Full floor aeration

Description

For some specific applications and for retrofit aeration systems, full aeration floors may be neccesary. Full floor aeration systems allow for air flow between 200-500 liters per cubic meter of storage, per minute (expressed as for example 200l/m³/min – or in US terms 0.2CFM/bu/min [0.2 cubic feet per minute per bushel])

Who should be using full aeration floors to aerate grain?
There are some considerations on you climate, but in Sub-Saharan environments, the rule of thumb is, if you are planning on storing grain in your silo, for a time exceeding 6 weeks, it’s probably a good idea to install a type of aeration system in your silos – to preserve the quality of the grain in the silo. Usually channel aeration systems would be sufficient for preservation of the grain.

Full aeration floors are typically installed in the following instances:
a) When you want to dry-aerate your grain
b) When you want to do in-bin drying of your grain
c) Installations as cooling bins – specifically for dryers that aren’t hot/cool type dryers – thus when your getting grain from your dryer at high temperature, and you need to cool that grain rapidly
d) Temperature control
e) Retrofit aeration systems – where the original silo did not have aeration installed – but there is now a need to install aeration. Full floor aeration systems are generally less expensive to install over an existing silo foundation, than cutting into, or recasting the existing foundation.
f) To save on civil works costs – or to make foundation construction easier – because full aeration floors don’t require channels in the silo foundation, the foundation design and construction is much simpler, and somewhat cheaper – sometimes making it an attactive option for especially clients who will be preparing civil works themselves, or in remote areas, where skilled construction workers are scarce.

How does the air flow in a full aeration system  
As with channel aeration systems – the air is blown in from the bottom of the silo, through the full aeration floor, and the air creates a cooling/drying front (depending on your application).
As the air is introduced across the entire floor, there isn’t a specific sideways movement of the grain in the bottom of the silo, as the pressure accross the entire surface of the silo will be equal, and thus the air immediately starts making the front through the grain in the silo. Typically full aeration floors will be installed with higher capacity fans, allowing higher airflow, and higher pressure, resulting in quicker front movement through the grain. 

Dry – aeration
Dry aeration is a process where the grain is dried in the silo, using no additional heating – by just blowing ambient air through the grain over extended periods of time. Depending on your climate, this is a cost effective way to be able to harvest your grain a bit wetter than usual, and having the ability to reduce the moisture content of the grain in the silo – adding flexibility to your faming operations.
When dry aerating – it is preferable to have an airflow of around 200 to 500 l/m²/min installed in the silo. Maximum harvesting moisture content should be in the region of 16% – and dry aeration will allow you to efficiently dry the grain to an acceptable 13.5% – drying times will differ – depending on the ambient air’s relative humidity during the drying time.

In-bin drying
In bin drying – is similar to dry-aeration, but with the addition of heat to the air that you’re using to aerate the silo. Air can be heated with LP or natural gas burners, diesel burners with heat exchangers, coal fired burners with heat exchangers, or biomass burners with heat exchangers. Heat exchangers are neccesary to prevent smoke and fumes entering the silo and contaminating the grain. Gas burners burn completely clean – and don’t require heat exchangers – and are widely used in Eurasia and the US – as those areas have abundant gas reserves. In Sub-Saharan Africa – diesel and coal are the most prevalent – while in South America – biomass is also popular.

When doing in bin drying – the handling depends on the air temperature that you are pushing into the grain – for low temperature drying – by heating the air to around 40ºC-50ºC – there is low risk of grain damage, by just blowing the air through the entire silo. When using high heat applications – for temperatures up to 90ºC – it is best to install stirring devices in your grain to prevent grain damage.

Stirring devices or sometimes call stirrators, are installed to stir around the grain in the silo, primarily bringing grain up from the bottom of the silo to the top, preventing the warm air from creating channels in the grain that it flows through ,heating those channels up excecssively – causing heat fractures in the grain.

Cooling silo
Most modern continuous dryers offered in the market today are hot/cool or mixed flow type dryers – which basically means that  the dryers have two compartments – the first compartment where the grain is heated up with large volumes of heated air – (up to 120ºC air temperature) where most of the drying happens in the grain – and then after the heating of the grain, the grain then enters the cooling compartment, where large volumes of ambient air is passed through the grain to cool the grain to ambient before exiting the dryer. In the case of dryers that don’t have a cooling section – it is neccesary to cool the grain after it exits the dryer – by offloading the grain to a cooling bin, and then aerating the grain to cool it down, to be suitable for storage.

Temperature control
Lower temperatures in the silo, keep biological activity in the silo controlled – specifically insect and fungal activities.
Aeration is a type of temperature control in the silo, to prevent moisture migration – most losses in silos are due to moisture migration. Grain is a good heat isolator, so it doesn’t cool off very quickly in the silo, especially in winter times, when temperatures outside rapidly drop. The air near the silo wall starts cooling off due to the colder outside temperatures, and the cold air starts to descend to the bottom of the silo (cold air descends – warm air rises – think of hot air balloons). As the cool air descends, it causes a convection stream of air through the silo – because as the cool air gets into contact with the warm grain in the centre of the silo, it heats up again, and starts rising.

As the heated air moves through the grain, it starts picking up moisture from the grain, and when the heated, moist air gets to the top of the grain, it comes into contact with the roof sheets (which are cold – from the cold outside temperatures) and the air starts cooling, causing condensation in the roof cavity of the silo. This condensation causes water to drip onto the top middle heap of grain in the silo, causing the grain to go bad.

See here to understand the typical airflow – causing the convection flows in the silo

The second place where you’ll find damage to your grain from moisture and condensation will be against the southern wall of the silo, which is caused by the hot air in the silo, coming into contact with the cold sidewall sheets, again causing condensation, resulting in grain losses. Sidewall grain losses like this – are probably the worst for the structure of your silo, as the rotting grain, attacks the galvanizing of your grain silo, resulting in the loss of the protective covering of the metal, and with the moist grain laying against the raw metal, rust starts occurring – causing permanent damage to the sidewall sheets, which may require replacement of the sidewall sheets that were damaged.

By aerating, you’re able to control the temperature in the silo – it’s not so much about cooling the grain – it’s about getting the grain inside the silo, to a similar temperature to the temperature outside the silo, to prevent the rapid cooling of the air against the silo sidewalls and roof, and preventing condensation inside the silo. It is suggested to aeration throughout the time that you’re storing the grain in the silo. If you haven’t installed temperature monitoring equipment in your silo, a rule of thumb is to aerate your grain at least once a week for around 2-3 hours – preferable in the early morning hours (04:00 – 07:00) when the ambient air is coolest and dryest (this will typically be after the dew point temperature has been reached throughout the night – allowing the air to discharge moist of it’s moisture)

To emphasise the importance of aerating grain – have a look at the below table – for the safe storage of grain (without aeration) – depending grain temperatures and grain moisture content in the silo – it also shows why it is so important to bring in grain at the correct moisture contents into your silo

How much aeration do I need?
The amount of aeration you need – depends on your requirements – for grain condition protection – airflows of beteen 80-150l/m³/min – are sufficient  – in which case – CHANNEL AERATION is the way to go, but for higher airflow requirements – between 200 and 500 l/m³/min full aeration floors are neccesary.

The airflow will depend on four factors:
a) The size and type of the fans that you install. ABC Hansen would always recommend installing high (2900RPM) centrifugal fans – for most applications. Centrifugal fans create enough pressure to be able to force air through the grain, and provide high air volume. While other type of fans – such as axial fans, can create high airflow, they are unable to create the pressure required for forcing the air through the grain in the silo
b) The type of aeration floor – full aeration floors are designed for certain airflows, with some floors having a higher opening percentage than other floors, to allow for higher airflow.  Make sure that the floor being offered to you is the correct floor for your required airflow –  which is dependant on your silo diameter, silo height and product type. Usually your silo supplier would calculate the required airflow – for your requirements – and design the aeration system for your model silo and requirements.
c) Amount of roof vents – this is an often overlooked item – it should be ensure that there are enough roof vents installed in the silo, to allow the air that you are blowing into the silo, is able to exit the silo – without creating extra pressure in the silo, that the fan has to try to overcome. The rule of thumb on roof vents is to have around 0.18m² roof vent opening per KW fans installed – for example a silo with a 22KW fan installed, will require a total roof vent opening of 3.96m² (22KW x 0.18m²). ABC Hansen’s standard roof vents have an opening of 0.2m² per vent – thus requiring 19 air vents (3.96m²/0.2m²)
d) On very high and narrow silos, it is sometime advisable to also install roof extractor fans – to create a negative pressure in the top of the silo, making it easier for the fans to push the air through the grain.

In our opinion it’s always better to plan for the worst – even if you’re planning on installing a silo for short term storage, the need may arise in the future to store for longer periods, and without an aeration system, it makes the proposition very risky. Aeration systems can also be retrofitted to existing silos though – so keep that in mind.