Controlled Atmosphere Storage Basics

Washington is the number one producer of fresh apples in the US. Oregon and Idaho also are significant apple producers. A large fraction of the apple and pear crop are stored in controlled atmosphere (CA). There are over 2000 controlled atmosphere rooms in Washington alone.

CA storages are specialized facilities designed for long-term storage of apples and pears. CA facilities are designed to quickly cool fruit from field temperatures to storage temperatures (approximately 30°F). The rooms are then sealed and are maintained with a high nitrogen concentration and a low oxygen concentration.

The goal of CA is to slow the ripening process of the fruit to extend its marketability throughout the year. The individual rooms may be opened between January and July. Once a room has been opened it, the fruit in that room is packed in boxes and is eventually shipped. After a room has been emptied, it is no longer refrigerated (and its evaporator fans are shut off) until the next season starts (or the room is used for another application). Most CA rooms operate about half the year.

A typical CA room would hold from 800 to 3000 bins of apples. Each bin contains approximately 900 lbs of fruit. Even at wholesale prices, there is a very high economic value to the contents of a single room. The dollar value makes it imperative that energy savings do not come at the expense of fruit quality.

Controlled atmosphere facilities typically consist of 4 to 50 separate rooms served by one or more centralized refrigeration systems.

Each CA room is equipped with one or more evaporators, which provide the refrigeration effect to cool the fruit. The evaporator are sized to handle the peak cooling loads associated with cooling fruit from orchard temperatures down to storage temperatures over the course of several days.

At other times, the refrigeration load in the rooms is comparatively small and is made up largely of the shaft power from the evaporator fans, heat of respiration from the fruit, and conduction through the walls and ceiling of the CA room.