FBA Issue 29: November / December 2009
Hammer mill operational factors, Part IV: Alternative Grinding Equipment
There has been a certain emphasis lately on alternative grinding equipment including so called "Airless Hammermills", vertical rotor hammermills, air swept pulverizers, disc mills, and more recently "vibrating screen" hammermills.
"Airless Hammermills" are so named as they are intended to operate without the benefit of aspiration air. Indeed, any hammermill can be operated without aspiration air and function as an airless hammermill, but production and product quality will be affected. In fact, in many dedicated fine grind applications, fine tuning of the aspiration air flow can be used as a technique to help control final particle sizing. Reduced aspiration air flow will naturally lead to a finer finished product produced, but at the cost of reducing through put, increasing the product temperature, and increasing the moisture loss during the grinding process.
Many of the modern vertical rotor hammermills are promoted as being "airless" machines indicating they will operate without the benefit of aspiration air. In fact, all new installations typically include a small fan, filter or cyclone unit to assist the grind product through the screen, and thus help control the temperature in the hammermill reduce the possibility of moisture condensing in the discharge stream.
The primary benefits of a vertical rotor hammermill seem to stem from its multiple product inlets. Most machines will have two or three inlets where the difference in velocity between the incoming product and the hammers is maximized, resulting in efficient grinding.
Because the inlets are typically small, vertical rotor hammermills will be limited in terms of the maximum particle size they can accept (i.e. solvent extracted meals with larger agglomerations) and less effective on materials with low bulk density as these low density products cannot flow efficiently into the grinding chambers.
The screens in a vertical rotor hammermill are full circle, and so do not have the ability to keep products from rotating within the grinding chamber. Hence, the bottom of the screen must be completely enclosed, normally with 2mm perforated material. Screen and hammer changes are relatively quick, with the grinding chamber being lowered pneumatically to give access to the grinding chamber. The grinding chamber is often advertised as being able to withstand an explosion.
As material enters the grinding chamber, it first contacts the side of the "top" hammers, and gradually accelerates as it passes through the grinding chamber. This reduces the effectiveness of the multiple inlets somewhat, as most efficient hammermill grinding occurs when the difference in velocity between the hammers and the products is greatest. To offset the affects of irregular wear on the hammers, there may be two or three different hammer types used in an AVRHM with some hammers being longer, thicker, or with additional hardfacing on the sides and body of the hammers.
"Air Swept Pulverizers" are called because they utilise high volumes of aspiration air to help convey products through the grinding chamber. In addition, this same air aspiration frequently functions as a built in classifier. By fine tuning the air flow, fairly precise control over the finished particle size can be maintained.
Different machines use different grinding mechanisms, but all rely on high velocity rotors that impact the material being ground. Some machines use an internal screen, while others are "screenless" and employ specially designed internal grinding plates to perform the desired particle size reduction within the machine.
As shown in the above illustration, coarse material is drawing into the pulverizer through the inlet (1), and is impacted by the internal fan assembly and rotor tips (2 and 3). The material also contacts wear liner or grinding plates (4) while lighter (i.e. finer) fractions are exhausted by the discharge fan (7). Heavier (i.e. coarser) particles are recirculated back to the inlet for further processing (8).
Advantages & disadvantages of air pulzerizers
The primary advantages of air swept pulverizers is their ability to product a very fine finished product under almost all operating conditions. Additionally, air swept pulverizers may be able to process materials with higher moisture or fat content due to the absence of any internal screens, and the high air flow rates that tend to scour the internal components of the machine while in operation.
The disadvantages of the air swept pulverizers include limited applications (useful only for fine grinding), high aspiration air flows, and high operating costs.
Because the air swept pulverizers operate at very high speeds, they are only suitable for producing very finely ground finished materials and so will only be appropriate for rations that require very fine finished particle sizing. If a feed production facility must produce a variety of finished particle sizes for different product requirements, it will be necessary to have multiple grinding machines of different types in the facility. Additionally, since they rotate at very high speeds, air swept pulverizers tend to generate very high noise levels and may require a separate enclosed grinding room to meet basic environmental requirements.
By design, air swept pulverizers utilize high aspiration air flow rates. High air flow, when combined with heat generated in the grinding operation, can lead to high moisture losses. This can produce a shrinkage of the raw materials, and can lead to problems with condensation in the associated duct work as well as bridging and poor flow characteristics of the finished ground meal.
Due to the high power consumption and large fans used in association with air swept pulverizers, the energy cost per ton is normally quite high. It is quite normal for an air swept pulverizer to achieve a capacity of 1 MTH for 100 HP (75 kW) connected. Based on an energy cost of $0.07/kWh, this amounts to more than $5.00 per ton, adding a 75 HP (55 kW) fan the total electrical cost per ton can easily exceed $9.00 per ton. In addition the high speed of the rotor assembly can result in high wear rates of the working parts, often exceeding $0.50/tonne.
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