What Is the Central Tube in an RO Membrane Element?
The central tube is the part everything else in a spiral-wound element is built around. Here is what it is, what it does, and why its size matters.
- Also calledPermeate tube
- MaterialABS / Noryl
- RoleElement core
- CarriesProduct water out
The central tube is the perforated plastic core that runs down the middle of a spiral-wound RO element. The membrane envelopes spiral around it and glue to it; permeate that reaches the core enters through holes in the tube wall and flows out the end as product water. It is both the flow outlet and the structural backbone of the element — also called the permeate tube or collection tube.
The Backbone of the Element
The central tube is a rigid perforated plastic tube — usually ABS, Noryl or PP. During production, the glued membrane envelopes and feed spacer are wound tightly around it, so it sets the centre point of the whole roll. A row of holes along the wound section lets permeate into the bore.
It is the same part people call the permeate tube, product water tube or collection tube. For full dimensions and materials, see the central tube data sheet.
From Membrane to Product Water
Feed water flows along the outside of the membrane; the part that passes through becomes permeate and is carried by the permeate carrier spiralling inward. When it reaches the core, it enters the central tube through the holes and runs out the end of the element as clean product water.
Because everything is built around it, the tube’s size is set by the element model. Match it with the element size reference, and see how it fits the build in the production setup guide.
What it gives the element
- A structural core to wind around
- A collection point for permeate
- The product-water outlet
- The connection to the next element
- A size set by the element model
Central Tube Materials at a Glance
Central tubes come in a few plastics. The right one depends on operating pressure, temperature and cost.
| Material | Key property | Typical use |
|---|---|---|
| ABS | Good strength at low cost | General household & commercial elements |
| Noryl (PPO) | Higher temperature & strength | Industrial & demanding conditions |
| Polypropylene (PP) | Strong chemical resistance | Specific chemistries / cleaning regimes |
Get the Core Right, or Nothing Else Fits
Because the element is built around it, the central tube is one error you cannot fix later in the build.
Sets the Size
An off-size tube means leaves do not glue cleanly and the diameter drifts.
Carries the Output
The bore and holes have to pass the full product flow without restriction.
Connects the System
It links to the next element, so the interface must match your vessel.
Need central tubes for your elements?
Tell us your element sizes and connection system — our engineers will match the right central tube free of charge and supply it to your build.
People Also Ask
What is the central tube made of?
Central tubes are typically ABS, Noryl (PPO) or polypropylene, chosen for strength at the element’s operating pressure and compatibility with the permeate.
Is the central tube the same as the permeate tube?
Yes. Central tube, permeate tube, product water tube and collection tube all refer to the same perforated core that collects permeate and that the element is wound around.
Why does the central tube have holes?
The holes let permeate that has spiralled inward enter the tube bore, where it collects and flows out the end of the element as product water.
Does the central tube size change with element size?
Yes. Its diameter, length and hole pattern are set by the element model (1812 to 8040) and the connection system. Use the element size reference to match it.