Grain Cooling Units

When analyzed on a yearly basis throughout the world, it is seen that grain production is in an increasing trend. According to the latest data of the World Food and Agriculture Organization (FAO) in 2021, grain production increased by 0.7% compared to last year and reached 2.796 million tons [1]. Consumption and storage data also follow production data in a similar way. As seen in Figure-1, when FAO examines the latest data, it can be understood that approximately one third of the produced grain is stored. Considering that production and consumption follow each other in similar amounts, it becomes very important to keep the stored grain in suitable conditions without losing quantity and nutritional value.

When analyzed on a yearly basis throughout the world, it is seen that grain production is in an increasing trend. According to the latest data of the World Food and Agriculture Organization (FAO) in 2021, grain production increased by 0.7% compared to last year and reached 2.796 million tons [1]. Consumption and storage data also follow production data in a similar way. As seen in Figure-1, when FAO examines the latest data, it can be understood that approximately one third of the produced grain is stored. Considering that production and consumption follow each other in similar amounts, it becomes very important to keep the stored grain in suitable conditions without losing quantity and nutritional value.

Figure-1: FAQ World grain production, consumption and storage forecast graph [1]

On the other hand, the development of agricultural technologies has reduced the time between harvest and storage of grain from weeks to a few hours. In this case, the grain may need to be dried and cooled according to climatic conditions. Another issue is the cases where the grain is transported and stored again for long and even intercontinental transport under harsh conditions. In this case, since the grain is exposed to conditions that are very different from the ideal storage conditions, the balance of nutrients and moisture may be disturbed

The life cycle of the grain, stored in the silo, continues. As the life cycle of grain continues, it produces heat and water vapor as a result of respiration. While this chemical reaction continues; on one hand, the nutritional value of the grain decreases, on the other hand, the temperature and humidity value in the silo increases. This situation also triggers the increase of insect and mold fungus that has entered the silo with the grain. The temperature increase in the silo facilitates the reproduction of insects. Considering that the insects in the silo breathe, they accelerate the increase in temperature and humidity. Generally, growth of mold fungus increases above 70% relative humidity. All these adverse conditions trigger each other, causing an increase in temperature and relative humidity inside the silo. As a result, significant losses occur in stored grain. Considering that the grain is a good insulator due to its very low heat transmission coefficient; released heat is kept inside the silo. As a result, the constantly heated environment creates an ideal condition for harmful organisms to reproduce. When the data based on observations and measurements are examined, temperatures above 21 °C generally provide ideal conditions for the reproduction of harmful creatures. It has been understood that there is a significant decrease in the growth rate of insect and other pest populations at temperatures below 16 °C (12 °C wet bulb and 65% relative humidity). Reducing the temperature inside the silo is an important factor in the long-term preservation of the grain in the silo, because it also slows down the life cycle of the grain.

The storage of the grain in the silo depends on many external factors such as ambient temperature, ambient relative humidity, exposure to sunlight, silo structure, as well as internal conditions. The change in ambient conditions during the day affects the stored grain positively or negatively.

Different methods are used for long-term preservation of the grain, stored in the silo. The main methods can be listed as; the use of chemical preservatives, natural ventilation with the help of a fan, and the cooling. Fan assisted ventilation is also called “natural or passive cooling”. It is one of the most widely used method. Briefly, in this method, outdoor air is directed into the silo from the lower part of the silo with the help of a fan. The main purpose of the ventilation method can be summarized as;

o Preserving the vitality of the grain,

o Reducing the internal moisture content of the grain,

o Slowing down insect growth,

o Prevention of mold and fungus formation,

o Reducing the use of chemical preservatives,

For homogeneous air distribution inside the silo, and easy cleaning, when necessary, appropriate grid and duct systems should be constructed on the bottom of the silo. Although, directing the outdoor air into the silo with the help of a fan seems to be an effective method, for ventilating the stored grain, and improving the storage conditions, it is a process that needs to be well managed. According to the environmental conditions, this method has some difficulties and negative effects. The most important ones are listed below:

• Ventilation times and conditions are directly dependent on environmental conditions. For this reason, fans can be operated for limited periods at appropriate ambient temperature and relative humidity values. Considering the recent climatic changes all over the world, these periods can be further restricted.

• Since the air rises upwards from places where the counter pressure is lower, temperature and humidity imbalances may occur inside the silo. The temperature variation within the silo is more constant as the silo diameter increases. However, if the stored grain temperature is higher than ambient temperature, it takes much longer to cool the larger silos. For this reason, smaller diameter silos should be preferred in cold climates and larger diameter silos should be preferred in tropical climates.

• While the grain is being stored in the silo, a slope angle is formed in the upper region depending on the grain type. In this case, the air ascending from the bottom of the silo will not be able to pass sufficiently over the grain in the top region.

• The temperature difference between day and night may cause condensation and dripping from the silo roof to the grain. As a result of the wetting of the grain in the upper region due to the result of dripping, these areas become ideal environments for mold and insect reproduction. In order to prevent this situation, the use of additional exhaust fans on the roof reduces this risk.

In order to improve the difficulties and negativities encountered during the ventilation of the silos with the help of fans, the vapor compression cycle (VCC) method can be used for cooling. In this method, with the help of the compressor, the refrigerant is circulated between the heat exchangers in a closed cycle, transferring heat to the environment, and the air passed through the heat exchanger, which meets the desired conditions, is directed into the silo with the help of a fan. The air taken from the outside is directed into the silo at the desired temperature and relative humidity. Thus, it is ensured that the stored grain is kept in ideal storage conditions for long periods of time. Environment air is conditioned, and directed into the bottom of the silo. The air moves upwards inside the silo and performs the cooling process inside the silo.   The heated air is exhausted back to the atmosphere through the vents on the roof of the silo. During the cooling of the air, the relative humidity value is also controlled.

The cooling capacity, performed using the vapor compression cycle method, can be calculated with the following equation:

W = Δh x Q x ρ_w

W :  Cooling Capacity (kW)

Δh : The enthalpy difference between the ambient temperature and the cooled air (kJ/kg)

Q :   Air flow rate directed into the silo (m³/hour)

ρw:  Density of outdoor air (kg/m³)

The relative humidity ratio is as important as the temperature of the air directed into the silo. Therefore, it must be controlled. If the relative humidity ratio of the air, in the area where the grain is stored exceeds, 70%, it can lead to mold growth, which also causes the temperature to increase due to mold respiration.

Grain is hygroscopic due to its ability to take in and give out moisture from the environment. Water is retained in the grain in three ways; absorbed water, adsorbed water and chemically bound water. The absorbed water is retained within the grain tissues by capillary forces. It is associated with water-soluble components such as sugars, mineral salts, organic acids, and some vitamins that the grain contains. Adsorbed water, consists of gas molecules of water trapped on the surface of porous particulate materials by electrostatic forces. Chemically bound water is present in association with grain components during the developmental growth and maturation of seeds. [4]

Each grain type has a value in which the intra-grain moisture is in equilibrium against the different relative humidity ratios of the air. The curve obtained by plotting these values is called the equilibrium moisture content (EMC) curve. Equilibrium values varies depending on the type of grain, the ambient air temperature, and the relative humidity ratio. EMC represents the moisture content of the grain at which it will eventually stabilize, if weather conditions remain stable for a long period of time. In this way, depending on the ambient conditions of the stored grain, the moisture values of the grain can be estimated or the moisture value of the grain can be controlled by changing the ambient conditions. For this process, psychometry diagram, on which moisture balance content (EMC) curves of different grains, can be used (Figure-2).

In addition, regarding with the most consumed grains, tables showing the EMC, according to the ambient temperature and relative humidity ratio have been prepared. [3], [7] As an example, tables for wheat, and barley are given in Table-1a and Table-1b. The moisture balance value of the grain can be seen from these tables. For instance, for wheat, it is read from Table 1a that the EMC is 14.5% in an environment at 15 °C and 65% relative humidity. The moisture balance value shows the ratio of the moisture mass in the wheat to the total mass.

Table-1a: Table, showing different equilibrium moisture content (EMC) values, ambient temperature and relative humidity values for wheat [3]

Table-1a: Table, showing different equilibrium moisture content (EMC) values, ambient temperature and relative humidity values for wheat [3]

 When air is passed over the grain inside the silo, 3 zones and fronts are formed in the silo. Figure 2 shows schematically the zones formed inside a silo, where the aeration process continues.

Figure-2: Schematic representation of the zones and fronts formed inside the silo as a result of the air directed into the silo [4]

The temperature and relative humidity values in the lowest region are the same as the values of the air directed into the silo (Zone A). At the top region, temperature and relative humidity values are the same as the initial conditions (before the air is directed into the silo) (Zone C). The region in between has a value between the values in the lowest and highest regions (Zone B). The movement of these zones moves in the same direction as the air flow. The fastest moving part of the region is called the leading edge and the slowest moving part is called the trailing edge. As long as the air is directed into the silo, after a certain period of time, the temperature and relative humidity values in the whole silo reach the air values directed into the silo.

Temperature and humidity fronts are formed in the area between Zone A and Zone C inside the silo. The reason for the formation of fronts inside the silo is that the temperature moves relatively faster than the relative humidity. Practically, the temperature advance rate is one thousandth of the air speed. (10^-3). The relative humidity advance rate is 100 times slower than the temperature advance rate. [4]

The variation of the equilibrium moisture content (EMC) for the stored grain in the silo depending on the weather conditions directed into the silo is explained through sample problems prepared by using the book named “The mechanics and physics of modern grain aeration management” [4].

Example 1:

Wheat is stored in the silo at a temperature of 35 °C and containing 12% humidity. If the silo is started to be aerated with air, having 11 °C (dry bulb temperature) and 90% relative humidity ratio, calculate the values of the 3 zones (A, B, C) that will form inside the silo?

Figure 3- Psychometry diagram with different moisture values and equilibrium moisture content (EMC) curves for wheat (2)

The temperature and relative humidity values in the zone C, are equal to the initial conditions in which the wheat was stored. Wheat in this region has a temperature of 35 °C and a relative humidity of 12%. Zone A has the same conditions with the air directed into the silo. When the condition of 11 °C (dry bulb temperature) and 90% relative humidity is marked on the psychometric diagram in Figure-3 (A), it is read that the temperature of the wheat in this region is 11 °C and the equilibrium humidity is 18%.

For 11 °C (dry bulb temperature) and 90% relative humidity, the wet bulb temperature is read 10 °C on the curve. This is also the saturation temperature of the air. In order to calculate the values of the grain in the B zone, the wet-bulb temperature curve in the diagram is matched with the grain moisture equilibrium curve. The temperature read at the intersection point is 15.5 °C. (B1) For a more detailed estimation, the rule that the grain internal moisture balance changes by 1% for every 28 °C temperature difference is applied. By applying this rule, the humidity of zone B is approximately 0.70% ((35-15.5)/28=0.70) lower than zone C is calculated. In this case, the equilibrium moisture content of the grain for the zone B is 11.3% (12-0.7=11.3). In the diagram, the dry bulb temperature is 16.7 °C for 11.3% on the wet bulb curve. Thus, it is seen that the temperature of the wheat for the B region is 16.7 °C and the equilibrium humidity is 11.3%.

With lower airflow rates, the cooling zone between the leading and trailing edges becomes thinner. When air flow rates are higher, the cooling zone becomes wider or thicker due to the reduced contact time of each air particle with an individual grain core.

Since the moisture front closely follows a constant air enthalpy (parallel to the wet bulb temperature), wet bulb temperature is considered a more satisfactory criterion than dry bulb temperature for the control of grain aeration systems. [4]

The cooling time of the grain, stored in the silo, to the desired temperature depends on many variables. However, a simple calculation method that can make an approximate time prediction was introduced by Navaro and Calderon [4]:

F= (M x ΔT x C) / (Q x ρ_w x CF x ΔH)

F: Cooling time (h)

M: Mass of stored grain (kg)

ΔT: The temperature difference between the initial condition of the stored grain and the cooled grain (°C)

C: Specific heat of stored grain (kcal/ (kg °C)

Q: Air flow rate over the stored grain (m³/h)

ρ_w: Average density of cooled air (kg/m³)

CF: Correction factor for enthalpy (0.4 ~ 0.5)

ΔH: Maximum enthalpy difference between the air entering and leaving the silo (kcal/kg)

Using this method, an approximate cooling time can be calculated for the grain, stored inside the silo. If it is proceed through the example; 

Example 2:

5,000 tons of soybean (12% moisture) were stored in the silo. While the outdoor temperature of the silo is 45 °C (30% rH), 25,000 m³/h air is supplied to the silo with 20 °C and 65% relative humidity. How many days will the soybean reach to the desired conditions?

Enthalpy differences are calculated on the psychometric diagram. The average specific heat value for soybean is read as 0.34 from the graph in Figure-4. If all values are substituted in the formula;

Figure 4 – Specific heat plot for different grains according to their moisture content [4]

F = [5,10⁶ x (45 – 25) x 0.34] / [25,000 x 1,225 x 0.5 x (20.5 – 10.5)]

F = 222 hours

F = 9.25 days

As a result of the calculations, it takes approximately 10 days, for the soybean to reach the target temperature value.

Aeration Fan Selection

The air directed into the silo is subjected to a high back pressure due to the stored grain. The back pressure value briefly changes depending on the type of grain, the height of the silo and the air velocity. The grain in the silo creates a porous environment due to its structure and the air moves upwards in this environment. For this reason, small grain products, such as oilseeds, create higher back pressure than large grain products, such as peanuts. When the air velocity increases, the friction losses in the duct also increase, which causes an increase at the back pressure. Increasing the height of the silo increases the counter pressure as it makes it difficult for the air to move in the vertical axis.

While choosing a fan, these factors should be decided. Centrifugal fans are the most suitable choice, as they are exposed to high back pressure values. Based on the observations, it is recommended that the optimum condition is 3 ~ 6 m³/h per ton in the selection of air flow rate. When the silo height exceeds 30 meters, this value can be taken as 2 ~ 3 m³/h per ton. High air velocity improves cooling times while increasing static pressure and electricity consumption. As a general rule; a two times increase in air flow causes a three times increase in back pressure and a four times increase in electricity consumption.

Grain cooler, operating with the vapor compression cycle (VCC) method, have been developed in order to cool the air taken from the outside to the desired conditions, and direct it into the silo. These air conditioners are systems that work with 100% outdoor air. In the vapor compression cycle, the refrigerant circulates in a closed cycle, providing heat transfer. Simple VCC is consisted of the compressor, evaporator, condenser, and expansion valve. In this cooling device, there are additional components such as heating coil and electric heater. The refrigerant in the superheated vapor phase compressed in the compressor is directed to the condenser, where it condenses into the liquid phase by dissipating heat to the external environment. The refrigerant then comes to the expansion valve, depressurized and directed to the evaporator. While the refrigerant flows through the copper pipes in the evaporator, the air taken from the outside with the help of the fan passes between the aluminium fins, and heat transfer from the air to the refrigerant is provided. The refrigerant, which evaporates by taking heat in the evaporator, moves towards compressor. As the temperature of the air passing over the evaporator decreases, the water holding capacity of the cooled air decreases, so some of the moisture in it is condensed and evacuated from the device. Although the absolute humidity in the cooled air decreased, the relative humidity value increased. Since the temperature of the air directed into the silo and the relative humidity are also important, the relative humidity must be controlled before the air leaves the device. For this reason, after the evaporator, the air is directed to the heating coil and the electric heater, respectively. First of all, in the heating coil, where the refrigerant in the superheated vapor phase is directed, the air is heated and the relative humidity is reduced. If this heating process is not sufficient, the air is heated a little more in the electric heater to reach the desired output air temperature and relative humidity. In order to reach the targeted air temperature and relative humidity value in the device outlet air, the air is cooled below the targeted temperature in the evaporator and the excess water in the air is condensed. Afterwards, the air is heated to reach the target temperature and relative humidity.

The air flow is controlled in order to achieve the targeted air output values in the grain cooler. The fan flow rate is increased or decreased according to the air outlet temperature. During the noon hours when the temperature high, the air flow may decrease at certain times, while it reaches the highest flow rates at night.

According to the cooling capacity, the use of one or two compressors in the grain cooler may be preferred. At coolers, having two compressors, energy saving is achieved by turning off a single compressor when necessary, depending on the outdoor conditions.

In order to achieve the targeted precise values, proportionally controlled valves, drivers and measuring instruments, that can be controlled via PLC are used. Grain cooler (grain dryer machine) can be easily adjusted by entering targeted temperature and additional heating for targeted relative humidity. Although it varies slightly according to the relative humidity value of the outdoor environment, it can be said that as a practical rule, every 1 °C temperature increase provides a 5% decrease in relative humidity. For a more precise calculation, the psychometric diagram can be used practically.

It is possible to operate only the fan of the grain cooler, without activating the compressor according to the outdoor conditions. In other words, the inside of the silo can be aerated with the help of a fan. Again, in cases where the grain is stored in the silo at high humidity values, the grain cooler can dry up to a certain temperature value. During this process, the air outlet temperature of the device is increased by directing the high temperature refrigerant to the evaporator.

Evaluation

In today’s conditions, as the need for food increases, the importance of preserving the produced products without loss has also increased. In this context, it has become a necessity to use the most effective methods to preserve the nutritional value of the grain, stored in the silos for a long time, to slow down or even stop the reproduction of harmful organisms inside the silo. The most commonly used method of aerating the silos with the help of fans includes the disadvantages mentioned in the previous sections. The most optimum solution to improve these negativities is the grain cooler (Grain chiller). Thanks to these devices, uninterrupted cooling can be provided regardless of external environmental conditions. After the grain cooler is connected to a silo, it can be operated for 24 hours according to the calculated period, ensuring that the grain in the silo reaches the desired temperature and relative humidity value. Since the grain is a good insulator, the grain can be stored for a long time under these conditions. Afterwards, the grain cooler can be connected to a different silo and the same process can be repeated. When necessary, these devices can be operated in only fan mode, which has the same function with the aeration fans. Additional chemical preservatives are not needed as the insect and mold formation inside the silo is slowed down or even stopped by cooling.

As a conclusion, if the proper selection is done according to the size of the silo, the type of the stored grain, and the targeted cooling times, grain coolers (grain dryer machine) are the most suitable solution that improves the storage conditions.

References

[1]         https://www.fao.org/worldfoodsituation/csdb/en/

[2]         P.Burrill, DPI & F “Aeration for grain cooling and drying: How to use the Psychrometric chart to select suitable air”, https://storedgrain.com.au/how-to-use-the-psychrometric-chart-to-select-suitable-air-burrill-dpiq/

[3]         “Equilibrium Moisture Content Charts for Grain Storage Management” Prairle Agricultural Machinery Institute (PAMI), https://pami.ca/wp-content/uploads/2021/10/Equilibrium-Moisture-Content-Charts-for-Grain-Storage-Management_rev2.pdf

[4]         S.Navarro, R.Noyes, “The Mechanics and Physics of Modern Grain Aeration Management”, CRC Press, 2002

[5]         D.W. Hagstrum, T.W.Phillips, G.Cuperus “Stored Product Protection” Kansas State University, 2012

[6]         www.storedgrain.com.au, “Aerating Stored Grain – Cooling or Drying for Quality Control”, 2013

[7]         S.Sadaka, R.Bautista, “Grain Drying Tools: Equilibrium Moisture Content Tables and Psychrometric Charts”, Agriculture and Naturel Recources, University of Arkansas