In the previous article, Feed Grinding-1, we explored the significance of particle size in feed production, especially focusing on Coarse Grinding, which plays a vital role in enhancing gut health and feed efficiency in poultry. We detailed the components of a grinding station, including the hammer mill, aspiration unit, and plenum chamber, along with engineering insights into equipment design to optimize coarse grinding for animal feed.

Building upon this base, this article will focus on FINE GRINDING—a crucial process for animal feed.  Fine grinding aims for a more uniform and smaller particle size, maximizing digestibility and feed conversion. We will explore the specialized equipment, operational techniques, and technical parameters essential for achieving effective fine grinding in animal feed production.

Particle size plays a critical role in determining the quality of grinding. In feed milling, whether it's for poultry, cattle, or aquatic species, the size of the ground particles influences feed efficiency, nutrient absorption, and overall animal health. Coarser particles are typically used in poultry and cattle feed to enhance nutrient retention, while finer particles are preferred for aqua feed. to enhance nutrient absorption, better digestibility for species with small mouths, Improved feed conversion ratio (FCR), Improved pellet quality and higher pellet density for sinking feeds.

Double Grinding with Two Hammer Mills

For finer grinding needed in feed production, “double grinding” systems can be employed. It uses two hammermills in series, with the first hammermill set to a larger screen size (1.5 to 3 mm) for a preliminary grind, followed by regrinding through a second hammermill with finer screens (0.4 to 1 mm) to achieve the desired fine particle size. Double grinding not only produces a finer and more uniform ground product but also improves overall grinding efficiency over single-pass systems.

Hammer Mill V/s Pulverizer

Compared to air swept pulverizers, hammermills provide high efficiency, low heating, and reduced aspiration requirements. With hard-faced beaters and  surface hardened screens, hammer mills typically result in lower moisture loss in the ground material due to reduced air flow and less product heating. When fine grinding for aquaculture feeds, hammermills typically lead to a moisture loss of only 0.5% to 2%, a lower rate than what is seen with air swept pulverizers.

Hammermills are versatile and less complex to install and operate than pulverizers, although they have some limitations regarding the particle size they can achieve. Conventional hammermills can produce ground material that is 95% to 99% less than 30 mesh (0.5 mm) with a mean particle diameter of 200µ to 300µ. In aqua feed milling, fine grinding is essential for producing water-stable pellets. Small ground particle sizes are necessary to create dense, well-formed pellets that are easier for aquatic species to consume and digest.

Pulverizers are generally preferred over hammer mills in applications where finer and more uniform particle sizes are required, especially for tough or abrasive materials. Pulverizers are ideal for achieving ultra-fine particle sizes (below 100 microns) & for more uniform particle size distribution. It can handle sticky or dense materials more effectively.  

HAMMERMIL vs. PULVERIZER

Tear-Shaped Grinding Chamber

A tear-shaped grinding chamber is essential in hammermill design for efficient and effective particle reduction. Its unique shape not only prevents material from re-circulating within the chamber but also promotes a smoother flow of material, which minimizes energy wastage and enhances overall grinding efficiency. This design allows for a more even particle size distribution, which is critical in applications such as animal feed production.

Additionally, the streamlined flow in a tear-shaped chamber reduces friction and heat build-up, preserving the nutritional quality and further supports a consistent beater-to-screen clearance, leading to improved capacity and reduced power consumption, making the tear-shaped grinding chamber a key feature in achieving high-performance, cost-effective milling.

Robust Rotor Support

In order to maintain the relative position of the rotor to the grinding chamber the foundation of the mill must be extremely robust. A solid, substantial structure positively maintains the clearances between the hammer tips and the screen through the full width of the grinding chamber This stout design must be accomplished without sacrificing the accessibility to the grinding chamber, as routine maintenance of the beaters and screens will be required.

Carbide Tipped beater

A vast range of beater styles is available globally from numerous suppliers, with distinct types suited to specific grinding requirements. For optimal durability and operational efficiency, beaters with hard-faced ends are recommended. Beaters come in single-hole designs, offering two working corners, or two-hole designs, which provide four grinding corners. Single-hole beaters are typically favoured to ensure rotor balance and reduce the risk of beater failure during operation for high-speed grinding.

These beaters provide superior wear resistance, maintaining sharp edges and reducing replacement frequency even under intensive use. While they may have a higher initial cost, the extended lifespan of tungsten carbide beaters leads to significant long-term savings by reducing maintenance costs and operational downtime. Only drawback that they are brittle & carbide tips chip off in the event of ferrous particle entering in the grinding chamber.

Split Screen

Hammermill screens are often split into two sections, with pads at the bottom to disrupt material flow within the grinding chamber. This split design allows screen sizing adjustments on both sides, maximizing productivity and product quality.

For fine grinding, sieves with 0.8 to 1.5 mm hole diameters are preferred to produce uniform particles, crucial for high-quality feed. These sieves undergo surface heat treatment, enhancing durability and wear resistance, which minimizes hole enlargement. The combination of optimal hole size and advanced surface treatment ensures long service life, reducing maintenance and improving grinding efficiency and operational effectiveness.

Hammer Mill Speed

Tip speed is determined by the hammermill’s diameter and motor RPM, calculated as:
Tip Speed (m/sec) = Rotor Dia.(Mtr) x 3.14 x RPM/60

Tip speed, along with screen size, greatly affects particle size in the finished product. Normally Tip Speed are kept at 85 – 100 m/sec for fine grinding, while lower tip speeds produce a coarser, more uniform granulation with fewer fines. Typically, smaller screen hole sizes are best paired with higher tip speeds, whereas larger hole sizes work effectively with lower tip speeds.

Replaceable Wear Components

An essential principle in effective hammermill design is ensuring that any component subject to wear is easily replaceable. Therefore, the wear parts should be constructed from durable, wear-resistant materials, designed to withstand heavy use and provide a long service life. Additionally, they should be designed for straightforward replacement to minimize downtime and maintenance complexity.

Full-Width Top Feeder

In contemporary hammermill design, incorporating a full-width top feeder is essential for maximizing operational efficiency and reducing costs. This design ensures optimal use of the 

entire screen area, allowing for even distribution of the workload across the full grinding chamber.

Top Feeder – Rotary & Screw Type

Rotary vane feeder with fluted/pocket roller offers better control of grindable having good flow characteristics (like grains). Screw-type feeders are ideal for handling materials with poor flow characteristics or those containing larger particles that may not move efficiently through a rotary wain feeder. However, screw feeders can sometimes cause surge in the material flow, limiting their use in high-efficiency grinding applications. A well-designed screw feeder, equipped with multiple screws and double flighting at the discharge end, can significantly reduce these issues, improving consistency and flow stability.

Air Assist – Pneumatic Conveying of Grinded Material

Air assist systems enhance hammermill efficiency by controlling the grinding chamber environment and facilitating product movement through screen perforations. Properly designed air assist systems can increase grinding efficiency, resulting in a more uniform product with less heat and reduced dusting. Typically, air-assisted grinding systems outperform non-assisted ones by 15-40%. Air flow is made of two main parameters.

• Airflow through the hammer mill screen: 5000 to 6000 m³/hr for 1 m² of screen area.
• Differential air pressure across the system: 700 to 1000 mm of water column

ENERGY CONSUMPTION AND COST-EFFICIENCY

The grinding process is energy-intensive, and optimizing the rotor speed, sieve perforation, and aspiration system can help reduce energy consumption. Fine grinding is more energy intensive, and is necessary for producing pellets for aquatic species and other applications where precise particle size is required. Balancing energy consumption with the desired particle size is key to achieving cost-effective feed production.

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