The short answer to when you should replace your air filters is when the cost of operating with clogged filters exceeds the cost to replace them. Compressed air filters are very important to the quality of air produced by the air compressor and dryer, and the condition of these filters can also effect the energy efficiency of the system.
The worst type of filter is one with no installed pressure differential gauge and no indication of any past maintenance date. The condition of a filter like this can be easily ignored (like the one in the photograph at right), leading to downstream pressure problems, sometimes incorrectly blamed on the compressor. And eventually the pressure problems will disappear when the filter fails internally, leasing to air quality problems. Filters like this need to be changed on a time schedule. And once done, the date of change displayed on the maintenance tag. The frequency of change depends on how much the filter is used, that is, how much air is flowing through it, but the manufacturer’s recommendations should be followed as a minimum.
Best bet is to have an accurate differential gauge installed across a filter, or a set of filters, to be used in detecting when change due to pressure differential is required. But, care should be taken in reading these measurements. The pressure differential of a filter varies exponentially with the flow through it, a square function. Thus, the reading shown on the differential gauge only really shows the true condition of the filter when the flow is near the filter rating, sometimes only occurring during short duration peak flows.
Sometimes coalescing filters will not ever develop excessive pressure differential across the element, but they will stop working properly as they age, allowing lubricant mists downstream to contaminate equipment. For this reason pressure differential should not be the only criteria in changing a coalescing filter.
How much does filter the differential cost? A typical clogged filter on a fully loaded 100-hp compressor could cause additional compressor power consumption of about ½% for every psi across the filter. This would consume about 4 cents/hr (at 10 cents per kWh electricity cost) or about $3,700 per year in extra energy for every extra psi of differential. Thus replacement of this filter might be wise once differential reaches 5 psid or above.
Additional costs would be incurred if the pressure differential also affects the control of the compressor, such as restricting the flow to a downstream main storage receiver. Further costs might be incurred if the filter has a drain installed that wastes compressed air.
Note that the main system filters are not the only important filters in a compressed air system, the filters inside the compressor and ones installed on compressed air operated machines must be maintained too. It is possible for a $20 filter element problem in a small critical machine to force the main compressor pressure setting to rise significantly to compensate, causing thousands of dollars in additional electricity costs.
And one last thought, it is important to ensure the filter actually needs to be there in the first place. Quite often routinely installed compressed air filters are not actually required in a system, but have been placed as part of an installed assembly or as part of a plant wide standard. Maintaining unnecessary filters is the worst waste of time and resources of all. Consider assessing your filtration system for appropriateness and removing redundant filters.