HUMIDITY MEASUREMENT IN COMPRESSED AIR SYSTEMS

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.

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 0ºC in the heat exchanger and upon the design of the heat exchanger itself. If the temperature drops below 0ºC 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 pre-determined 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 iterally hundreds of compressed air users, manufacturers and compressed air equipment manufacturers worldwide.