11 Aeration of Grain

14 Pages·2012·0.59 MB·English

11 Aeration of Grain
Shlomo Navarro
Ronald T. Noyes
Mark Casada
Frank H. Arthur
Aeration is the forced movement of ambient air by Substantial storage losses can be caused by micro-
fan power through a grain bulk to improve grain flora that flourish in moist grain and insects that can
storability. Aeration is primarily used for cooling, but be destructive if preventive measures are not taken.
additional objectives are to equalize grain tempera- These losses should be considered a result of interac-
ture throughout the bulk, promote limited drying, tions between components of the ecosystem, affected
and remove fumigant residues and odors. Aeration by grain and ambient weather conditions. Interac-
is distinguished from “passive” or “natural” ventila- tions between damaging pests, the grain, and other
tion, which takes place in corn cribs, where sidewall physical components of the system form a dynamic
wind pressures force ambient air through the grain, infrastructure, with each component continuously
causing slow natural drying of damp unshelled corn, affecting the others. The role of aeration in this eco-
or in grain bins where roof wind forces create suction system is to uniformly “condition” the stored grain to
convection currents between roof vents and base a desirable low temperature and maintain desirable
fan openings. Aeration flow rates should be distin- conditions in the grain bulk by moving the sufficient
guished from recirculated fumigation, which uses air volumes of suitable quality through the grain
very low airflow rates, and from drying, which uses mass (Navarro and Noyes 2002a).
very high airflow rates compared to aeration.
The purpose of this chapter is to guide grain manag-
Aeration is widely used in stored grain management ers on the concept of using aeration to preserve grain
programs in the United States. Pioneering engineer- quality and manage insect populations in conven-
ing work of U.S researchers such as Foster (1953), tional farm and commercial grain storages.
Robinson et al. (1951), Shedd (1953), and Holman
(1966) and research on technological aspects of aera- Objectives of Aeration
tion by Hukill (1953), and more recently by Cuperus
et al. (1986), Arthur and Casada (2005, 2010), and
The objective of aeration is to maintain the quality of
Reed (2006), form the basis of modern grain aera-
bulk grain in storage. Although aeration can improve
tion systems. Aeration technology is used to modify
storage conditions, aeration does not improve intrin-
the grain bulk microclimate to reduce or eliminate
sic quality attributes of grain.
the development of harmful or damaging organisms
in the grain by reducing and maintaining grain tem- Cooling the grain bulk for pest suppression –
peratures at safe levels below humidity levels which Cooling grain is the primary objective of grain
support microflora activity. Aeration helps sustain aeration (Reed and Arthur 2000, Reed and Harner
favorable storage conditions for the safe preservation 1998a) when discussing pest suppression.
of grain quality.
Stored Product Protection 1
Part II | Management: Prevention Methods
Stored grain insects are of tropical or subtropical an efficient method for controlling mold, at lower
origin and require fairly high temperatures, typically grain temperatures, mold damage is reduced.
75° to 90°F (24° to 32°C) for development. Grain-
Maintenance of seed and grain quality –
infesting insects are sensitive to low temperatures.
Stored product insect development is generally Low kernel temperatures are desirable for better
stopped below 60°F (16°C); there is little insect maintenance of seed and grain quality. Studies have
survival above 110°F (43°C). In the southwestern shown that the lower the temperature (within certain
and south-central U.S., temperatures of wheat, rice, limits), the longer the seeds maintain full viability. A
and sorghum at harvest can range from 90° to 110°F rule of thumb (Harrington 1973) states that a seed’s
(32° to 43°C), depending on the specific crop and life span in storage is doubled for each 9°F (5°C)
location. During fall harvest in the northern U.S., decrease in temperature (within the range of 32° to
grain temperatures around 50° to 65°F (10° to 18°C) 122°F (0° to 50°C) and for each 1 percent decrease
are typical. in seed moisture (within the range of 5% to 14%).
Seeds are commonly stored with equilibrium rela-
At temperatures below 70°F (21°C), population tive humidity from 30% to 40% with good results.
growth of most storage insects is significantly sup- For extended storage times of seeds, Vertucci and
pressed. Grain temperatures of 60° to 70°F (16° to Roos (1990) recommend the best storage moisture
21°C) are considered “safe” for insect management, content is between 19% and 27% equilibrium relative
because feeding and breeding are slow. Complete life humidity.
cycles at these temperatures take three months or
Equalization of temperature through-
more, so insect population growth remains insignifi-
out the grain bulk –
cant. Insect damage caused under these low temper- Because of self-insulating
ature conditions is minimal (Flinn et al. 1997). properties, grain placed in storage during summer
harvest retains initial harvest temperatures for a long
The crucial control parameter for mite pests is not time before cool weather arrives in the fall (except
temperature, but establishing an equilibrium relative for grain near bin walls, exposed conical base, or the
humidity (ERH) below about 65%. About 12.5% surface). It is recommended that harvest heat be
moisture content (MC) for wheat at 77°F (25°C) removed by nighttime suction aeration as soon as
suppresses mite development (Cunnington 1984, ambient temperatures are 15° to 20°F below internal
Navarro et al. 2002). Temperatures required to sup- grain mass temperatures to minimize insect activ-
press mite development in damp grain ity at or near the grain surface. The initial cooling
(14% to 16% moisture content wet-basis) are obtain- should be followed by additional aeration when gen-
able in temperate climates. Maintaining low uni- erally lower ambient temperatures will allow cooling
form grain temperatures is too expensive at the bulk the entire grain mass below 70°F.
periphery when mean ambient temperatures are
Prevention of moisture migration in the
favorable for mite development. Although cooling
grain bulk –
moist grain is unlikely to prevent moderate mite As the ambient temperature drops
infestation, aeration is expected to minimize “hot during the cool season, the surface (and peripheral)
spots” and heavy mite populations associated with layers of the grain become considerably cooler than
them. the internal grain mass. Temperature gradients
are established in the grain bulks that can lead to
Suppression of micro floral growth –
Low convection currents that circulate air through the
temperatures are required to prevent mold damage intergranular spaces. In large bulks, the cold dense
in moist grain. Temperatures below 40°F (5°C) are air settles along the outer walls. The warmer air
needed for the suppression of most mold develop- (which contains more moisture than cool air) moves
ment. For suppression of Penicillium molds, tempera- toward the colder upper surface of the grain bulk.
tures must be below 0°C. Most fungi do not grow at When the warm air reaches the cool layers of the
relative humidities below 70%, which is equivalent grain bulk, moisture condenses and creates wet layers
to roughly 13% moisture content for cereal grains at or spots in the grain. Recent studies (Montross et
typical storage temperatures. The moisture content al. 2002, Montross and Maier 2001) suggest a new
threshold is lower for oilseeds. In practice, mold moisture equilibration theory for the mechanisms
growth is dependent mainly upon interstitial air involved in this moisture movement in a non-
humidity. Although cooling grain may not seem like aerated grain mass. Using the finite-element model
2 K-State Research and Extension
Chapter 11 | Aeration of Grain
they developed, additional large-scale trials will be bulks containing oil rich seeds such as cottonseeds,
required to demonstrate the effect of significant soybeans, and sunflower seeds at sufficiently high
temperature gradients on moisture condensation due moisture conditions, very high temperatures are
to convection currents that carry moisture into the generated and “spontaneous combustion” can occur,
cool layers of the grain bulk. On the other hand, the starting a fire. Do not operate aeration fans if fire is
traditional natural convection hypothesis suggests detected (by the smell of smoke or burning grain in
that the natural convection currents in the grain bulk the exhaust air stream) in a grain bulk.
alone are sufficient to cause large amounts of mois-
Limited grain drying by aeration –
ture to “migrate” to cooler layers or the cooler surface A small,
grain, where the air cools to “dew point” and deposits but significant drying effect (from 1/4% to 1/2%
excess moisture, slowly increasing the grain moisture moisture loss per aeration cooling cycle) is typically
content in the upper parts of the grain bulk. experienced, and during long-term aeration (mul-
tiple cooling cycles) up to 2% moisture reduction
Prevention of head-space and down spout
may occur while cooling large grain bulks. Because
condensation –
Under-roof condensation is of the very low flow rates during aeration, the drying
a different natural process than moisture migra- front moves slowly, and this small drying effect is
tion within the grain bulk. Condensate that drips usually limited to the grain near the entrance of the
on the grain involves moisture in humid air, which aeration air. This grain moisture loss is reflected in a
accumulates in the head-space above the grain bulk corresponding shrinkage or market weight loss in the
and condenses on the under-surface of the bin roof. grain bulk. This must be considered in grain manage-
Bins with sufficient roof vents and open eave gaps ment as a cost for keeping grain safe for marketing.
(spacing of 1/2 to 1 inch) between sidewall and roof, Aeration moisture shrinkage as well as “invisible”
generally have enough natural ventilation to avoid handling loss will affect facility records significantly
under-roof condensate. Condensate is especially and should be considered when grain receipt and
problematic in bins with eave gaps that are perma- delivery records from storage facilities or sites do not
nently sealed to prevent fumigant gas losses and easy tally.
grain access for insects.
Removal of fumigant residues and odors –

Prevention of biological heating of dry
The release of or desorption of fumigants at the end
grain –
In grain bulks where infestation is localized, of a fumigation can be achieved with relatively low
insect populations develop in small pockets of grain. air flow rates. The aeration system can be operated
The lesser grain borer and the three primary weevil intermittently (in pulses) to flush gas vapors from
species found in grains in the United States — the the grain bulk and storage. Aeration systems can be
rice weevil, the maize weevil, and the granary weevil operated for 10 to 15 minutes every two to three
— are characteristic species that develop local- hours to allow interstitial air space to reach equilib-
ized infestations in bulk grains, creating hot spots. rium with the concentration of the fumigant in the
Temperatures of heavily infested grain undergoing grain. Thus, the aeration system can be operated sev-
widespread heating are typically about 100° to 110°F eral times to ventilate the storage. Storage odors also
(38° to 43°C). When heavy infestations are discov- can develop in a grain bulk due to hot spots contain-
ered, the grain should be fumigated immediately to ing insects or moldy grain. Sour odors result from
stop insect activity. Then aeration should be used to anaerobic activity in the process of fermentation
cool the grain bulk. at high moisture contents (above 18% for cereals).
At moderate moisture levels (14% to 18% moisture
Prevention of spontaneous heating of moist
content for cereals), musty odors in grain are usually
grain –
In warm moist grain (equilibrium relative caused by the growth of certain molds. Other odors
humidity greater than 70%), respiration can become occasionally found in grain are considered commer-
very intensive due to mold development. High cially objectionable foreign odors (COFO) because
levels of respiration produce a phenomenon called they are odors that are foreign to grain and render it
“spontaneous heating.” Heating of the grain bulk is unfit for normal commercial usage. Most odors can
detrimental to grain quality. In spontaneous heating, be reduced using aeration; however residual odors
hot spot temperatures can easily reach 135°to140°F may linger after repeated aeration cycles. Commer-
(57° to 60°C) creating steep temperature gradients cial applications based on pilot laboratory studies
between heated and surrounding cool grain. In
Stored Product Protection 3
Part II | Management: Prevention Methods
have used aeration combined with ozone treatment and airflow resistance for a specific fan is called the
to reduce off odors in grain (personal communica- system curve. Fans with certified (measured) fan per-
tion Carlos Campabadal). formance curves should be used for designing grain
aeration systems. The performance of similar size
Aeration System Design fans from different manufacturers can vary widely.
For example, against a resistance of 2.4 inches of
water (600 Pa), fan A provides a measured flow rate
In a typical aeration system, the basic components
of 1,695 cfm (800 L/s), fan B, 2,755 cfm (1300 L/s),
are a bin with perforated in-floor or on-floor ducts; a
and fan C, 5,509 cfm (2600 L/s), which at this air-
fan connected to the plenum or duct system to force
flow resistance is more than three times higher than
the air through the grain; and one or more roof vents
the airflow rate of fan A (5,509/1,695 = 3.25). A
for exhaust or air intake. Many variations of the typi-
high-speed vane-axial fan may be suitable for corn,
cal aeration system are used in practice.
but a low-speed centrifugal fan may be needed for
Resistance of grain to airflow – Cereals, oil- sorghum or wheat on the same size bin because of
seeds, and granular animal feeds have an intergranu- higher static pressure required for the airflow rate for
lar porosity or void space that ranges between 35% which it was designed.
and 45% of the bulk volume. Two different grain
types may have similar porosities but the surface area Aeration System Design
per unit volume for small-seeded grain would be
Considerations
larger than for the large-seeded grain, e.g., sorghum
seeds are smaller and the kernel surface area is larger
Airflow rates –
than for maize. At the same superficial airflow For upright storages (concrete
rate (i.e., the same cfm/bu), the specific air veloc- silos and tall steel bins) airflow rates of 0.05 to 0.10
ity through sorghum is much higher than through cfm/bu (3 to 6 (m3/h)/tonne) and for horizontal
maize, which has large intergranular void openings storages airflow rates of 0.10 to 0.20 cfm/bu (6 to 12
and shorter interstitial path lengths for airflow. The (m3/h)/tonne) are typically used. Higher airflow rates
increased velocity over larger surface areas and the (0.20 to 0.25 cfm/bu), which will cool grain faster,
longer air paths through smaller interstices cause are needed in southern regions with limited cool
the higher resistance for sorghum than maize even weather conditions. Central U.S. systems may find
though the percent air volumes in the masses are that 0.15 to 0.20 cfm/bu works well, while 0.1 to
about the same. In a typical aeration operation, the 0.15 cfm/bu in northern states may be sufficient due
resistance (expressed in inches of water static pres- to early long periods of cool weather.
sure) to airflow through the grain is the most signifi-
Aeration speed is analogous to grain quality insur-
cant design factor.
ance. Slow cooling may be cheaper, but if grain
Airflow path in the bulk – Many of the recom- spoils, slow cooling is false economy. Good aeration
mendations on design and operation of ducts for economy is what provides grain managers with high
grain aeration systems are empirical rules for duct quality grain in any geographic location.
spacing and air velocities in the ducts. The aim is to
Because airflow and power requirements for grain
keep air paths through the grain as nearly equal in
depths exceeding 100 ft (30 m) become excessive,
length as possible. If there is a path that is signifi-
reduced airflow rates of 0.03 to 0.05 cfm/bu (2 to 3
cantly shorter than the others, an excessive amount
(m3/h)/tonne) may be required. Doubling the airflow
of air will flow through the shorter air path. The
rate triples the required static pressure while fan
longest path should be less than 1.5 times the length
power is increased by over four times.
of the shortest path, though larger variations in path
lengths may be used with satisfactory results in small
An excellent alternative to consider on concrete silos
dry grain bins.
with strong roof structures is to use a two-fan, “push-
Fan characteristics – pull’ system. With a roof-mounted fan pushing air
The performance of fans
down and a duplicate-base mounted fan pulling
is graphically represented by plotting airflow rate
air down, each fan only has to overcome the resis-
on the ordinate, and static pressure on the abscissa.
tance of half the grain depth. Higher airflow can be
The graph of this relationship between airflow rate
achieved at reasonable static pressures and costs.
4 K-State Research and Extension
Chapter 11 | Aeration of Grain
Air duct velocities –
To minimize friction loss power requirements (hp/1,000 bu) vs. depth (ft) for
in ducts, a compromise between duct diameter and wheat, maize (shelled corn), sorghum and soybeans,
air velocity is made. In aeration ducts, maximum respectively (Navarro and Noyes 2002a).
velocity should be at or below 2,000 ft/min (600 m/
min). For transition and supply ducts up to 20 ft (6 A Windows program called FANS (Minnesota
m) long, velocity could be 2,500 ft/min (750 m/min) Extension Service 1996), provides valuable design
or less. Transition ducts should have a taper (slope) assistance for fan type, size, and power selections and
of 20° or less. For 45° to 90° elbows, the centerline static pressure required based on desired airflow, bin
radius of curvature should be at least 1.5 times and diameter, grain depth and grain type. This software
preferably 2.0 times duct diameter. Joining two 45° contains performance data on over 200 fans listed
elbows to make a 90° elbow is acceptable practice. by manufacturer and fan horsepower. The National
Institute of Agricultural Technologies of Argentina
Air distribution systems –
The ratio of length of (INTA) has also developed software, named AireAr,
the longest airflow path to the shortest airflow path for sizing and selecting grain aeration fans (Bartosik
should be 1.5:1. Positive pressure systems have a et al. 2009). The user can select, round flat bottom or
more uniform airflow distribution and are preferred coned bottom, and between leveled grain surface or
over negative pressure systems in horizontal storages. grain peak, and enter its dimensions as well as the
The exit velocity from the perforations should not grain depth.
exceed 30 fpm (9 m/min).
Operating Aeration
Intakes and exhaust –
In general, roof vents
Systems
should be equally spaced around the circumference
of the roof at about 1/3 to 1/2 the distance up the slope
Direction of air flow –
from the lower edge. Bins with sealed eaves need The question of whether
roof ventilators spaced around the roof , which pro- air should be pushed or pulled (sucked) through
vide at least 1 square foot of roof vent opening per grain is a subject of controversy that has caused
800 to 1,000 cfm of airflow with a minimum of two much discussion. As with most processes, there are
vents per bin. Bins should have at least one vent near significant advantages and disadvantages in selecting
the peak to provide natural ventilation from lower a specific aeration method. The designs of aeration
vents to upper vent. This will minimize moist air systems involve many variables, so it is important
accumulating in bin peak and going up downspouts. to recognize when the advantages of up flow versus
Downspouts should have gravity flap valves to down flow, or pressure versus suction, outweigh the
minimize moist air entry during pressure (up-flow) disadvantages. Either pressure or suction airflow
aeration, which causes condensate dripping into the could be used in most grain storage structures, and
grain. One or more vents should be located near the most aeration systems can be adapted for pressure or
peak to minimize moist air condensation in down suction airflow depending on the specific situation.
spouts used for filling the storage. The vent cross sec-
tion area should be sized preferably for an air veloc- There are two conditions where pressure airflow
ity of 1,000 ft/min (300 m/min), with a maximum should be used: (1) in regions where aeration roof
velocity of 1,500 ft/min (450 m/min). The pressure vents can become iced over because of freezing rain
difference between the headspace of a storage bin or or heavy snow and (2) when warm grain has been
silo and outside should not exceed 0.12 inch water loaded on top of cool grain. Suction systems are not
column (30 Pa) during either pressure or suction used in the central and northern U.S. Corn Belt
aeration. Higher pressure differences may cause because of the many roof collapses that occurred
structural damage and is an indication of inadequate from 1950 to 1970 before the grain industry recog-
exhaust area. nized that suction airflow was not satisfactory.
Estimate static pressure and fan power
Situations that are frequently encountered conform
requirements –
To select the proper aeration fan to the following guidelines:
for the system to be operated at a specific airflow
rate [cfm/bu - (m3/h)/tonne], knowledge of static • Suction airflow provides quick early cooling of
pressure requirements is essential. Figure 1 provides the top of grain where insect populations are
static pressure (inches of water column) and fan heaviest.
Stored Product Protection 5
Part II | Management: Prevention Methods
4)5)5)8)10) 15) 4)5)5)8)
1/1/1/1/1/ 1/ 1/1/1/1/
8 (7 (2 (8 (3 ( 9 ( 8 (0 (8 (7 (
50730 6 3952
22111 0 2111
30.0 0.0.0.0.0. 0. 23.0 0.0.0.0.
0.052 (1/20) 20.0
Maize
20.0 0.095 (1/10)
Wheat (shelled corn)
1.196
10.0
of water 1089...000 0.718.1477 0.1108.007.0826 (1/40) of water 7689....0000 0.5402.722 00..006438 ((11//2300))
nches 76..00 0.588 0.039 nches 45..00 0.361
ure, i 45..00 0.392 ure, i 3.0 0.253 0.036 0.024 (1/40)
s s
res 3.0 0.275 0.020 res 2.0 0.181 0.018
p p
c c
ti ti 0.011
ta 2.0 ta 0.108
S S
1.0
0.9 0.072
cfm/bu 0.8
hp per 1,000 bu 0.7
1.0 0.6
0.9
0.8 0.5
13 20 30 40 50 60 7080 100 150 10 20 30 40 50 100 150
Grain depth (ft) Grain depth (ft)
4)5)5)8)10) 15)20) 4)5)5)
1/1/1/1/1/ 1/1/ 1/1/1/
9 (9 (6 (3 (0 ( 8 (0 ( 9 (9 (6 (
49630 65 496
21111 00 211
15.0 0.0.0.0.0. 0.0. 20.0 0.0.0. 0.133 (1/8)
Grain sorghum 0.025 (1/40) Soybeans 0.100 (1/10)
10.0
0.113
9.0 0.756
8.0 10.0
r r 9.0 0.756 0.066 (1/20)
wate 76..00 0.567 0.076 wate 78..00 0.567
of 5.0 0.038 of 6.0 0.050 (1/30)
s s 5.0
e e
ch 4.0 0.378 ch 4.0 0.378
re, in 3.0 0.265 0.019 re, in 3.0 0.265 0.003.0825 (1/40)
u u 0.019
s s
pres 2.0 0.189 pres 2.0 0.189
c c 0.011
ati ati 0.008
St St 0.113
1.0
0.9 0.076
1.0 0.8
0.9 0.7
0.8 0.6
0.7 0.5
10 20 30 40 50 60 80 100 150 13 20 30 40 50 607080 100 150
Grain depth (ft) Grain depth (ft)
Figure 1. Static pressure developed at different airflow rates (solid line, cfm/bu) and fan power requirements (dashed line,
hp/1,000bu) for aerating wheat and soybeans (bulk density 60 lb/bu), shelled corn, and sorghum (bulk density 56 lb/bu). A fan
static efficiency of 50% was assumed in the calculation of fan power (compiled from Navarro and Calderon 1982).
6 K-State Research and Extension
Chapter 11 | Aeration of Grain
• Suction airflow should be used to aerate warm fans are either suction or pressure). The payback on
grain when aeration is started during cool such a low-cost ($500 to $1,500) aeration controller
weather, for grain stored in metal bins, or to is usually less than one year.
prevent excess condensation under the headspace
roof. For systems where grain has to be dried in storage
• Suction airflow should be used in tropical or (in-bin drying), conditioned to a specific end use
subtropical humid climates when cool weather (e.g., popcorn to optimize popping volume) or mar-
conditions are marginal for insect control. ket moisture content (e.g., soybeans harvested too
• Pressure airflow should be preferred in large flat dry), or where weather conditions are highly variable,
storages for uniform airflow. a microprocessor-based aeration and low-tempera-
• Pressure airflow is required when loading warm ture drying controller is preferred. The payback on
grain on top of grain already cooled such as (a) such a controller ($1,500 to $3,000) is usually less
when loading warm grain from a dryer on top of than one year when critical end-use quality factors
aerated grain in a storage bin or (b) when load- are considered.
ing warm grain delivered to an elevator on top of
Operating aeration based on humidity controls may
a bin that has been previously cooled.
reduce the aeration fan operating time excessively. If
• Pressure aeration can usually be performed
humidity control is used, the aeration management
regardless of the air humidity because the
plan must provide adequate fan operating time to
mechanical fan compression heat reduces the
complete the aeration cycle within a target time; fan-
relative humidity of air entering the grain mass
operating time should be monitored and the control
somewhat, depending on storage and fan sys-
scheme modified as needed during the aeration
tems. Heat of compression adds about 0.75 to
season to insure adequate, timely grain cooling.
1°F per inch static pressure increase.
• Pressure airflow minimizes or eliminates the risk Monitoring ambient air and use of comput-
of roof collapse from icing of aeration vents. er aid to predict aeration system perfor-
mance –
Aeration control equipment – One reason automatic aeration controllers
Essentially,
have often been abandoned by stored grain managers
aeration controllers are electrical system control
soon after installation is the inadequacy of the fan
devices designed to provide automatic starting
control strategy to accommodate local weather con-
and stopping of aeration fans based on selected tem-
ditions. Before implementing any automatic control
perature and humidity levels deemed suitable
strategy, local historic weather records should be
for the aeration program. Existing control systems
evaluated to determine whether a planned strategy
may be categorized as follows: simple mechani-
guarantees sufficient fan operation to achieve desired
cal time controllers; thermostats without relative
control objectives. Ten years of historic weather
humidity control; complex electro-mechanical
records are a minimum for evaluation; 20 to 30 years
controllers with humidity control; temperature dif-
is recommended (Arthur et al. 1998, Arthur and
ference controllers; wet bulb temperature controllers;
Siebenmorgen 2005).
proportional time controllers; and microprocessor
and computer-based temperature monitoring and
Computers are an ideal platform with which to
aeration control systems.
model grain storage management systems and strate-
Selecting aeration controllers – gies (Arthur et al. 2001). Computer models can be
Use of auto-
utilized to study the physical and biological param-
matic aeration controllers that minimize exces-
eters involved in grain storage and establish realistic
sive aeration will result in savings by more precise
operating parameters to implement best stored-
minimum cooling cycles, which will reduce grain
grain-quality management practices. Numerous
market weight loss, grain damage due to spoilage
computer programs have been developed throughout
(self-heating) and insect infestation, end-use quality
the world for this purpose.
loss, and aeration fan electrical operating costs (Reed
and Harner 1998b). As long as grain temperature Time required for cooling –
A family of curves
control is the primary objective, a simple low-cost
to describe several variations of temperature change
electromechanical aeration controller may suffice to
from 77°, 86°, 95°, and 104°F (25°, 30°, 35° and
control all the fans at one installation (assuming all
40°C) to ambient temperatures of 50°, 59°, and 68°F
Stored Product Protection 7
Part II | Management: Prevention Methods
400
From 77° to 50°F
From 77° to 59°F
350
From 77° to 68°F
300
From 86° to 50°F
From 86° to 59°F
From 86° to 68°F
n
o 250
i
t
a
r
e
p
o
n
a From 95° to 50°F
f
f 200
o
s From 95° to 59°F
r
u
o From 95° to 68°F
H
150
From 104° to 50°F
From 104° to 59°F
From 104° to 68°F
100
50
0.017 0.05 0.10 0.15 0.20 0.25
Airflow rate (cfm/bu)
Figure 2. Calculated family of curves showing the aeration time needed for reducing wheat (at 12% moisture content wet-basis)
temperature from 77°, 86°, 95°, and 104°F to ambient temperatures of 50°, 59°, and 68°F at 64% relative humidity (Navarro
and Noyes 2002a).
(10°, 15° and 20°C) at 64% relative humidity is pre- cially in geographical regions with marginal ambient
sented in Figure 2. This family of curves clearly indi- temperature conditions. If airflow rates are increased
cates that by reducing or increasing the airflow rate above 0.15 cfm/bu [10 (m3/h)/tonne], cooling capac-
beyond certain limits, the aeration time needed to ity becomes progressively less effective. At higher
cool grain satisfactorily may exceed practical limits. aeration airflow rates, which are needed where the
hours of cooling weather are marginal, for each
At a low airflow rate, below 0.017 cfm/bu [1.0 increment in airflow rate, the cooling time becomes
(m3/h)/tonne], the aeration time will exceed 600 to less pronounced (the lines are asymptotic).
700 h, which is not practical for grain cooling, espe-
8 K-State Research and Extension
Chapter 11 | Aeration of Grain
Chilling Grain with
The initial grain temperature and ambient air con-
ditions are the primary factors that influence the Refrigerated Air
curves shown in Figure 2.
Steel bin roof venting – There are some storage situations where ambient air
Moisture condenses
conditions are not suitable to cool grain. For these
inside cold spouts and runs back onto the surface
situations, refrigerated air units for chilling grain
grain. Installing one or two vents close to the center
have been developed for commodities that justify the
fill point will help minimize condensation in the bin
added expense of refrigerated aeration. In refriger-
fill pipe. In pressure aeration, the roof vent system
ated aeration, ambient air is passed through the
must be designed with sufficient cross-sectional
evaporator coil and a secondary reheat coil of the
area to allow adequate exhaust or inlet air volume
refrigeration unit, and then is blown into the grain
to maintain vent throughput velocities of 1,000 to
bulk using the existing aeration system. Passage
1,300 fpm (300 to 400 m/min).
through the secondary reheating coil adjusts the air
The vent opening area should be divided into several relative humidity to 60% to 75% to match the target
equally spaced vent units based on the customary moisture content of the dry grain. The amount of
design practices in the area. reheating and the final air temperature are adjust-
able by the operator to achieve the desired aeration
Roof exhaust fans to minimize condensa-
conditions.
tion –
To minimize humid exhaust air roof con-
densation during up-flow or pressure aeration, high Evaluation of Aeration
volume propeller type roof exhaust fans can be
System Efficiency
installed. Roof exhausters should be sized to provide
a total air volume of 1.5 to 2 times the aeration fan
system airflow in order to draw in excess ambient air Aeration efficiency includes uniform air distribu-
to dilute moist exhaust air, lowering the dew point tion through the stored product, sufficient airflow
of the total air mass exhausting through the roof to maintain temperature and moisture, and mini-
fans. When roof fans are used, fresh air is pulled mal energy loss due to improper selection of fans,
in through the roof vents, mixed with the cooling motors, and ducts. Aeration systems may perform
air moving upward through the surface grain and less efficiently than originally planned; low system
exhausted through the roof fans. Thus, the drier, efficiency often goes undiscovered until long periods
diluted air mass that contacts the under side of bin of aeration have failed to produce the desired cooling
roof sheets is less likely to experience condensation. results. Many factors may be involved in the mal-
function of an aeration system. The main problems
If roof vents are mounted about 1/3 of the roof slope are faulty system design, improper system operation,
distance from the peak, roof exhaust fans should excessive dockage accumulation in certain regions
be spaced about 2/3 to 3/4 of the roof slope distance of the grain bulk, faulty fans, rusted out sections of
from the peak, and mounted symmetrically around transition ducts causing air leaks, molded grain layers
the roof. If two fans are used, they should be placed from moisture migration which restricts airflow or
opposite each other on the roof. Three fans should gradual duct blockage by foreign material and fines.
be spaced 120 degrees apart, four fans, 90 degrees
apart, six fans at 60 degree intervals and eight fans, Aeration system efficiency should be tested when a
45 degree angular spacing. new installation is first operated or any time mea-
sured cooling times are longer than those calcu-
A major problem can occur when roof fans becomes lated initially. Aeration system efficiency should be
imbalanced, the vibration can cause serious structural rechecked after any major change, such as installing
damage to steel bin roofs, causing water leakage and a new fan, improving aeration ducts, or when storing
grain spoilage. Roof mounted fans must be checked grain different than the type or quality of the grain
for fan blade balance and vibration before each stor- for which the aeration system was designed. Mea-
age season, as well as periodically during the aeration surement of the airflow rate and static pressure of the
season. system are important procedures in evaluating the
aeration system efficiency.
Stored Product Protection 9
Part II | Management: Prevention Methods
Measurement of static pressure – The U-tube the unit of grain volume (bushels = 1.244 ft3) of the
manometer is probably the simplest device for mea- commodity will give the airflow rate in cfm/bu.
suring static pressure. The U-tube is a glass or plastic
tube partially filled with water or special gauge oil A straight section of the supply duct, at a speci-
(for low temperatures) in which the pressure is read fied distance (usually in numbers of pipe diameters,
directly in inches, cm or mm of water column. The e.g. 10 pipe diameters of straight pipe) downstream
reading is taken by measuring the difference in the from the fan provides a preferred airflow measure-
liquid levels of the two parallel tubes to determine ment position. But, in practice, convenience governs
the aeration system resistance pressure. The internal the position at which measurements are made to
diameter of the tube should be at least 0.2 to 0.24 determine airflow rate; air velocity readings can
inch (5 to 6 mm), and the walls perfectly clean. A also be taken in front of the fan entry orifice, a roof
small diameter hole (0.06 to 0.2 inch) (1.5 to 5 mm) door opening or roof vent in vertical bins. Thermo-
should be drilled in the side of the airflow transition anemometers (also called hot-wire anemometers),
or connection duct (Figure 3). This static pressure if properly calibrated, are suitable for airflow mea-
access hole should be connected to the U-tube with surement. They are suited primarily for measuring
a flexible connecting tube. One end of the U-tube relatively low velocities such as 10 to 2,000 ft/min.
must be open to atmosphere when reading static Windmill or rotary vane anemometers are also used
pressure. for taking a series of grid pattern readings across fan
openings to determine the average air velocity enter-
ing or exiting the fan; the average air velocity multi-
plied by the fan opening cross-section area gives an
estimate of air volume.
Fan efficiency –
Although fans are selected on the
basis of performance ratings and the recommended
Supply duct
fan selection range is supplied by most manufac-
turers, their “installed” operating performance and
efficiency may be substantially different than that
listed in the manufacturer’s fan performance charts.
Flexible tube Therefore, during the first operating stages of a new
installation fan efficiency should be evaluated. With
12
the difficulties and inaccuracies that may occur in
8
determining fan efficiency under field conditions,
4 early fan performance testing provides an excellent
12 inch water
0 initial evaluation to ensure that the fan performs as
column
designed. Such evaluations may be performed in an
4
installation where the required power to operate the
8
system is significantly greater than those specified in
Water
12 Figure 1. Standard fan performance data obtained
from tests conducted at officially approved “certifica-
tion” laboratories are sometimes available and should
be more accurate than field evaluations under similar
static pressures.
Figure 3. Static pressure measurement using U-tube
manometer.
Using modern aerodynamic science and technology,
manufacturers have developed, high performance
Measurement of airflow rate –
For conve-
fans with efficiencies as high as 80%. Conversely,
nience in the United States, the unit of measure for
poorly designed, improperly manufactured, or poorly
airflow used here will be (ft3/min)/bu (cfm/bu). The
selected fans may have efficiencies as low as 15% to
volume of airflow may be determined by multiplying
20%. Low fan efficiency will result in aeration system
the average velocity (ft/min) by the cross-sectional
failure and serious monetary losses.
area (ft2), at the same point of airflow measurement.
The unit volume of air, ft3/min (cfm), divided by
10 K-State Research and Extension