Air dryers are used to remove moisture from compressed air, There are various types of compressed air dryer available to remove moisture from compressed air.
Compressed air dryer are classification based on the method of separating moisture from compressed air
- Refrigerant Air Dryer
- Dessicant air Dryer.
- Deliquescent dryers
- Membrane type air Dryer.
- Refrigerated type air dryer:-
Refrigerated type air dryer is most popular and common for general purpose application.The compressed air is cooled in an air-to-refrigerant heat exchanger to about 4 Deg C, at this temperature moisture in air get condense and drained though moisture separator. Leaving air at lower temperature is passed though incoming air for heat exchange to reduce load in refrigeration system. With refrigeration system pressure dew point is achieved as 4 Deg C.
2. Desiccant Air Dryer:-
These dryers use a desiccant, which adsorbs the water vapor. A distinction needs to be made between adsorb and absorb. Adsorb means that the moisture adheres to the desiccant, collecting in the thousands of small pores within each desiccant bed. The composition of the desiccant is not changed, and the moisture can be driven off in a regeneration process by applying dry purge air, by the application of heat, or a combination of both. Absorb means the material that attracts the moisture is dissolved in and used up by the moisture. Absorption takes place in a deliquescent desiccant type dryer.
Regenerative desiccant dryers normally are of twin tower construction.One tower dries the air from the compressor while the desiccant in the other tower is being regenerated after the pressure in the tower has been reduced to atmospheric pressure. Regeneration can be accomplished using a time cycle or on demand by measuring the temperature or humidity in the desiccant towers or by measuring the dew point of the air leaving the on-line tower.
Bases on regenerationDesiccant air dryer can be classified as:-
a. Heat Less Desiccant air Dryer.
No internal or external heater is used. Purge air requirement can be upto 18% of total air flow. In this type of dryer at 7 Kg/cm2(g)has pressure dew point -40 Deg C, in this type of air dryer pressure dew point upto -70 Deg C can be achieved.
b. Heat reactivated air Dryer. (Internal or External)
Internal or External heat is applied by heater in this dryer for the recharging of desiccant, In internal heat type dryer, Steam or electricity may be used in heater embedded in the desiccant bed. This reduced the amount of purge air loss requirement to less than 10%. The purge air plus normal radiation is used to cool the desiccant bed after regeneration to prevent elevated air temperatures going downstream.
In externally heated regenerative desiccant dryers,the purge air is heated to an elevated temperature and then passes through the desiccant bed. The amount of purge air is approximately 5-10% of the air flow through the dryer. The purge air from the compressed air system can be eliminated if a blower is used for the circulation of heated atmospheric air through the desiccant bed. To protect the desiccant bed from oil contamination from the air compressor, a coalescing filter is required upstream of the dryer.To protect downstream equipment from desiccant dust or “fines”, a particulate filter downstream of the dryer also is also recommended.
c. Heat of compression air dryer:-
Heat of compression type air dryer can be used with oil free air compressor. This type dryers are no purge loss dryer. For Air dryer regeneration heat of compressed air is used. With HOC dryer Pressure dew point of -40 Deg C can be achieved.This type of dryer can be most economical type of regenerative dryer.
3. Deliquescent air dryers
A deliquescent dryer typically consists of a pressure vessel filled with a hygroscopic medium that absorbs water vapor. The medium gradually dissolves—or deliquesces—to form a solution at the base of the pressure vessel.The liquid must be regularly drained from the vessel and new medium must be added to keep system operating. The medium is generally in tablet form.
Deliquescent dryers have no moving parts and don’t require electrical power for operation. Common applications therefore often involve remote,hazardous, or mobile work sites. This type of dryer can be used for removing moisture from compressed air, natural gas, waste or any other type of gases.
The performance of a deliquescent dryer is depended on air temperature,cooler the air temperature better the performance of air dryer.
4. Membrane air Dryer:-
Membrane dryer refers to a dehumidification membrane that removes water vapor from compressed air. the compressed air is first filtered with a high-quality coalescing filter. This filter removes liquid water, oil and particulate from the compressed air. The water vapor–laden air then passes through the center bore of hollow fibers in the membrane bundle. At the same time, a small portion of the dry air product is redirected along the outside surface of the fibers to sweep out the water vapor which has permeated the membrane. The moisture-laden sweep gas is then vented to the atmosphere, and clean, dry air is supplied to the application.The membrane air dryers are designed to operate continuously. Membrane air dryers are quiet, reliable and require no electricity to operate.