Quick answer
In short
A flat spiral or pancake coil is a tube wound in one plane, each turn larger than the last, like a watch spring. It is defined by the outside diameter, the clear opening at the centre, the number of turns or the spacing between them, the tube size, and the way the inner end is brought out past the turns. Because it is only one tube diameter thick, it fits shallow spaces such as the bottom of a tank or against a flat surface. Uniinox forms flat spiral 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 |
|---|---|---|
| Dₒ | Outside diameter | Measured over the outside of the tube |
| Dᵢ | Clear opening at the centre | Inside the innermost turn |
| d × t | Tube outside diameter × wall | For example Ø12 × 0.8 mm |
| N | Number of turns | Or the gap between turns |
| — | Inner and outer ends | Direction, length and end type of both connections |
When a flat spiral fits better than a helix
A flat spiral covers a round area evenly while staying one tube diameter thick, so it is chosen when the space is wide but shallow. A helical coil of the same tube length would be much taller.
The inner end has to leave the coil. It is usually bent up and over the turns, or out of the plane, so state which side each end must come out on and in which direction.
Spacing between turns
For evenly spaced turns, the radial pitch is the difference between the outermost and innermost centreline diameters divided by twice the number of turns. If it is smaller than the tube outside diameter, the turns would touch. The calculator on this site checks this for you.
Where both connections must leave at the outer edge, two spirals can be stacked and joined at the centre. Show this on the drawing, because it doubles the thickness.
Tube length
L ≈ N × π × (D_out + D_in) / 2 + legs
D_out = outside diameter − d and D_in = centre opening + d are the outermost and innermost centreline diameters. The formula treats each turn as a circle at the average diameter, which is close for evenly spaced turns. 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.
- Outside diameter, centre opening and number of turns or turn 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 flat spiral (pancake) coils
What is a pancake coil?
A pancake coil is another name for a flat spiral coil: a tube wound outward in one plane so that it is only one tube diameter thick.
How is the inner end of a flat spiral brought out?
It is usually bent up and over the turns, or out of the plane of the spiral. Mark the direction and length of both ends on the drawing.
How much tube does a flat spiral coil need?
Approximately the number of turns × π × the average of the outermost and innermost centreline diameters, plus the connection legs. The coil tube length calculator works this out from the outside diameter, centre opening and turns.