Heaterless Type (Pressure Swing Dryers)
Dual tower desiccant air dryers are used to produce dewpoint temperatures below the freezing point of water, as well as reduce the moisture content of compressed air used in critical process applications. Typical dewpoints produced by these types of dryers are -40 o F to -100 o 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 airlines are exposed to low ambient temperatures, below 32 o F, and in critical applications such as electronic component manufacturing and transporting and packaging of pharmaceuticals. Desiccant 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:
1. Activated Alumina
2. Silica Gel
3. Molecular Sieve
The process of adsorption 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 the concentration of water vapor in the air is greater than 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.
In summary, in a desiccant dryer, adsorbing the water vapor onto the large, dry surface area of a highly porous material called desiccant dries the compressed air. This adsorption occurs as water vapor in the compressed air moves into the pores of the dry desiccant and is attracted to the surface of the pores.
As water vapor is adsorbed, heat is released. The water remains on the surface of the desiccant until it is stripped off, and finally, desiccant can be regenerated and used again and again.
Dual tower desiccant dryers offer a continuous supply of dry compressed air by 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.
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.
The standard cycle time for a heaterless air dryer is 10 minutes, for a -40 o 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. The dry air is then directed by a system of check valves to the air outlet.
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.
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 after changeover.
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 present in the air stream to separate out in the bottom of the tower. The collected contaminants remain there until they are flushed from the system as the tower is depressurized.
The average purge air requirements of a heaterless 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 the release of heat 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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