This document provides three section on problems
of and solutions for Cool Storage -
General Remarks on Cool
Evaporative Cooling Aggregates have the disadvantage
to pull moisture out of the goods to be cooled - which means loss of quality as well as of
If you are not familiar with this effect put
a fresh, juicy apple into a refrigerator without any wrapping. In a few days the originally
smooth and shiny skin of the apple will become wrinkled and unattractive - the apple will
not invite you to eat it. For your private use this may not be a too great loss - you know
how long you have stored it in the refrigerator.
Whatever can happen in your refrigerator will
happen in cool storage on a large scale .... and will have here a devastating effect, as
the stored goods will lose value with each day in storage.
To avoid this deleterious effect it suffices
to maintain the humidity in the storage area very near or at saturation; this means the air
surrounding the stored goods is so saturated with water vapor that it cannot take up more
water (= 100% relative humidity).
If no water is taken up, no water can be extracted
from the stored goods !
Managers of cool storage facilities who cannot
use climatization techniques try to reach this objective by sprinkling water on the free
floor space to increase the relative humidity. In many cases this method is not sufficient -
mainly because most storage areas provide not much free floor space, and the method is not
easily kept consistent. Moreover a lot of manpower and time is used for a result which is
at best a compromise.
Utilization of a fogging aggregate can solve
this task very easily and cost effective. Due to the low temperatures in cool storage the
need for humidification is usually very low. Laws of physics dictate that cold air - contrary
to warm air - can take up only a very small amount of water vapor, so that saturation can
be attained very fast.
The relative small investments needed for humidification
of cool storage stems from the fact that any measurement of humidity can be omitted. Even
if we now receive protesting calls from manufacturers of hygrostats we are convinced that
affordable measuring of humidity in a cool storage is simply not possible. This opinion stems
from the fact that the measuring cell has to dry off after reaching saturation which will
take much too long at these low temperatures. In the mean time the humidity will drop off
again to undesired values until re-fogging will be triggered.
Experiences from existing installation have proved
that a simple interval-setting can provide a far more stable humidity level than any other
For the needed investments of a humidification
plant the following estimate can be given:
Cool Storage 10 x 20 x 9 m (resulting
volume 1.800 m³)
Storage Temperature 2 to 4°C
Goods some 500-600 tons (Carrots)
Estimated Investment € 4.000,-- excl. VAT,
including setup, excluding shipping and/or travelling cost
This means for a single storage area [ as being
used more and more by bio-farmers and/or nurseries] investments of some 2,20 € per cubic
meter are to be expected. The guarantee times of plantfog installation run from 3 to 5 years
with an expected lifetime exceeding 10 years if they are maintained regularly. Operating
cost for the example above can be estimated at some 50 Cent/day.
If several storage units are united in a block
of Cool Storage of if larger units are discussed investments are getting much more reduced
as the capacity on a single pump feeding the fogging circuits is more than enough to feed
humidification of several blocks or a large storage facility while controlling each unit
or room individually.
Sometimes our customers object by arguing that
inserting "warm" water into cool storage will increase the cooling effort and therefore
cost. This can be easily refused as firstly the inserted amount of water is quite small and
secondly adiabatic cooling from the evaporation of the hovering fog-particles will actually
increase the cooling efforts.
A renowned asparagus producer in eastern Austria could
dismantle his ice-water facility used formerly for humidification with which granted him
a nice benefit cost-wise.
Last but not least we want to address water quality.
Generally drinking water quality should be sufficient. However, if fruit or vegetable is
stored in open boxes and not otherwise the hardness of the water should be taken into regard.
If storage times run to weeks and months instead of days ugly grey deposits can form on the
stored goods if water hardness is too high. As this purely aesthetic effect degrades saleable
quality we recommend a maximum of some 5°dH (deutsche Härte) for the humidification
Water softener plants, for instance ion exchangers, are nowadays affordable at
interesting prices for the small amounts needed for fogging (estimated investment some 1.200
to 1.500 Euro, operating cost some Euro 25 a year !)
Storage of Fruit and Vegetable
If we look at cool storage of fruit and vegetable
we can easily see that the break-even point for an investment in to Plantfog-Humidification
can be reached within some month of operation - simply by calculating the prevention of weight-loss.
Keeping up the quality of the goods is not yet regarded in this preview. !
If you remember
the experiment "apple in the fridge" a cool storage which is not sufficiently humidified
can lose contiuously money. The stored goods are usually re-sold by size or weight and any
loss of water results in a degradation. Depending on goods and quality of the cooling aggregates
this weight loss can amount as much as 2 to 7% each month.
(It is no question that storage areas used solely
for fruit need additional care, e.g. reduction of oxygen, etc. However, more and more "mixed"
storage areas are installed by nurseries and/or bio-farmers)
To conclude - the main objective to install a
humidification aggregate in a cool storage facility is the possibility to retain the quality
at harvest time for a considerable timespan.
As scientific tests have shown (please refer
to the table of Preservation
Times of Stored Vegetable) the possible storage time
of fresh vegetable can be double or even longer than with conventional cool storage. While
vegetable farms without a proper cool storage have to transport and sell their goods within
a few days following harvest, farmer with a state-of-the-art facility can store their goods
for considerable lengths of time. As the time of harvest is usually the same for all competitors
the need to market the goods fast means usually a deterioration of prices while farmers with
a humidity-controlled storage facility can wait until price-levels normalize again and offer
goods with harvest- quality also at later points in time with better pricing.
Cool Storage of decorative Plants
With Cold Storage of decorative plants the return
on investment calculations are not as easy as with the other examples. Main criteria are
quality (prize) of the plants and reduction of dropouts by desiccation and have to be evaluated
for each case.