Does Coil Coating Reduce Heat Transfer?
Key facts
| Film thickness | Typical efficiency impact | Main risk |
|---|---|---|
| 5–15 μm | < 2% | Uneven coverage at edges |
| 15–25 μm | 2–4% | Partial bridging on fine fins |
| 20–40 μm | 3–6% | Fin bridging, airflow restriction |
| > 40 μm | > 6% | Bridging and insulation effect |
Why coatings affect heat transfer
A coil moves heat from refrigerant, through copper tube and aluminium fin, into the air. Any layer added to the fin surface adds a small thermal resistance and can change how air flows between fins.
For a thin, uniform film the added resistance is very small compared with the air-side boundary layer, which dominates the total.
Film thickness matters most
Thin sol-gel and waterborne films of 5–15 μm are tested at under 2% efficiency loss. Thicker air-dry and epoxy-type films of 20–40 μm and above can show measurable losses, especially when applied by brush or uncontrolled spray.
Ask for a dry film thickness range, how it is measured, and a heat-transfer test result for that range.
Fin bridging on high-density fins
On coils with narrow fin spacing, excess coating can span the gap between fins. Bridged fins block airflow, and this causes more efficiency loss than the film itself.
Dip coating with controlled drain-off and air-knife clearing reduces bridging. QC should include a visual bridging check on every batch.
18 fins per inch · 0.10 mm fins · 1.31 mm bare gap · drawn to scale
At 18 fins per inch the gap between fins is about 1.3 mm. A 10 μm film on each face closes it by roughly 1.5%, a 40 μm film by roughly 6%. Coating that bridges the gap closes it completely.
How it is tested
Heat-transfer loss is measured by testing the same coil, or matched coils, before and after coating under identical airflow and refrigerant conditions. PG-S1 is tested to GB/T 8446.2.
What this means for your project
Specify a film thickness range rather than a coating type alone. Request the heat-transfer test report and confirm that fin bridging is part of the batch QC. For high-density fins, prefer thin-film systems applied by dip.
