Used
in various industrial applications, compressed air dryer is a component of
compressed air systems. These systems are used for generating compressed air
and effectively removing all the lubricants, moisture, impurities from the
air coming over from other compression processes.
Compressed Air Dryer in Gas Plants
Compressed air dryer is an integral part of any gas plant. Compressed air
dryers in gas plants are used to remove water and other contaminants from
compressed air. Different technologies are used to remove contaminants from
air and produce the final output, which is different gases like oxygen,
nitrogen, hydrogen, helium, natural gas, CNG, LNG etc. The technologies used
are :
- Refrigeration: Removes the water by cooling the air.
- Desiccant adsorption: It adsorbs the water/impurities in the air
with granular material like activated alumina, silica gel, zeolite and
carbon molecular sieves.
- Membrane filtration: The air is dried in single or multiple stages
which restricts tool corrosion and other problems related with water.
Working Principle of Compressed Air Dryer
Though different varieties of driers have their unique operating
principles, the main system of working of compressed air dryer is more or
less the same, which we are explaining below in figures 1 and 2:
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Compressed air enters the compressed air dryer and is moved to
Tower 1 by valve (A), which moves to the air dryer outlet through
shuttle valve (B). A portion of the dried air is limited to near
atmospheric pressure by using orifice (C). This is extremely dry, low
pressure air. It passes through and regenerates the desiccant in Tower
2. This process is exhausted by using purge/re-pressurization valve (D)
and exhaust muffler (E) to atmosphere. After the set time is fixed,
there is the automatic solid state timer which shuts the
re-pressurization valve (D). This leads to Tower 2 to re-pressurize
slowly. After few minutes valve (A) shifts and purge valve (D) re-opens.
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| In the second figure, the main air flow is now dried by Tower 2
while Tower 1 is being regenerated. |
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Types of Compressed Air Dryers
Refrigerated Compressed Air Dryers:
These type of dryers make use of a mechanical refrigeration system to cool
the compressed air and at the same time condense water and hydrocarbon
vapor. Most of these dryers cool the compressed air to a temperature of
around 1.6°C to 10°C which leads to a pressure dew point range of
0.5°C to 3.8ºC. This is the preferred range because it allows the
pressure dew point to fall within the range that is attainable with common
refrigeration system controls. Refrigerated compressed air dryers are
available in two basic styles:
- Non-cycling Refrigerated Dryers: Non-cycling compressed air
dryers cool the compressed air in an evaporator or an air-to-refrigerant
heat exchanger. Operation of the refrigeration compressor is continuous
or non-cycling and hence the name. The warm compressed air flows into
one side of the evaporator while low pressure, liquid refrigerant moves
into the other side of the evaporator.
- Cycling Refrigerated Dryers: Cycling compressed air dryers
cool the compressed air by using an intermediate heat exchanger medium,
which can be sand, metal, or a fluid. A compressed air chiller, two heat
exchangers, and refrigerant evaporator are fitted inside a tank. This is
filled with a thermal conducting fluid. The refrigeration system removes
heat from the fluid and the chilled fluid removes heat from the
compressed air. Since here the system is used to only cool the fluid,
once the fluid temperature is chilled to the required point, the
refrigeration compressor is closed. On an average, cycling compressed
air dryers can lead to 50% savings when compared to equally sized
non-cycling designs.
Heat
Less Compressed Air Driers:
These dryers are capable to take advantage of the natural tendency of
desiccants to set up equilibrium with their surrounding areas and to produce
dry and clean air. The system consisting of two towers with beds of
activated alumina absorbent desiccants, completes the filtering process.
From the compressor the wet air enters the tower, loses all its moisture
contents and the dry air moves up to the top of the tower. All the
desiccants are saturated. There is an automatic timer which moves the
incoming air to another tower. When one tower is drying the air, the other
tower regenerates the desiccants. This is an automatic process, which
utilizes a purge of already dried air to pass through the desiccant bed.
This Purge air removes the absorbed moisture from the absorbent desiccant.
Desiccant Air Dryers
Desiccant
air dryers make use of chemicals beads, called desiccant, to adsorb
moisture, impurities, water vapor from compressed air. There are three
different types of desiccants used commonly:
- Silica gel: This silica is in amorphous form with very good
water vapor adsorbing capacity.
- Activated Alumina: This is a porous form of aluminum oxide
with silicon dioxide. It has very good water vapor adsorbing capacity
and is the preferred desiccant for heatless air dryers.
- Molecular Sieve: This is a porous form of zeolites. They are
desiccants formulated specially to remove specific vapor or gas
molecules.
Buyer's Guide: Specifications
Specifications to consider for purchasing compressed air driers are:
- Dimensions, Connections and Weight
- Inlet air temperature
- Ambient air temperature
- Dew point requirement
- Operating pressure
- Airflow · Available utilities
- Drying capacity
- Maximum pressure
- Motor power
Compressed
Air Dryer
