Quick answer
In short
A serpentine coil is a tube bent back and forth in parallel straight passes, each joined to the next by a 180° return bend, so the tube covers a flat rectangular area. It is defined by the straight length of each pass, the number of passes, the centreline bend radius, which sets the spacing between passes, the tube size and the positions of the two ends. Uniinox forms serpentine coils in stainless steel or copper tube to the customer's drawing.
Dimensions to specify
Mark these dimensions on a sketch or drawing. Say which ones control the fit and which are only for reference.
| Symbol | Dimension | Notes |
|---|---|---|
| S | Straight length of each pass | Between the start of the bends |
| n | Number of straight passes | An odd number puts the two ends on opposite sides |
| R | Centreline bend radius | Pass spacing is 2R for a simple 180° bend |
| d × t | Tube outside diameter × wall | For example Ø9.52 × 0.7 mm |
| — | Ends and mounting | End positions, orientation and supports |
Where serpentine coils are used
A serpentine panel covers a flat rectangular area, so it can lie along the floor or a wall of a rectangular tank or bath, or sit in a slot where a round coil would not fit. Several panels can be stacked or joined into one assembly.
State how the coil will be mounted. If it carries steam or must drain completely, the orientation decides whether the passes can fall toward the outlet.
Bend radius and pass spacing
With a simple 180° return bend, the centre-to-centre spacing between passes is twice the centreline bend radius. Tighter bends put passes closer together but make the tube harder to form without flattening or thinning it, so the smallest radius depends on the tube diameter, wall and material and is confirmed against the drawing.
Return bends can also be made as separate fittings joined to straight tubes, as on the finned cores shown on this site. For a coil formed from one length of tube, show the bend radius on the drawing.
Tube length
L = n × S + (n − 1) × π × R + legs
n straight passes of length S, joined by n − 1 return bends of centreline radius R, each π × R long. The result is the coil length only; connection ends and forming allowances add to the tube that is cut.
What to send for a quotation
A dimensioned sketch is enough to start. Photos of the space or of an existing coil help if you do not have a drawing.
- Straight length, number of passes and bend radius or pass spacing
- Tube material, outside diameter and wall thickness
- Working medium inside the tube and the liquid or air outside it
- Operating temperatures and pressure, and any test requirement
- Connection ends, direction and length of the legs, and supports
- Quantity and any packing or cleanliness requirement
Frequently asked questions
Questions about serpentine (u-bend) coils
What is the difference between a serpentine coil and a helical coil?
A serpentine coil lies in one plane, with straight passes joined by 180° bends. A helical coil winds around an axis and rises with each turn.
How close together can the passes be?
For a simple 180° bend, the pass spacing is twice the centreline bend radius. The smallest radius depends on the tube size, wall and material and is confirmed against the drawing.
Can a serpentine coil be made from copper tube?
Yes. Serpentine coils can be formed in stainless steel or copper tube to the customer's drawing.