Standard air line filters for distribution lines generally remove particles down to about 50 micron. Finer filters can be used for better protection, where required, but it is better to use these for second stage filtering at individual take-off points. Filters can be placed in four categories:
Rough filters for distribution mains, capable of removing particles down to 50 micron.
• Medium efficiency filters in the range 5 to 50 micron.
• Fine filters in the range 1 to 5 micron.
• Ultra-fine in the range down to 0.1 micron.
Fine and ultra-fine should be considered for second and third stage filtering and should be protected by a coarser filter to protect the finer filter from gross contamination.
Air Iiue filters
A well designed air line filter of the correct size will effectively remove water but cannot affect the vapour content of the air. If the air is subject to further cooling down stream of the filter, more water may condense out. If a complete absence of water is required, the air must be treated so that its dew point is lowered below any temperature reached by the air in use.
The operation of an airline filter is simple. Air entering the filter passes through louvres which directs the air in a swirling pattern. Centrifugal force throws the liquid and particulate matter outward to the inside of the filter bowl, where it falls down to the bottom of the bowl to be drained away. A baffle prevents the turbulent air from picking up the Compressed Air Purification Systems
liquid and returning it to the system. As the air leaves the bowl it passes through a filter element which removes more of the remaining impurities.
Airline filters can be equipped with an automatic drain to save manual operation of the drain valve. All filters should be mounted vertically, with a 5 mm drain line. A filter of this kind is often followed by a line lubricator to add the correct lubricant to the air line when used for tools or circuit elements.
When selecting the appropriate filter for an application, a performance and cost should be properly balanced. The initial cost may be low, but if the element needs to be replaced frequently, replacement costs may exceed the initial saving. A filter with a high efficiency rating may cause an unacceptable pressure drop, which may further increase the operating costs. Excessive pressure drop may result from incorrect pipe sizes or excessive flow through the filter. Never select a filter on the basis of the pipe size alone; they should be chosen on the basis of maximum flow and system pressure.
In order to prevent failures in air operated devices, monitoring of the filter for element replacement is necessary. Some filters rely on pressure drop through the filter as an indication of element saturation. Others rely on a visual check of the element, or observation of a colour change.
In coalescing filters, element life is determined by pressure drop, in absorption filters, element life is governed by saturation. It is worthwhile changing the filter elements at prescribed periods whether or not the pressure drop indicates it to be due. Elements should be changed at least once a year.
Element life depends on the type of compressor supplying the system. An oil-free rotary compressor has a minimum of oil carry-over; a reciprocating compressor is next best.
In a rotary flooded compressor, an oil separator is fitted as part of the system to minimize oil carry-over. The remaining aerosols in the delivery line are of the order of 0.1 to 0.5 micron. A sub-micron filter is necessary to deal with these. If there should be a separator failure or there is poor maintenance, large quantities of oil can be carried over and cause immediate failure of the filter element.
There are occasions when it is worthwhile installing a filter test set in order to assess the efficiency of a filter element. In most cases tests should be performed by the filter suppliers, but equipment is also available for laboratories who wish to perform their own tests. Such a test set is illustrated in Figure 10. This can be used for batch testing or for the checking of an individual element. The unit generates its own aerosol and measures the subsequent concentration of the test aerosol by a laser photometer.