APPLICATION STORY
  
Humidity measurement in compressed air systems.
 
'The only way to be certain that the fryer is functioning
correctly is to measure the moisture content of the outlet
air directly using a suitable hygrometer.'

Many manufacturing processes require a constant supply of
compressed air. The air may be used for many different reasons
- for example the actuation and control of pneumatic valves,
cylinders and controllers; operation of process machinery or
tooling; as a transport medium for bulk materials; or as a purge
gas.

The compressed air supply will contain some moisture which,
if uncontrolled, could potentially cause damage to the process,
pneumatic controls, tooling, finished product, or indeed the
components of the air distribution system itself.

To reduce the likelihood of these problems occurring, compressed
air supplies are often conditioned to remove or significantly
reduce the moisture content, before the air is consumed by
the process.

 
 

 Michell Instruments'
 Cermax portable dewpointmeter




 
Refrigerant dryers are commonly used to reduce the moisture
content of compressed air. In principle the compressed air is
cooled in a heat exchanger to a temperature just above the
freezing point of water, causing much of the moisture in the
air to condense out to liquid, which can then be drained off.
Efficient operation of the dryer is dependent on maintaining
good temperature control at close to 0 Deg.C in the heat exchanger
and upon the design of the heat exchanger itself. If the temperature
drops below 0 Deg.C ice will form on internal surfaces and dryer
performance will be reduced. Refrigerant dryers are normally
equipped with temperature sensing devices to monitor the outlet
air temperature. This measurement will give a first indication of
the operation of the dryer, but does not guarantee the quality of
the air at the dryer output. The only way to be certain that the
dryer is functioning correctly is to measure the moisture content
of the outlet air directly using a suitable hygrometer.

Regenerative desiccant column dryers can offer a very efficient
method of air drying. These dryers are normally configured in
twin columns, allowing one column of desiccant to be regenerated
whilst the other is performing the air drying process.


There are two common types - pressure swing and heat-regenerative. A pressure swing dryer uses a sudden drop in pressure of the column under generation to evaporate moisture from the desiccant, which is then flushed out of the column to atmosphere by a back purge of air from the column under load. In the case of heat-regenerative dryers, large electrical or steam heaters are used to regenerate columns of saturated desiccant material and in such cases energy efficiency is of prime importance. As the demand made on the dryer can be variable, it follows that it would be advantageous to control the dryer cycle period according to the output performance required - rather than simply on a fixed time interval, as is often the case. By monitoring the moisture content of the common dryer outlet using a hygrometer, this information can be used, not only as a quantitative measurement, but also to initiate the dryer cycle change-over point at a predetermined moisture level. This system is often referred to as "dewpoint dependant switching" or-DDS and it provides the user with significant energy savings as well as providing a consistent quality of dry air output. Michell Instruments develops and manufactures a wide range of humidity measurement instrumentation for the Compressed Air industry. Michell products are used by Domnick Hunter, Atlas Copco, Air Products and literally hundreds of compressed air users, manufacturers and compressed air equipment manufacturers worldwide. ________________________________________________ For more information, please contact:- Michell Instruments Ltd. Tel: +44(0) 1223 433800 Fax: +44(0) 1223 434895 E-mail: info@michell.co.uk Website: www.michell.co.uk April 2003
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