Meal conditioning is more of an art than simply a mechanical process. Years ago, formulations were much different than they are today, with a far broader range of materials being used from a much wider geographical area.
Three basic elements still apply, however, when we think of effective conditioning, namely that the meal being conditioned requires temperature, time and movement to ensure it arrives at the die in a suitable form to make durable pellets.
To take these three elements in turn, let’s first look at temperature, probably the most important of the three elements. All meal needs to be raised to a suitable temperature. Whether it is in order to control salmonellae or simply to make cereal endosperm pliable, the temperature of the meal needs to be raised. The most common way to do this is by use of turbulator conditioners. These have remained the same since their inception, with the only major change being their size and subsequent dwell time of material brought about by the greater holding capacity.
Initially, the turbulators were short, relatively small-diameter items with simple paddle arrangements to move the material and a manifold at the side where steam and liquids could be added. These have now grown to multiple shafted machines with much larger motors and an assortment of screws and paddles internally that allow material to be retained for greater periods of time. The first machines only allowed retention times of a few seconds, barely enough for steam or liquids to be absorbed. Compensation of these inefficiencies was made by increased pellet mill horsepower, greater die thicknesses and the use of tapered inlets to give greater compression. Modern-day machines are usually fed with a variable speed screw feeder arrangement often in tandem with a similar device at the turbulator outlet that enables maximum retention times to be achieved.
The use of automated pellet mill control systems as well as better valve actuation for steam and liquids injection means that modern-day installations can be operated remotely without the presence of a dedicated pellet mill operative. This does not mean that the pellet mill should be ignored and left without any attention — far from it. The pellet mill is probably the key machine within the feed mill when it comes to influencing pellet quality, and many a long day has been spent dealing with complaints caused by inattention to pellet mill performance.
As for the care and maintenance of turbulator conditioners, these need to be examined regularly as blades and screws will wear and effective barrel diameters will decrease. As paddles wear, power consumption will increase and dwell times will decrease quite significantly.
Good quality steam is an essential part of good conditioning, whether turbulators or kettles are used. The correct steam pressure is required according to the type of meal being blended and conditioned. Most poultry feeds made as three-millimeter pellets from a high cereal base will require relatively high pressure steam being injected at the conditioner. Pressures of around 35 to 40 psi are common, whereas high cellulose feeds with relatively high levels of added liquids, such as ruminant feeds with high molasses levels, will require low steam pressures of around 10 to 15 psi.
Good quality steam comes from a correctly sized boiler that is large enough to provide all the steam that is required without water droplets being scavenged from the boiler water surface, as is the case when the boiler is undersized for the amount of steam required. Pellet quality suffers in many mills simply due to additional pellet mills being added without proper care being paid to the boiler size. The positioning of valve trains, traps and strainers is also very important when it comes to influencing steam quality. Correctly positioned valve trains are in a direct line to the pellet mill conditioner, with no sharp bends and no rapid reduction of pipe diameters. A long straight run of adequate size enables an even supply of steam at the correct and regular pressure to be injected at the conditioner. Time spent planning any new installation of steam piping is time well spent.
The second element of conditioning is time. The longer the meal can spend conditioning the better the condition it will be in when it enters the die. With turbulator conditioners, the conditioning time is relatively short, no matter whose equipment we use, hence turbulators are generally operated at relatively high temperatures. Kettles were the norm just after the war years, when most ingredients were fibrous and required extensive conditioning. Such kettles, familiar to all readers, allow anywhere up to 15 minutes conditioning time, subject to the size of pellet press they are feeding. This length of time spent conditioning gives nutritionists great scope when formulating, since it allows them to incorporate ingredients that would prove difficult to handle through turbulator conditioners.
Of even greater flexibility are the live bins seen occasionally in Holland and Belgium which are steam-jacketed and compartmentalized. These allow up to 30 minutes of conditioning, but the power required to keep the product moving is quite high and these types of conditioning media are not seen outside of the Netherlands and Benelux countries.
These machines do highlight the third element of good conditioning, which is movement. Keeping meal on the move and presenting coarse particles to the steam injection ports does assist conditioning. Gristing correctly also assists conditioning. Not only fine grist, but one with a range of particle sizes will enable good conditioning to be affected and allow for adequate compression to be achieved at the die.
As a general rule, the following points may be of value to millers seeking to improve their conditioning and pelleting systems.
When making rolls or pellets in excess of 9 millimeters diameter for feeding to outdoor livestock, molasses is invariably used and steam requirements are quite low. This size of pellet or roll, usually 20 millimeters diameter, is used to prevent birds from carrying them away and also so livestock can identify them amongst low-grade vegetation.
Die specification and conditioning principles usually go hand in hand. Thicker dies, where greater compression can be achieved, can often compensate for poor conditioning in terms of pellet quality. The downside of this, however, is the increased power usage. Good conditioning will assist in efficient use of the pellet mill and will save on die wear and roll wear, and it will enable optimum outputs to be achieved at sensible power consumption costs.
As mentioned earlier, grist spectrum is important not only for good compression in the die but also for meal conditioning. While the bulk of material should be equally distributed on 1,000-, 500- and 250-micron screens, you should aim for throughs of not less than 20%. Mixing grinder screen sizes and ensuring that hammermill beaters are regularly turned — end for end and corner for corner — will assist in maintaining a good grist spectrum.
Good boiler maintenance is important to maintain boiler efficiency. Boilers should be inspected at least annually, and regular checks should be made on steam traps and separators. “Wet” steam will soon become apparent at the press and is invariably due to either trap failure or incorrect steam pressure. Occasionally in new installations, pipe diameters may be inadequate, but it is hoped that pipe diameter and length calculations are properly checked at the design stage of any new work or major repairs.
I generally work on the basis that 100 pounds of steam is required for each one tonne per hour of feed being produced. By using this as the basis for steam volumes, it is relatively easy to check the pipe diameter by referring to the lowest steam pressure you will require and checking velocities and volumes with your local steam pipe supplier.
One final point is that when you have a series of pellet mills in a row or fed from the same boiler, you must always ensure that steam supplies are separate to each pellet press in order to avoid pressure and volume fluctuations when stopping or starting presses while others are in operation. While this may seem very obvious, you would be surprised at the number of mill installations that have not allowed for this, particularly when it is determined that one press will only operate for a limited period of time. There are more items that need to be looked at when it comes to meal conditioning, but the aforementioned points are the most significant. A regular walk around the mill each day will tell you many things and allow you to follow seasonal trends. The use of a plant diary is invaluable and often makes very interesting reading once you have a few years’ worth of observations to follow.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J B Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at: firstname.lastname@example.org .