What are recirculation and bypass losses in an enclosure?
Two invisible flows steal delivered cooling: recirculation (exhaust air leaking back to the fan inlet, raising its inlet temperature) and bypass (cool air that short-circuits past the heat source without picking up heat). In unsealed enclosures these commonly waste 10–25% of nominal flow — sometimes more — which is why a fan selected from CFM alone underdelivers.
Recirculation — the fan drinks its own exhaust
Any gap between the fan's exhaust path and its inlet lets hot exhaust re-enter. The fan then works on hotter, less dense air, and every downstream component sees a higher inlet temperature. Classic causes: fan mounted in a free hole with open corners instead of a shroud, missing gasket around the fan frame, and inlet and exhaust ports on the same wall.
- Gasket or shroud the fan frame — a free-hole mount is the worst case.
- Separate inlet and exhaust walls where possible; add a divider baffle otherwise.
- Measure the fan's actual inlet temperature: if it is several degrees above room, recirculation is present.
Bypass — cool air that skips the work
Air takes the path of least resistance: any unobstructed channel around a heat sink or card cage carries flow that never touches the heat. Fixes are physical — seals, blanking plates over unused slots, directed outlet nozzles, and ducting from fan to hot zone. The energy-balance ΔT method exposes bypass instantly: a large flow with a small measured ΔT means much of the air is bypassing the load.
| Loss path | Typical magnitude (unsealed) | Primary fix |
|---|---|---|
| Recirculation at fan | 5–15% of flow | Gasket, shroud, port separation |
| Bypass around load | 10–20% of flow | Blanking plates, ducting |
| Combined worst case | 25%+ | Full airflow architecture review |
How to find and verify the losses
Smoke or tissue-trace at the fan inlet and exhaust with the unit closed; thermally image the interior to find hot pockets where bypass starves flow; and compare the ΔT-method flow against the fan curve prediction — a large shortfall (≫ margin) means losses dominate. Re-run the same checks after sealing: the flow gain from gaskets and blanking plates is routinely double-digit percentage points, at near-zero cost.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
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