Why Bypass Occurs
Air follows the path of least resistance. A densely packed pallet or product stack presents significantly higher flow resistance than the open space above or beside it. Given the choice, airflow preferentially takes the easy path.
The bypass effect is amplified by:
- Dense stacking — Minimising air gaps between products to maximise payload creates maximum resistance to through-flow
- Solid containers — Boxes, bins, and wrapped pallets prevent air penetration
- Poor loading geometry — Gaps above cargo create low-resistance bypass channels
- Inadequate baffles — Missing or poorly designed air guides allow bypass routing
Typical Airflow Distribution
In a refrigerated loadbox with standard product loading:
60-70% — Bypasses over the top of product stacks, never contacting cargo
20-30% — Contacts the front face of product stacks only
10-20% — Actually penetrates through product to provide internal cooling
Implications for Temperature Control
The bypass effect explains why
return air temperature can show excellent compliance while product temperatures lag significantly. The air returning to the evaporator is predominantly the 60-70% that took the easy path—air that was already cold and stayed cold because it never absorbed heat from product.
Product in the centre of dense stacks receives minimal forced convection and warms from accumulated metabolic heat (in produce) or infiltrated heat (in frozen goods) while return air readings show apparent success.
Mitigation Strategies
Loading practices — Leaving air gaps between product rows, using spacers, and avoiding solid wall loading allows circulation paths through cargo
T-bar floor systems — Channels in the loadbox floor create forced return paths under product, pulling air through rather than over cargo
Open containers — Wire mesh crates and vented bins allow air circulation that solid containers prevent
Air curtains and baffles — Physical guides that force airflow through intended paths rather than allowing bypass
Related Concepts
The bypass effect compounds
dead zone formation and interacts with
thermal stratification. See our technical article on
dead zones and temperature distribution.]]>
