Channel Aeration

Description

For most applications, channel aeration floors are sufficient for grain condition protection. Channel aeration floors allow for air flow of between 80-150 liters per cubic meter of storage, per minute (expressed as 80l/m³/min – or in US terms 0.8CFM/bu/min [0.8 cubic feet per minute per bushel])

Who should 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 aeration in your silos – to preserve the quality of the grain in the silo. There is a rule in grain storage, that says that grain leaving your silo, should always be in better condition than it was when it entered the silo – and you achieve that by conditioning your grain.

A commonly asked question is – “how does the air get to all the grain with a channel aeration system? It looks like the bottom grain isn’t going to get any air – because the channels aren’t blowing over the entire floor?”
The answer lays in the physics of the air flow – as it is more difficult for the air to flow through all of the grain above it, the first path that the air chooses to flow is sideways – untill the pressure in the bottom of the silo is equal to the pressure to move upwards – when it is equally difficult for the air to move sideways as it is to move upwards, it starts moving upwards, creating what is called the cooling front – this the grain in the silo will always start cooling from the bottom, and work itself up. The following graphic explains it rather well. In terms of reaching all the grain in the silo with the airflow, there is no difference between a channel aeration and a full floor aeration system – both systems are able to get the air where it needs to go.

Why do we aerate? 
Temperature control is the short answer – but here is a better explanation, so that you understand what’s going on in the silo:

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 – which is what channel aeration floors are designed to give you. If you want to be able to achieve in bin grain drying, or if you have extremely high fluctuations in temperatures – full floor aeration may be required – to see more information on full aeration floors – see here: FULL FLOOR AERATION 

The airflow will depnd 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 size and type of aeration floor. Make sure that your aeration channels are wide and long enough to provide enough surface area on your aeration planks to ensure that there is sufficienet space for the air to be able to get through the planks and into the grain. Aeration planks typically have an effective opening of around 20% (surface area of holes compared to the plank). Usually your silo supplier would calculate the required airflow – for your requirements – and design the aeration system and the channels in the silo foundation according to the calculation.
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.