Hammermills versus roller mills
March 1, 2010
When it comes to size reduction of principal feed ingredients, recent practice has been to use hammermills as the main grinding machines.
This wasn’t always the case. In the early days of compound feed milling, when raw materials were homegrown and power sources were either wind or water, the effort needed to grind or flake cereals into a form where animal uptake was optimized dictated that roller milling was a more economic and popular means of size reduction.
Thus, roller milling was the traditional method of preparing cereals and fodder for on-farm consumption by livestock. Today, millers have the option of using either method, or both, and there are many factors that impact their choice.
First, and in some people’s eyes the most important consideration, is power consumption per tonne of grinded product. In this case, I am referring to general processing of cereals and proteins, and my comments do not relate to the specialist grinding of micro-ingredients and high-fat raw materials, which both need careful and specific attention when being ground.
In recent years, hammermill diameters have gradually increased, which obviously allows for greater peripheral beater tip speed at lower revolutions. This has meant the impact effect on cereals at the outer extremities of the grinding chamber is increasingly severe. Consequently, power consumption levels in such hammermills have been reduced to a minimum. This is partly due to a combination of increased screen-hole diameter that complements the increased peripheral beater tip speed by accelerating the impact of individual particles between beater and screen.
This increased diameter of hammermill grinding chambers has led to the adoption of machines with greater throughput capacity, and there has been a progressive shift toward the adoption of post-grinding techniques in most mills built today. In this case, post grinding refers to the positioning of the hammermill after the blending stage as opposed to pre-grinding positioning, when hammermills are placed at the early stage in the mill flow, before ingredients are combined together.
ADVANTAGES AND DISADVANTAGES
There are distinct advantages to adopting the post-grind position for hammermills. Building layout is simplified, overall bulk ingredient storage capacity is reduced, and capital costs are thus minimized in new installations. There are some disadvantages, however, as millers are aware, particularly those who have been called out in the early hours of the morning when a hammermill has broken down and there has been no reserve of ground product for manufacture through the pellet mills while the hammermill is being repaired.
Essentially, hammermills rely on the impact of screens and beaters on the product being ground to reduce it to the desired granularity for incorporation into a balanced ration. Roller mills simply “roll” or “crush” product between two revolving cylinders. This latter process has the distinct advantage of requiring considerably less power, although it is not possible to achieve the fineness of final grind through a roller mill that can be achieved through a hammermill. In a hammermill, the screen-hole diameter controls the maximum finished particle size of any ground product. When using roller mills, there is no screen being used, and unless the product is sifted and the coarse fraction reprocessed, the resultant particle size is purely reliant on the miller’s skill in setting the roller mill effectively.
Roller mills, particularly single pass installations, require more care and attention than hammermills in order to achieve a consistent and accurate grind. Ensuring the feed is spread thinly across the face of the roller mills can present some problems as mechanical feed gates can easily become obstructed, impairing the smooth and regular flow of product into the “nip” of the roller mill. Variability of raw material also needs regular adjustments as opposed to the all-encompassing grinding nature of the hammermill.
The available capacity is also a major consideration when using roller mills as there is a need for machines of considerable size or number to achieve the similar capacity as that of hammermills in the same circumstances. There are other general considerations that may affect capacity such as the cleanliness of the grain and the presence of foreign objects that may restrict flow through the roller mill feed mechanisms.
However, there are some circumstances when roller mills have the edge and it is not completely desirable to reduce ingredients down to a very fine particle size. Ruminant animals prefer to consume flaked cereals, as do horses and outdoor pigs. In such instances, the roller mill comes very much to the fore, particularly where “coarse” or “open” rations are being produced and fed. In the case of beef lots, where the finished feed is not required to be pelleted for purposes of cost-effective transportation, the roller mill can be used quite effectively and can be a key part of reducing power consumption at the mill.
One advantage of using flaked cereals is that the ability to incorporate liquid ingredients into a ration is enhanced. The greater surface area presented by a flake allows for greater absorption of liquids. At the very least, it allows for coating of a greater surface area if absorption is not fully achievable with such ingredients as molasses and some fats and oils. In the brewing industry, a standard grist is required that has been proven to allow optimum application and absorption of enzymes into the mash stage of the process. This can only be achieved by the use of roller mills, often triple roller mills where product is ground twice to achieve the desired grist spectrum.
It should be stressed that the roller mill, when equipped with fluted or corrugated roll chills, can achieve a relatively fine grind, particularly when moisture content of cereals is optimized. The use of differential roll drive arrangements, which create a sheer effect between the chills, not only allows for a finer particle size output, but the sheering effect the roller mill has upon starch granules in cereals is advantageous to the nutritionist when compiling rations. This is especially true for young stock, such as baby piglets and veal calves, where the digestive tract is undeveloped and its sensitivity needs to be respected and treated kindly in early stage diets. The use of HTD belt drives to achieve differential roll speeds of up to 2.5:1 is now well proven, and as a result of such engineering technology there is little need for lubrication of the modern roller mill.
One of the biggest disadvantages of using roller mills is that when the roll chills become worn, replacing them with new chills and subsequently recorrugating the old chills is a major endeavor in terms of time and expense. The good news, however, is there are no screens that can burst or become damaged.
Another positive aspect of using roller mills is that they require little or no air flow to operate effectively due to the fact that, with the rollers being mounted horizontally, product passes through by gravity. Hammermills require a steady and balanced airflow in order to operate efficiently and to keep screens clear and unimpeded. The cost of moving that air, the capital cost of filters and fans, and the space requirement must all be borne in mind when drawing comparisons between grinding techniques.
Recent hammermill designs have been quite innovative, and we have seen the combination of roller mill and hammermill technologies begin to emerge. By using a roller mill, or adopting roller grinding principles as part of the feed mechanism on entry to the hammermill, the raw material is partially ground at that point, which then allows the hammers and screens in the grinding chamber of the hammermill to be fully effective, with often excellent grinding efficiency results.
Not only is a finer grind achievable with far less power consumption, but the control the miller has on the resultant particle size of the grinded ingredient is enhanced tremendously. By partial preparation of the product between the rollers in transit to the hammermill grinding chamber in such an arrangement, the best of both worlds is achieved.
As power consumption becomes increasingly important, you will likely see greater use of roller milling technology as part of overall grinding techniques. Rolls of up to eight inches in diameter are being adopted as feed mechanisms with differential drives and variable gap settings. Compared to conventional, straight forward hammermilling, these new hybrid arrangements can reduce power consumption by around 15%, which cannot be ignored in these stringent times.
The key to successful size reduction, however, is diligence and, as with all aspects of mill management, attention to detail is paramount. The daily walk around the mill, keenly observing minor daily changes in operations, will always prove to be the best defense against rising costs.
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: email@example.com .