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 CHAPTER 4. AUXILIARY EQUIPMENT
Section 1. INTAKE FILTERS
1 DESCRIPTION. Air filters are provided on air compressor intakes to prevent
atmospheric dust from entering the compressor and causing scoring and
excessive wear. There are two types of air filters, the dry type and the
oil-wetted type. Generally, dry type filters are more efficient than
oil-wetted types in trapping and removing very fine, solid particles from the
incoming air. However, dry type filters must be cleaned and replaced more
often than oil-wetted types. Oil-wetted types are often used where there are
heavy dust concentrations present in the atmosphere.
1.1 Dry Type Filter. Dry filters employ many different materials for the
filter media. Paper, polyester felt, and fine wire mesh are a few examples.
The filter media can be folded, wrapped, and layered in many configurations to
achieve the desired efficiency. Although the dry filter is more efficient
than the wetted type filter, the pores in the dry filter media become clogged
and result in a pressure drop across the filter. Dry type filters cannot be
used successfully where intake air contains moisture or vapors in amounts that
would cause disintegration of the filtering media. The main advantages of the
dry type filter, when used in an approved application, is its high efficiency
and ease of maintenance.
1.2 Oil-Wetted Type Filter. Wet filters have filter elements that are coated
with a film of oil. The oil film catches airborne particulates before they
reach the actual filter element media. Wetted type filters are of two
designs, oil-wetted and oil-bath filters. In an oil-wetted filter, a coating
of oil is deposited on the filter element, which is usually made of layers of
wire mesh. The oil coating is intended to adhere to the element for a fairly
long service period. The airborne particulates are impinged or trapped on the
filter element which has been covered with a film of oil. In an oil-bath
filter (figure 4-l), the same viscous impingement principle is employed.
However, the airflow is directed through the oil sump, carrying oil with it to
the filter element where the oil collects and washes the impinged particles
down to the oil sump, forming sludge. The self-washing aspect of the oil-bath
filter extends the time between maintenance routines.
2 INSPECTION. Air filter inspections are to be performed when any of the
following conditions exist.
(a) Prescribed time interval on the maintenance schedule has elapsed.
(b) Pressure drop across the filter element indicates a maintenance
requirement.
(c) One-fourth inch of sludge has built up in the oil sump of the
oil-bath type filter.
4-l
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