In the early 90s’ major fruit exporters investigated having perishable produce like apples and bananas carried under a low oxygen atmosphere to extend the shelf life and improve the quality of the fruit when arriving at the market destination. Due to the fact that the fruit is a living organism “breathing”, lack of oxygen would be damaging to the cargo as well.
Modern reefers designed for the fruit trade use Controlled Atmosphere (CA) Systems either as fixed installed systems or based on systems built into standard ISO containers. The CA systems has a control system to accurately control the oxygen concentration in the cargo holds and automatically compensate with injecting either Nitrogen or Air to compensate for leakage or loss of oxygen due to the respiration of the fruit.
For reefer container ships Wilhelmsen Ships Equipment can provide Modified Atmosphere (MA) systems where the reefer containers are continuously being flushed with a known Oxygen/Nitrogen mix. The basis for MA systems are Nitrogen Generators. Wilhelmsen Ships Equipment has developed standard CA systems built into containers complete with compressor(s), Nitrogen Generator including an accurate CA control system, as well as fixed systems completely integrated into the ships.
Features and benefits
- containerised or fixed solution
- custom design based on standardised components
- easy to install
- sophisticated filtering system
- unique feed air dehydration solution
- long life of the membrane modules
- applying most advanced fibre technology
- PLC control of Nitrogen generator parameters
- advanced automatic atmosphere control system
- easy operation
- minimum maintenance
Technical Design
Modified or Controlled Atmosphere systems require sufficient amounts of nitrogen to create and maintain optimal storage conditions for the cargo. Basic elements of the system are: feed air source, air pre-treatment system, N2 purity control arrangement and for CA systems, atmosphere monitoring and control.
Feed air is delivered by oil filled screw compressors with modulating control, the air is saturated with water and contaminated with oil and particles and has constant pressure of about 14 bar.
In order to secure good performance of the membranes sophisticated filtering systems remove free water, oil vapour and particles from the air. Most of the fruit loading ports are located in hot and humid regions so for containerised units removal of the heat from the container and water from the feed air is critical. In Unitor containerised systems specially designed dehumidification arrangement ensures proper water content in the air, even in the most difficult ambient conditions.
Parallel sets of membrane modules separates the air stream into pressurised product (Nitrogen) and oxygen enriched air which is evacuated to permeate discharge at atmospheric pressure.
Differential pressure controller located downstream of the membranes provides oxygen content control either by manual adjustment or automatically, based on a signal from the oxygen analyser.
In Modified Atmosphere systems Nitrogen will flow into the containers at a constant rate while in the case of Controlled Atmosphere systems a balance between O2 and CO2 content will automatically be maintained in the cargo holds.
Depending on selected operation mode, PLC based atmosphere control system will direct Nitrogen from the N2 generator or Air from feed air compressor(s) to each of the protected zones. During the voyage the control system is monitoring atmosphere inside the cargo holds regularly by taking gas samples from protected compartments.