Spiral dryer fabric: what 820A, 831B and the grams mean

Spiral dryer fabric is a belt of plastic coils. The coils lie across the machine, their turns interlock and are held together by pintles. Such fabric is fitted in the dryer section of a paper or board machine: it presses the paper web against the dryer cylinders and lets steam through. In designations like 820A or 831B the plant encodes the coil size and whether the fabric has a filler. The grams are the weight of one square meter of fabric.

Where the fabric runs and what it does

In the dryer section of a paper machine the web zigzags over heated cylinders. The dryer fabric presses it against the cylinders so that heat is transferred evenly and the sheet does not wrinkle. Between the cylinders "pockets" form, where steam leaves the paper. The fabric must let this steam through, otherwise the pocket is locked by moisture and drying gets worse.

Dryer felts used to stand in the dryer section. Spiral fabrics are fitted in their place. The catalog says exactly that: replacement of dryer felts in the dryer section.

How spiral fabric is built

Each coil is a plastic monofilament, most often polyester, wound into turns. Neighboring coils are shifted so that their turns interlock, and a pintle is passed through the common channel. Row after row makes a belt of the required length.

Such fabric has no weave knuckles, as a woven one does, so the surface is smoother. It is easy to make endless: the ends are joined with the same pintle right on the machine. For the same reason a damaged section can be cut out and a new one inserted; the fabric makers note this too.

What a filler is

An open spiral fabric lets a great deal of air through. For heavy board this is good: steam leaves quickly. For light paper too strong an airflow is bad: the web begins to flutter in motion and may tear or get a mark.

So a filler is inserted inside the coils: a flat or shaped monofilament. It closes part of the voids, and air permeability drops. The denser the filling, the less air passes. That is why fabrics with a filler are heavier: in the catalog they weigh from 1480 to 1530 g/m², and without a filler from 1050 to 1350 g/m².

How to read 820A, 821B, 830B, 831A

These are the plant's designations, not an international standard. At another manufacturer the same fabric may be named differently. The catalog records show this pattern:

It works out like this: 82 is fabric without a filler, 83 with a filler. The third digit 0 is a small coil, 1 a medium one. The letter A on three items comes with the note "imported raw material"; items with the letter B have no such note. What exactly the letter means at this plant, clarify when ordering: a single pattern in the catalog is not a statement.

The coil size affects how coarse the mesh is and how smooth the surface is. It is chosen for the kind of paper or board and for the machine speed.

What the grams mean

Grams per square meter show how much a square meter of fabric weighs. Weight depends on the monofilament thickness, the coil size and the filler. By weight you can compare two fabrics of the same type, but you cannot choose by weight alone. What matters most for drying is air permeability. It is given in CFM, cubic feet per minute. If your old fabric has a passport with CFM, give this number when ordering.

How to take the size

1. Width. From the machine passport or from the old fabric. The fabric must not extend past the edge of the cylinders and must not be narrower than the web.
2. Length. This is the length of the endless belt along its path on the machine. It is taken from the dryer group passport or measured with a tape along the fabric path with the tension roll in mid position. Measuring an old removed fabric is unreliable: it stretches.
3. How many dryer groups there are and in which group you are changing the fabric. Conditions in the first and the last group differ.
4. What you are running: kind of paper or board, web weight, machine speed.
5. Photos of the old fabric close up, with a ruler: the coil and the filler are visible.

When to change the fabric

The fabric is changed when it has worn through, torn, stretched so that the tensioner is at its limit, or clogged with fibre and pitch so that washing does not help. The life depends on the machine, the speed and the kind of paper; there is no general figure. A spare fabric is kept rolled up, dry, away from sun and heat.

How we do it

• Forming felts and screens are made by the Xinhuibo plant, and its items are in the catalog.
• By the dimensions, photos and passport of the old fabric, we look for the item at the manufacturer. What we find, with the price, photos and specifications, we send before the order. If the plant's designation is different, we tell you how the item differs.
• We inspect the lot at the plant before shipment: part number, dimensions, lot, packaging.