Inside its production facility in Apulai (Sachim Srl), Arrigoni Spa develops its products measuring their performances concerning light transmission, resistance to stress and deformation and air permeability.
This last characteristic is analysed using equipment designed specifically by Giuseppe Starace, the head of the research team. The air permeability test aims at creating a “classification” based on the resistance opposed to the air that passes through different nets or fabrics. It is easy to understand the usefulness of this if you think that, for example:
- Ventilation must be guaranteed in a netted greenhouse;
- It is necessary to make sure open field crops transpire but are protected at the same time;
- In the construction sector, nets protecting scaffolding may have to deal with strong winds.
The air permeability test
The air permeability of a fabric or net is defined as the volume of air that passes through a square metre in a second depending on the pressure. Air permeability can also be expressed as the speed (e.g. km/h) that air reaches when it passes through a net placed between two areas that have different pressures.
The air permeability of Arrigoni fabrics is measured in a pipe with a suitable shape and size, analysing the air capacity that passes through (figure 1).
Figure 1. Functioning diagram of the system to measure the air permeability of Arrigoni fabric. The fabric separates two areas with a different pressure (p1 and p2), measuring the speed (in [km/h]) or volumetric capacity (in [dm3/(m2·s)]) of the airflow.
The Arrigoni laboratory can carry out permeability tests placing the net or fabric at different inclinations with respect to the direction of the airflow (figure 2). This means that the information is not limited to a “unique” “lab” case (net perpendicular to the wind), but also keeps into consideration what happens on fields, where wind changes its intensity and direction.
Figure 2. Section of the measuring device, showing how the inclination of the net can be varied with respect to the axis of the pipe.
ARRIGONI can also analyse “inclination zero” conditions, i.e. when the fabric and the direction of the wind are parallel. This makes it possible to measure the quantity of air let through in certain pressure conditions if the wind hits a net tangentially.
Porosity and air permeability
Two fabrics with the same percentage of empty area (i.e. porosity) can have different air permeability. This depends on the type of thread and on how they are woven.
Arrigoni carried out some tests measuring both parameters (porosity and air permeability) and proved its connection and entity.
The indicators to determine permeability performances
The Arrigoni research team identified performance indicators that express net permeability in absence of the net itself, i.e. that express how the net influences air speed just for having been installed (think of it as a “windbreak”).
The useful indicator in these cases is called “air flow reduction” and measures the percentage of wind speed reduction obtained by placing the net. In case of crops that are protected against insects, nets should actually let air through to avoid rotting. In this case, the indicator is called “air passage” and is defined as the airflow that remains despite the net.
Examples and results on some Arrigoni nets and fabrics
Charts 1, 2 and 3 report some of the results obtained by Arrigoni nets. There is a correlation between porosity and permeability, but different fabrics with the same porosity present a different permeability.
These results make it possible to see how choosing the correct net is important to enable air circulation also considering the forces transmitted onto a structure and the capability to affect the microclimate of the environment to be protected.
Air passage indicator: it is defined as the ratio between the air passing through the net during tests and the flow inside the testing apparatus without the net. Air speed: it is the permeability value expressed in [km/h]. It indicates the wind speed that causes a 50 [Pa] pressure drop on the net. Porosity: it expresses the percentage of empty area per surface unit (complementary with the coverage).
Flow reduction indicator: it is defined as the ratio between the quantity of air blocked by the test piece and the flow inside the testing apparatus without a net.
Air speed: it is the air permeability value expressed in [km/h]. It indicates the air speed that provokes a 50 [Pa] pressure drop. Covering: it expresses the full area percentage per surface unit (complementary with the porosity).
Porosity: it expresses the ratio between empty and full area per unit of net surface.
Air passage indicator: the ratio between the air passing through the test sample and the flow inside the testing apparatus with no net.
Presenting the results to the scientific community
Many of the results achieved with the experimental campaign on Arrigoni fabric, carried out in cooperation with the University or Bari and University of Salento, have been published at an international level1 at GreenSys2015 - International Symposium on New Technologies and Management for Greenhouses held in July 2015 in Evora, Portugal. Further developments will be published on an international journal in cooperation with the same universities.
1 S. Castellano, L. De Pascalis, M. Lippolis, G. Scarascia Mugnozza , G. Starace - Evaluation of HDPE nets performance in micro wind tunnel - GreenSys2015 - International Symposium on New Technologies and Management for Greenhouses - Evora (Portugal), 2015
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