World Grain - April 2008 - 63

GRAINOPERATIONS

Shelled maize final kernel moisture content
(Equilibrium Moisture Content = EMC)
% Relative Humidity
Air temp F
20
30
40
50
60
70
80

35%
11.2
10.8
10.5
10.1
9.7
9
8.3

40%
11.7
11.3
11
10.6
10.2
9.7
9.1

45%
12.7
12.2
11.7
11.3
10.9
10.4
9.8

50%
13.7
13.1
12.5
12
11.6
11.1
10.5

55%
14.5
13.9
13.3
12.7
12.1
11.5
10.8

60%
15.1
14.6
14
13.3
12.7
12
11.2

65%
16.2
15.5
14.8
14.1
13.4
12.8
12.1

70%
17.1
16.4
15.5
14.8
14.2
13.5
13

75%
18
17.4
16.6
15.8
15
14.5
13.9

80%
19.6
18.7
17.8
16.9
16
15.4
14.8

85%
21.1
20.2
19.4
18.6
17.8
16.8
15.8

90%
23.5
22.5
21.5
20.5
19.5
18.5
17.4

95%
25.8
25
24.2
23.4
22.6
21.3
20

Source: SafeGrain

degrees F/9 to 12 degrees C and a 1/5
(0.20) CFM system will require about
40 hours of continuous fan operation for
the same temperature change.
Warming the grain above 48 degrees
F/9 degrees C will increase potential
mold growth opportunities. Moving
grain with a greater than 15 degrees F/9
degrees C air/grain temperature differential increases condensation or sweat
potential. Choosing the lesser of two
evils is a management decision. Monitoring the grain temperature will tell the
operator if the grain is stable or not.
Exhaust fans may be run without main
fans on hot summer days to prevent condensation caused by temperature differentials of over 15 degrees F/9 degrees C
between the ambient temperature and the
dead air space inside the silo. Condensation is completely avoided if the dead
air temperature and the roof temperature
are the same. Wheat arriving at the facility during the summer harvest seems to
benefit from aeration even when cooling

is not taking place, as insects do not like
an environment with moving air.
EFFECTS OF RAIN AND HUMIDITY
Questions about operating aeration
fans while it's raining are often asked.
The answer is it depends on how long
the rain is expected to continue. Keep
on aerating with a positive system and
do not start with a negative system if the
rain is expected to last less than 12 to
24 hours. If negative air is being used
when the rain starts, the aeration should
be stopped if the rain is expected to last
more than 6 to 12 hours. The negative
aeration fan system should be restarted
as soon as possible after the rain ends.
The cause for concern is much lower
with positive aeration systems, since a
positive aeration fan will add a 2 to 5
degree F/1 to 3 degree C heat rise to the
air entering the bin. The air molecules
are compressed, creating friction (thus
heat) between the air molecules as a factor of the tunnel velocity of the air. The

temperature increase will cut the relative humidity of the air entering the bin
under pressure. The warmer air will thus
cause a slight drying effect.
A general rule is that humidity is cut
in half with every 14 degrees F/9 degrees
C temperature increase, so it is recommended to continue aeration in a positive
system even if it starts to rain if the ambient temperature is expected to remain
stable. Negative aeration fans do not add
any heat to the air pulled into the bin, and
it really becomes a management decision
to continue or stop aeration if extended
rain is in the forecast based on the expected duration of high humidity. The
flexibility of positive aeration systems
with roof exhaust fans in rain is just another reason to justify the added expense
WG
of a roof exhaust fan system.
Scott Chant is president of Safe-Grain/
Maxi-Tronic Inc. He can be reached at
schant@safegrain.com.
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