TECHNICAL ARTICLE Testing of Glass Fiber Coalescing Filters G. Belforte, T. Raparelli, and A. Trivella Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Torino, Italy Keywords Efficiency Air – Oil Filters, Coalescent Filters, Filter Media, Fiber Glass, Fibrous Filters Correspondence A. Trivella, Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Torino, Italy Email: andrea.trivella@polito.it Received: October 20, 2011; accepted: December 19, 2011 doi:10.1111/j.1747-1567.2012.00806.x Abstract The paper presents an experimental study of coalescing filters for separating oil from compressed air in industrial systems. A test bench for measuring filter efficiency was set up which reproduces several typical operating conditions of pneumatic systems. Tests were carried out on commercial products, using filters of different sizes as well as several borosilicate cartridges of similar size and shape. Preliminary analysis of cartridges indicated significant differences in glass fiber dimensions and binder composition. Test results made it possible to compare performance achieved by the different configurations in terms of efficiency and pressure drop. Further measurements were repeated with different cartridge supply system geometries. One of the tested filter-cartridge- supply system configurations was then used to investigate system behavior while varying certain operating parameters individually: air velocity, oil concentration, filter supply pressure, and operating time. Results are presented in statistical form. Introduction The compressed air in a pneumatic system is normally moist and contaminated, containing by-products of wear from moving parts, metal particles, and oxides, and, often, quantities of oil resulting from compres- sor lubrication or production needs. Coolers and driers are used downstream of the compressor to reduce condensation, while filters of varying grade are provided along the line to retain solid particles of different sizes. Removing oil is essential both in order to ensure that the air is safe to breathe in the area where it is released, and to guarantee the cleanli- ness required for many applications, for example, food processing, pharmaceuticals, metrology, or electronics industries. ISO 8573-1 classifies the cleanliness of air for industrial use into several purity classes, according to the concentrations and particle sizes of the solid and liquid contaminants contained in the air. In particular, permissible oil concentrations range from 5 to 0.01 mg/m 3 ANR (volume of air second the ‘‘Atmosfere Normale de Reference’’ conditions) with purity classes from 4 to 0. Considering that oil concentration at the compressor outlet can vary from 0.2 to 2 g/m 3 , depending on machine wear, the importance of the task performed by purification filters is clear. Coalescing filters are normally used to separate oil from compressed air. These filters consist of a random mass of inorganic fibers embedded in a binder fea- turing a high porosity of around 90%. Various kinds of binder are used: phenolic, epoxy, acrylic. As the binder is chemically inert and nonabsorbent, it is capa- ble of maintaining sufficient structural integrity over time. 1 The volume actually occupied by the fibers as a ratio of the total volume, that is, the packing frac- tion, is small, which makes it possible to achieve low pressure drops as air crosses the filter. Liquid particles are captured by the fibers and slide toward the outer surface of the filter, forming larger and larger droplets as a result of coalescence until they fall by gravity into the filter’s collection chamber. Coalescing filters can also operate as particulate filters, taking advantage of the fibers’ ability to capture solid contaminants. Filtration efficiency, or the ratio of the concen- tration of liquid retained and the amount entering the filter, is determined by a number of mechanisms involved in contaminant capture by the fibers. 2–4 These mechanisms are heavily dependent on the relative dimensions of the particles and fibers, packing fraction, and flow conditions. Experimental Techniques (2012) © 2012, Society for Experimental Mechanics 1