FeCrAl Spiral Coil Supplier China: How to Specify the Correct Coil

Ordering a custom spiral heating coil for an industrial furnace requires more information than simply providing its outside diameter. Many coil-specification problems trace back not to a missing dimension but to an ambiguous one: a number that was technically provided but could be read two different ways by the person actually winding the wire.

When contacting a FeCrAl spiral coil supplier China, the most common and avoidable source of a mismatched coil is exactly this kind of ambiguity. This article focuses specifically on the handful of dimensions that are routinely miscommunicated between buyer and factory, and how to word them so there is only one way to read them.

Inside Diameter or Outside Diameter: Say Which One

A specification should state whether the quoted coil diameter refers to the inside diameter or outside diameter, because this single ambiguity causes more rejected coils than almost any other specification error.

For example, if a coil uses 2 mm wire and the required internal diameter is 20 mm, the outside diameter will be approximately 24 mm. Providing only “20 mm coil diameter” without specifying ID or OD can therefore result in a coil that does not fit the intended ceramic tube, groove or support, even though the factory produced exactly what the number on the order said.

The safest practice is to write both figures on the purchase order explicitly, for example “ID 20 mm / OD 24 mm, wire diameter 2 mm,” rather than relying on the reader to infer which one was meant. Drawings are strongly recommended for custom orders for this reason alone.

Pitch: Wound-Tight or Already Stretched?

Pitch describes the spacing between consecutive turns of the spiral, and it is another dimension where the number alone can be read two ways. A tightly wound coil may be supplied with adjacent turns touching during manufacturing and intended to be stretched to its working length on installation, or it may be produced with the final spacing already established at the factory.

A specification that states only a pitch value without saying whether that value describes the as-shipped condition or the installed condition leaves room for the factory and the buyer to assume different things. State explicitly which condition the pitch figure describes, and if the coil ships pre-stretched, say so on the drawing.

Active Length, Stretched Length and Terminal Length Are Three Different Numbers

Many furnace coils have straight terminal sections that connect the heating element to the electrical supply, and these sections are not part of the active heated spiral. Treating “coil length” as a single number, when the factory actually needs three separate figures, is a common source of elements that are the right total length but the wrong proportions.

A complete specification should separately state the active coil length (the heated spiral section), the stretched spiral length (the active section at its installed pitch), and the straight terminal length at each end, along with the terminal diameter and any required bends or connection features. This becomes especially important when an element has to fit an existing furnace without modification, since a coil that is correct in total length but wrong in the split between active and terminal sections may not position the heated zone where the furnace needs it.

Put the Electrical Target on the Same Drawing as the Geometry

Geometry and electrical requirement are often communicated in separate conversations, which creates its own ambiguity: a factory that receives dimensions without a target resistance has no way to flag that the two don’t actually match. Include supply voltage, required power or target resistance, and whether that resistance is the expected operating value or the cold resistance to be checked before installation, directly on the same drawing as the dimensional specification.

This lets the factory, or your own engineering team, catch a mismatch between the stated geometry and the stated electrical target before production rather than after a finished coil fails incoming inspection.

Assign a Drawing Number Before the First Production Run

Once a specification has been clarified and a sample approved, assign it a drawing or part number immediately, rather than after the first few reorders. Referring to “the coil we ordered in March” six months later reintroduces exactly the kind of ambiguity this article is about; referring to “drawing FC-114, revision B” does not.

This single habit does more to prevent repeat-order mismatches than almost any other step in the specification process, because it forces the ID/OD, pitch condition, and length breakdown questions above to be answered once, in writing, rather than re-derived from memory on every reorder.

Conclusion

Most coil-specification problems are not caused by missing information but by information that could be read more than one way. Inside versus outside diameter, as-wound versus as-installed pitch, and the active, stretched and terminal portions of overall length are the dimensions most often left ambiguous, and each one has a specific, simple way to remove the ambiguity.

Buyers who resolve these specific ambiguities on the drawing before production, rather than leaving them to be inferred, see far fewer coils rejected for not matching what was actually needed.