Fin density (fins per inch, FPI) is the inverse of pitch. It is the number most often written on a purchase order and the one most often misunderstood.
FPI and pitch
| FPI | Pitch (mm) | Typical service |
|---|---|---|
| 7 | 3.6 | General air cooling |
| 8 | 3.2 | Most air coolers |
| 9 | 2.8 | Clean air, compact bundles |
| 10 | 2.5 | HVAC, refrigeration |
| 11 | 2.3 | Maximum area, clean air only |
| 5 | 5.1 | Light fouling |
| 4 | 6.4 | Flue gas, moderate dust |
| 2–3 | 8.5–12.7 | Heavy dust, soot blowers |
Why FPI alone is not a specification
Two coils with the same FPI can differ by 50 % in surface area if their fin heights differ. Always specify fin height, pitch and thickness together, plus the base tube OD. A purchase order that says only ’10 FPI’ leaves the supplier free to deliver almost anything.
Density and cleaning
Above about 9 FPI, water washing becomes difficult and compressed-air cleaning is the practical option. For installations where cleaning access is limited, 7–8 FPI is a safer choice even if the initial calculation favours denser fins.
Convert your FPI to a full specification
Tell us the tube diameter, fin height, fin pitch, materials and quantity — we reply with price and lead time within one working day.