Dual tower desiccant air dryers are used to produce dewpoint temperatures below the freezing point of water and reduce the moisture content of compressed air used in critical process applications. Typical dewpoints produced by these types of dryers are -40° F to -100° F, although lower dewpoints are possible.
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These types of dryers are typically used to dry instrument air and process air, as well as applications where air lines are exposed to low ambient temperatures below 32° F, and in critical applications such as electronic component manufacturing and transporting and packaging of pharmaceuticals.
How Desiccant Air Dryers Work
Desiccant air dryers lower the dewpoint of compressed air by adsorbing water vapor onto the surface of a desiccant.
The three basic types of desiccant used in dual tower regenerative air dryers are:
- Activated Alumina
- Silica Gel
- Molecular Sieve
The adsorption process begins as the water vapor, which is more highly concentrated in the compressed air stream, moves into an area of lower water vapor concentration in the pores of the desiccant. Once inside the pores, a natural attraction of the vapor molecules to the solid surface of the desiccant causes water vapor molecules to build up on the surface of the desiccant.
As enough molecules gather, vapor changes phase and becomes a liquid. The process continues as long as water vapor concentration in the air exceeds the concentration in the desiccant pores. The water remains on the surface of the desiccant until it is stripped off. This is called reactivating or regenerating the desiccant.
By doing this, the desiccant may be used again and again.
How Dual Tower Desiccant Dryers Work
Dual tower desiccant dryers offer a continuous supply of dry compressed air using two identical towers, each containing a bed of desiccant beads. While one tower is on stream drying the compressed air, the other tower is off stream so the desiccant in that tower can be regenerated.
Pressure Swing Desiccant Dryers
In a pressure swing type regenerative dryer, the regeneration of the desiccant bed is accomplished by expanding some of the dried air to near atmospheric pressure and directing it across the wet desiccant bed. This swing in pressure produces expanded air with a very low water vapor concentration.
Because the water vapor concentration of this expanded or purge air is less than the water vapor concentration in the pores of the desiccant, water vapor moves from the desiccant back into the purge air stream. The purge air stream then carries the desorbed water out of the dryer.
Pressure swing dryers are often called heaterless or heatless dryers because no outside heat is added for the regeneration of the desiccant.
Standard Pressure Swing Drying Cycle
The standard cycle time for a pressure swing air dryer is 10 minutes for a -40° F dewpoint. (Other variations exist for varied dewpoints.)
The main air stream enters tower A at the left inlet switching valve and is dried by the desiccant. A system of check valves to the air outlet then directs the dry air.
While this occurs, the desiccant is regenerated in the off-stream tower by throttling a portion of the dried air to near atmospheric pressure by means of an adjustable purge rate valve and orifice. This expanded air, or purge air, flows through and reactivates the desiccant in tower B.
The purge air is then exhausted through the right purge re-pressurization valve and muffler to the atmosphere. After 4 minutes, the right purge re-pressurization valve closes, allowing tower B to slowly re-pressurize.
At 5 minutes, the left inlet switching valve closes, and the right inlet switching valve opens. The main air stream is directed across the regenerated desiccant in tower B. As this occurs, the left purge re-pressurization valve opens, allowing tower A to depressurize.
Purge air now flows through tower A, allowing the desiccant in this tower to be regenerated. The bulk of the vapor is removed in the lower part of the dryer, and the upper section is used to polish the air (to remove the final grains of moisture, to arrive at the final pressure dewpoint). All dryers of this type work essentially the same way.
Dual Tower Air Dryer Considerations
Desiccant Dusting Prevention
The flows through the dryer must be precisely controlled to prevent bed movement. If the air velocity were too great, the desiccant would begin to float or fluidize. This would cause the desiccant beads to rub together and degrade as desiccant particles wear away. This desiccant dusting reduces the useful life of the bed as well as increasing wear and tear on the valves.
Dusting also causes frequent maintenance of downstream filtration. To prevent bed fluidization, manufacturers design their dryers to have the off-stream tower slowly and completely re-pressurized before tower change over. This prevents bed jolting as well.
Optional Contaminant Separation
On some dryers, the inlet air enters the bottom of the tower and flows upward through the desiccant bed. This allows any liquid water and large contaminants in the air stream to separate at the tower's bottom. The collected contaminants remain there until they are flushed from the system as the tower is depressurized.
Purge Air Requirements
The average purge air requirement of a pressure swing type air dryer is 15-20% of the rated capacity of the dryer. (Not the compressed air system capacity.) The purge air requirement varies with the different manufacturers&#; requirements and designs.
Heat Release
Another important aspect of adsorption is heat release as water vapor is adsorbed onto the desiccant. This heat of adsorption is produced by the change in the energy state that occurs as water vapor is attracted to the desiccant and condenses.
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How Do Air Dryers Work? &#; Membrane, Desiccant, and Refrigerated Dryers
Compressed air dryers are essential in a host of industrial facilities that require a regular supply of dry, compressed air. These dryers are critical in industrial operations because the presence of moisture in industrial air can damage pneumatic systems, cause freezing in pipelines, promote corrosion in metallic parts, etc.
A wide range of instrument air dryer types are available on the market today. Understanding the different types of air dryers and their principles of operation will let you choose the ideal compressed air dryer for your unique application.
What Is a Compressed Air Dryer?
A compressed air dryer is a piece of equipment designed to separate water vapor or moisture (de-humidify) from industrial process air. In the typical system, a compressor draws in humid air and compresses it, which raises the air temperature and then cooling the air condensing water vapor out of the unit.
However, there are other ways to achieve separation. The main types of air dryers include the refrigerated, desiccant-type, and membrane type air dryer.
Refrigerated Air Dryer Working Principle
Refrigerated air dryers are one of the most common types used in industrial applications due to their ease of maintenance and relative cost-effectiveness. They are suitable for air-drying applications without special requirements such as a minimum dew point.
The refrigerated air dryer working principle is based on de-humidifying air by rapidly cooling it, condensing it, and draining off the moisture. The operation is like that of a domestic refrigerator or home air-conditioning system.
Refrigerated air dryers de-humidify air through the following phases:
- Warm, moist air enters the dryer, which cools it rapidly to about 3°C (37.4°F) in a refrigeration unit. At this temperature, the water vapor in the air condenses into pure water, which collects in a water trap and fed into discharge lines. The warm, gaseous refrigerant is cooled and regenerated in a condenser.
- The dry air in the chamber is re-heated to room temperature and fed through an outlet.
Moreover, refrigerated air dryers are available in two variants &#; cycling and non-cycling.
- Cycling dryers are 100% duty cycle machines that can maintain the dew point at a constant temperature.
- Non-cycling dryers work by intermittently shutting down and restarting to maintain the required temperature.
Both variants are useful in different applications, although cycling types tend to ramp up maintenance costs.
Desiccant-type Air Dryer Working Principle
A desiccant air dryer system consists of two towers, one for drying the air and the other for regenerating the desiccant. The drying tower contains a porous desiccant material that inhibits water molecules as compressed air from the inlet passes over it.
A less common variant, the single tower desiccant air dryer, comprises a single tower containing a desiccant material that dries air coming in from the environment. Single-tower dryers also contain no mechanical parts and do not require electricity to operate. They are suitable for use in hazardous and corrosive environments.
These instrument air dryers use a desiccant material, hygroscopic substances that exhibit a high affinity for water as the de-humidification agent. These types of compressed air dryers achieve a low dew point, making them suitable for use in colder climates and industries that require super dry air.
There are three main types of desiccant dryer systems &#; heated, heatless, and heat of compression desiccant air dryers. Let&#;s take a closer look at these below.
Heated desiccant air dryer
Heated desiccant air dryers utilize a source of heating within the drying tower to heat the desiccant material sufficiently to minimize the need for purge air. Typical dew points in heated dryers range from -40°C to -73.3 °C (&#;40 to &#;100 °F).
Heatless regenerative desiccant air dryer
Heatless desiccant dryers do not contain a heated system in the regenerating tower; instead, they utilize &#;purge air&#; to expel moisture from the tower. Typical dew points in heatless dryers range from -40°C to -73.3 °C (&#;40 to &#;100 °F).
Heat of Compression
A heat of compression air dryer uses both towers simultaneously to maintain a uniform temperature. This design allows for lower operating costs but with a less consistent dew point.
Membrane Air Dryer Working Principle
Membrane dryers use permeable membranes similar to nitrogen separation membranes or CO2 separation membranes to extract water vapor from process air. These systems are convenient to use, more cost-effective, and require less maintenance since they have no moving parts. They are suitable for low volume air separation.
GENERON Has Air Drying Solutions for Diverse Applications
GENERON is a world-class manufacturer of compressed air systems. We have over 50 years of experience providing a full range of air-drying solutions to the commercial, industrial, and environmental facilities worldwide.
Contact us online today for more information on our compressed air dryer solutions or to request a quote.
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