Updated: Aug 26
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Multi-Compartment and Multi-Temperature ATP Equipment: Design, Testing and Approval
Understand the additional engineering challenges of ATP multi-compartment and multi-temperature equipment, from reinforced insulation and dividing walls to remaining refrigerating capacity and airflow.
25 August 2026 · Testing & Measurement
KEY TAKEAWAY
Multi-temperature equipment cannot be assessed as several independent small boxes. The outer body, internal partitions, refrigeration unit, evaporators and simultaneous compartment loads form one coupled thermal system.
IN THIS GUIDE
1. What makes multi-temperature equipment different
Multi-compartment equipment divides one insulated body into separate cargo spaces that may operate at different temperatures. Heat can enter from ambient conditions and can also move between compartments.
The refrigeration system must therefore manage simultaneous and interacting thermal demands.
The warm compartment can become a heat source for the frozen compartment.
2. The outer body requires reinforced performance
ATP multi-temperature dimensioning uses a reinforced outer-body basis. The Handbook describes an outer-body K-value of 0.40 W/m²·K or less for the multi-compartment equipment covered by the procedure.
This controls external heat gain before the additional complexity of internal partitions is considered.
3. Dividing walls have their own thermal role
Internal partitions separate compartments operating at different temperatures and create additional heat-transfer paths. Their area and thermal properties therefore enter the dimensioning process.
Partition design should also maintain durability and sealing under repeated operational use.
4. Capacity demand is calculated by compartment
Each compartment has a thermal demand based on its external and internal heat-transfer paths and the most unfavourable operating condition relevant to the calculation.
The design should not assume that total refrigeration-unit capacity can be distributed arbitrarily among compartments.
5. The 1.75 capacity factor remains central
The ATP Handbook describes the nominal refrigerating capacity of the multi-temperature unit as at least the heat loss through the outer body multiplied by 1.75. It also applies the 1.75 factor to the relevant compartment-demand checks for remaining effective capacity.
This reserve principle makes simultaneous-performance verification more demanding than simply summing catalogue capacities.
For multi-temperature equipment, total capacity and usable remaining capacity at each evaporator are both important.
6. Remaining effective capacity matters
When one compartment is controlled at a different temperature, the capacity available to another evaporator can change. ATP procedures therefore address remaining effective refrigerating capacity in multi-temperature operation.
This is one reason a mono-temperature unit capacity result cannot automatically justify a multi-temperature configuration.
7. Airflow and control strategy are part of the system
Evaporator position, return-air paths, partitions, fans, defrost and control logic influence whether each compartment can maintain its target condition.
Control interactions should be evaluated under simultaneous operating cases rather than one compartment at a time only.
8. Build the approval evidence before final design freeze
Multi-temperature approval benefits from early modelling and configuration control.
Define each compartment geometry and required temperature class.
Establish outer-body and partition thermal properties.
Calculate worst-case compartment heat loads.
Verify nominal and remaining effective refrigerating capacity.
Check evaporator and airflow arrangement.
Freeze unit, partition and control configuration for evidence generation.
Retain the dimensioning and conformity documentation with the ATP file.
Engineering perspective
Multi-temperature projects should be treated as thermal-system integration. A compartment-by-compartment heat-balance model, tied to tested unit performance and controlled geometry, is the best foundation for a smooth ATP approval path.
Primary references
The ATP Agreement and its Annexes are the primary legal reference. Handbook comments support interpretation and harmonization but are not legally binding on Contracting Parties.
Related Technical Guides
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