Machine Design

Air Dryers

Water vapor is contained in high-pressure air. In vapor form, the water does little damage in most components. But if the water is allowed to condense in the system, it can rust pipes, freeze up actuators, and damage a process, tool, or instrument.

Manufacturers of pneumatic components normally specify that air of a certain dryness be supplied for best operation. For example, some pneumatic instruments and logic elements require ultradry air; at the other end of the scale, most pneumatic hand tools require only that moisture not actually turn to liquid during the air-expansion process.

Main-line pneumatic system dryers are installed close to the compressor, usually between the air receiver and distribution lines. Such a unit dries the whole pneumatic system in a plant and prevents a host of filtration problems throughout the system. Individual dryers may be used directly upstream of specific components that require dry or ultradry air. For providing either mainline or point drying, three types of dryers can be used.

Deliquescent dryers are simply large pressure vessels filled with a chemical having an affinity for water -- salt, urea, and calcium chloride are common. As the compressed air passes through the vessel, the salt dissolves in the water vapor and drips to the bottom of the tank where it is drained. The dried air is then discharged through the outlet port at the same temperature at which it entered.

Deliquescent air dryers are inexpensive and simple; however, they require that the salt be replenished regularly. In addition, the corrosive salt solution can cause drain traps to clog. Also, a fine salt mist can be entrained in the air and be carried downstream to corrode system components.

Deliquescent air dryers can suppress the dew point by only about 20°F below the inlet temperature. Newer models are somewhat more efficient, but these types are limited to inlet air temperatures of 70°F or lower.

Refrigerated air dryers remove moisture from air by cooling it so that water vapor precipitates out. Where dew point requirements are above 32°F, these dryers are the most economical choice because of their low initial cost. They are available for a wide range of dew point requirements -- from 33 to 100°F -- and require little operator attention. Refrigerated dryers cannot suppress dew point below 33°F because ice forms and clogs the heat exchangers.

Generally, refrigerated air dryers use two heat exchangers in series to condense entrained moisture and reheat the outlet air. Most dryers of this type precool the incoming air before it reaches the refrigeration chiller. Precooling reduces the load on the chiller so that smaller, less-expensive heat exchangers and compressors can be used.

The precooling arrangement is usually an air-to-air heat exchanger that uses outgoing cold air to cool the inlet air. Since this setup requires cold air to precool inlet air, the dryer has a start-up delay before it reaches the required dew point.

Where slow start-up cannot be tolerated, dryers employing air-to-refrigerant heat exchangers are available. Although more costly than standard designs, these dryers provide a nearly constant dew point, even with fluctuations in airflow and ambient temperature.

Desiccant dryers use nonconsumable chemicals such as silica gel or active alumina, and can remove almost all the moisture from compressed air. A desiccant dries air by adsorbing moisture on its surface and holding the water as a mono or biomolecular film. The method of regeneration, the process of removing adsorbed water from the desiccant, is the primary distinguishing feature among the various types of desiccant dryers.

Most regenerative desiccant dryers are dual-chamber systems with one chamber on-stream drying the compressed air while the other is off-stream being regenerated. There are three ways to regenerate a desiccant: with air, internal or external heaters, or a heat pump.

Heaterless dryers regenerate desiccant by passing a quantity of the dried air through the offstream vessel. These dryers are the most expensive desiccant types to operate because they require high purge airflows to regenerate the desiccant. For example, a pressure dew point of -40°F uses about 14% of the compressed airflow for regeneration. To minimize air usage, some dryers are equipped with controls so that purge rate can be adjusted to accommodate variations in ambient temperature.

Heaterless dryers produce dew points down to -150°F. Maintenance costs are low because they have few moving parts, and desiccant can last 10 to 15 years if the air is prefiltered to prevent oil contamination.

Heat-regenerative dryers are similar to heaterless dryers except that the desiccant vessels contain heating elements. Depending on the amount of heat applied, these types still require 2 to 6% purge airflow to produce a pressure dew point of -40°F. The less purge air used, the higher the cost of power to run the heaters.

Heat-regenerative dryers have the same dew-point range as heaterless types. However, since high regenerative temperatures can damage equipment and desiccant, maintenance costs and downtime can be higher.

Heat-pump dryers combine the best features of heaterless desiccant and refrigerated dryers. These dryers use refrigeration cooling to remove most of the incoming moisture and to cool the onstream vessel for maximum adsorption efficiency. Thermal energy from the inlet air then is used to heat the offstream vessel and a small amount of purge air for regeneration.

In this type of dryer, a -40°F pressure dew point requires 1.5% purge airflow to regenerate the desiccant. Thus, heat-pump dryers have considerably lower operating costs than the other types.

Heat-pump dryers produce stable dew points down to -100°F. Also, because regeneration takes place at a lower temperature, the hazards and maintenance problems of heat-regenerated dryers are eliminated.

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