Tubular filter primarily intended for filtration of liquids

A tubular filter which is primarily intended for filtration of liquid flowing substantially radially towards the interior of the filter. The filter comprises two layers (10, 11) of mineral wool. An interior layer (10) has the fibres oriented substantially concentrically to the tube axis and an outer filter layer (11) of mineral wool has the fibres oriented substantially radially to the tube axis. The material of the tubular filter must be arranged so that its density increases from the outer portion of the tubular filter towards its inner portion.

This invention relates to a tubular filter primarily intended for 
filtration of liquids, the liquid flowing substantially radially towards 
the interior of the tubular filter. 
I.a. in connection with separation of oil from water it is previously known 
to utilize so-called tube filters, which are composed of several layers of 
mineral wool. Thus, U.S. Pat. No. 3,210,229 describes as an example such a 
filter (cf. FIG. 1 in the patent) consisting of two inner layers of 
substantially radial fibre direction and an outer layer with fibres which 
are oriented substantially transversely to the direction of flow. The 
known filter is in the first place intended for use with a liquid passing 
radially outwards from the interior of the filter. The density of the 
filter will then decrease in each specific layer as seen in the direction 
of flow even if the density may increase from layer to layer. 
It is now the object of the invention to provide a tubular filter of the 
type indicated above which has a good depth effect as well as good 
pressure indicating properties. The filter is to have a long life and 
should be especially suited for filtration of water which is to be used 
e.g. as drinking water or bath water. However, the use of the filter is by 
no means restricted to water but may also comprise other liquids and 
gases, also. 
According to the invention the new filter is primarily characterized in 
that it comprises in combination an inner filter layer of mineral wool, 
the fibres of which are substantially concentrically oriented to the tube 
axis, and an outer filter layer of mineral wool, the fibres of which are 
oriented substantially radially to the tube axis. It is especially 
advantageous to arrange the material of the tubular filter in such a way 
that its density will increase from the outer portion of the tubular 
filter towards its inner portion, the increase being continuous in the 
outer mineral wool layer. 
Tests have shown that a filter built according to the principles of the 
invention has a very good depth effect in the outer mineral wool layer 
which facilitates a rapid passing flow due to the radial fibre 
orientation. As the material in the outer layer of the tubular filter has 
increasing density in inward direction big particles will get stuck more 
closely to the outside of the filter whereas finer particles will 
penetrate further into the filter and get stuck more closely to the inside 
of the outer layer or in the inner layer. Due to the fibre orientation in 
the inner layer the filter liquid will be braked and, moreover, 
redistributed peripherically to a certain extent, as it is more difficult 
for the liquid to penetrate into the inner layer. In this connection it 
can be worth mentioning that twice as great a flow per time unit is 
normally obtained through a layer in which the fibres are oriented in the 
direction of flow as through a layer in which the fibres are oriented 
transversely to the direction of flow, even if the layers have the same 
density. If the inner layer hos now a higher density than the outer one 
this will result in a not inessential pressure difference between the 
inner and outer portions of the filter. This pressure difference can be 
determined by means of conventional water pressure gauges which, thus, 
will show reference values in relation to the clogging of the filter. In 
this way an indication of the suitable time for replacement of the filter 
is obtained.

The tubular filter illustrated on the drawing comprises as main 
constituents two filter layers 10, 11 of mineral wool, which preferably 
consist of glass fibre. 
The inner filter layer 10 has preferably a density of 40-100 kg/m.sup.3, 
preferably 55-75 kg/m.sup.3, and consists of a tube made from glass fibre. 
The fibres are preferably concentrically oriented relative to the 
longitudinal axis of the tube. 
The outer filter layer 11 has preferably a density of 20-70 kg/m.sup.3, 
preferably 30-50 kg/m.sup.3, and is made from a glass fibre disc with the 
fibres oriented perpendicularly to the disc plane. This disc of mineral 
wool is built from a conventional glass fibre disc with fibres oriented 
parallelly with the disc plane, from which rods or strips of the intended 
length have been sawed. These rods or strips have been joined and glued 
together so that they form said disc with fibres oriented perpendicularly 
to the disc plane. 
The glass fibre disc with fibres oriented perpendicularly to the disc plane 
is adapted to the previously described tube which was made from the disc 
with parallelly oriented fibres, the disc edges of the outer layer 11 
facing each other being glued together (at 12). Thus, an outer filter 
layer 11 is obtained in which the density of the material increases 
towards the centre of the filter, the density .rho..sub.x in a layer at 
the distance x from the inner surface being determined by the density 
.rho..sub.o of the starting material according to the formula 
EQU .rho..sub.x =.rho..sub.o (r.sub.i+d /r.sub.i +x) 
wherein r.sub.i is the inner radius and d the difference between outer 
radius and inner radius, i.e. the thickness. 
A binding layer 13 is glued onto the outside of the outer layer 11, which 
binding layer preferably consists of glass tissue. The binding layer 13 
can be a mesh net, e.g. approximately 30 meshes per cm.sup.2, and is 
illustrated only partially on the drawing. 
In practical tests with tubular filters made according to the principles of 
the invention it has been found to be advantageous to select the ratio of 
the thickness of the inner layer to that of the outer one within the range 
of 1/3-1/50. The length of the tubular filter is not critical. 
The tubular filter described above can be modified within the scope of the 
appended claims. Thus, it is e.g. possible to supplement the tubular 
filter construction described above with additional glass fibre layers 
having a radial and/or transverse fibre orientation.