Besides being a single felt like the traditional Pick-up, in fact, it also performs a forming function for the paper sheet, receiving a huge amount of water in addition that will essentially dispose by centrifugation through the forming fabric that wraps externally the felt in correspondence to the Forming Roll. The dry content after the landing on the felt is only 0.2%, meanwhile after centrifugation through the forming fabric, the dry content of the paper sheet is 12%, therefore, at the formation, a 16 g/m2 sheet loses a quantity of water equal to 7866 g/m2. Like all felts for tissue machines, it must therefore be featured by: a fast start-up (24-48h); low thermic and electrical consumption; an acceptable duration (30-60 gg). The quality and quantity of the production are directly linked to the felt and therefore to its performances, both at the press suction area and in the Nip area.
From our knowledge it turns out that just 1g/m2 more of water in the sheet is enough to lose 1% dry and 4% speed, to say 72 m/min for a machine running at 1,800 m/min.
The initiation of the project for such a felt should start from the acquaintance of the airflow which is sucked by the suction press. It should also take into account the air characteristics, i.e. temperature and humidity of the air intake. In fact a hot and dry air is much more effective, from the point of view of dehydration of a cold, moist air.
In this regard there is the study of the systems to suck instead of ambient air, a preheated air conveyed in front of the suction zone of the press from particular boxes, as the old steam boxes.
As said, it is important to know the capacity of the pump connected to the suction press.
In fact, it will be the air molecules that cross the sheet + felt sandwich to get loaded with the water that they come across along their way and they
will load as much as they can hold. So, how much more air we can withdraw through the Felt, greater is its hygroscopicity and greater will be the sheet dryness at the ingoing side of the pressing area, and at the exit of the Nip, thanks to a lower rewetting as a consequence of a drier felt.
Obviously, the amount of air extracted is often a fixed parameter for each machine and as well tending downwards due to the high energy costs associated to the engines of the pumps that can be estimated approx. 500 €/year/Kw installed and 1.0-1.5 Kw/m3/min of vacuumed air.
Consequently, the felt is asked to take charge of the “transformation” of the air quality in order to make rather insignificant molecules from a hygroscopic point of view, become extraordinary moisture “eaters”. Transformation will be carried out by the so-called “Vacuum”. And the vacuum is produced by the felt structure that determines a loss of charge of air flow when it is crossed by this. This loss of charge is directly proportional to the specific surface of the fibres (finer fibers, greater specific surface), to the thickness of the felt, to the speed of the flow and, in reverse, proportional to the open area of the various layers of the felt. The equation that describes the loss of load of a fluid in a porous medium is the equation of Darcy, reshaped for modern needled felts in the Kozeny equation:
∆ P = z * 2.25 * K * Μ * V * S * (1-ε) 2/g * ε3
where K (Kozeny constant) is a constant value without dimensions that represents the tortuosity of the capillaries that has variable values, from 3 to 6, g is the well-known force-mass conversion factor, ε is the empty fraction of the felt element. S is the specific surface of the fibres and can be calculated with the following expression:
S = 4 * d/d2
where d is the diameter of the fibers that compose the Batt.
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Felt engineering: tech support
All these complex equations must be applied to each layer of batt in order to subsequently rebuild the total load loss and from this the permeability of the felt in the canonical CFM.
Obviously, this is practically possible only with the use of a specific software known as our FELT ENGINEERING® at present exclusively owned by Binet sul Liri. Returning to the molecules of the air, these, thanks to the resistance encountered in the crossing of the felt and the consequent depression that is generated, compared to the atmospheric pressure, they dilate increasing their hygroscopic capacity by loading the water deposited between the sheet and felt fibers to the passage. The physical law describing the phenomenon is Boyle’s law, enungated for perfect gases, but it is valid with good approximation also for air, so at constant temperature.
P1 * V1 = P2 * V2
V2 = P1 * V1/P2
If P1 = 1 atm and P2 = 0.5 atm we will have that
V2 = 2 * V1
That is, with a vacuum of 0.5 atm = 5 m of water column, the volume of the air molecule doubles its volume and if its relative humidity was in the typically humid environment of the paper mill of 70-80%, becomes of 35-40% and therefore highly hygroscopic. This is the reason why a low vacuum does not take water away, simply because the air molecules are not sufficiently hygroscopic.