Making steam

by Emily Wilson
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The conditioning process is, without doubt, the most important component of any feed pelleting system, at least as far as quality is concerned. It also may be the least understood component of the feed manufacturing process by pellet mill operators, most plant managers, and even equipment suppliers.

Conditioning includes any processing or additions made to the mash after it leaves the mixer and before it reaches the pellet die chamber. It may include steam and/or water addition, expanding, compacting, pre-pelleting, ripening, and so on. Whatever type of conditioning is employed should be optimized to give the best pellet quality at a reasonable rate without significant destruction of available nutrients or feed additives.

Steam conditioning is often overlooked and poorly understood by many in the feed processing industry. It is one of several factors, along with formulation, grind, die selection, and cooling and drying, that control pellet quality. In recent years, there have been numerous changes that have influenced the feed industry’s perception of steam conditioning requirements.

For example, many contemporary animal diets are formulated with less grains and more by-products compared with diets of a few years ago. Many by-products, such as animal protein meals, corn gluten meal, recycled bakery waste, and even whole "fuzzy" cottonseed, find use in current animal diets. Some of these ingredients do not respond to steam conditioning as well as ground grains and soybean meal, causing higher motor loads, reduced production rates and poorer quality pellets.

In addition, nutritionists often include high (>1.5%) levels of fats or oils in the animal diet, with little or no understanding of how pellet quality may be impacted. Many feed plants are forced to produce pelleted feed volumes well in excess of rated system design to satisfy sales or animal feeding demand.

The bottom line is that many input requirements can have a negative impact on pellet quality, yet the feed manufacturer is supposed to produce pellets that will survive anything and still land in front of the animal intact. A good understanding and appreciation of the steam conditioning process and its control will go a long way in allowing the production of the highest quality pellets possible at the required production rate.

STEAM HEAT. Steam is used in the pelleting process primarily because of its unique ability to carry and transfer heat through condensation.

If only moisture was needed to optimize the pelleting process, a garden hose attached to a faucet would be a much more economical source than steam. Similarly, if heat was all that was needed, a direct gas fired burner would be cheaper than a boiler. However, the conditioning process needs a great deal of heat and moisture targeted at a very precise location — namely, the surface of each particle in the pellet mash. Steam is the only practical way to do this.

As the relatively cool mash particles are placed in close contact with the steam, the heat from the steam is transferred to the particle causing its temperature to rise. For each 970 BTU of heat transferred from the steam to the mash, one pound of water is condensed onto the surface of the mash particles. This phenomenon of condensation is not unlike the condensation of water vapor from humid air to the surface of a cold drink can.

Once the condensation to liquid
takes place on the surface of the particle, both the heat and moisture begin to
migrate into the particle because of a
gradient (differential) difference between the surface and the interior of
the particle.

Because grains, protein meals, and other common ingredients are typically good insulators, the process of heat and moisture migration is relatively slow. This, then, brings into focus issues related to optimum atmospheric conditioning: mash particle size and retention time.

The smaller the particle size, the more thorough heat and moisture can penetrate to the core of the particle in a given amount of time. Conversely, if a coarse particle is used in the mash, the heat and moisture will not fully penetrate the particle. A hard, dry particle core will
not elasticize enough for ideal pellet

As the average particle size of the mash is reduced, the surface area of the mash is increased geometrically. Because the steam condenses on the surface, a larger surface area means more steam can be condensed per unit of mash weight.

Pellet quality is often improved with fine grinding is directly related to particle size (heat/moisture migration) and surface area (steam condensation). To optimize atmospheric conditioning, use the finest practical grind.

Because most feed ingredients have high insulating values, it takes time for the heat and moisture to penetrate into the core of each particle. The time available is limited to the time it takes for a given particle to move through the conditioning chamber. This is referred to as "retention time."

Retention time is not easily or precisely measured and, in reality, represents an average amount of time spent in the chamber. It can be crudely estimated by simply turning off the feeder and starting a stop watch at the same time. When the load on the pellet mill begins to drop, read the watch. This will give some idea as to the average retention time in the chamber.

Other techniques involve injecting dye into the feed throat and collecting timed samples off the conditioner every two seconds. The visual color intensity will increase then decrease with progressive samples. The time at which the most intense color is seen is taken as the average retention time. Similar results can be obtained using iron tracers.

The objective of measuring or estimating retention time is that to optimize conditioning operations, retention time must be optimized. In order to do that, one has to know where the starting point is.

Most research suggests that both pellet quality and throughput are improved if conditioning time is in the range of 30 to 90 seconds. Most conditioners have a demonstrated retention time in the range of 5 to 10 seconds, therefore, there are real opportunities to improve if appropriate changes are made.


INCREASING RETENTION TIME. The rate at which mash passes through the conditioner is controlled by pick (paddle) angle and shaft speed. Both can be adjusted to optimize retention time.

As a rule, original equipment manufacturer (OEM) conditioners are factory-set at a 30° to 45° forward angle. As the shaft rotates, all picks move the mash toward the discharge. The pick angle can be reduced to a more neutral position (85° to 75°) if the shaft speed is high (>150 rpm), meaning the pick angle can be set to a position nearly perpendicular with the shaft. This has the affect of reducing the "pumping" action of each pick and increasing retention time.

In slow speed conditioners (80-100 rpm), the picks can be set more parallel with the shaft (at 0 to 15° angle to the shaft). This setting will allow the picks to lift the mash and carry it part way around the barrel.

Setting pick angle is a "trial and error" exercise at best. Pick angles at the feed throat should be retained at their factory setting for about the first one-fourth of the conditioner. This will ensure that the mash is moved rapidly forward into the conditioner and provides a void area for the steam to enter the chamber.

Pick angle adjustment should be done in about the middle 50% of the conditioner length. As a suggestion, pick settings should be such that the mash level in the conditioner is about 70% of the available volume. If the conditioner is overfilled, there is risk of choking the feeder and creating mechanical damage.

Increasing the retention time of the mash will increase the load on the conditioner drive motor and may result in overload. Checking the current draw of the motor under load before adjustments are made will clarify the situation.

Before addressing changing shaft speed, a discussion of the two prevailing philosophies is in order. Some engineers subscribe to "stirred bed" conditioning while others follow "fluidized bed" conditioning. The basic difference is the speed at which the shaft turns.

A high shaft speed (fluidized bed) results in the mash being lifted and aerated as it moves down the barrel. The idea is to force mash particles to the top of the conditioning chamber where free or excess steam tends to lay. By placing mash particles in the free steam, it follows that more steam will be condensed and become used.

A slow shaft speed allows the mash to settle to the bottom of the conditioner and be "gently" pushed along the barrel. This obviously allows for longer retention time but leaves the upper part of the barrel open for steam to move freely without being utilized.

Various equipment suppliers have gone from the slow speed, common 30 years ago, to high speed, common 10 to 15 years ago, and back to the slow speed more common today.

The critical part of the design of a conditioner is to provide for the introduction of the steam so that it is in close and immediate contact with the cooler mash so that instant condensation occurs. This often requires multiple entry ports or an elongated slit in the shell of the conditioner.

Regardless of how it is done, the openings must be kept clear so that steam velocity at the entrance is low and the steam is not forced through the mash too quickly. As far as adjusting shaft speed is concerned, there are no particular rules except that the speed should be great enough to provide good agitation and movement down the conditioner. Speed less than about 80 RPMs should be avoided.

Water is a critical component in the bonding that takes place during pellet formation. In typical pelleting processes, the only water added is in the form of steam.

Depending upon the formulation, optimum conditioned mash moisture is in the range of 16% to 17.5%, with 4 to 5% coming from conditioning. There are times when a feed processor simply can’t reach target temperatures before the upper moisture level is met.

Other times, when the grain is dry and warm, he can’t get enough steam into the mash without exceeding target temperatures. Late in the crop year, it is often advantageous to add 1 to 2% water during conditioning to improve pellet quality and production rate.

Recent studies at Kansas State University, Manhattan, Kansas, U.S., have shown that moisture addition at the mixer can be highly accurate and can result in substantial improvement in pellet quality. The equipment for precise moisture addition was perfected in applications to the steam flaking operation and are easily adapted to feed milling operations. The best option will have to
be determined locally through experimentation.


OTHER OPTIONS. Double and triple pass conditioners, jacketed conditioners and pressure conditioning are examples of other conditioner design options, as well as more "exotic" conditioners, such as compactors and expanders. Each has its own advantages and disadvantages but because of space constraints will not be discussed in this article.

However, it is refreshing to note that after 50 or 60 years of little change in the pelleting process, new ideas and concepts are beginning to challenge the
status quo. A U.S. extrusion equipment manufacturing company recently introduced a highly modified system that takes advantage of most of the positive characteristics of the extrusion process while controlling or eliminating most of the negatives, including high capital and maintenance costs and low capacity.

The system is basically a combination of a highly sophisticated atmospheric conditioner and a short residence time, modified extruder.

In operation, the conditioner provides the residence time and exposure to optimize pellet quality while the modified extruder component provides the force necessary to form the mash into a pellet by passage through a plate die with the appropriate die hole size.

This concept has some very distinct advantages over a comparable traditional pellet mill/expander combination yet results in similar pellet quality enhancement. High levels (>70%) of gelatinization are possible resulting in excellent pellet quality and, perhaps, increased digestibility. High levels of fat (> 10%) can be tolerated in the mash and still result in an acceptable pellet.

Perhaps the feature that is most attractive to many commercial feed manufacturers is the speed and ease with which die changes can be accomplished. In most instances, a change from one pellet size to another should take 10 to 15 minutes so. While the cooler is being cleared, a complete die change can be accomplished. This, in affect, results in zero down time to change pellet size.

This is obviously new technology that will require a great deal of thought and study to refine applicability, but it is a good example that shows that the industry is becoming more dynamic.

There is no single conditioning option that is best for all applications and situations. But remember, all factors involved in pellet quality are interrelated and must ultimately be addressed if the process is to be successful.


Keith C. Behnke is a professor in the Department of Grain Science and Industry at Kansas State University, Manhattan, Kansas, U.S. This paper was presented at the American Feed Industry Association’s convention earlier this year in Indianapolis, Indiana, U.S.