The mixer plays a vital role in the feed production process, with efficient mixing being the key to good feed production. In fact, only the perfect mixing of the feed ingredients will ensure uniform distribution of nutrients, vitamins and minerals, which will result in a homogeneous nutrient content in each feed pellet. Further, it will ensure optimum growth of the animals.
Particle size may be the most important factor in causing this phenomenon. In general, the smaller and the more uniformly sized the ingredients are prepared, the more nearly they will approach random distribution during mixing. In many formulae a decrease in particle size is necessary to attain a sufficient number of particles of an essential additive (vitamin, mineral, medication) for dispersion in each daily feed unit. This may require the particle size to be the diameter of dust, from 10 to 50 microns.
The result of mixing dry solid ingredients may be a definite pattern of particle segregation. This is due to differences in the physical properties of ingredients and the shape and surface characteristics of the mixer. Segregation or poor blending can have many implications, including: rejected product, variable color, look or taste, excessive blend times, customer complaints, erratic dosage, product or process delays, inconsistent particle size, poor quality control.
Batch process means discontinuous. Venting spigot must always be open to vent air in a filter, preferably slightly under pressure; filling through the product inlet, with the unloading door closed: mixing, with unloading door closed, product inlet closed or open only if connected with a closed feeding hopper; emptying of the mixed product through the unloading door. Batch mixing can be done on an open flat surface with shovels or in containers shaped as cylinders, half-cylinders, cones or twin-cones with fixed baffles or moving augers, spirals, or paddles.
In the continuous process the machine is an open system. The production sequence consists in a flow of product that enters from the loading inlet, than it is mixed and exits from the unloading outlet. Both inlet and outlet doors are always open. Therefore, it’s important to remember that the final product quality depends on the permanence inside the mixing chamber and on the volume of product in the processing phase.
Continuous mixing proportions by weight or volume, is a technique best suited for formula feeds with few ingredients and minimal changes and no (or minimum) liquids addition.
Depending on the different products to be mixed, you can use one the following mixing tools:
- Ploughshare: are composed by two symmetrical plough-shaped faces. The space between the two faces is closed with a specially shaped metal sheet to prevent the wedging of the product. This tool works on the principle of mechanical fluidisation obtains excellent mixing quality in a very short time.
- Mix paddle: require a longer mixing time than ploughshare, but are easier to clean. Paddles are normally used with pastes and moist or sticky products.
- Screw: the fourfold screw ribbon is used for non intensive and gentle mixing or to keep the product agitated to avoid lumps. It’s used also to homogenise different batches obtained with ploughshare or paddle intensive mixers in a single batch.
Talking about mixers, speed and quality are two qualities customers prioritise. Furthermore, a good mixer should be easy to clean and maintain.
Mixing is very important and it’s essential to know what the feed formulation is and that all the ingredients remain intact during the process. The formula has to be mixed properly, so the animals get exactly the ingredients they need to grow.
A good mixer must meet the following specifications:
- Homogeneity after as short a time as possible
- Careful treatment of mixed material
- No residues remaining in the mixer
- Possibility to add liquids
- Short filling and discharge time
- Variable rate of admission
In most of cases, the main problem with achiving maximal efficiency of pellet mill is quality of steam.
Condensed steam works as a grease for material coming through holes in die. In theory every particle is covered by a thin surface of moisture and that reduces friction. Reduction of friction contributes to increase efficiency and lengthens life of die and rollers. Because of using steam and higher temperature starch is partly transformed (gelatinized), what makes pellets more tight and dense.
Gelatinization is a process of changing starch into monosaccharides under the influence of time and proper level of humidity.
Both time of staying in mixer and temperature are usually enough to come to this transformation.
From our experience we know, that best temperature needed to reach best results, for most kinds of fodder, it is about 75°C or more. The humitidy level of raw material’s should reach 17-18%. Those values are best for most kinds of fodder.
More amount of steam added even at longer intervals clogs the die and consistency of the raw material gets dough.
The solution is to increase the temperature without increasing the humidity. It is very important while projecting pelleting lines.
Also extremely important it is to assure constant supply of steam – with possibly highest temperature, dry and unchanged pressure. And most of all the steam must not be humid and condensed.
In case when steam contains too much water, the mixed raw material is too humid to reach the needed temperature.
In some countries like in USA this is not so important because cereal, which is used there, has low humidity
Mixing conditions are different depending on the recipe of pellets and different content of proteins, starch and molasses.
To explain some terms, which we use here, we attach a diagram which shows how the steam is produced.
The diagram shows 3 stages, while water is transformed into superheated steam.
The vertically axis of temperature starts with freezing temperature. The second point it is both 100°C and boiling point (at atmospheric pressure).
This point grows every time when the pressure grows and decreases when the pressure diminishes. The last temperature point replies to superheated steam
The horizontal axis of the diagram shows three stages of warmth content in steam, "h" is a sensible heat of water, "l" is a next stage - latent heat needed to change the state of matter - transformation from water to steam, without temperature increase.
Latent heat depends on temperature and while the pressure increases the latent heat insignificantly diminishes.
At the third stage, which replies to superheated steam, every growth of warmth is connected with direct increase of temperature.
Relationship between humidity and steam - it is often used term - relates to proportion of latent steam fraction.
If the humidity level amounts to zero then the steam is dry - and this is that what we need and any amount of heat added in this moment would bring about transformation into superheated steam.
If steam contains some fractions of humidity it means, that water is delivered as suspension and we deal with wet steam.
We have already stressed that it is needed to use dry steam or superheated steam, but absolutely avoid wet steam.
Practically, temperature in connection with pressure let us know with what kind of steam we deal.
We know from our experience, that if we want to reach temperature about 75°C-90°C in press and when we use the proper steam then it affects increase of humidity of the raw material to about 5% (for example from 12% it grows to 17%).
That's why it is estimated that demand for steam in pelleting process is 5% of maximum production of pellet mill.
Knowing the maximal efficiency of pellet mill it is easy to count the demand for steam.
But if the efficiency of pellet mill is unknown we can suppose that the efficiency amounts to 100kg/h and multiply by power (Hp) of the main motor.
This computation gives us a maximum value, but it is always better to make bigger than smaller mistake.
From experience we also know that, if we increase for 10°C the temperature of raw material, then the humidity will increase for 1%.
If we already know how much steam do we need, we can count how big boiler will we need. Specific feature of steam systems is fact, that the steam is absorbed by the raw material and that's why those systems have condensing circulation relatively short.
It means, that to the boiler is delivered a big amount of supplying water, and temperature of this water is lower than temperature of water delivered to device that produces steam in which drip retrieval is big.
That's why water treatment should be more regular and has better quality than in case of system with big drip retrieval.
The amount of steam is defined in kilos produced of supplying water in temperature of 100°C
But in our specific case steam applying would be quite dangerous, because supplied water has much lower temperature and the steam is not 100°C hot, and it would mean that our steam is produced in atmospheric pressure.
The efficiency of boiler varies depending on supplying water temperature and production pressure. When you choose a boiler you should buy a 25% bigger one than it results from theoretical demand for steam.
The best kind of boilers for pelleting demand are smoke boilers. For smaller machines it is recommended to install a timer, which enables turning on in advance, before start of new shift.
If some machine (practically or theoretically) is not able to produce enough amount of steam, then the temperature of water should be increased. It is easy to make using fuel economizer and thermally isolate supplying water container. This is a very good way of eliminating waste of warmth. We can't forget that waste of warmth is very expensive.
We should also mention that drip is to get back to supplying water container to decrease to minimum consumption of fuel.
Another practical advice says, that increasing temperature of water for 6°C lets decrease for 1% consumption of fuel needed to transform water into steam.
Often reason of wet steam producing is too high level of water inside boiler. It is recommended to check regularly this level.
It is very important to remember, that in usual European pelleting conditions the steam must be hot, dry and should be added under stable pressure.
The advices given below are very important and should be always remembered:
General rule for steam addition lines is to create many dewatering points, especially in vertical parts of piping.
Piping should be sloped downward according to transportation direction. The level difference should be 0,5cm in 1m section. If the piping is very long we can save on height using siphon bottles.
It is recommended to use cumulative pipe for steam carrying off branches. Cumulative pipe can also serve as drip separator. The diameter of the pipe should be big enough to slow down the speed of steam for separating the transported water and for discharging by dewaterer placed at the end of the cumulative pipe. Also the cumulative pipe should be sloped downward and be located below the level of pellet mill, so that water could freely strain “upward” from a pipe.
Moreover, planning the size of cumulative pipe we should also think about its future application like for heating of containers with molasses (treacle) or fat
It is obvious, that all valves and piping should be properly insulated in its whole length, and distance from conditioner to boiler should be as shot as possible.
In the past sometimes was used a following practical solution: steam piping and molasses piping were built simultaneously and were insulated together. This was how the molasses was heated. But it unfortunately decreased the warmth of pelleting steam. Nowadays it is suggested to design separated line for molasses heating using warmth coming from cumulative pipe. This solution the warmth of pelleting steam is stable.
Please consider while designing steam line:
it is recommended to use safety-valve, suitable for the line
quick closing-valve with automatic dewaterer supplying pellet mill with steam, could be the first step to automatic control of pelleting process
dewaterers are most important devices of steam distribution system
It must not be forgotten that during start-up dewaterers must be able to reduce excessive amount of water. The whole amount of steam used during the start-up is created without overheating and contains water.
in start-up phase there appears significant quantity of water; it is very important to install adequately big container, where the water will be gathered; it should be located outside the steam system. Then the water from the container is piped away to supplying water container
cumulative pipe is as important as dewaterers
remember that behind the drips separator should be placed following devices: dewaterer, steam movement indicator, cut-off valve (especially when we want to increase level of drips or we expect, than in piping can be antipressure created.
Steam movement indicator lets us control if drip unloading is in order. Indicator must be placed in very available location.
The level of condensed steam can be increased by pressure of the steam, but we should remember that 1 atmosphere answers to 10 meters. Diameter of pipes should be so wide to transport steam without pressure decreasing. It has been proved that little diameters of pipes are often the cause of limitations in steam line and decrease productivity of pellet mill.
It has been proved many times that pressure differently affects to different production formulas. It is very hard to determine what pressure value is best. Usually 3,5 bar is a satisfactory value.
It has been also checked that depending on pressure the productivity of pellet mill can insignificantly change.
The high pressure is preferred, though it is always connected with problems with safety.
It is sure, that best results can be achieved performing many tries with different values of pressure.
Changes in range from 1 to 5 bars should let us find optimal pressure.
It is very important to provide the steam system with transporting pipes with proper diameter for a given pressure
The amount of air needed for cooling depends on diameter of pellet:
pellet, diameter 2 - 4 = 1.2 m3/kg/h
pellet, diameter 5- 7= 1.4 m3/kg/h
pellet, diameter 8 - 11 = 1.5 m3/kg/h
pellet, diameter 12 - 16 = 1.6 m3/kg/h
Time of staying pellets in cooler depends on dimensions of pellet. For example, for a pellet of diameter about 5-7mm it is 15 min.
It is very important to remember, that speed of air flow wouldn’t be more than 1 m/s for small pellets and 1,2 m/s for big pellets.
Nowadays in fodder industry are used 3 types of coolers: horizontal, vertical and counterflow. Every type has its different advantages, but the theory is for all of them the same.
How the pellet is cooled
The cooler influences to pellets in two ways. In a moment, when pellets gets into cooler, the temperature and humidity are reduced (the degree is exactly determined). Lack of humidity and warmth affects cooler’s productivity.
Basic parameters which are valid in production process are also valid in cooling process, so if we decrease temperature by 11°C we can expect humidity reduction by 1%. Cooler is able to reduce most of warmth and moisture added during conditioning and warmth from the main motor.
Step by step – what is going on with pellets
a) steam in raw materials in conditioner in condensing and causes increasing of humidity of the raw material by 3-5%. During steam condensation a lot of warmth is produced. Then the raw material is pelleted and also during this process a lot warmth is produces.
Fresh pellet has temperature about 60-94°C. To reach finally good quality product, the pellets must be now cooled and dried.
b) When pellets leave pellets mill, it has relatively fibrous structure and this is a cause of moisture absorption. It happens due to capillary action. This is the same effect as with water absorption by paper napkin
c) The construction of the cooler lets flow the surrounding air as close as possible to pellets. This air, which is not in 100% saturated picks up the moisture from surface of pellets. The moisture is taken away by the process of evaporation, which results in cooling.
d) The warmth taken away from pellets makes air warmer. In consequence increases the air’s ability to water absorption. Moreover this warmth is needed to avoid condensation in air system, which could cause bigger amount of moisture.
If temperature of air in cooler is 70°F (20° C) by relative humidity 85%
and flowing through pellets would be heated up to 120’F (48’ C) then its ability to carry the moisture would be 5 times bigger than in initial conditions. But when the moisture in cooler was taken arises gentle balance between warmth and humidity.
e) Pellets is in unbalance state when the cooled humid surface in not entirely surrounded by air. Then the humidity level is higher inside a pellet than on the surface. It results in strange behavior of pellets which behave like wick. It is a cause of migration of steam along the pellets together with warmth. This moisture then is ready to be taken away by cooling air.
f) This process lasts until most of moisture coming from conditioning process is removed together with warmth. The humidity of pellet after cooling is usually the same (or a little higher) as humidity of mixture coming into conditioner.
This level of mixture humidity can’t be removed cooled in normal conditions. When into cooler flows big amount of very dry air then it comes to decrease of humidity or to shrinking of pellets.
Sometimes it is possible, that water is added to mixture before it gets into conditioner and we can’t reach high enough temperature to remove this moisture. It such a situation the humidity of pellets will be very high.
g) During cooling process pellets coming out from cooler will be always 5-8oC warmer than before loading into cooler. It means that if air flowing into cooler has temperature about 60’F (15’C) then discharged pallet will have temperature between 70’ F (20’C) do 75’ F (23’C).
Day by day check following parts of pellet mill
Check it every week
Every month maintanance. Don't forget to check it at the end of every month
Automatic lubrication system consists of a pump equipped with
Inner blades, which eliminate air in oli
Pumping group driven by motoreducer
Transparent plastic tank
Sensor connected with pressing piston’s stick which indicates when level of grease gets low
Control panel equipped with
Inside the control panel are 2 timers, which make easier grease and pause time controlling.
Caution: Greasing and pause time are already adjusted by us, but any time can be changed acoording to your experience and needs.
The advantages of using pelleted feed are:
- The heat generated in conditioning and pelleting make the feedstuffs more digestible by breaking down the starches.
- The palatability of the feed is increased.
- The segregation of ingredients in a mixing, handling or feeding process is prevented. By feeding a pelleted feed, the animal is more apt to receive a totally mixed ration than one that has separated through these processes.
- Waste is minimized during the eating process. When pelleted feed is fed, each animal receives a well-balanced diet by preventing the animal from picking and choosing between ingredients.
- Bulk density is increased, which enhances storage capabilities of most bulk facilities. Shipping facilities are also increased, thereby reducing transportation costs.
- Feed in pellets forms reduces natural losses because it reduces the formation of dust.
- The time and energy consumption for the prehension of the food is decreased.
By combining moisture, heat and pressure on feed ingredients, a degree of gelatinization is produced which allows animals and poultry to better utilize the nutrients in these ingredients. Consequently feed conversion will be improved.
There are many types of pellet mills that can be generally grouped into large-scale and small-scale types:
- Large scale pellet mills are usually used to produce animal feed pellets, wood pellets, and fuel pellets for use in a pellet stove. It is mainly used in large scale commercial production with the features of long working life, high output and low consumption.
- Small-scale mills are usually applied for home use with the advantages of low cost and easily move. By working principle, it can be divided into screw presses and hydraulic presses. The same basic process is used for both types. A die, also known as a mold, holds the uncompressed powder in a shaped pocket. The pocket shape defined the final pellet shape. A platen is attached to the end of the screw (in a screw press) or the ram (in a hydraulic press) which compresses the powder.
Adopting vertical principle, in the flat die pellet mill the raw materials drops down by the its own weight into the pelletizing chamber where they are compressed between the rollers and die to form pellets by going through the die holes.
Advantages of flat die pellet mills:
✔ They’re small in size and light in weight because of the compacted structure design.
✔ They’re easier to be cleaned and maintained and cheaper because of the design of portable rollers and die.
✔ They’re more convenient to monitor the process of pelletizing by watching the pelletizing chamber, which make the experiment of indeterminate materials for raw materials expansion easier.
✔ They are known for reliable operation, greater mobility, low noise, low energy consumption.
✔ They are far cheaper than that of the ring die pellet mills.
✔ The pressure of flat die pellet mills is adjustable. The diameter of rollers in the flat pellet mill is not restricted by the diameter of the pellet die so that the roller pressure can be increased by enlarging the inside bearing space, adopting a big bearing which also expands the working life.
Disadvantages of flat die pellet mills:
✔ They’re not suitable for large scale pellet production.
✔ The gap between the roller and the die is tricky which should be adjustable according to the species of the raw materials, especially the hardness of the raw materials.
✔ They provide relatively low productivity, comparing with ring die pellet mills (around 80 to 1000 kg /h).
In the ring die mill the basic pelleting principle is a simple operation where feed mass is distributed over the inner surface of a rotating, perforated die ahead of each roll, which compress the feed mass into the die holes to form pellets.
Advantages of ring die pellet mills:
Disadvantages of flat die pellet mills:
Pellet mill capacity is one of the most discussed topics at compound feed production.
Provided that conditioning with saturated steam is optimized, the following points are decisive for the press capacity (throughput):
active die surface (open area)
installed power drive
requested pellet quality: for example, a pellet mill running at 30 t/h with broiler feed in the US or South America produces 15-20% fines, but in Western Europe only 3% fines (PDI 97%) are accepted, resulting in a throughput of only 25 t/h
Press capacity depends substantially on pellet quality (stability according to the PDI, Pellet Durability Index). Durability of pellets shall be determined by tumbling the test sample for 10 seconds at 500 rpm, in a dust-tight enclosure. PDI is a number that describes the percentage of remaining pellets that are not reduced to fines after being agitated under controlled time and conditions by various mechanical means.
Feed distribution is the most overlooked factor in a pellet mill operation.
The distribution must be uniform on the working track of the die. Otherwise, the rolls work badly and, in addition to the loss of productivity, there is the risk of clogging the die with the consequent lockup of the machine. The rotary feed cone and the correct positioning of the deflectors are fundamental for the correct distribution of the product.
When feed distribution is controlled properly, spreading material across the entire working track, the production capacity of the pellet mill is increased. An uneven distribution causes irregular wear of the die and consequently a productivity lower than expected and a premature wear.
Knowledge of particular materials properties lets machine operator predict the effect of density, pressure and warmth on the formula so that operator could determine the best conditions of machine work to pellet feed.
Feed and constituent's properties are determined by following factors:
The quality of pelleted feed changes when the characteristics of raw materials changes, due to this and constant pellets quality, properties and quality of raw materials should be controlled.
In case of big differences look for additional methods of compensation.
Protein and weight
The efficiency of machine and determined formula pellets quality can predominantly be predicted on protein content and weight.
Ingredients with high protein content become plastic (warmth effected) and in consequence high quality of pellets is achieved.
High density ingredients or feeds make possible to achieve high efficiency. When an ingredient or feed contain lots of natural protein and have high density we can expect high efficiency and high quality of pellets; but when there is low protein content and high density we can expect high efficiency but worse quality. Pelleted maize is an example.
Ingredients or feed with high protein content and low density results good pellets quality, but lower efficiency.
For example: crushed ears of maize with wheat grains, cotton seed shells, wheat gluten meal.
Weight of raw materials is an important factor determining the efficiency of pelleting press.
Light material, with high fibre content results the less production (efficiency)
Milling / Crushing / Grinding
Fine milling to small particles we achieve higher density and following benefits:
- better quality
- increase of production efficiency
- increase of volume density
- higher HP efficiency
- longer matrix life
Particularly important is that steam penetrates small particles, makes them softer and more plastic but is not able to go into large ones.
Besides large particles can cause cracks in pellets. It is recommended to use the sieves with mesh diameter smaller then 3 mm in hammer mills.
By means of finely milling the efficiency of pelleting press increases and less power is used to pelleting. It's worth to mention that milling, reducing wipe properties of material, results increase of matrixes life.
Fiber decreases productive efficiency of pelleting press, because it's difficult to compress fiber into pellet. However at the same time, thanks to natural bindings appeared in fiber high quality pellets is obtained.
"Fat content" means both natural and supplemented fat concentration. Both natural and supplemented fat contribute to increase of productive efficiency.
However excessive fat concentration (usually not more then 2%) can show negative effect on pellet's quality. Supplemented fat can be derived from animal and vegetable.
Pelletometry may be thick, thin, medium.
Finely milled raw materials have larger area of steam humidity absorption. It results better “lubricatation” of material and higher production efficiency, because of greater number of small particles being exposed on steam activity.
Density may be increased by finely and midly milled materials mixing. Negligent material crushing causes cracks in pellets.
It's difficult to product strong pellets from starch rich ingredients.
Starch glue property become active in high temperature and high humidity. Gelatinized material functions as lepiszcza and helps to achieve expected quality of pellets.
Important: if for any reason natural starch contained in feed is gelatinized we should expect worse quality of pellets.
Example is cereal grain drying in high temperatures, which causes pre-gelatinisation.
Sometimes it's necessary to add some moisture before pelleting process for good quality of product. Many of actually used ingredients are very dry and besides added moisture we should add steam, especially when earlier moisture was absorbed before mixture get into conditioner.
When expected quality of pellets can't be achieved nor by steam conditioning or using proper matrix we can use bonds.
Two of usually used are: bentonit, lignin sulfate. Using of lignin sulfate is expensive so should be the last solution.
Organic and inorganic acids used by feed industry.
Acids are usually used in feed industry as antibacterial, anti-mould or acidifying factors.
Phosphoric acid: inorganic, liquid in natural, no sour smell, corrosive
Acetic acid: organic, liquid in natural, sour smell, corrosive
Formic acid: organic, liquid in natural, sour smell, corrosive
Propionic acid: organic, liquid in natural, sour smell, corrosive
Lactic acid: organic, liquid in natural, nice smell, corrosive
Citric acid: organic, crystalic or powdered in natural, nonintensive smell, less corrosive
Usually organic and inorganic acids are not used by feed industry as pure forms because they are corrosive. To eliminate the problem there are available liquid or solid-dry forms properly buffered (buffered saline) to weaken corrosive property.
Mentioned recombined forms are more expensive then pure acids.
There are acid additives less expensive but more corrosive.
It's recommended to use products from checked, trustworthy firms who offer good quality, non-corrosive acid additives.
Feed additives are mixed with mixture to increase the quality, disease prevention or feed conservation; short list as following:
Medicine used to prevention from parasite infections.
A kind of medicine synthesized by alive organisms (mould, fungi, bacteria, plants) having antiseptically properties, like: penicillin, streptomycin etc.
Added to feed for reduction of oxidation and fat go off.
Pro-vitamin A, usually provided with chlorophyll, but synthetic form can also be supplemented to feed.
Substances added to soluble for adapt to pH changes. Used in ruminants nutrition for optimal pH : 6,2-6,5, which lets to digest feed and synthesize microbial protein: rumen microbial: bicarbonate, bentotin, magnesium monoxide.
Water soluble vitamin associated with fat metabolism and transport. Common poultry and hogs feed additive. Most ruminants synthesize enough of choline but choline can be supplemented in cows at the first stadium of lactation diet.
Medicine used in coccidiosis treatment. Coccidia is intestinal protozoa occuring in cattle and poultry intestines.resulting in diarrhoea and big wastage.
Fish digestive system is very short (for example: it is 10 times shorter that swine digestive system) and that is why fish feed must be light. Both dimensions and shape of pellets must be adjusted to size of fish. Besides, in water pellets must behave according to habits of fish: sink or float on surface of water. And this feature of feed depends on pellet density.
Quality requirements of fish feed
There are carnivorous fish (salmon, trout), omnivorous fish (carp) or herbivorous fish (crucian). Selecting a proper formula for a given kind of fish, we should pay attention to their nutrition habits. Bed fish usually consume food at the bed of water basin and eat 6 times slower than carnivorous. The food for them should not move in the water. Raptorial fish catch the food during slowly fall onto bed and that is why this kind of fish does not eat close to the bed. Shrimps and other crustaceans nourish only at the bed of basin. This is the reason why their food must not dissolve in water within few hours.
Kinds of food for fish
Depending on product density we can get food with different properties, which are appropriate for their nourish habits, which are explained below: food floating on surface of water – for warmwater fish slowly sinking food – for raptorial fish, which like cold water sinking food, water resistant, for warmwater fish sinking food, extremely water resistant, for shrimps and other crustaceans In growth phase fish can need pellet up to 12 different sizes, from 0,05 mm to 12 mm. If the given food is too big or too small, fish will not eat it.
Particle size reduction is the first step in the feed manufacturing process and, in terms of pellet mill operations, has a great influence on pellet quality (between 15% and 20%).
Fine particle structures are favorable for:
- mixing homogeneity,
- process ability,
- fat and liquid absorption,
- low wear.
At the same time, high contents of fines are not favorable for:
- healthy nutrition,
- flow ability,
- feed intake,
- dust formation and explosion safety
Grinding, or particle-size reduction, is a major function of pellet manufacturing for several reasons, all of them improving the ease of handling ingredients and their storability:
- clumps and large fragments are reduced in size;
- some moisture is removed due to aeration;
- additives such as antioxidants may be blended.
Within a feed mill two solutions can be adopted: pre-grinding and post-grinding.
Pre-milling occurs when each individual component is milled individually before mixing, instead post-milling takes place when the components are weighed and mixed before being milled.
Pre-grinding allows you to grind better and faster, saving time and energy. However, the product already ground into flour is more complex to handle and requires the use of appropriate silos that facilitate the extraction. For these reasons, now in the most cases the feed industry in Europe is shifting on post-grinding, although this system requires more effort in balancing the ingredients. In modern plants that use post-grinding, a separator is inserted before the mill that bypasses the fine parts, making the process more efficient.
Both the pre-milling and post-milling systems can be:
- direct with pre-sieving
Grain size analysis: decreases
Productivity of hammer mill: decreases
Amount of hammers
Grain size analysis: decreases
Productivity of hammer mill: decreases
Diameter of holes in net
Grain size analysis: increases
Productivity of hammer mill: increases
Thickness of net
Grain size analysis: decreases
Productivity of hammer mill: decrease
Density of holes in net
Grain size analysis: increases
Productivity of hammer mill: increases
Area of inner shell
Grain size analysis: increases
Productivity of hammer mill: increases
Amount of air
Grain size analysis: increases
Productivity of hammer mill: increases
Hammer mills are mostly impact grinders with swinging or stationary steel bars forcing ingredients against a circular screen or solid serrated section designated as a striking plate. The material is held in the grinding chamber until it's reduced to the size of the openings in the screen.
The number of hammers on a rotating shaft, their size, arrangement or sharpness, the speed of rotation, the wear patterns and the clearance at the tip relative to the screen or striking plate are important variables in grinding capacity and the appearance of the product.
Heat imparted to the material, due to the work of grinding, is related to the time it is held within the chamber and the air flow characteristics. Impact grinding is most efficient with dry, low-fat ingredients, although many other materials may be reduced in size by proper screen selection and regulated intake.
Two types of arrangements exists in the hammer mill:
Horizontal hammer mill: it’s the most common and consists of a horizontal drive shaft, which suspends vertical hammers to crush any friable and fibrous dry materials containing less fat.
Vertical hammer mill: in this mill, the drive shaft is positioned vertically and screens and hammers are positioned horizontally. Material successfully reduced in size to the diameter of screen holes or smaller, are carried by gravity outside the mill and thence by air or conveyor to storage in "make-up" bins. Oversize particles, not easily broken, drop through the mill and may be recycled or discarded. Thus, foreign materials, such as metal and stones, are discharged before they are forced through the screen causing damage.
The productivity of hammer mill can be approximately counted on the basis of the formula below
G (kg/h) = kW * D * JkW
G = productivity of hammer mill in one hour
KW = power of main motor
D = diameter of holes in the net
JkW = coefficient of milling characteristic of each product
JkW coefficients for some products
Rice bran: 15
Flour of coconut extract: 80
Flour of sunflower extract: 50
Flour of soybean extract: 70
Remains after barley purifying: 5
Beet pulp: 11
Salt (NaCl): 75
Each product has its own optimal milling speed [m/s].
Chaff of oats: 115
Below you can find some coefficients of milling:
Flour of soybean extract: 5
Capacity (counted in m3/min) of sucking air for hammer mill should be twice bigger than maximal capacity of hammer mill counted in m3/min.
Decreased airflow can be a reason of:
✔ decreased productivity of hammer mill
✔ finer final product
✔ bigger demand for energy
✔ higher milling temperature
✔ quicker wearing
✔ bigger dusting
In the case of roller mills occurs a combination of cutting, attrition, and crushing.
There are smooth or corrugated rolls rotating at the same speed set at a predetermined distance apart with material passing between the two. A tearing action may be added by operating the rolls at different speeds and by corrugations which are different for each roll (for example, the top roll may have off-radial spiral corrugations and the bottom roll lateral corrugations).
Roll grinding is economical but limited to materials which are fairly dry and low in fat.
The most common grinders are the hammer mills and the roller mills. They have been applied to the task of particle size reduction or grinding in feed milling applications.
Roller mills have been used in the processing of common feed materials for years. The earliest roller mills used in the feed milling were abandoned flour milling roll stands, used primarily to produce coarse granulation of friable materials. Over time, roller mills have been used to perform a wide variety of tasks related to the production of animal feeds.
Hammer mills have traditionally been used to produce the finer grinds for pelleting and many mash (meal or non-pelleted) feed applications as well. The hammer mill is a relatively simple machine and requires a fairly low degree of skill in regard to both the operation and maintenance. However, recent significant changes in the industry have caused many to reassess their approach to particle size reduction. Increasing energy costs, increasing customer awareness of feed quality and environmental concerns all challenge the validity of the hammer mill as the only choice for particle size reduction applications.
The introduction of large diameter hammer mills has limited the noise these machines produce naturally during operations.
HORIZONTAL HAMMER MILL
|VERTICAL HAMMER MILL||ROLLER MILL|
|moderate investment costs||moderate investment costs||high initial cost|
|universal application (any friable material and fiber)||less contents of fines, more structure||careful treatment of the product|
|easy shifting of the particle size (wider range)||easy shifting of the particle size||uniform particle size distribution|
|simple operation/maintenance||simple operation/maintenance||expensive maintenance|
|greater particle size variability||aspiration system not necessary (fan integrated in the heavy particle separator)||limited particle entry size|
|high specific power requirement||lower energy demand||reduced specific power requirement|
|high throughput rates||higher throughputs can be achieved (depending on product)||reduced heating of the product|
|robustness and plainness||particle size tend to be irregular in shape|
|high fine content||no effect on the fibrous product|
To produce pellets of acceptable quality the particle size of the ground materials must be correct. Finer grinding will result in a better-quality pellet or extruded feed, increases the capacity of the pellet mill or extruder, and reduces wear of the pellet mill or extruder working parts such as dies, rollers and worms. --
Because animal needs vary considerably, the degree of processing for various diets also must vary.
Ruminant animals such as cattle and sheep have rather long, complex digestive tracts and so require a less processed feed material. On the other hand, many of the ingredients used in ruminant feed pellets consist of low protein, high fiber material so fine grinding may be required to achieve a reasonable pellet quality.
Swine have a short, simple digestive system (much like humans) and therefore benefit from a more highly processed feed, while poultry have a short but rather complex digestive system and, depending on the makeup of the diet, can efficiently utilize feed stuffs less highly processed than swine. Although it has been postulated that finer grinding increases substrate availability for enzymatic digestion, there is evidence that coarser grinding to a more uniform particle size improves the performance of birds maintained on mash diets (and in lower but still significant way in pellet diets).
This counter-intuitive effect may result from the positive effect of feed particle size on gizzard and gut development. A more developed gizzard is associated with increased grinding activity, resulting in increased gut motility and greater digestion of nutrients.
The size and the age of the animals also affect the dietary requirements so far as particle size is concerned. Younger animals require a finer, more highly processed feed than do older, more developed livestock. Factors such as moisture content of the grain, condition of the hammers and/or screens (hammer mill) or the condition of the corrugations (roller mills) can produce widely varying results. In addition, the quality of the grain or other materials being processed can have a dramatic impact on the fineness and quality of the finished ground products.
An optimal grinding requires the right amount of intake of air, which is cleaned in the nozzle filters. This also guarantees an economic use of the hammer mill. The air is necessary to quickly transport the fine product from the area of the hammers through the grinding sieve and, at the same time, to cool the product.
By the ventilator, mounted in front of the filter a vacuum is generated and air is sucked into the air inlet funnel of the hammer mill in feed device, passes through the milling chamber and milling screen, thus keeping the screen perforation clear. On its way to the filter, which is of automatic self cleaning type, the aspiration air carries dust along which settles at the outer surfaces of the filter pockets. Compressed air burst is injected into the inside of the filter pockets thus reversing the airflow for a split second and removing the settled dust from the filter cloth.
The intervals of these are controlled by the rotary air distributor which releases the compressed air from the pressure tank via air valves to the injector nozzles mounted in front of the open filter pockets. The dust is collected in a hopper with an airlock or directly dumped into following bin or conveying system.
The filtering surface must be evaluated considering a filtering ratio = 2
This means that the airflow in m3 /min is twice the filtering surface in m2.
For example, in order to filter 90 m3/min of air aspirated from the mill we’ll need a 45 m2 filter.
The volume of the air used to clean the sleeves in Nl/min is equal to 5-8 times the filtering surface in m2
According to the ATEX rules, these are the solutions adopted for the filters for hammer mills:
● A flame catcher must be installed to prevent the effects of a possible explosion inside the building. Sometimes you have to add bottle with CO2, that opens automatically very quickly.
● Another possibility is to built a channel and guide the explosion gas out of the building.
Power of a motor given as in a producers specification is actual for work without breaks by nominal power supply and frequency in temperatire of environment maximum 40°C at heigth 1000m. In case of different environment conditions, the power of motor should be changed according to data in the table.
If the temperature and heigth vary at the same time, proper values should be multiplicated by values given in % in the table.
Temperature of environment °C<30° power % = 106
Temperature of environment °C 40° power % = 100
Temperature of environment °C 45° power % = 97
Temperature of environment °C 50° power % = 93
Temperature of environment °C 55° power % = 90
Temperature of environment °C 60° power % = 87
Heigth <1000 : power % = 100
Heigth 1500 : power % = 97
Heigth 2000 : power % = 93
Heigth 2500 : power % = 90
Heigth 3000 : power % = 87
Heigth 4000 : power % = 79
Simultaneously with changes of power are changing features like Ma/Mn.
If power will decrease more than 15% then a special winding should be used.
Usually motors are equipped with F Class insulation, for which standards let for 105oK temperature increase (because of overload). In outside environment if temperatutr is not higher than 40oC and heigth is 1000m
Impregnation is made of high quality resin – Class H (whole 180oC). Due to this winding is resistant against moisture and high temperature.
Wires made of copper, Class H insulation are covered with polyester enamel, that increases more the termal winding safety
Caution: Motors can incessantly work even if voltage differs from the nominal by 5%, but the frequency must be the same as nominal.
Technical data characterize relationship between Ia/In, Ma/Mn i Mmax/Mn for direct start-up at nominal voltage.
For the star-delta start-up this relationships come down about 30%.
It is used if there’s no limits for power supply and the machine tolerates without damages big amount of steam during start-up.
His kind of start-up is often used with machines which are started-up with full duty (load) and for which moment of force is stable
but the speed varies (conveyor belts, lifts)
This kind of start-up can be used when moment of force is 50% smaller than nominal motor’s power. It is used usually with centrifugal machines (fans, pumps).
Warmth produced at start-up is independent on used system only in case of a little moment of force, otherwise during star-delta start-up bigger amount of warmth is produced and it results in bigger overheating than at direct start-up.
During frequent start-ups or high inertia moment of the load the thermal protection of control meters is often not sufficient. In cases like this one the machine can be equipped with motors with thermal protection.
The thermoprotecotor is placed in winding according to one of following systems:
Thermoprotector with bimetal disc
When winding’s temperature reaches the same temperature as excitation temperature of bimetal transmitter, its contact (which is usually closed)
opens and directly breaks coil of control meter, what results in its opening.
Thermoprotecotor with thermistor (PTC)
When excitation temperature grows then inner resistance suddenly increases. It results in break of meter coil and its opening. It happens by agency of “detaching device” – we don’t have it our offer.
Thermal protection for motors is useful in following cases
Exciting temperature of thermoprotectors depends on kind of motor insulation:
155oC for „F” Class, 180°C for „H” Class.
In case of insufficient intensity of airflow check following sense of rotation; remember, that fan with radial rotor, when spinning in wrong direction, pushes out some amount of air from circulation spinning speed and tension of belts determine intensity of airflow using Pitot head determine static pressure of air suction and blow-out.
The difference between static pressure of air suction and blow-out determines static pressure of fan.
check, if tension and pressure answer to values from specification
If tension is low and pressure is equal or higher than in specification the causes of unstable work
are connected rather with a machine, than with fan.
In this case check losses of tension in circulation, but at the same time control the static pressure
of most important parts of machine.
If intensity and static pressure are low then there is a problem with a fan and accompanying devices
In this case
✔ check for presence of foreign bodies
✔ check if some flexible connections are damaged (maybe they are cause of narrowing of air channels)
✔ check leaking
✔ check if the fan has a proper suction measuring nozzle
✔ check for possible obstacles in air sucking (curves, rapid changes is airflow direction)
✔ check for possible obstacles at air outlet (curves, rapid changes is airflow direction)
Change of rotation speed
Intensity changes relatively to rotation; Intensity V = V1 x (n/n1)
Changes of pressure: Pt = Pt1 x (n/n1)^2
Changes of power: P = P1 x (n/n1)^3
If the noise comes from movement of airflow, the cause of noise is connected with foreign bodies laying in suction point or air outlet. The solution is to remove them.
But more frequent the cause of noise is connected with inappropriate selection of fan.
In this case we suggest to replace the fan with other one, which is more quiet (usualy with bigger diameter and lower speed) or use some silencing material.
If noise is of mechanical origin the cause is connected with physical contact between moving parts, wrong bearing or vibrations of metal sheets.
If noise is of electric origin the cause is connected with eccentric between rotor and stator or vibrations of winding.
In each of these cases we suggest using antivibration supports (pendulum-type vibration damper supports) and antivibration connections made of rubber, both in suction point and air outlet.
Vibrations can appear because of lack of balance or inappropriate construction of supports or because of both reasons.
The construction can be strenghtened or change its resonance grequency adding some ballast.
Very effective solution is to use antivibration supports (pendulum-type vibration damper supports).
How to recognize if a bearing is damaged?
It is not easy to identify the reason of damage only on the basis of the view of bearing. In many cases we are able to draw conclusion on the basis of shapes of signs on bearing.
We will not be able to prevent from further damages if we don't know conditions of work, greasing and constructional properties of the bearing.
Besides we have to know symptoms of damage and all other events, that accompanied the damage.
What should we do before disassembling?
If we want to properly recognize the problem, before disassembling, we have to precisely examine, fragment by fragment given below characteristic points and write down the results.
If the base of bearing will be disassembled, and the bearing and covers will be washed then all the information will be lost.
Let's take a look at the general view of the machine, and especially at parts of machine close to bearing. What do we see? Are there impurities or production reminders?
Is it possible, that water, solutions, oil or steam get to the nest of bearing? What amounts?
It grease leaking possible? To verify that we can't only check the level of oil via control hole and check seal at main shaft's entrance. Also don't forget to inspect
all closings between cover and mounting, oil pipes seal, screws at unloading point and control hole.
Noise during work
Often symptom of damage of bearing are varied noises, which are different than those which we hear during normal work. In this case we have to determine and describe those sounds,
for example like permanent, pulsating, regular, periodical, whistling, din or bangs.
If there is possibility of non-coaxiality between the main shaft and the holder it is recommended to use bearings which are best in situation of wrong alignment.
The best bearings are transverse self-aligning ball bearings, transverse roller bearings and thrust self-aligning roller bearings.
The reasons of bearings non-coaxiality can be for example bend of the main shaft because of big load, when bearings are placed on supports which are on two different and distant bases or when it was impossible to synchronize at the same time proper bearing's cover in mounting.
Bearings of "Y" type have spherical outer ring in order to prevent from non-coaxiality during assembly stage. Non-coaxiality of this kind sometimes comes to being in agricultural machines.
The maximal load and maximal tightening torque for screws with threading in accordance with ISO rules with big pitch of thread
Maximal load (kg) = Pmax
Maximal screw homing moment kgm = Mmax
Screws class 8,8
M 10 pitch of thread = 1,50 Pmax= 2600 Mmax kgm= 4,97
M 12 pitch of thread = 1,75 Pmax= 3780 Mmax kgm= 8,46
M 14 pitch of thread = 2,00 Pmax= 5160 Mmax kgm=13,46
M 16 pitch of thread = 2,00 Pmax= 7020 Mmax kgm=20,4
M 18 pitch of thread = 2,50 Pmax= 8600 Mmax kgm=28,4
M 20 pitch of thread = 2,50 Pmax=11000 Mmax kgm=39,6
M 22 pitch of thread = 2,50 Pmax=13600 Mmax kgm=53,0
M 24 pitch of thread = 3,00 Pmax=15900 Mmax kgm=70,0
M 27 pitch of thread = 3,00 Pmax=20600 Mmax kgm=101,0
M 30 pitch of thread = 3,00 Pmax=25200 Mmax kgm=138,0
Screws class 10.9
M 10 pitch of thread = 1,50 Pmax= 3660 Mmax kgm= 7,0
M 12 pitch of thread = 1,75 Pmax= 5320 Mmax kgm=11,9
M 14 pitch of thread = 2,00 Pmax= 7250 Mmax kgm=18,92
M 16 pitch of thread = 2,00 Pmax= 9900 Mmax kgm=28,80
M 18 pitch of thread = 2,50 Pmax=12100 Mmax kgm=40,00
M 20 pitch of thread = 2,50 Pmax=15450 Mmax kgm=55,60
M 22 pitch of thread = 2,50 Pmax=19100 Mmax kgm=74,50
M 24 pitch of thread = 3,00 Pmax=22300 Mmax kgm=98,00
M 27 pitch of thread = 3,00 Pmax=28900 Mmax kgm=142,0
M 30 pitch of thread = 3,00 Pmax=35400 Mmax kgm=193,0
1 m = 3.28 ft = 39.36 in
1ft = 12 in = 0.305 m
1 in = 25.4 mm
1 m2 = 10.764 sq.ft
1 sq.in = 6.45 cm2 = 645 mm2
1 kg = 2.205 lb
1 lb (pound) = 0.454 kg = 16 oz
1 kg/cm2 = 1 at = 10 m di H2O = 14.2 psi
1 psi = 0.0703 kg/cm2
1 kgm = 7.233 ft.lb
1 lb.ft = 0.138 kgm
Caution: Mt in kgm (N in Hp, n in g/min)
1 kWh = 860 kcal
1 kcal = 0.0011627 kWh
1 kcal = 3.968 B.T.U.
1 B.T.U. = 0.252 kcal =107.65 kgm
1 B.T.U. = 0.0049 kWh
1 kcal = 427 kgm
1 Hp = 75 kgm/s
1 Hp = 0.736 kW
1 kW = 1.36 Hp
1 kW = 102 kgm/s
1 B.T.U. = 0.252 kcal
1 kWh = 860 kcal
1 kcal = 3.968 B.T.U.
1 lb./cu.ft. = 0.01602 kg/dm2
1 kg/dm2 = 62.43 lb./cu.ft.