Water vapour
Water enters the compressed air system through the compressor intake as a
vapour (or gas). The ability of air to hold water vapour is dependent upon its
pressure and its temperature. The higher the temperature, the more water vapour
that can be held by the air, the higher the pressure, a greater amount of water
vapour is squeezed out. As large volumes of air are draw into the compressor
and compressed, the temperature of the air increases significantly. This allows
the heated air to easily retain the water vapour in the atmospheric air.
Unlike water in a liquid or aerosol form which is removed from compressed air
using a coalescing filter; water vapour (water in a gaseous form) can only be
removed using a dryer.
Adsorption (Desiccant) Dryers
Adsorption dryers remove water vapour by passing air over a regenerative adsorbent
material known as desiccant which strips the moisture from the air. All adsorption
dryers remove water vapour using this method, however the adsorbent must be
periodically regenerated to ensure a continuous supply of dry air, and a number
of different regeneration methods are available.
Pressure dewpoint is the term used to describe the temperature at which condensation
will occur and the water removal efficiency of a dryer is expressed as a pressure
dewpoint (written as a temperature). Adsorption dryers are extremely efficient
and typically provide pressure dewpoint’s of -40°C or -70°C. This
means for water vapour to condense into a liquid, the air temperature would
need to drop below -40°C or -70°C respectively. The actual air temperature
after an adsorption dryer is not the same as its dewpoint.
Typically, a pressure dewpoint of -40°C is used in most applications as
compressed air with a dewpoint below -26°C will not only prevent corrosion,
it will also inhibit the growth of micro-organisms within the compressed air
system.
Refrigeration Dryers
In general purpose applications, refrigeration dryers are often used in place
of adsorption dryers. Refrigeration dryers remove water vapour by cooling the
compressed air and collecting the condensed liquid. For this reason, they are
limited to positive pressure dewpoint’s to prevent freezing (typically
+3°C, +7°C or +10°C). This type of dryer will remove free water
from the system and slow corrosion, however they will not inhibit bacterial
growth and are not suitable for installations where piping is installed in ambient
temperatures below the dryer dewpoint i.e. systems with external air receivers
and piping.
Are all compressed air dryers the same?
Compressed air purification equipment is essential to all modern production
facilities. It must deliver uncompromising performance and reliability whilst
providing the right balance of air quality with the lowest cost of operation.
Today, many manufacturers offer products for the filtration and purification
of contaminated compressed air, which are often selected only upon their initial
purchase cost, with little or no regard for the air quality they provide, the
cost of operation throughout their life or their environmental impact. When
purchasing purification equipment, delivered air quality, the overall cost of
ownership and the equipment’s environmental impact must always be considered.
The Parker domnick hunter Design Philosophy
Parker domnick hunter has been supplying industry with high efficiency filtration
and purification products since 1963. Our philosophy "Designed for Air
Quality & Energy Efficiency" ensures products that not only provide the
user with clean, high quality compressed air, but also with low lifetime costs
and reduced CO2 emissions.
Cost Effective System Design
The quality of air required throughout a typical compressed air system will
vary depending upon application. Treatment of the compressed air at only one
point alone, for example the compressor room; is not enough and it is highly
recommended that the compressed air is first treated prior to entry into the
distribution system (to a quality level suitable for protecting air receivers
and distribution piping) and then at the point of use, with specific attention
being focussed on the application and the level of air quality required. This
approach to system design ensures that air is not “over treated”
and provides the most cost effective solution to high quality compressed air.
The use of the ISO 8573-1:2001 Air Quality Standard provides the system engineer
with a simple method of specifying the air purity required at each point in
the compressed air system. Using this standard allows Parker domnick hunter
to quickly and easily select products from its comprehensive range of purification
products to exactly match your system requirements and ensure both your capital
and operational costs are kept to a minimum.
The following table highlights the Parker domnick hunter filtration and drying
products required to achieve each air purity classification shown in ISO 8573-1:2001.
For further information regarding the ISO 8573 air quality standards, please
refer to publication reference 17 400 4765.
