Tubular membranes are not self-supporting membranes and they are classified as tubular, capillary and hollow fiber membranes. They are located on the inside of a tube, made of a special kind of material. This material is the supporting layer for the membrane. As the location of tubular membranes is inside a tube, the flow in a tubular membrane is usually inside out. The main cause for this is that the attachment of the membrane to the supporting layer is very weak. Tubular membranes have a diameter of about 5 to 15 mm. As of the size of the membrane surface, plugging of tubular membranes is not likely to occur.