In most of cases, the main problem with achiving maximal efficiency of pellet mill is quality of steam.
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.
Steam conditioning depends a lot on the formulation to be pelleted, and in practice the formulations can be grouped into 6 different categories:
and more specifically:
High Grain Feeds (High Starch) = Poultry, broiler, and turkey feeds are in this group.
High temperatures and high moistures are necessary to gelatinize the starches in the grain. The gelatinized material acts as a binder to produce tough pellets.
To partially gelatinize the starches, the meal moisture must approach 17-18% moisture and the temperature must be at least 85°C (180°F).
The hotter the meal, the greater the degree of gelatinization.
Normally low pressure (1.5 bar) steam is used.
High Natural Protein Feeds
This group includes supplements, concentrates and some steer and dairy feeds. Heat is more important than moisture to plasticize the protein.
These feeds require more steam than the heat sensitive feeds but less than high starch formulas.
Normally high pressure steam is used (2.0-2.5 bar)
Molasses Feeds
The amount of steam that can be added to this group is directly proportional to the per cent of molasses in the formula.
Since molasses is approximately 26% water, the quantity of steam that can be added must be reduced or the meal will become too wet.
This condition will result in choke-ups.
Normally high pressure steam is used (2.0-2.5 bar)
Complete Dairy Feeds
This group is usually between 12-16% in protein.
This group contain large amounts of fluffy, roughage-type ingredients and are also low in grain content.
These ingredients have a low ability to accept moisture
Steam addition should be low to keep the meal temperature below 60°C (140°F) and the maximum moisture level at 12-13%.
If above levels are exceeded pellets expand and crack after leaving the die.
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 super heated steam.
The vertical 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 super heated 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 at this moment would bring about transformation into super heated 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 super heated 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. A specific feature of steam systems is the 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 the temperature of water delivered to the 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 a 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 it was sometimes used in 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 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
Steam conditioning depends a lot on the formulation to be pelleted and in practice the formulations can be grouped into 6 different categories:
and more specifically:
High Grain Feeds (High Starch) = Poultry, broiler, and turkey feeds are in this group.
High temperatures and high moistures are necessary to gelatinize the starches in the grain. The gelatinized material acts as a binder to produce tough pellets.
To partially gelatinize the starches, the meal moisture must approach 17-18% moisture and the temperature must be at least 85°C (180°F).
The hotter the meal, the greater the degree of gelatinization.
Normally low pressure (1.5 bar) steam is used.
High Natural Protein Feeds
This group includes supplements, concentrates and some steer and dairy feeds. Heat is more important than moisture to plasticize the protein.
These feeds require more steam than the heat sensitive feeds but less than high starch formulas.
Normally high pressure steam is used (2.0-2.5 bar)
Molasses Feeds
The amount of steam that can be added to this group is directly proportional to the per cent of molasses in the formula.
Since molasses is approximately 26% water, the quantity of steam that can be added must be reduced or the meal will become too wet.
This condition will result in choke-ups.
Normally high pressure steam is used (2.0-2.5 bar)
Complete Dairy Feeds
This group is usually between 12-16% in protein.
This group contain large amounts of fluffy, roughage-type ingredients and are also low in grain content.
These ingredients have a low ability to accept moisture
Steam addition should be low to keep the meal temperature below 60°C (140°F) and the maximum moisture level at 12-13%.
If above levels are exceeded, pellets expand and crack after leaving the die.