge gas. When the purge gas exits the drying tower, it thus draws the vapour along with it. The purge gas may be: A dry gas with specific characteristics. A proportion of the dry air obtained via another drying tower. A proportion of the actual compressed gas to be dried. Atmospheric air. At the outlet of the drying tower, the purge gas may be: Released into the atmosphere, in which case a continuous supply of purge gas is required throughout the entire regeneration process. Recovered for repeated use as a purge gas. Recovered to be used after it has undergone a drying process in another tower. This is the case when a proportion of the actual compressed gas to be dried is used as a purge gas. The regeneration process may be carried out: Without heating, thus making it necessary for the purge gas to be very dry and the half cycles to be very short, so as to enable the vapour to acquire the same heat upon being desorbed as it relinquished when it was adsorbed. The heat in question remains in the desiccant for a certain amount of time. With heat being provided by means of a heater immersed in the desiccant bed. With heat being provided via the purge gas itself, which involves warming it by means of a heater outside the drying tower. Heating has two purposes. The first is to provide the heat of desorption that the vapour adsorbs upon being released, while the second is to reduce the moisture level of the purge gas, especially in the case of it being wet gas. In adsorption dryers in which regeneration takes place without heating and with a short tower inversion process, the stream to be dried, from which drawn condensation will previously have been decanted, is channelled to one of the two towers, where it undergoes a process in which water vapour is adsorbed using an inert, solid desiccant. The reactivation of the desiccant saturated during the previous half cycle takes place simultaneously in the second tower. Such regeneration is carried out at atmospheric pressure with a proportion of the compressed gas that has already been dried and decompressed using a calibrated metering valve. The extremely low water vapour tension gives rise to the desorption of the water retained in the saturated desiccant, thus restoring the desiccant to an ideal condition for it to be used to dry the compressed stream in the subsequent half cycle. No other source of energy is used. In adsorption dryers in which regeneration takes place using internal electric heating and with a long tower inversion cycle, the stream to be dried, from which drawn condensation will previously have been decanted, is channelled to one of the two towers, where it undergoes a process in which water vapour is adsorbed using an inert, solid desiccant. The reactivation of the desiccant saturated during the previous half cycle takes place simultaneously in the second tower. Such regeneration is carried out using an internal electric heater in each tower, positioned inside a heat sink in such a way as to ensure that there is no direct contact between the desiccant and the heating element, and to allow for its removal without having to empty out the adsorbent load. The water vapour released during the regeneration stage is drawn to the exterior of the dryer by means of a small stream of dry gas. There are also dryers with three and as many as four drying towers, which are used in certain special cases. Nonetheless, the most common configuration is the twin drying tower system. Technical specifications adsorption dryer regeneration via heating Technical specifications adsorption dryer regeneration without heating        When the conditions of the gas to be treated at the inlet of the drying equipment are not suited to the adsorption process (as a result of a high temperature and/or a high level of relative humidity), preliminary treatment is carried out on the gas. The gas to be treated usually undergoes preliminary drying. To that end, a cooler (water exchanger) is used to reduce the temperature and expel the condensation that has formed with the lowering of the dew point. However, to obtain better performance from adsorption equipment, it is possible to use a refrigeration dryer to perform preliminary drying, thus leaving the gas to be treated in optimal conditions as regards temperature and relative humidity for the adsorption process. Units of the kind in question are therefore called combined adsorption dryers, as the gas is first treated using another type of system.        Polígono Industrial El Pedregar. c/ Progrés, nº 6 - 8. 08160 Montmeló (Barcelona) • Spain Telf. (34) 93 572 12 22 • Fax. (34) 93 568 45 56 • airsec@airsec.com