Candle precoated filter for residual volume filtration

A candle type precoated filter for residual volume filtration has a pressure vessel having a cylindrical upper part and a conical base, a plurality of filter elements arranged substantially vertically and including at least one residual filter element and other filter elements formed so that the at least one residual filter element is lengthened toward the base and is longer than the other filter elements and a separating wall which separates the residual filter element in an upper portion of the pressure vessel from the other filter elements so as to be tight against media and to project out in a lower portion of the pressure vessel beyond a length of the other filter element.

BACKGROUND OF THE INVENTION 
The present invention relates to a candle precoated filter for residual 
volume filtration. 
More particularly, it relates to a candle type precoated filter for 
residual volume filtration which includes a cylindrical pressure vessel 
with a conical base and filter elements which are arranged vertically at 
collecting tubes and in which at least one residual filter element is 
lengthened toward the base. 
Such candle filters have a cylindrical housing with a conical lower part in 
which are installed a plurality of filter candles which are generally 
combined to form registers. The filter candles include a carrier tube on 
which a rigid or flexible filter medium is arranged. The filtration is 
effected by flowing through the filter layer from the outside to the 
inside, a filter cake being formed in a tubular shape on the filter 
medium. The filtrate flows through the carrier tube to the outlet spout. 
Cleaning is effected by a brief, percussive reverse flow through the 
filter medium. 
A candle type precoated filter is known from CH-A-658 401 in which one or 
more filter candles are provided with an enclosing tube which is connected 
with the filter candle so as to be open at the bottom and sealed at the 
top. Enclosed and unenclosed filter candles are provided with separate 
flow-out valves. If the filter inlet and outlet of the unenclosed filter 
candles are closed and the filter housing is acted upon by a gas, the 
filter contents are filtered through the enclosed filter candles until the 
level of the impure liquid has reached the lower edge of the enclosing 
tube. 
However, the known candle filter has disadvantages. For example, when the 
flow-out of the unenclosed filter candles is closed, the filter cake, 
depending on its composition, can fall out of the unenclosed filter 
candles as soon as the filter housing is acted upon by gas due to the 
absence of differential pressure. The filter cake is then pressed into the 
narrow annular cross section between the enclosing tube and filter candle 
and very quickly stops up the latter. This leads to unreasonably long 
residual filtration times. Even if the filter cake which has dropped down 
is not pressed into the filter tubes (because the lower edge of the 
enclosing tubes is arranged quite high in the filter housing), problems 
occur when cleaning the enclosed filter candles. The filter cake adheres 
to the inside of the enclosing tube so that the enclosing tube is entirely 
or partially stopped at least after repeated cleaning. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a candle 
type precoated filter, which avoids the disadvantages of the prior art. 
More particularly, it is an object of the present invention is to improve a 
candle type precoated filter in such a way that, on the one hand, no 
residues impede the residual volume filtration so that residual filtration 
times are achieved which are distinctly shorter than the main filtration 
time and on the other hand the residual volume remaining in the filter can 
be varied in configuration and thus adapted to the cake volume. 
This object is met according to the invention in that a separating wall 
separates the residual filter element in the upper portion of the pressure 
vessel from the rest of the filter elements so as to be tight against 
media and projects out beyond the length of the main filter elements in 
the lower portion. 
Since the free cross section around the filter candles is practically 
unrestricted by the separating wall, there is no risk of stoppage either 
during the residual volume filtration or when cleaning the filter candles. 
The filter candles in the residual filtration space and in the main 
filtration space have separate outlet valves. After closing the feed of 
impure liquid and the outlet of the main filter candles and after the 
admission of gas to the main filtration space, the entire volume of impure 
liquid in the filtration space can be filtered off until the lower edge of 
the separating wall via the residual filter candles. The gas penetrates 
and filtration stops only when the level of impure liquid falls below the 
lower edge of the separating wall. 
It is advisable to provide at least one externally closable opening in the 
region of the lower part of the separating wall. However, it is also 
possible to arrange one or more externally closable openings one below the 
other. Here the advantage consists in that the residual volume in the 
filter housing can be varied in configuration when the gas penetrates and 
accordingly adapted to the cake volume. An arrangement having a slot in 
the separating wall which can be closed externally by a slide in the 
manner of a flood gate is particularly advantageous. When residual 
filtration begins, the slide in the separating wall is first fully opened. 
The filter cake dropping down from the main filter candles collects in the 
cone of the filter housing and only the impure liquid standing above the 
latter is pressed into the residual filter space. After the start of gas 
penetration, the slide in the separating wall is closed slowly so that 
practically minimum residual volume can be achieved. 
In its simplest construction the separating wall can be a planar wall which 
projects into a symmetrical cone. But it can also be constructed as a 
central tube or double separating wall in a filter with a 
symmetrical-conical lower part. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to FIG. 1, the filter has a pressure vessel 1 with a conical base 
2. A plurality of filter elements 3, 3' are arranged at collecting tubes 
in the interior of the pressure vessel I and closed by a valve 10 outside 
the vessel wall. Residual filter elements 4 are connected with a valve 11 
outside the vessel wall via a collecting tube. A separating wall 5 is 
welded in the pressure vessel and sealed against a cover 8. A pressure 
line with a valve 12 is provided in the cover 8. An outlet valve 9 and a 
line with a valve 13 for the impure liquid feed are arranged in the apex 
of the cone 2. The level of the residual volume occurring when the filter 
is acted upon via the valve 12 is designated by position 14. 
According to FIG. 2, openings which are closable from the outside by valves 
6, 6' are installed in the lower portion of the separating wall 5. The 
level of the residual volume 14, 14' can be changed in stages from the 
upper edge of the uppermost opening to the lower edge of the separating 
wall. The volume of the residual filtration can be reduced in stages with 
these valves. Instead of the valves, a slot can also be inserted in the 
separating wall and closed from the outside by a slide. The volume of the 
residual filtration can accordingly be reduced in a continuous manner and 
hence exactly adapted to the filter cake volume possibly dropping from the 
main filtration space. 
According to FIGS. 3 and 3a, a slot 15 is inserted into the separating wall 
5, which slot 15 can be covered (closed) in a continuous manner by an 
externally actuated slide 7. The level of the residual volume remaining in 
the housing after the filter is acted upon by compressed gas can 
accordingly be lowered in a continuous manner from the upper edge of the 
slot 15 to the lower edge of the separating wall 5. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
candle precoated filter for residual volume filtration, it is not intended 
to be limited to the details shown, since various modifications and 
structural changes may be made without departing in any way from the 
spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.