Tubular membranes are not self-supporting 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. Because 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.
With capillary membranes the membrane serves as a selective barrier, which is sufficiently strong to resist filtration pressures. Because of this, the flow through capillary membranes can be both inside out and outside in.
Hollow fiber membranes
Hollow fiber membranes are membranes with a diameter of below 0.1 µm. consequentially, the chances of plugging of a hollow fiber membrane are very high. The membranes can only be used for the treatment of water with a low suspended solids content. The packing density of a hollow fiber membrane is very high. Hollow fiber membranes are nearly always used merely for nano filtration and Reverse Osmosis (RO).
Hollow fiber membrane