Winegrowers Supplies  -  Sheet Filtration - 'depth' filtration
 

Setting up / inserting the filter sheets between the plates:

Ensure the appropriate seals are fitted to each filter plate. Different makes and grades of filter sheet vary in thickness, so it's possible that seals of a different thickness may be needed; this normally only applies to 20 x 20 cm filters where 3.5, 4.0 and 5.0 mm thick seals are available.

If steam sterilisation is not available, then:-
All plates and seals should be dipped (for at least 10 seconds) in 2% Free SO2 solution.
The filter inlets/outlets should be swabbed/sprayed with 2% Free SO2 solution, or sprayed with special alcohol sterilizing spray.

A filter sheet should be inserted carefully (centrally) between each of the filter plates.
The smooth side of the sheet is the discharge side.
Seitz and Beco sheets are printed on the smooth side.
Cordenons sheets are printed on the coarse side.

With 40 x 40 cm filters: normally, starting at the input end, the smooth side must face outwards (against the filter end-plate/chamber), the other sheets are then placed alternately between the intermediate plates, so that a smooth side always faces a smooth side, and a coarse side always faces a coarse side.
There may be an even or odd number of filter plates, according to the model of filter and whether a single or double filtration (with changeover plates) is being carried out.

With 20 x 20 cm filters: the flow is different from normal 40 x 40 cm filters: starting at the input end, the coarse side must face outwards (against the first filter plate/chamber), the other sheets are then placed alternately between the intermediate plates, so that a smooth side always faces a smooth side, and a coarse side always faces a coarse side.

When all the sheets and plates have been inserted the centre-screw-spindle is screwed in just enough to hold the 'filter pack' sufficiently firmly to stop the sheets slipping.
Water is then poured over the filter pack to thoroughly moisten the edges of the sheets.

Steam sterilisation:

Not all types of filter plates are suitable for steaming. Noryl (black) plastic is a suitable material, even better is stainless steel.

Ideally steam (saturated steam, maximum bar pressure) from a steam generator should be used to sterilise the filter and bottle filler, the steam must be fed in to the outlet (normally the upper connection) of the filter.
The inlet of the filter is normally the lower connection point.
Bringing the steam in from the lower channels (inlet side) could have the effect of washing off and thereby damaging sheets on the feed side.

The bottle filler and connecting tubing must also be steam sterilised, by connecting to the inlet of the filter. The nozzles of each filling tube must be propped open to allow steam to flow freely from them.

The condensation water accumulating in the filter should be allowed to drain off freely at the bottom, by having the drain taps slightly open.
It is important that each side-branch in the filter (pressure gauge etc) has a drain-cock through which steam can flow during sterilisation.
The thermal load during sterilisation somewhat reduces the strength of the filter sheet.
The steaming process must be carried out with the filter pack slightly loose. This not only provides space for thermal expansion but allows the free flow of condensation water in the area of the sealing edges.
All valves must be slightly open initially during the steaming process, and steam must flow for at least 20 minutes after it starts to flow out of the filler tubes.

Filter sheet rinsing/conditioning:

The filter pack must then be rinsed/flushed/conditioned in order to eliminate soluble particles and the taste of 'cardboard/paper'. To achieve this it is necessary that cold pure water flows over and through the entire filter area, from inlet to outlet, for about 20 minutes at a rate of 1 to 2 times the filtration rate.
To achieve even greater purity a 1% citric acid solution can be circulated in order to remove acid-soluble particles prior to filtration; this should not be used for red wine filtration.

The rinsing process must be carried out with the filter pack slightly loose, to allow some free flow of water in the area of the sealing edges. When rinsing, special care must be taken to fully remove/vent the air from the filter pack. If this is not done, sludge particles deposited during filtration can be squeezed through the filter sheet with escaping gas bubbles. Vents are located at the highest points of the channels/chambers.

Procedure: use appropriate tubing to connect the pump inlet to a vat of clean water, the pump outlet to the filter inlet, and the filter outlet back to the vat of water (with the tube carefully positioned so that the bubbles do not circulate back to the inlet; direct the tube inside a jug placed under the water).
Prime the pump if necessary, switch on the pump with the bypass closed, immediately the pump starts to function - open the bypass; the pressure difference between inlet and outlet must not exceed bar even momentarily. Adjust the pump bypass until the flow is as specified above, continue circulating for at least 10 minutes until the sheets are saturated with water and all air has been vented, then for a further 5 minutes with the outlet going to waste.
Gradually turn/tighten the centre-spindle-screw sufficiently to eliminate leaks; this is easier when the pump is switched off; a lot of force is needed to fully tighten the screw. Food-grade grease (or Vaseline) must be applied to the screw thread in order to prevent wear and make it easier to turn.
Before stopping the pump always open the pump bypass so that the pressure is low.

Filtration:

The pump inlet should now be connected to the tank of wine to be filtered.
Start the pump and begin to filter slowly.
After a while examine/taste the wine from the filter outlet drain valve, do this continually until the water/wine mixture is sufficiently wine then stop the pump and direct the filter outlet tube over the top of the source tank, re-circulate the wine to the source tank for several minutes to ensure it is homogenous; at least 30 litres should be re-circulated.
Connect the filter outlet to the bottle filler and re-start the pump; with a T-piece and two valves it is possible to redirect this without stopping the pump. Care should be taken to prevent pressure surges.
Gradually close the bypass to increase the pressure and flow rate to the required level:-

Fine/clarifying/coarse filtration: maximum rate = 625 to 750 litres/hour/square metre of sheet;
i.e. maximum rate = 100 to 120 litres/hour per 40 x 40 cm filter sheet used,
      maximum rate = 25 to 30 litres/hour per 20 x 20 cm filter sheet used.
maximum pressure difference (inlet over outlet) = 3.0 bar.

Sterilising filtration: maximum rate = 375 litres/hour/square metre of sheet;
i.e. maximum rate = 60 litres/hour per 40 x 40 cm filter sheet used,
      maximum rate = 15 litres/hour per 20 x 20 cm filter sheet used.
maximum pressure difference (inlet over outlet) = 1.5 bar (preferably 1.0 bar maximum),
for spritzig/petillant wine the maximum should be between 0.5 and 1.0 bar.

With a higher filtration rate and increasing pressure difference the danger of sludge breakthrough (and hence contamination of the filtrate) increases.


Using changeover (crossover) plates:-

The three changeover plates (two end plates and a central stainless steel plate) are inserted somewhere near the middle of the pack. Below is a sketch-drawing for a 40 x 40 cm filter, it is the same principle for 20 x 20 cm.

The plug supplied has to be inserted in order to change the position of the outlet, from one end of the filter to the other.

The liquid passes through the first set of (coarse) sheets, each sheet filters simultaneously (i.e. in parallel not in series) from the inlet to the outlet side. Then the flow changes direction and passes through the second set of (fine or sterilising) sheets. The holes in the stainless steel changeover plate must be on the correct side to allow the flow through; if the plate faces the wrong way it will block the flow!

It's very important not to filter too quickly (maximum flow as stated above) or the flow can push fine particles through the sheets. To maximise flow rate it's best to use more sterile grade sheets than coarse sheets, for example with 20 x 20 cm use 18 sterile and 12 coarse sheets; allowing a flow up to 270 litres/hour.
 

           


Filter sheet regeneration:

Sterile-grade filter sheets should be used for one day only.
Re-use/regeneration of other grades of sheet is generally only economical on a large scale. Used filter sheets can be regenerated by rinsing with water flowing in the direction of filtration, as, due to a change in the pH value and the solute balance, separated constituents (colloids and particles) are washed out of the filter sheets.
Rinsing at a rate of 1 to 2 times the filtration rate consecutively in three temperature ranges has proved successful; 2 minutes with cold water, 3 minutes warm rinsing at about 45 C, approx 10 minutes hot rinsing at 70 to 80 C at 1 to 1 bar counter pressure.
All rinsing and sterilizing processes must be carried out with the filter pack loosened. This not only provides space for thermal expansion but allows the free flow of water in the area of the sealing edges.

For beer filtration, Beco recommend rinsing with water flowing in the opposite direction to that of filtration, for some peculiar reason.

Types of filter plate:

Moplen: white plastic, for filtering any drink containing alcohol up to over 40 %vol. They should not be steamed.
Noryl (steamable), black plastic, with plastic or stainless steel inserts; these are only suitable for alcohol up to 35 %vol.
Noval (steamable plastic), similar to Noryl.
Hydronalium (stove-enamel finish).
Stainless steel Perforated Plate.
Stainless steel High efficiency Filter Chambers.

Filter seals:

In the course of time seals may lose their flexibility and become brittle, and the sealing effect is unsatisfactory.
For 40 x 40 cm filters: lenticular silicon seals (6 mm thick, yellow) are highly recommended for flexibility and sealing performance.
For 20 x 20 cm filters: rubber seals are normally 3.5 to 4.0 mm thick; except for the two at the filter ends, which are about 1 mm thick. 5.0 mm seals are available for use with thicker filter sheets (e.g. some Seitz).
If, because of poor seals, excessive pressure is applied to the filter pack, this reduces the free space between the filter plates/chambers. The filter sheets are compressed excessively and the filter rate is thereby reduced.

Filter manufacturers:

Seitz: Orion, Zenit, Original: 2 outset top channels and 2 inset bottom channels
Seitz: Ariston

Schenk: similar to Seitz

Strassbourger: special plates with single central channels at top and bottom

Spadoni: special end and intermediate plates, 4 channels

The following use the same type of plate (2 outset top and 2 outset bottom channels):-
Friedrich Freres:
Elva Cristal: requires special plates at each end
Hilge: the end chambers act as end-plates, so an odd number of plates is used
Carlson:

Pressure gauges for filters:

A simple pressure gauge consists of an internal curved Bourdon tube closed at the far end. Wine entering this hollow tube can lead to infection, it cannot be sterilised by steam because back pressure prevents steam from entering the tube. It is therefore preferable that any outlet pressure gauge is of the diaphragm type, having a flexible rubber (heat sterilisable) diaphragm which transmits the pressure to the tube via high-density glycerine.

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