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Radial external wire

These tubes are composed of a surface profile wire that is spirally wrapped around axial support profiles and attached to each other by resistance welding.

There are 2 types:

slot tube
Radial external wire - FOTI
slot tube
Radial external wire - FITO


The direction of the filtering flow determines the position of the surface profiles in relation to the support profiles.

  • The flat side of the surface profile is on the outside of the tube when the flow is from out to in (FOTI)
  • The flat side of the surface profile is on the inside of the tube when the flow is from in to out (FITO). This is also called a reversed profile.


The designation of a slot tube with radial external wire describes its construction as follows:

Surface wire / Support wire x  number of support wires
Example: 12S/Q25 x 8

This means that the profile wire is 12S. The support wire is Q25 and there are 8 support wires, equally spaced around 360°

Standard constructions (1)

Profile Support wires Min. diameter Min. slot width (2)
in mm in mm
4S Q22 or Q25 20 0.015 (3)
5S Q22 or Q25 20 0.015 (3)
6SC Q22 20 0.015 (3)
10S Q25 20 0.015 (3)
11S Q22 or Q25 21 0.015 (3)
12S Q25 21 0.015 (3)
Q35 30 0.05
18S 22S or Q35 30 0.05
Q25 25 0.03
D45 300 0.2
10x3 300 0.2
22S 22S or Q35 45 0.05
D45 or 10x3 or 25x3 300 0.2
28S Q35 215 0.2
D45 or 10x3 or 25x3 300 0.2
34S Q35 300 0.2
D45 or 10x3 or 25x3 300 0.2
42S Q35 or D45 300 0.2
10x3 or 25x3 300 0.2

(1) Special constructions are possible on request
(2) The minimum slot will increase when the diameter becomes bigger
(3) Depending on the slot tube diameter

Standard finish of the slot tubes:

  • Grinding of the end surfaces
  • Removal of loose profile ends

Standard tolerances:

Diameter < 164 mm +/- 0,3 mm
Diameter >= 164 mm - <= 301 mm +/- 0,5 mm
Diameter > 301 mm +/- 2,5 mm
Length for OD <= 301 mm +/- 2 mm
Length for OD > 301 mm +/- 4 mm

Special features

Variable slot openings are possible on all diameters

Continuous cylinders: by using a lot of support profiles, an axial slot can be obtained. These filter elements are called continuous cylinders.

Optional finish for the slot tubes:

  • Degreasing and pickling
  • Fastening of loose profile ends
  • Post machining for tighter tolerances

Slot tubes with end pieces:

Welding ring and closure plate
Resistance welded closure
End ring
Threaded fitting
Welding ring and threaded fitting

Standard end pieces are always welded onto the support profiles. Other custom made end pieces can be fabricated on demand.

To protect slot tubes against collapse, reinforcements are optional.

Reinforcing a slot tube is possible through:

  • More or stronger support wires
  • By adding internal reinforcement
slot tube with reinforcement spiral
Reinforcement spiral
slot tube with reinforcement
Reinforcement cross
slot tube with perforated tube inside
Perforated reinforcement tube also used as a distributor of the fluid over the slot tube.
support tube

Support elements

If the slot tubes are used as a carrier for another filter medium, for example textile, round wire can be applied. These support elements can be welded with 1, 2 or 3 wires next to each other.

Technical information

  • Diameter: ± 25 to 40 mm
  • Standard profiles
    • Spiral wire: Ø 0.8 mm
    • Support wire: Ø 2 mm

Compared with conventional support elements (such as perforated tubes, wire mesh,…) Trislot support elements have the following advantages:

  • A smaller contact between the support tube and the filter media results in a more active filter area, which creates a larger active filter media and a lower pressure drop.
  • Thorough cleaning by back flushing is possible.
  • Strong mechanical constructions
  • Larger open area