Agriculture Reference
In-Depth Information
influencing biological processes which, in turn, influence the conductivity to
water vapour of the onion skins and the heat output of the stored bulbs.
These biological changes can feed back and affect the physical conditions in
the store. To counter these changes, the internal environment of the store must
be monitored by sensors, and heat and water vapour must be removed or
introduced as necessary, using heating, refrigeration and the mixing of external
air with the air circulating through the stored bulbs. The rate at which heat or
water vapour can be added to, or removed from, the bulbs will depend on the
temperature and vapour pressure differences between bulb surfaces and the
circulating air, and the boundary layer resistance to heat and water vapour
transfer. These will depend on the rate of air flow through the bulbs, and the
temperature and vapour pressure of, and the rate of exchange with, ventilating
air. By mathematically modelling such a complex and interacting system of
variables, it is possible to combine information on the processes involved and to
design storage systems that optimize storage conditions. This approach has been
applied to the design of high-temperature bulk stores in Brazil (de Matos, 1987).
LOW
-
TEMPERATURE STORAGE
The harvesting and curing systems used prior to
cold storage are described in Chapter 6. Onion stores can contain bulbs stacked
to a depth of 5 m and need load-bearing walls and ducting, plus under-floor
ventilation, to force fan-driven, conditioned, ventilating air upwards through
the stack of the bulbs at an appropriate rate (see Fig. 7.14).
Fig. 7.14.
Diagram of a typical bulk onion store (from MAFF, 1978. Courtesy of the
Department of Food and Rural Affairs (DEFRA) (UK)).
