World Grain - April 2022 - 50

GRAIN OPERATIONS
In temperate climates, grain temperatures in the periphery
are lowest during winter and highest during summer but are
generally not captured because of the lack of cables placed
near the silo wall. The silo wall and about a 3 foot (1 m) layer
of grain closest to the wall experiences the greatest temperature
fluctuations during a typical fall to late spring storage period.
This demonstrates that stored grain managers essentially
have no temperature management control over the periphery
layer of a grain mass.
The question arises as to the purpose of temperature monitoring
when the grain mass in the periphery (and near the surface)
where most of the temperature fluctuations occur is not
equipped with temperature cables.
Cable supply companies have similar recommendations for
placement of temperature cables as a function of bin diameter.
However, upon closer examination, the placement of cables
nearest the wall shows little consistency as bin diameter increases
(see table l, bottom of page 48).
For example, when grain mass depths are assumed the same
as bin diameters (i.e., 36 to 105 feet; 11 to 32 m diameter),
distances of the periphery cables to the outer wall range from
7 to 10 feet (2.1 to 3 m), resulting in radii fractions of 1/2 to
5/6, periphery grain volume to bin wall surface area ratios of
5.8 to 8.7, and periphery grain volumes per cable increasing
as bin diameter increases.
For the largest commercially available bin diameters (135
and 156 foot diameter) at maximum level grain depth, values
were extrapolated by assuming periphery cable distance to
bin wall was 8.5 feet (2.6 m) and periphery grain volume per
cable was the same value as for the 105 foot (32 m) diameter
bin. This would result in placing 19 and 22 periphery cables at
59 feet (18 m) and 70 ft (21.3 m) from the center, respectively,
and result in the same periphery grain volume to bin wall surface
area ratio of 8.0.
As the ratio of volume to surface area increases, the
dampening effect of grain insulation to the cyclical temperature
changes of the grain nearest the bin wall increases.
This reduces the potential for managers to obtain useful information
from temperature cables placed too far of a distance
from the bin wall and grain surface during the storage
period.
On the other hand, if the primary value of cables from the
center along the radius to the periphery is gained from monitoring
the advancement of the cooling front during aeration,
then fewer cables with optimally spaced sensors should suffice.
Given that grain often is left peaked and un-cored (especially
during the bin filling period), there is little doubt in the
value of always placing a temperature cable in the center of a
bin for monitoring the progress of the aeration cooling front
compared to shallower periphery layers, especially as bin diameter
and height increase.
Summary
Monitoring grain during the cool-down (aeration) and storage
(non-aeration) periods using temperature sensors is
critical for managing and maintaining quality. However, the
documentation for recommending the number of sensors
on a cable and the number of cables and their placement
in a grain mass is not readily available. Nor do the current
recommendations reflect what may suffice for stored grain
managers to extract useful information from temperature
data recorded. Therefore, temperature cable manufacturers
and suppliers should consider revising (and justifying) their
current recommendations for the optimal number and placement
of temperature sensors and cables needed. This would
make monitoring of stored grain quality more effective while
substantially reducing the costs of cable-based temperature
monitoring systems for end users.
In the next article in this series, we will evaluate the effectiveness
of temperature cables for detecting increasing
temperatures in the grain mass due to biological activity that
results in self-heating and spoilage.
Dirk E. Maier is a post-harvest engineer with the Iowa
Grain Quality Initiative at Iowa State University.
He may be reached at dmaier@iastate.edu.
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World Grain - April 2022

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