What Are Dual Tower Regenerative Desiccant Air Dryers?
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.
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.
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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