Reefer Setpoint: -18 C or -25 C?
Most frozen cargo holds at -18 C. Going to -25 C costs payload, fuel and pull-down time. How to decide by product, dwell time and ambient temperature.
-18 °C or -25 °C: what actually changes in the truck
Choosing -25 °C instead of -18 °C does not mean dialling a lower number on the controller. It changes the refrigeration unit model, the compressor duty cycle, the panel thickness that makes economic sense, the door and seal specification, and the defrost cycle. A body built for -18 °C running continuously at -25 °C will lose temperature in the rear third, ice up the evaporator and raise its power draw. A body specified correctly for -25 °C costs roughly 8–15 % more than the same body at -18 °C, and consumes more fuel per hour in service. The decision therefore belongs to the product you intend to carry, not to the brochure.
What temperature does your cargo actually need
| Cargo | Carriage temperature | Notes |
|---|---|---|
| Chilled meat, dairy, fresh produce | 0 to +4 °C | Airflow and humidity matter more than deep capacity |
| Fresh fruit and vegetables | +2 to +12 °C, by product | Some products are damaged below +10 °C |
| Pharmaceuticals | +2 to +8 °C | Requires temperature logging and alarm, not just cooling |
| Frozen meat, frozen fish | -18 °C | The standard frozen chain; the usual reference point |
| Ice cream, some seafood, some vaccines | -25 °C and below | Requires a different unit, thicker panels, stronger door seals |
Why a body rated for -18 °C cannot simply be set to -25 °C
Three physical limits appear at the same time. The first is evaporator capacity: the unit’s rated cooling capacity is stated at a specific evaporating temperature, and pulling the box 7 °C lower moves the operating point beyond what the compressor was selected for. The second is heat ingress: the temperature difference to ambient grows from roughly 53 K to 60 K in a 35 °C market, which increases heat flow through the panels by about 13 % — on the same panel thickness. The third is defrost: at lower box temperatures the evaporator ices faster, and a defrost cycle that is adequate at -18 °C will leave ice that progressively blocks airflow. The visible symptom is a box that holds temperature empty and fails when loaded.
Panel thickness and unit selection, matched
Panel thickness is where the two specifications diverge most in cost. For a -18 °C box in a hot market, 75 mm walls and 100 mm floor and roof are a common working specification. For -25 °C, 100 mm walls and 120 mm floor and roof are typical, and the roof is where skimping is most often attempted and least forgivable, because it receives direct sun. The refrigeration unit must then be selected at the correct evaporating temperature, not from its headline capacity figure.
The panel-and-unit calculation behind these limits is set out in insulated panel thickness and unit matching.
What -25 °C costs you in service
Expect higher fuel or engine-hour consumption, a shorter compressor service interval, more frequent defrost cycles in humid climates, and less tolerance for door openings. In multi-drop distribution, a -25 °C box that is opened twenty times a day will struggle in a way a -18 °C box will not. If the route involves frequent door openings, consider a unit with a larger evaporator and a divided or curtained body rather than simply paying for more compressor.
How to specify it so the quotation is comparable
Give the supplier five numbers and nothing else: cargo type, required box temperature, maximum ambient temperature in the destination market, number of door openings per day, and expected loaded hours per day. A supplier who quotes from “we can do -25 °C” without asking those five is quoting a number, not a body. Ask for the unit model and its capacity at the stated evaporating temperature, the panel thickness by surface, and the defrost method. Three quotations can then be compared line by line.
Our bodies are built at 50, 75, 100 and 120 mm, with the refrigeration unit matched to the stated duty rather than to the chassis size. If you send the five numbers above, the reply will state the panel schedule and the unit model that meets them, and will say so if a requested combination cannot hold temperature in your ambient conditions.