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Dryer Types Used for
Air Compressors
The most common types of dryers include
the refrigerant dryer (which is most commonly used), cycling dryers,
non-cycling dryers and heat of compression dryers.
The refrigerant dryer has low initial costs, as well as relatively low
operating costs. It is the most used dryer in the industry. However, it
is not suggested for use in sub-freezing temperatures as the moisture
contained in the compressed air may freeze and, thereby, damage the
dryer. A precooler/reheater is included on most refrigerated dryers so
that the dried compressed air can be reheated with an air-to-air heat
exchanger by using the incoming hot air. By utilizing this process, the
temperature of the incoming air will be lowered prior to passing through
the refrigerant mass and to the air heat exchanger. This causes
decreased heat on the refrigerant system. The purpose of reheating the
dried air is to avoid condensation on the exterior of the compressed air
piping which can occur in warm humid environments. In order to prevent
freezing, the evaporator temperature must stay at 32 degrees F or
higher. It is usually possible to obtain an air pressure dew point of 35
degrees F or higher for air exiting the dryer. The refrigerated dryer is
capable of lowering the dew point of the air to approximately the same
temperature as that of the air leaving the refrigerant evaporator. One
or two thermostats control refrigerant-type dryers so that it can shut
off the refrigerant compressor when it’s not needed.
Cycling dryers contain a storage medium, such as a heat sink, thermal
mass or chilled media that cools compressed air indirectly. Non-cycling
dryers directly cool compressed air in a refrigerant-to-air heat
exchanger. A thermal storage medium such as a heat sink, prohibits quick
cycling of the refrigerant compressor. Thermal storage mediums may
include powdered metal, glycol and water, sand, steel or aluminum. The
features of such mediums are that they have high specific heat and high
coefficient of heat transfer.
Heat-of-compression dryers are inexpensive regenerative, water-absorbing
dryers that use heat produced during corrosion. The amount of
compression to achieve desiccant regeneration of the medium is
determined by the temperature. There are two types of
heat-of-compression dryers: the single vessel and the twin tower.
The cooling switch of towers is achieved from the evaporation of a
liquid refrigerant rotating drum contained in a single pressure vessel.
As a result the moisture in the air will condense, dividing the moisture
into two separate air streams. One stream is removed from the process
and drained by a separator. This stream is part of the hot air. A hot
gas bypass valve controls the air temperature before it becomes
evaporated.
The regenerative-desiccant dryers use an absorbent regeneration of the
desiccant bed. It absorbs moisture by collecting the left-over air
dispersed from its pores making a large amount of the air compressor to
be preserved compared to a smaller amount of the after-cooler. This air
then passes through a drying desiccant. In some cases, it passes through
a regeneration cooler in which a desiccant type will use it before
merging it with the main air stream by a special drying process.
A large particle is used in a twin-tower dryer while a small particle
operation is used for final drying, although it’s quite similar to the
twin-tower process. Compressed air that needs drying exits by the air
compressor with all the air flow through one tower only. The other
portion passes through the second tower before regeneration and the
process is completed when the air flows into the drying tower. The air
purged can be reheated at a very high temperature either inside or
outside of the dryer to produce less regeneration, making the use of a
purge air required. The purge air can be abounded by a centrifugal or
lubricant-free blower.
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