BCAS 2018

www.bcas.org.uk 31 Technology Guide housed in a similar or identical housing to a coalescing filter. They are usually matched to the pipe size of the system and therefore have a small volume of adsorbent material inside. This requires frequent element changes to constantly achieve the desired air quality. This type of filter is best suited for small flow, point of use applications, not compressor rooms. Sterile air filters Micro-organisms in compressed air can be a serious problem. Not normally visible to the naked eye (some can be as small as 0.04 μm), they are drawn into the compressor intake in huge quantities and multiply rapidly in the compressed air system under the right conditions. The passage of even a few viable organisms into a clean area, process or system causes contamination. This can result in reduced product quality, complete rejection or serious infection. Some applications, typically in the food, beverage or pharmaceutical industries require a degree of control over micro- organisms or even sterile compressed air. For this a combination of dry air (-40°C pressure dewpoint to inhibit the control of micro-organisms) and point of use sterile air filters are used. The growth of micro-organisms can be controlled and low compressed air dewpoint’s are known to inhibit the growth of micro-organisms. For example, the BCAS Food and Beverage Grade Compressed Air Best Practice Guideline 102 recommends a pressure dewpoint of -40oC for direct contact applications or in-direct applications where micro-organisms may still present a risk. By controlling the multiplication or growth of micro-organisms within the compressed air system, high efficiency dry particulate filters can be used to remove almost all as a particulate. For critical applications needing 100% particulate removal and applications needing sterile compressed air, sterile air filters can also be used. The benefit of dedicated sterile air filters is that they not only provide sterile compressed air, they are designed such that they can be sterilised in place with steam. Energy consumption associated with filtration In a compressed air filter, pressure losses are a combination of fixed pressure loss and incremental pressure loss. Fixed pressure losses are designed into the filter from the beginning and come from the filter housing and element endcap designs. Many manufacturers offer filters with flow management built into the design and whilst costing a little more to purchase, they offer significant savings when in operation. Whilst the initial performance of the two filters may look similar or identical in literature (both quoted 200mbar wet dp), one filter has an element with a higher dirt holding capacity than the other and would therefore have lower operational costs. Point of use filters & pressure loss Point of use filters are necessary for almost all compressed air systems. They deal with the contamination that is already in the distribution piping (rust, pipescale, particulates, oil vapour and micro- organisms), especially in ageing systems. When it comes to high pressure losses in existing systems, it can often be traced back to point of use filtration. As facilities change or expand (this applies to point of use equipment and applications also), demand for air increases and system pressure subsequently drops. Compressed air filters must be sized for the minimum system operating pressure (remember lower pressure = more air volume = larger filter). If they are not, they will have too great an air flow which significantly increases pressure losses (dp), increases costs significantly and results in contamination carryover to the application. When sizing point of use equipment, always check how pressure changes when equipment is operating. It is not uncommon for pneumatically operated processes to drop the system pressure significantly when using compressed air. Never size the equipment based upon the compressor discharge pressure. Correctly sizing a point of use filter can save significantly on pressure losses and energy consumption. The information provided above will enable the designer to arrive at the correct purification equipment specification for the facility. Please remember to ask a specialist to review your filtration requirements. Filtration