In recent decades, several technologies have been developed to secure grain storage. Sensor, cable, radar, gas detector and more. As with everything, technological solutions have their advantages and also their limitations.
I often tell people: “Just because you're given a scalpel doesn't mean you're a surgeon. Knowledge first, then tools!” Why? Because some technologies have their limits, like cable monitoring systems, and if you're not careful, you could very well have storage problems.
Increasingly present on the market, monitoring cables for grain silos are taking their place. This technology can measure grain temperature, estimate moisture and approximate the amount of grain in inventory.
In this article, we'll take a look at the physical properties of corn, its benefits, and the challenges that lie ahead if you're not careful.
Planning to adopt a monitoring technology to track your storage bins? We'll introduce you to two. First, temperature cables, then CO2 sensors.
Finally, we'll present a working method you can implement to avoid storage problems.
Our hope is that, after reading this article, you'll be better informed about how to ensure successful storage, year after year..
But first, a little science!
What is thermal conductivity?
Thermal conductivity is a physical property of a material that describes its ability to transfer heat. It is represented by the letter λ (lambda) and expressed in W/(m-K) (watts per meter-kelvin).
What is corn's thermal conductivity?
The thermal conductivity of grain corn is relatively low, making it a good thermal insulator. This property is important to consider during storage, as it limits heat dispersion throughout the kernel mass.
Approximate values of thermal conductivity (λ) of corn in watts per meter-kelvin (W/(m·K))
• Dry corn (12-15% moisture): approx. 0.12 to 0.15 W/(m-K)
• Wet corn (30-35% moisture): approx. 0.18 to 0.25 W/(m-K)
In comparison with other products :
Product | Thermal conductivity (λ) |
Air | 0.024 W/(m-K) (Very good thermal insulation) |
Mineral wool | 0.03 - 0.04 W/(m-K) |
Wood | 0.12 - 0.20 W/(m-K) (Varies according to wood type and moisture content) |
Plastic | 0.1 - 0.5 W/(m-K) (Varies according to plastic type) |
Water | 0.6 W/(m·K) |
Concrete | 1.2 - 2.5 W/(m·K) |
Galvanized steel | 50 W/(m·K) |
Aluminum | 205 - 237 W/(m-K) (Highly thermally conductive) |
Negative points to consider:
• During storage, low heat dissipation limits the ability to track the temperature of the grain mass.
• Low heat dissipation also limits the spread of hot spots and favors localized thermal runaway.
A positive point to consider:
• When the grain is dry and properly tempered, corn's insulating capacity helps protect the grain mass during hot summer periods.
What is an R1 insulation factor?
The R1 insulation factor refers to the thermal resistance (R) of a material expressed in m²-K/W (square meter-kelvin per watt).
Definition of thermal resistance (R)
Thermal resistance is a measure of a material's ability to slow heat transfer. It is calculated by the formula :
Where:
• R = thermal resistance (m²-K/W)
• e = material thickness (in meters)
• λ = thermal conductivity of the material (W/(m-K))
What does “R1” mean?
• “R1” means that the material has a thermal resistance of 1 m²-K/W.
• The higher the R value, the better the thermal insulation.
• A value of R1 is moderately insulating; for example, a 4 cm thick mineral wool slab with a thermal conductivity of 0.04 W/(m-K) would have approximately R = 1 m²-K/W.
Examples of thermal resistances:
Product | Distance (e) | Thermal conductivity (λ) | Thermal résistance (R) |
Mineral wool | 4 cm | 0.04 W/(m·K) | R = 1 m²·K/W |
Expanded polystyrene | 3 cm | 0.03 W/(m·K) | R = 1 m²·K/W |
Corn | 15 cm | 0.15 W/(m·K) | R = 1 m²·K/W |
Corn | 91.44 cm | 0.15 W/(m·K) | R = 6 m²·K/W |
Concrete | 175 cm | 1.75 W/(m·K) | R ≈ 1 m²·K/W |
Compared with the various materials commonly used in construction to insulate buildings, grain is a good thermal insulator.
How long does it take to propagate heat through 3 feet of grain?
The time required to propagate heat through a bed of grain corn can be estimated using thermal diffusivity (α) and the thermal diffusion equation.
1. Calculating thermal diffusivity (α)
Thermal diffusivity is defined as :
where:
• α = thermal diffusivity (m²/s)
• λ = thermal conductivity = 0.15 W/(m-K)
• ρ = apparent density = 680 kg/m³.
• Cp = mass heat capacity = 1.35 kJ/kg-K = 1350 J/kg-K
2. Characteristic heat propagation time
The characteristic time for heat to propagate over a distance L of 3 feet (0.9144 m) is given by :
where :
• t = thermal diffusion time (s)
• L = depth of corn kernel = 0.9144 m
• α = thermal diffusivity calculated above.
To recap:
It would take approximately 59 days (or 5,117,099 seconds) for heat to spread across 3 feet (0.9144 meters) of corn at 14.5% moisture by pure conduction. However, in actual storage, air convection and moisture accelerate this process considerably, especially if a fan is used for drying or aeration.
CÉROM experiment and heat transfer observation
In 1997, the Centre de recherche sur les grains (CÉROM) published a technical bulletin entitled “La conservation des silos de grains sans ventilation de réchauffement printanier”, in which the insulating factor of grain is highlighted.
We can see that it takes nearly 3 weeks for outside heat to penetrate 30 cm (1 foot) of grain and raise the temperature by 10 degrees Celsius. We can also see that at 2.5 meters (8 feet), the temperature remained the same from April to July (3 months).
The limits of cable technology
As mentioned in the introduction, every technological tool has its advantages and disadvantages. Before adopting such a solution, it's important to understand how the grain in storage behaves, so as to know how to use it, as well as its limitations, because you could be in for some nasty surprises.
Cable silo tracking technologies can help you do just that:
• monitoring the cooling front
• estimating the moisture content of stored grain
• estimating your inventory
But they have their limits, because :
• monitoring coverage is limited and localized
• slow response time
• they are prone to breakage
Monitoring the cooling frontt
As you cool your grain, a cooling front gradually propagates upwards. The cables allow you to see the cooling progress. Once the entire mass has cooled, the air leaving your bin's air intakes will be cold.
IMAGE (silo avec sonde et front de refroidissement)
To estimate the cooling time of your bin, you can also use the following formula:
Cooling time of a bin = 15 / unit flow rate [CFM/BU].
Use the University of Minnesota's online calculator to determine your unit flow [CFM/BU].
Estimating the moisture content of stored grain
Contrary to what you might think, the cables don't measure grain moisture, but rather estimate it. Using the principle of equilibrium between air and grain moisture (thanks to the famous equilibrium charts), cables are able to give you an idea of the moisture content of your grain. By measuring air moisture, which is in equilibrium with grain moisture, cable tracking systems enable you to estimate the moisture content of your grain.
As a general rule, the estimates are relatively accurate and provide good control over your grain's moisture. Remember, you'll need a good moisture tester to get a more accurate and official measurement before you sell your grain.
CAUTION! While you're aerating, the air is no longer in equilibrium. You'll have to wait a few hours, or even a few days, to get a new moisture reading for your grain.
Estimating your inventory
Because grain is insulating, there will be a temperature difference between the grain mass and the air above your plenum. Using this principle, you can get a rough idea of your inventory. Although this estimate isn't very precise (5% to 10% of the bin's capacity), you'll be able to get an idea of where you are in your inventories.
Limited, localized monitoring coverage
Due to the low conductivity of grain, moderately fast detection coverage can be expected over only about 2% to 5% of the grain volume. To increase coverage, there is no other option than to add cables, which, of course, drives up the investment cost considerably.
Unfortunately, the cable doesn't measure its entire length, as there is only one probe every 3 feet.
Slow reaction time
Because the distance between cables is relatively large (10 to 16 feet), a problem located halfway between two cables may take several weeks or even months to be detected.
WARNING Many people think that installing an automated aeration system with cable monitoring “makes their storage 100% safe”. But this is absolutely false! Failure to aerate sufficiently, to inspect your bins and to monitor your system exposes you to the risk of loss. In other words, it's as if you'd done nothing at all.
Subject to breakage
The traction exerted by the grain on the cables when emptying your bin is particularly intense. Over time, fatigue sets in, and sooner or later, maintenance will be required.
Not all bins are built to withstand this kind of pull. In fact, the roof of your bin is under intense strain, and if your bin isn't strong enough, it could collapse. Think again.
Secure your storage with CO2 monitoring
All biological activity releases carbon dioxide (CO2). When your grain tries to germinate, it consumes its starch, releasing energy, moisture and CO2. It's the same process when your grain ferments or heats up.
When air circulates in your bin, it transports CO2 to the outside. This air enables the CO2 sensor to detect any abnormal concentration of carbon dioxide and take prompt action. Acting early protects your income.
Working methods to avoid storage problems
Aeration. Yes, aeration. This task, underestimated, neglected, unloved, denigrated - in short, we'll go on telling you about it for weeks, until you're fed up, irritated, sick of it. Be aware that this regular action is in fact the magic wand, the key to your success.
No, it's not sexy, yes it's boring, and yes you have 1 million other things to do, but taking the time to perform this action on a daily basis:
evens out your grain's moisture content
maintains your grain's temperature at a safe level
and improves the overall quality of your grain.
Don't have the time? You've got $500,000 in inventory in 1 bin, and you're not worried? Never forget that your grain is alive, and that in spring, it only takes a few days and a little heat to germinate your grain.
Step #1 - Filling and removing the bin core
Normally, people take out the center of their bin (Silo Core) 4 weeks or more after filling their bin. BAD IDEA! It may already be too late! Do it as soon as possible.
Step #2 - Cooling the grain mass
Have you just put your grain in the bin? Aerate non-stop, rain or shine, to get the heat out of your grain.
Step #3 - Aerate according to grain balance curves
These famous charts are your best friends. They'll help you even out your grain's moisture content without drying it out or dampening it.
Don't have them? Contact us and we'll send them to you free of charge!
Étape #4 - Aerate, aerate, aerate
Basically, you will:
Cool your grain as soon as it's stored in the silo (80-100 hours) Or cool during drying (approximately 240 hours)
Cool your silo in stages during autumn (3x 80-100 hours)
Warm your silo in stages during spring (3x 80-100 hours)
Perform maintenance aeration (30 days at 10 hours)
Save energy? Not really! A 15 hp fan costs about $1.30/hour. So cooling a 2000 metric ton silo costs between $100 and $130.
If your corn is worth $250/MT, is it worth saving $130 and putting $500,000 worth of grain at risk? Not really! If you do this exercise 10 times a year, $1,300 on $500,000 is still very low. And yet, that's a lot of hours of aeration!
In summary, spending between $0.7 and $1.0/MT on electricity is an investment in your company's financial health. Nothing more.
Conclusion
Thinking that a cable system "will tell you when you're going to have a storage problem" is exposing yourself to risk, because as we have seen, grain is insulating and prevents the natural propagation of temperature.
However, regular aeration will make all the difference, as it will help both to standardize the moisture of your grains and remove moisture pockets that could create a storage problem. It will also allow you to control the temperature of your grains and secure your storage.
In the end, if you had to remember just one thing, it's:
Aeration= Security
Complementary Blog: Is your bin properly assembled? What to look out for