The pelleting and cooling portion of the milling operation is the heavy engineering end of things where results of technical adjustments are tangible and improvements have the greatest effect on mill performance.
The scope of pelleting and cooling includes the generation of process steam and the screening and storage of bulk finished products. Let’s start at the beginning with the prepress bin. Often badly designed and too close to the point at which steam meets the pelleting process, pre-press bin bottoms are invariably covered with “hammer rash,” where operators have struggled to release meal that has become damp and bridged across the outlet of the bin. Good design, good discharge arrangements, allowance for escaping process steam and good access to both discharger and bin outlet are all matters that should be given due consideration at the outset. Unfortunately, they rarely are and feed millers have to live with the consequences.
The next phase in the pelleting operation, once the meal leaves the pre-press bin, is the choice of conditioner. The options include turbulator or kettle, expander or compactor. Most millers go for the conventional turbulator, albeit in different forms and sizes but basically incorporating the same principles in which the meal temperature can be raised significantly to generate an amalgam of chewing gum-type viscosity that is pliable and can be formed into a suitable pellet through the die. You often see the addition of liquids at this point, either a small amount of fat on rare occasion or more likely a large amount of molasses if the price is right and the category of livestock being fed is appropriate, usually ruminant diets.
Maintenance of turbulators by inspecting barrels and blades to ascertain when replacements are needed due to wear and tear is vital. Reduced blade length will lead to a buildup of meal on the inside of the barrel, which reduces the diameter of the barrel and consequently reduces the dwell time available to the operator. This leads to variable moisture levels of products as the meal does not have the right amount of time to optimize the absorption of steam, causing pellet quality to suffer as a direct result. Regular weekly checks are beneficial.
It is most useful to incorporate feeders before and after the turbulator to ensure the turbulator runs constantly full. It also allows the pellet mill to draw feed from the feeder after the turbulator according to full-load amperage demand. This is followed by a corresponding change in feed rate into the turbulator that enables a much more stable condition to be maintained. Auto pellet mill controllers can adequately control this arrangement and be easily installed by all the reputable pellet mill manufacturers and suppliers.
As for other types of conditioners, the kettle has its advocates, especially where high levels of cellulose material are incorporated into diets. Long dwell times of 15 minutes or more can be achieved and pellet quality, particularly where large diameter cobs and rolls are being produced, is greatly enhanced. The difficulty with kettles is that they
require considerable headroom and cannot easily be fitted into many mills. They are also classed as pressure vessels and require regular inspection by outside inspectors that can be expensive and time consuming.
Expanders and compactors each have their place and role to play, particularly in aquafeed production. Compactors have found favor once again in Europe, having been very popular at the time when byproducts were abundantly available. With power costs being paramount these days, the cost per tonne of finished feed is being examined more and more. The art of double pelleting, the forerunner to compactors, has virtually disappeared purely because it is too expensive, even though the quality of pelleted feed derived from such double pelleting arrangements is exceptionally good.
As for steam requirements, dry steam, with a good, straight pipe work arrangement adequately trapped with good separators and condensate return, is always advocated. Controlling pressure appropriate to the ingredients being used is usually where most millers get it right or wrong, and it is the pressure and volume of steam that can have the largest effect on finished pellet quality.
Usually, the higher the cellulose content, the higher the pressure. Each miller will have his own set of parameters, but there are some who unfortunately see steam as a complete mystery and, apart from an on/off valve, they do not see it as a weapon in their armory when controlling pellet quality. Regular boiler inspection and maintenance, water treatment checks and controls, and checks on condensate return will also show benefits and should be encouraged.
Probably the most debated aspect of pellet mill operation is die design. When deciding on the die specification to be used, one needs to take into account the grist spectrum from the blending and mixing system.
Daily checks on grist spectrum and some basic trials will demonstrate to the miller what ideal spectrum suits the diets being processed with the ingredients available to the buyer at the time. Die specifications will continually change depending on the raw materials being used, and there can be no panacea for all circumstances. There needs to be a regular inspection of what suits the circumstances at the time, and there might be occasions when it becomes economically viable to change dies before they are completely worn out on the basis that raw materials have changed and power costs need to be taken into consideration.
Refurbishing dies and skimming them down are not recommended, but many millers do this and offer sound economic arguments for adopting the practice.
The increased internal diameter of the die that results from refurbishing and skimming it down, the extra wear and stress it puts on the rolls and the stresses it puts on the press bearings and housings can, at worst, lead to unforeseen expense and, at best, reduced product quality. However, some modern-day presses are being engineered in recognition of the practice being more widely adopted. Casings, housings and bearings are all being strengthened to accommodate the increased die diameter and the increased demand placed on the mechanics of the pellet press. No doubt, this topic will be debated at length along with choice of die specification.
COOLING AND STORAGE
After pelleting comes cooling, and all millers are aware of the developments in cooling technology, from the initial inception of vertical coolers to the use of horizontal coolers and rotary coolers, and, finally, to the modern-day counter-flow box coolers being almost universally adopted in today’s mills. The current coolers are efficient, adaptable, easy to install and allow for rapid product changeover. More attention is being paid to the internal workings to reduce power consumption, and emissions to
the atmosphere are also being monitored with closed-circuit applications being experimented with in some quarters. You will certainly see further developments in cooler technology over months and years to come.
Cooling times are associated with the temperature of pellets leaving the press and, perhaps more significantly, the moisture of pellets, since it is just as much the pellet moisture that needs to be controlled as the temperature.
To avoid mold growth, you need to reduce both of these parameters, which cannot be done in the few seconds it takes to pass through a cooler. Correct air flow during storage is just as important and often neglected. Negative pressure applied to bulk storage for finished products can show significant benefits. As millers who have used them will testify, the use of tote bags demonstrates that the amount of moisture given up by pellets after cooling is quite remarkable. Placing pellets in a tote bag with a closed top for 24 hours will show just how much water is evacuated from pellets in the 12 hours or so immediately after processing.
Seeing this makes one appreciate how important adequate storage bin ventilation really is. Alternatively, when pelleted feed is kept on a vehicle overnight, you may be surprised the following day how much moisture and mold has accumulated on the underside of the vehicle cover sheet, again demonstrating the point that pelleted feed needs considerable time to cool, and no matter how much you try to rush nature, it will take its own sweet time.
Effective cooling is required, but good storage conditions and proper bin ventilation after the event is just as important.
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?.