Tubular membranes have several advantages. They can handle viscous liquids with high levels of suspended solids and can be chemically or mechanically cleaned. The tubular polymeric membranes tend to be housed in modules of stainless steel or plastic. Spiral membranes, as the name suggests, consist of tightly packed filter material sandwiched between mesh spacers and wrapped in a small tube. The high packing density significantly increases their surface area compared with tubular membranes.
Spiral membranes require careful pre-filtration to avoid blocking if suspended solids are present.
However, developments in spacer designs are helping to increase the number of applications to which spirals are suited. The spacers provide the channels through which the process fluid flows as it traverses the membranes surface, and therefore have a major influence on the ability to resist blocking from solids, as well as on the pressure drop along the length of the element. Spacer developments have included increasing the channel height from about 0.7 mm to over 2 mm, and changes to the geometric form of the spacer to give smoother flow channels. Polymeric spiral membranes are generally used when a high throughput is required, while polymeric tubular membranes are more suited for low maintenance operations, for highly viscous products or for fluids with suspended material. Hostile processing environments, high levels of solvents and other process considerations may dictate the use of ceramic membranes.