(Extracted from IBPGR 1976)
Selection of site
There are a number of important factors to be considered in the selection of a site for a long-term storage facility. Most of these are obvious, but it may be prudent to state some of the major factors:
Socially stable area within reach of security personnel;
Reliability of mains electricity supply and voltage stability;
Suitability of substratum for foundations, adequate drainage, and absence of flooding;
Situation away from dangerous chemical or fuel storage areas; and
Easy access to the area where seeds of active collections are threshed, dried and cleaned; although these processes which produce heat and dust should not be in the same building.
Seed bank construction
As envisaged, a seed bank will consist of a main building containing a standard cold room and another partitioned space for ancillary facilities. The construction of the outer protection shell will be dictated by the local climatic conditions, building regulations and the availability of materials.
However, when only small collections are to be stored, other methods are suitable (Appendix 4).
Design of the cold storage room
It has already been pointed out that there need be no special provision to control relative humidity in the cold store, providing accessions are stored within sealed containers. Nevertheless, in order to prevent icing of the evaporator, and to maintain the integrity of the insulation, it is essential that ingress of moisture into the store is minimized. Furthermore, in order to prevent rusting of any ferrous materials, the relative humidity should not exceed 70 %. Consequently, the insulating materials should be adequately sealed against moisture ingress, and in this respect prefabricated panel construction is considered preferable to, safer than, and in most circumstances cheaper than locally built structures. In addition, an air-lock should be provided at the entrance to the cold room. With efficient management it should not be necessary to control the temperature or humidity of the air-lock since the store would only be entered a few times each week, at most. If it were envisaged that the store would be entered much more frequently than this, then in humid tropical climates it would be essential to control the relative humidity of the air-lock at about 30 %. This could be conveniently achieved with a lithium chloride humidity control device or some other appropriate system. But it would be preferable to ensure limited frequency of entry through efficient management.
If these precautions were taken, relative humidity in the store would automatically be controlled below 70 % and, using conventional compressor refrigeration techniques, possibly as low as 40 %. As mentioned earlier in this report, it would only be necessary to consider specifically controlling relative humidity at a value as low as 40 % if there were some doubt about the reliability of the sealed containers. If it were decided to do this, then the RH could be controlled at a guaranteed value of 40 % or less by using a small secondary refrigeration plant in which the refrigerant was evaporated at a lower temperature than the main refrigeration plant.
The thickness of insulation for -10°C to -20°C should be 100 – 130 mm (4 – 5 inches) with a thermal conductivity of about 0.017 W/m deg. C; such insulation should meet the economical running requirements of both temperate and tropical countries. A reliable type of insulation should be used so that it retains its thermal properties, resists the attack of pests and vermin, and contains a fire retardent.
Doors should be heated to prevent icing, and seals should remain effective without shrinking or warping. Floors should either be ventilated or heated to prevent frost heave. All electrical wiring and fittings should be accessible and designed for low temperature operation.
In view of its well-tried reliability, availability, and ease of maintenance, it is recommended that conventional direct or indirect vapour-compression refrigeration should be used. Such systems can be obtained as package units, factory tested systems for operation under tropical conditions.
The refrigerant used should be one that is common and readily available in order to facilitate recharging should there be a leakage.
Standard commercial refrigeration units and direct expansion, forced ventilation room coolers are recommended. Air-cooled condensers should be used rather than water-cooled condensers, since if water-cooled condensers should be used failure of the refrigeration plant could occur through failure of the water supply or because of impurities in the water.
It is recommended that a cold store should be provided with two separate refrigeration units capable of maintaining the desired storage temperature while operating for 18h daily under extreme temperature conditions. Because the maximum electricity load is governed by the large starting current of the compressor motor(s) the units should be switched on sequentially using current limiting starting techniques. In addition an automatic standby generator should be installed to provide sufficient power for at least one refrigeration unit and other essential services should there be a failure in the mains supply.
A three-phase alternating current power supply will be required. The estimated total connected load would be 20–45 kVA for the store sizes considered in this report.
If it is assumed that circulation spaces are left within the store, then recirculation of air in the cold room to prevent unacceptable temperature gradients will be achieved using fans and ducting as conventionally employed in cold stores. The chilled air recirculation rate should be between 5 – 10 changes per hour. More sophisticated air-circulation systems should not be necessary, since a variation of 2°C within the storage space would be quite acceptable.
In addition to providing two refrigeration systems and a stand-by generator, it is recommended that four devices should be incorporated in order to ensure that the temperature of the cold store operating at -18°C is maintained at the desired temperature. These are:
a continuous temperature recording device;
a temperature-sensitive switch which triggers an audible and visual alarm should the temperature rise or fall by more than 5°C from the control temperature;
a temperature-sensitive switch which cuts off the fans and compressors should the temperature rise or fall by more than 10°C from the control temperature; and
a wax cartridge which switches off the fans and compressors should the temperature rise to 40°C.
In all cases the temperature-sensitive elements should be sited in the upper regions of the store. Together these safety devices should not cost more than $ 3,000. In the case of the large stores, it is also recommended that a smoke detector be included in the air circulation system.
In areas where earth tremors are probable, the store should be constructed on a raft or some other device which minimizes the possibility of damage; and in such areas nothing should be constructed above the cold storage room.
These precautions should be entirely adequate since if, in spite of them, there should be a failure in the refrigeration plant, the thermal time constant (i.e. the time taken for the temperature to rise through 67 % of the range from the operating temperature to ambient) is likely to be 4 – 5 days in the larger stores. Furthermore, change in temperature per se does not affect seed viability: the viability period is a function of the integration of temperature with time. Thus, for example, if there were a complete failure so that the temperature rose to ambient for two weeks, this would not be more serious than an initial delay of two weeks in putting the dried seeds into store on arrival.
In order to ensure the safety of people using the store, it is essential that the door is designed so that it can be opened from the inside. An indicator light should be placed on the outside of the store in a prominent position which is in parallel with the internal lighting circuit of the store. It is also recommended that a notice is placed on the door which indicates that nobody should enter the store without telling some other responsible person.
It is recommended that specifications for tender be based on the standards of the American Society of Heating and Refrigeration Engineers (ASHRE) since these are considered the most helpful and are universally accepted.
Ancillary rooms and equipment for the operation of long-term seed stores
In addition to storing the seeds, there are a number of activities which need to be carried out in order to operate long-term storage facilities: the seeds need to be cleaned, dried, packaged, tested for viability, purity and moisture content; and records need to be kept. In some institutes where seed banks are being established there may well be existing laboratories, offices and equipment which could be used or modified for these purposes, but in others, all the appropriate facilities may have to be newly provided.
Irrespective of the size of the storage facility the same operations will need to be carried out at each seed bank, and thus the requirements are likely to be relatively independent of the size of the seed store. The following recommendations for minimum space requirements are based on this assumption, although it is realised that for a small establishment it may be possible to reduce these allocations without drastically affecting the efficiency of the organisation.
|Machinery room (refrigeration plant for cold store)||20% of area of cold store (but not less than 10 m2)|
|Seed drying room||15 m2|
|Seed cleaning room||30 m2|
|Seed testing laboratory||40 m2|
|Offices and records room||40 m2|
|Store rooms||40 m2|
|Toilets and service room||20 m2|
|Total (excluding machinery room)||185 m2|