Protected Cultivation and Secondary Agriculture
Table of Contents
Important Engineering properties such as physical, thermal and aero hydrodynamic properties of cereals, pulses and oilseed,
Their application in PHT equipment design and operation.
Physical properties:
These properties describe the Particle Size and Shape, volume, surface area, weight, Bulk Density, porosity and Coefficient of Friction of the grains. These properties affect how the grains fit, pack, flow and occupy space in machines and equipment. They also affect how the grains interact with heat and moisture during processing operations.
- Bulk Density:
- This refers to how densely packed the agricultural material, like cereals, pulses, and oilseeds, is. It helps determine the storage capacity and how efficiently they can be transported.
- The density of grains can range from 600 to 1200 kg/m3 for cereals, 800 to 1400 kg/m3 for pulses and 900 to 1500 kg/m3 for oilseeds.
- Particle Size and Shape:
- The size and shape of individual grains impact how they flow and pack together. Proper particle size distribution ensures uniform handling and storage.
- The size and shape of cereal pulses and oilseeds vary depending on the type of cereal crop, the growing conditions, and the processing methods. They can range from a few millimetres to several centimetres in length and width and have different shapes such as round, oval, flat, or elongated.
- Porosity:
- This represents the number of empty spaces or pores within the crop material. It affects the ability to retain air and moisture, which is crucial for maintaining quality during storage.
- The porosity of grains can range from 30 to 50% for cereals, 35 to 55% for pulses and 40 to 60% for oilseeds.
- Coefficient of Friction: This determines how much resistance grain particles experience when moving on surfaces. It's essential for designing equipment used in grain handling and processing.
Thermal properties:
These are the properties that describe the amount of heat required or released by the grains during heating or cooling processes. They also describe the rate of heat transfer between the grains and the surrounding air or water during processing operations. These properties depend on the moisture content, composition and structure of the grains.
- Thermal Conductivity:
- This refers to how quickly heat passes through the material. Understanding this property helps design drying and storage systems that can effectively control temperature and moisture levels in the crops.
- The thermal conductivity of grains can range from 0.1 to 0.3 W/mK for cereals, 0.15 to 0.4 W/mK for pulses and 0.2 to 0.5 W/mK for oilseeds. The latent heat of vaporization of grains can range from 2200 to 2500 kJ/kg for cereals, pulses and oilseeds.
- Specific Heat Capacity:
- This is the amount of heat required to raise the temperature of a unit mass of the crop material by one degree Celsius. It influences the energy needed for drying and heating processes.
- For example, the specific heat of grains can range from 1.2 to 2.5 kJ/kg K for cereals, 1.4 to 3.0 kJ/kg K for pulses and 1.6 to 3.5 kJ/kg K for oilseeds.
Aero-hydrodynamic properties:
These properties describe the motion and behaviour of the grains in air or water streams. They also describe the force exerted by air or water on the grains during pneumatic or hydraulic transport, separation and cleaning processes. These properties depend on the shape, size, density and surface characteristics of the grains.
- Terminal Velocity: This is the maximum speed at which a grain particle falls through air or water. Understanding this property helps design equipment for grain cleaning and sorting.
- Aerodynamic Coefficient: This affects the resistance experienced by grain particles when exposed to airflow. It's crucial for designing air distribution systems in grain dryers and aeration bins.
- Moisture Diffusivity: This determines the rate at which moisture moves through grains during drying and storage. Knowing this property is vital for designing efficient drying and moisture control systems.
- For example, the drag coefficient of grains can range from 0.4 to 0.8 for cereals, pulses and oilseeds. The terminal velocity of grains can range from 1 to 10 m/s for cereals, pulses and oilseeds. The Reynolds number of grains can range from 10 to 10000 for cereals, pulses and oilseeds. The friction coefficient of grains can range from 0.2 to 0.6 for cereals, pulses and oilseeds.
Their application in PHT equipment design and operation
- Separating and grading: The size and shape of the grains affect the size and shape of the sieves or screens. The density and drag coefficient of the grains affects the air velocity and pressure for aspiration or blowing. The terminal velocity and Reynolds number of the grains affect gravity separators' settling or rising behaviour. The colour and surface characteristics of the grains affect the sensitivity and accuracy of colour sorters.
- Handling and storing: The weight and bulk density of the grains affect the capacity and power requirement of conveyors and elevators. The porosity and thermal conductivity of the grains affect the airflow and heat transfer rate in bins and silos. The moisture content and specific heat of the grains affect the storage temperature and humidity conditions.
- Drying and dehydration: The moisture content and latent heat of vaporization of the grains affect the drying time and energy requirement. The thermal diffusivity and thermal conductivity of the grains affect the temperature distribution and moisture gradient in the grains. The drag coefficient and terminal velocity of the grains affect the fluidization behaviour and bed height in fluidized bed dryers.
- Temperature and Moisture Control: Knowledge of thermal properties enables operators to control temperature and humidity conditions in drying and storage equipment, ensuring the safety and quality of the produce.
- Airflow Management: Aero-hydrodynamic properties help operators optimize airflow distribution in drying and aeration equipment, ensuring efficient and uniform moisture removal during drying and storage.
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