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Air permeability ,an important factor in the manufacturing process
2019/06/13
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Air Permeability Also known as Air Pressure, Air Leakage or Air Tightness Testing – the procedure measures the uncontrolled flow of air through gaps and cracks in the fabric of the building.
Air permeability is defined as the volume of air in millilitres which is passed in one second through 100s mm2 of the fabric at a pressure difference of 10mm head of water.The specimen is clamped over the air inlet of the apparatus with the use of rubber gaskets and air is sucked through it by means of a pump as shown . The air valve is adjusted to give a pressure drop across the fabric of 10mm head of water and the air flow is then measured using a flow meter.
Air permeability testing plays a key part in establishing the energy efficiency of a building and as regulations tighten it will be increasingly necessary to make design considerations for air tightness early on in a build.
Air permeability is an important factor in the performance of such textile materials as gas filters, fabrics for air bags, clothing, mosquito netting, parachutes, sails, tentage, and vacuum cleaners. In filtration, for example, efficiency is directly related to air permeability. Air permeability also can be used to provide an indication of the breathability of weather-resistant and rainproof fabrics, or of coated fabrics in general, and to detect changes during the manufacturing process.
Performance specifications, both industrial and military, have been prepared on the basis of air permeability and are used in the purchase of fabrics where permeability is of interest.
Construction factors and finishing techniques can have an appreciable effect upon air permeability by causing a change in the length of airflow paths through a fabric. Hot calendaring can be used to flatten fabric components, thus reducing air permeability. Fabrics with different surface textures on either side can have a different air permeability depending upon the direction of air flow.
For woven fabric, yarn twist also is important. As twist increases, the circularity and density of the yarn increases, thusreducing the yarn diameter and the cover factor and increasing the air permeability. Yarn crimp and weave influence the shape and area of the interstices between yarns and may permit yarns to extend easily. Such yarn extension would open up the fabric, increase the free area, and increase the air permeability.Increasing yarn twist also may allow the more circular, high-density yarns to be packed closely together in a tightly woven structure with reduced air permeability. For example, a worsted gabardine fabric may have lower air permeability than a woolen hopsacking fabric.
For woven fabric, yarn twist also is important. As twist increases, the circularity and density of the yarn increases, thusreducing the yarn diameter and the cover factor and increasing the air permeability. Yarn crimp and weave influence the shape and area of the interstices between yarns and may permit yarns to extend easily. Such yarn extension would open up the fabric, increase the free area, and increase the air permeability.Increasing yarn twist also may allow the more circular, high-density yarns to be packed closely together in a tightly woven structure with reduced air permeability. For example, a worsted gabardine fabric may have lower air permeability than a woolen hopsacking fabric.
On completion testing measures the effectiveness of your air leakage strategy by establishing routes of leakage in the finished envelope.
If a test fails or there is more leakage than planned, a good test engineer will be able to indicate leakage areas, and in most cases suggest suitable remedial measures.
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