The replaceable element is made of speciallytreated convolutions (wrinkles) to increase itsdirt-holding capacity. The element is noncleanableand should be replaced with a new filter elementduring maintenance inspections.MAGNETIC FILTERS.— Some hydraulicsystems have magnetic filters installed at strategicpoints. Filters of this type are designed primarilyto trap any ferrous particles that may be in thesystem.PNEUMATIC GASESClean, dry gas is required for the efficientoperation of pneumatic systems. Due to thenormal conditions of the atmosphere, free airseldom satisfies these requirements adequately.The atmosphere contains both dust and impuritiesin various amounts and a substantial amount ofmoisture in vapor form.Solids, such as dust, rust, or pipe scale inpneumatic systems, may lead to excessive wearand failure of components and, in some cases,may prevent the pneumatic devices from operating.Moisture is also very harmful to the system. Itwashes lubrication from moving parts, therebyaiding corrosion and causing excessive wear ofcomponents. Moisture will also settle in low spotsin the system and freeze during cold weather,causing a stoppage of the system or ruptured lines.An ideal filter would remove all dirt andmoisture from a pneumatic system withoutcausing a pressure drop in the process. Obviously,such a condition can only be approached; itcannot be attained.Removal of SolidsThe removal of solids from the gas ofpneumatic systems is generally done by screening(filtering), centrifugal force, or a combination ofthe two. In some cases, the removal of moistureis done in conjunction with the removal of solids.Some types of air filters are similar in designand operation to the hydraulic filters discussedearlier. Some materials used in the constructionof elements for air filters are woven screen wire,steel wool, fiber glass, and felt fabrics. Elementsmade of these materials are often used in the unitthat filters the air as it enters the compressor.Porous metal and ceramic elements arecommonly used in filters that are installed in thecompressed air supply lines. These filters also usea controlled air path to provide some filtration.Internal design causes the air to flow in a circularpath within the bowl (fig. 9-16). Heavy particlesand water droplets are thrown out of the airstreamand drop to the bottom of the bowl. The air thenflows through the filter element, which filters outmost of the smaller particles. This type of filteris designed with a drain valve at the bottom ofthe bowl. When the valve is opened with airpressure in the system, the accumulation of solidsand water will be blown out of the bowl.An air filter that uses moving mechanicaldevices as an element is illustrated in figure 9-17.As compressed air passes through the filter theforce revolves a number of multi-blade rotors athigh speed. Moisture and dirt are caught on theblades of the rotors. The whirling blades hurl theimpurities by centrifugal force to the outer rimsof the rotors and to the inner walls of the filterhousing. Here, contaminating matter is out of theairstream and falls to the bottom of the bowlwhere it must be drained at periodic intervals.Removal of MoistureThe removal of moisture from compressed airis important for a compressed air system. If airat atmospheric pressure, even at a very low relativehumidity, is compressed to 3000 or 4500 psi, itbecomes saturated with water vapor. Somemoisture is removed by the intercoolers andaftercoolers (see glossary). Also, air flasks,receivers, and banks are provided with low pointdrains to allow periodic draining of any collectedmoisture. However, many uses of air require airwith an even smaller moisture content than canbe obtained through these methods. Moisture inFigure 9-16.—Air filter.9-12
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