The structure feature of HEPA filter bag is that it is composed of layers in grades. Inner layer is the thickest to provide the best pre-filtration and largest collection ability. While the middle and outer layers are with high density and micro-fiber. It can provide a better filtration precision. There are at most 12 layers in a HEPA filter bag in different density and precision .This can lead a stable filtrating efficiency and dust-collection  ability. It also will increase the lifetime of filter bag (according to the total flowing ability). Moreover, HEPA filter bag owns the ability of hydrocarbon (oil) absorption because of its special structure.HEPA Filter Bag realizes the lost cost and high efficiency of replacement in the industrial filtration.