How Many Types of Reefer Container Are There, and How to Set Temperature and Ventilation

"Reefer container" is often used as if it describes one uniform product, but the term actually covers a small family of designs built for different jobs. Two shipments both moving in "a reefer" can involve meaningfully different equipment — one relying purely on a built-in electric compressor, another actively managing the gas mix inside the box, another running off a portable generator for a leg of the journey with no fixed power supply.

Getting the type of reefer right is only half the job, though. Even the correct unit will damage or degrade cargo if it's set up wrong — and "set up" means two separate decisions: what temperature to hold (the setpoint), and whether to exchange air with the outside world or seal the box shut (the ventilation setting). This article walks through both: the equipment types available, and how setpoint and ventilation actually work together to protect a specific cargo.

What a Reefer Container Is and How It Works

A reefer container is a fully insulated steel box with a self-contained refrigeration unit built into one end wall. Unlike a passive insulated container, which simply slows the rate at which outside temperature affects the inside, a reefer actively generates cooling (or, in some units, limited heating for cold-climate protection) and circulates air through the cargo to hold it at a target range.

The unit draws air from the top of the container (return air), passes it across a refrigeration coil, and pushes the cooled air down through channels in the floor (the T-bar or grated floor most reefers use), so it rises up through the cargo stack from below. This underfloor airflow design is why cargo stowage pattern matters — blocking the floor channels or stacking cargo flush against the walls restricts airflow and creates uneven temperature zones inside an otherwise correctly set container.

Power comes either from a fixed source — the vessel's reefer plug points, a container yard's reefer socket, or a truck's onboard reefer power — or from a portable generator when no fixed source is available for a given leg.

Types of Reefer Containers by Design

The figure below breaks these down, but the practical distinction is this: an integrated electric-standby reefer is the default choice for the large majority of cold-chain sea freight, and it's what most shippers picture when they hear "reefer container." It relies on a fixed or generator power source and controls temperature and, on many modern units, humidity.

A controlled-atmosphere (CA) reefer goes further, actively managing the internal atmosphere — reducing oxygen and controlling CO2 levels — to slow the biological processes (ripening, respiration) that continue in certain fresh produce even under refrigeration. This matters most on longer voyages where standard refrigeration alone wouldn't keep cargo in acceptable condition for the full transit time.

Genset-powered reefer setups aren't a different container so much as a different power arrangement — the same integrated unit paired with a portable generator, used when a shipment moves through a leg without fixed reefer power, such as certain rail or road segments. The porthole/clip-on design, which relies on an external cooling machine rather than a self-contained unit, is largely a legacy design in general commercial shipping today, though it still appears on some older equipment or specialized routes.

How the Temperature Control System Works: Setpoint, Return Air, and Supply Air

Three numbers matter when reading a reefer's performance, and confusing them is a common source of misunderstanding. The setpoint is the target the unit is instructed to hold — a single number entered into the control panel. Return air temperature is what the unit reads coming back from the cargo space, and supply air temperature is what the unit pushes back out after cooling. The unit works to bring return air down to setpoint by adjusting supply air; it does not directly measure the temperature at the core of a pallet or box deep inside the stack.

This distinction matters practically: a reefer holding its setpoint perfectly on the return-air sensor can still have cargo in the center of a poorly stowed load sitting several degrees off target, because airflow never reached it evenly. This is why cold-chain protocols for sensitive cargo often specify both a machine setpoint and a stowage pattern — the setpoint alone does not ensure uniform cargo temperature.

Ventilation: Why Some Cargo Needs Fresh Air Exchange

Many reefer cargoes are biologically inactive by the time they're loaded — frozen meat, frozen seafood, and most pharmaceuticals aren't undergoing any ongoing biological process that produces gas. For that cargo, the ventilation setting is normally closed, because there's no benefit to exchanging air and a real cost: every liter of outside air pulled in has to be cooled down to setpoint, adding load to the refrigeration unit for no purpose.

Fresh produce is different. Fruits and vegetables continue to respire after harvest — consuming oxygen, producing CO2, and in many cases releasing ethylene gas, which accelerates ripening in the produce itself and in anything nearby sensitive to it. In a sealed box, CO2 and ethylene build up, which can push some produce toward over-ripening or spoilage faster than temperature alone would explain. Opening the ventilation to a specified position allows continuous exchange with outside air, clearing that buildup, at the cost of extra refrigeration load to cool the incoming air.

The ventilation setting is typically expressed as an opening size (a percentage or a specific measurement depending on the unit), not a simple open/closed switch, which lets the setting be tuned to the specific respiration rate of the cargo rather than treated as all-or-nothing.

Matching Setpoint and Ventilation to Cargo Type

The practical exercise for anyone booking a reefer shipment is matching both variables — setpoint and ventilation — to the specific cargo, rather than defaulting to whatever setting was used on a previous, different shipment. Frozen protein wants a low, steady setpoint and closed ventilation. Chilled fresh produce wants a setpoint just above freezing and a ventilation opening sized to the specific product's respiration rate — different fruits and vegetables respire at different rates, so the correct opening size genuinely varies by product. Pharmaceuticals typically want a narrow, tightly held setpoint with closed ventilation and continuous monitoring, since the tolerance for deviation is much tighter than for food cargo. Ventilated dry cargo, such as certain green coffee or cocoa shipments, may barely need active cooling at all — the reefer unit is used mainly to move air and manage humidity rather than to chill the cargo significantly.

Getting this wrong in either direction causes real damage: too little ventilation on respiring produce risks gas buildup and accelerated spoilage; unnecessary ventilation on frozen or pharmaceutical cargo wastes refrigeration capacity and can introduce temperature instability. Confirming both settings with the shipper's own product knowledge — not just a generic "reefer" instruction — is what separates a correctly configured shipment from one that arrives in poor condition despite the container technically working as designed.

Pre-Cooling, Loading, and Airflow Around the Cargo

A reefer unit is designed to hold a temperature, not to rapidly bring down the temperature of warm cargo loaded into it. Best practice is to pre-cool the container to setpoint before loading begins, and to load cargo that is already at or close to the target temperature. Cargo loaded warm places a heavy, often unrealistic burden on the unit to remove that heat during transit, and in the meantime, cargo near the door or at the edges of the stack — furthest from consistent airflow — is the most exposed to temperature drift.

Stowage pattern matters as much as pre-cooling. Cargo needs to be loaded to preserve the airflow channels the unit depends on: not blocking the floor T-bar grooves, not packing cargo flush against the side walls where it can restrict circulation, and not exceeding the container's load height, which can obstruct the path back to the return-air intake at the top. A well-set unit moving air through a poorly stowed load still produces uneven results, which is why loading instructions for reefer cargo typically go beyond "keep it cold" and specify how the cargo should physically sit inside the box.

Monitoring and Data Logging During Transit

Most modern reefer units log temperature data continuously throughout the voyage, and many carriers offer remote monitoring that flags deviations while the container is still in transit rather than only revealing a problem at unloading. For temperature-sensitive cargo, requesting a copy of the temperature log — sometimes called a reefer download or a PTI (pre-trip inspection) and temperature record — as part of the shipment documentation gives an objective record that both the shipper and consignee can refer back to if a condition dispute arises.

What the log typically shows is return-air temperature over time, not necessarily the core temperature of cargo deep in the stack, which circles back to the stowage point above: the log is strong evidence of how the machine performed, but it's not a substitute for physical spot-checking of cargo condition on arrival for genuinely high-value or high-risk shipments.

Example

Consider a Thai exporter shipping fresh mangosteen to an overseas buyer. Given the produce is still respiring after harvest, the forwarder books a standard integrated electric-standby reefer set to a chilled setpoint just above freezing, with ventilation opened to a position suited to mangosteen's typical respiration rate, based on guidance from the produce's own handling notes. The container is pre-cooled to setpoint before loading begins, and cartons are stowed with gaps preserved along the floor channels and side walls to keep airflow reaching every layer of the stack.

On a separate booking the following month, the same exporter ships frozen seafood to a different buyer. This time the forwarder books the same type of integrated reefer, but sets a much lower setpoint and closes the ventilation completely, since the frozen product has no ongoing respiration and any air exchange would only add unnecessary refrigeration load. Both shipments use the same underlying equipment type, but the setpoint and ventilation configuration is deliberately different because the cargo itself is fundamentally different.