This invention relates to a filter for a liquid more particularly
liquid fuel, and of the kind which is intended to be located between a reservoir
and the inlet of a pump, the filter incorporating a filtering medium which acts
during operation of the pump to retain particulates suspended in the liquid.
The filter has an inlet which is connected to the reservoir and an
outlet which is connected to the inlet of the pump and in operation the pressure
at the filter outlet will be lower than that at the filter inlet by the amount
of the pressure drop across the filtering medium. The pressure drop which occurs
depends upon a number of factors such as the nature of the liquid, its viscosity
and flow rate and the pore size of the filtering medium. The liquid which is contained
within the reservoir may be aerated and as the liquid is drawn through the filter
medium the air will tend to be trapped by the filter medium. Air pockets will
form and will eventually pass through the filter medium. In the case of a fuel
filter for an engine fuel system there is the possibility that the sudden release
of the air to the pump will result in an interruption in the flow of fuel to the
engine leading to engine starting and running difficulties.
In the accompanying drawings:-
- Figure 1 is a sectional side elevation through part of a filter element incorporating
- Figure 2 is a view showing a modification to the filter medium shown in Figure
- Figures 3 and 4 show in perspective, the filter medium of two other forms of
filter element, and
- Figure 5 is a side view of a portion of filter paper as modified in accordance
with the invention.
With reference to Figure 1 of the drawings the filter element comprises
an outer sheet metal casing of cup shaped form and having a side wall 10 and a
base wall 11. Extending axially within the casing is a centre tube 12 this being
located at its lower end upon an apertured raised portion of the base wall and
at its upper end by a location defined in an apertured closure plate 13. The outer
peripheral edge of the closure plate is rolled over the edge of the side wall 10.
The base wall 11 is provided with flow apertures 14 and although not shown in
the construction of the apertures 14 the metal removed to form the apertures is
arranged to form a support for the filter medium of the element.
The filter medium which is located in the space defined between the
internal surface of the side wall 10 and the outer surface of the centre tube 12
comprises in the example of Figure 1 two sheets 15, 16 of filter paper which are
wound about the centre tube. In the example the sheets are wound four times about
the tube but in practice there will be more turns. During the winding process
adhesive is applied to say the strip 15, on one side of the paper adjacent one
longitudinal edge and to the other side of the paper adjacent the other longitudinal
edge. As the strips are wound together the adhesive secures the strips together
so that in section and as seen in Figure 1 the strips adopt a zig zag configuration
defining a series of pockets 15A. In the use of the filter element it is mounted
so that its upper end fits into a mounting and its lower end is engaged by an
end closure. The mounting and the end closure define passages so that the liquid
to be filtered flows down the centre tube 12 through the apertures 14 and then
into the pockets and housing the strips of filter paper and out through the holes
in the plate 13. The flow of liquid is indicated by the arrows. Contaminant in
the liquid is trapped on the surface of and in the filter paper.
The liquid in particular if the liquid in fuel for compression ignition
engine will contain air which since the filter paper is "wetted" by the fuel will
tend to be trapped on the upstream side of the paper. As the air collects it will
tend to form air pockets which will locate at the upper ends of the pockets formed
by the strips of paper. Eventually the pockets of air will be drawn through the
paper and as described, may influence the operation of the engine. The pump will
have an air venting system but this system may not be able to deal with substantial
quantities of air drawn into the pump but will be able to deal with small quantities
of air contained in the fuel.
In order to prevent the accumulation of large quantities of air into
the filter it is proposed to provide zones in the filter element which will allow
the passage of air between the upstream and downstream sides of the filter medium.
In the example of Figure 1 the aforesaid zones are formed by a series
of small holes 17 along the length of the strips 15, 16, the holes being positioned
adjacent the upper ends of the pockets formed by the strips.
In a particular example the roll of filter medium has inside and
outside diameters of 20mm and 70mm respectively and a height of 60mm. Such a filter
has a fuel filtering capacity of between 40 and 50 litres/hour and the holes 17
are able to allow an air flow of about 500cm³/hour.
The element which is shown in Figure 2 is substantially the same
as the element shown in Figure 1 except that the holes 18 are formed along joint
line between the strips of filter paper.
The filter element which incorporates the filter medium shown in
Figure 3 comprises a perforated centre tube not shown, about which the filter medium
is wrapped. Around the filter medium is a perforated cover and the assembly is
completed by a pair of end caps which are glued to the opposite ends of the filter
medium. The filter medium itself comprises a length of filter material which is
folded transversely of its length to form a series of pockets 19 into which the
liquid to be filtered is directed as indicated by the arrows. The filter element
is mounted with its axis substantially vertical so that any air will collect at
the upper ends of the pockets. Holes 20 in the folded portions of the medium allow
the air to escape to the down stream side of the filter element.
The filter element which incorporates the filter medium seen in Figure
4 is a flat type of filter and the assembly will include sides which extend around
the four sides of the filter medium. The medium is folded as in the example of
Figure 3 and the end edges and side edges sealed to the aforesaid sides of the
assembly. The filter element is intended to be mounted horizontally with the liquid
flow being as indicated by the arrows. Pockets 21 are formed as a result of folding
the medium and the folded portions of the medium are provided with holes 22 to
allow the air to escape to the downstream side of the filter element.
In Figure 5 there is illustrated part of a strip 23 of filter paper
which can be incorporated into the element construction described in the earlier
figures. The strip 23 has an upper treated zone 24 and a lower untreated zone
25. The untreated zone 25 provides for the filtration of the liquid and the treated
zone 24 provides for the passage of air to the downstream side of the filler element.
The treated zone 24 may be formed by providing tiny holes or otherwise by modifying
the paper to facilitate the passage of air. Mechanical, electrical, hydraulic
or chemical means may be used to modify the paper. As an alternative the treated
zone 24 may be formed from paper having a different filtration characteristic
to the paper forming the untreated zone 25. In this case the edges of the lengths
of paper are secured together along a glue line 26.