evaporator unit. Point-source discharge creates predictable airflow patterns—and predictable dead zones—due to the physics of momentum decay and buoyancy stratification.
How Point-Source Discharge Works
In most transport refrigeration and small commercial freezers, a single evaporator unit discharges cold air from one location—usually roof-mounted at the front of the space. The cold air jet travels outward from this single point, spreading as it moves away from the discharge.The Velocity Decay Problem
Air velocity decreases with distance from a point source due to:- Jet spreading — The air stream expands into a cone, distributing momentum over increasing area
- Entrainment — Moving air pulls in surrounding stationary air, slowing the overall flow
- Wall and product friction — Contact with surfaces absorbs momentum
Predictable Dead Zone Locations
Point-source discharge from a roof-mounted unit creates predictable dead zones: Upper rear corners — Furthest from discharge, receive lowest velocity air that has already fallen due to buoyancy Areas behind product stacks — Airflow velocity too low to penetrate around obstacles Floor corners opposite the evaporator — Cold air accumulates at floor level but lacks velocity for effective heat transferAlternatives to Point-Source Discharge
Ducted systems — Channels that distribute cold air to multiple discharge points, extending reach and improving uniformity T-bar floor returns — Channels in the floor that create forced return paths, working with buoyancy rather than against it Multi-evaporator designs — Multiple smaller units positioned throughout the space (rarely used in transport due to cost and complexity) Ceiling discharge patterns — Perforated ceilings or distributed ceiling jets that provide more uniform dischargeDesign Trade-offs
Point-source discharge is standard because it’s:- Simple — One unit, one installation point
- Inexpensive — Minimal ducting, fewer components
- Space-efficient — Evaporator mounted out of cargo space