Advances In Windrow Turning

From BioCycle
Journal of Composting &Organics Recycling
July 2001, Page 63

INNOVATIONS AND TRENDS
ADVANCES IN WINDROW TURNING
Developed almost 30 years ago, windrow turners have evolved to meet the varied biological, mechanical and economical demands of composting operations.
Robert Rynk and Nora Goldstein
Part I

TURNING compost isn’t easy. Think of what a turner must do –- drive through a long pile of dense, heterogeneous materials with varying particle size and moisture, leaving the pile in roughly the same shape as it started. This pile presents a harsh environment for machinery — abrasive, moist, hot, dusty and corrosive. What’s more, the environment can change drastically, being sometimes wet, cold or dense and sometimes dry, hot or light. The pile may contain large and unyielding objects, such as a large rock or a fence post (steel or wood). Or, it may contain troublesome tangle-ready items like steel cable or plastic bags. The machine must travel over uncertain ground that is often uneven, slippery, muddy or riddled with potholes. Furthermore, users want turning equipment to accomplish this undertaking on large piles, with a minimum of wasted space, quickly, reliably, and with little maintenance, downtime and fuel use. It is a tall order and, therefore, not surprising that several approaches, and many equipment choices, have developed.

BioCycle’s annual composting systems and equipment directory demonstrates the growth in the marketplace. Ten years ago, the directory (April, 1991) listed seven companies in the windrow turner category. All were U.S. manufacturers (two of which no longer market turning equipment). This year, 18 companies are on the list; five are based in Europe.

Information for this article was gathered by surveying manufacturers of windrow turning equipment. More specific details from those responding will be summarized in Part II of this report.

TURNER ORIGINS

Unlike screens and grinders, which have their origins in other industries such as mining and timber, the windrow turner is a piece of equipment developed specifically for composting. Some of the earliest turner manufacturers — Scarab (1972), Wildcat (1973) and Resource Recovery Systems of Nebraska (1975) — have agricultural roots (two to compost feedlot manure and the other built at the request of a local farmer). Brown Bear entered the compost market in 1977 while cooperating with the University of Nebraska in composting research. Double T Equipment got into the business in 1978, designing and manufacturing composting equipment primarily for the mushroom composting industry. Other vendors whose equipment grew out of the agricultural field include Doppstadt, Frontier, HCL and Komptech. Frontier’s first unit was built for a dairy operation. Its second style of turner was designed to turn grass seed straw.

Allu and Scat got into the market via requests for equipment to compost municipal biosolids. Midwest Bio-Systems and Sandberger (Autrusa) both approached turner design and development based on biological parameters. Backhus was operating its own composting facility and developed a turner based on its operating needs.

TYPES OF TURNERS

Windrow turners can be categorized in several ways, e.g., by turning mechanism, orientation to the windrow, power source, mode of travel. But more than anything else, the mechanical action of the turner is its defining feature, as that is what agitates the compost. This article categorizes turning mechanisms into four groups: drum, auger, elevating face and trapezoid.

Drums: The majority of turner models in this category employ a horizontal spinning shaft with paddles, roughly one to three feet in diameter, in a windrow-shaped housing (Figure 1a and 1b). The shaft is sometimes referred to as a rotor or a drum (we’ll refer to as drum). The drum is oriented across the windrow width and sits slightly above ground level. The housing above the drum envelops the compost as it agitates and reshapes the windrow. Paddles, teeth, tines, knives or hammers are attached to the drum (we refer to as paddles). Paddles are oriented on the shaft to move the compost in the desired direction — inward, upward or outward. Most models move material toward the center of the windrow. A few use an auger to achieve this movement.

Manufacturers of spinning drum turners include Allu, Backhus, Double-T, Frontier, HCL, Komptech, Midwest Bio-Systems, Resource Recovery Systems of Nebraska, Sandberger, Scarab and Wildcat. The paddle design is among the more varied features between different brands of turners. Some paddles agitate and toss the material vertically with as little bruising as possible. Others are intended to be more aggressive, imparting a shredding action as well as tossing. Most leave the windrow in roughly the same location as it started.

The majority of drum turner models straddle the windrow as they move down its length (Figure 2). The entire width of the windrow is turned in a single pass. Thus, the windrow width must conform to the length of the drum. One company, Wildcat Manufacturing, makes a turner that turns a windrow in two passes. The drum extends to the middle of the windrow, turning half the width at a time.

Some drum-type turners rely on a tractor for both power and travel. Others have an engine but still require a tractor or loader for travel (Figure 3). With loader driven units, the turner is either towed or mounted on the front forks. Pushing or pulling a turner with a tractor or loader requires an aisle between windrows or pairs of windrows for the tractor or loader to travel. The aisle is unproductive space and reduces the capacity of the site. Therefore, many models of drum turners are self-propelled and self powered (Figure 4).

Auger: A second type of turning mechanism uses an auger, or auger-like paddles, to lift and move the windrow contents. A few auger turners have a shaft with a continuous flight, like a screw. Other machines achieve a similar turning effect using a set of large slanted paddles, arranged in a helical fashion (Figure 5). Auger-type turners usually mount on the front of a skid-steer loader, or wheel loader. Some units hitch on the rear of a tractor but they are rarely used for turning compost. The turning mechanism for most models is powered by the tractor’s or loader’s hydraulic power transmission system. Brown Bear, the primary manufacturer of this type of turner, does sell self-contained models. Scat and Sandberger also offer auger-like turners.

Auger turners work much like a snow blower. As the turner moves down the length of the windrow, it shifts the material to one side, nearly eliminating the need for space between windrows. It can turn a windrow or a strip of a larger pile.

Elevating Face: A third type of turning device is known as the elevating face turner. The only manufacturer of this design is Scat Engineering. It is essentially a wide backwards-sloping conveyor with cleats or teeth that lifts the compost up and over itself (Figure 6). The teeth help to break apart bags but otherwise do not reduce particle size. Additional conveyors on the back of the turner place the compost in the desired location.

An engine powers the turning mechanism. Some models require a tractor or loader for travel; others are self-propelled. Like the auger type, the self-propelled elevating face turners minimize aisle space and can turn a entire windrow or a slice of a large pile.

Trapezoid: Three manufacturers –- Backhus, Doppstadt and Fecon — have a windrow turner that works in a different orientation altogether. Referred to by some manufacturers as trapezoid turners, these machines rest on the sloping side of a large windrow or pile (which naturally forms a trapezoidal shape). As the turner moves along the side, rotating plates or teeth shave off a layer of the compost. The compost is lifted from the existing windrow and conveyed to the side across an aisle where it is added to the side of a new windrow or pile (Figure 7). The conveyor moving the compost determines the location and height of the new windrow. The turning mechanism mounts on the hitch of a tractor or loader and is powered by the tractor or loader. The vehicle, which can push or pull the turner, travels in the aisle between the two piles. The plates or teeth of the turning mechanisms can be changed to provide more or less shredding action and to suit the material in the pile.

TURNING OUT MORE AND BETTER MACHINES

The changes and innovations to windrow turning equipment have been numerous, driven by the evolving needs and trends in the composting industry as well as manufacturers' efforts to improve machine performance. The modifications have resulted in turners that are more varied, durable, require less maintenance (or reduce the effort), provide the operator more control and more comfort, produce a desired effect from the turning action, accommodate large or varied windrow sizes, travel and transport more easily, and perform additional functions like watering, materials handling and covering piles with fabric (Figure 8).

The current selection of turners can handle windrows that are smaller and larger than the previous norms. For example, in addition to their large turners, both Sandberger and Wildcat have equipment for windrows less than nine-feet wide. On the other end of the scale, one of Allu’s turners can handle a 26-foot wide windrow in a single pass. Frontier, Komptech, Resource Recovery and Scarab models also handle windrows of roughly 20 feet in width or larger. Nearly all of the manufacturers have widened their product selection. Those who originally offered tractor-assisted models now have self-propelled units, and vice-versa.

Operator control and comfort appear to have become a priority for turner designs. Cabs of self-propelled turners are now air conditioned and heated; some have air filter systems. Tractor-assisted models also have changed to accommodate operator comfort. Advances have given operators more control from within the cab and improved the ergonomics of the equipment controls.

In an effort to increase efficiency, many facility operators have sought to process more compost on a site of a given size. Turner manufacturers have moved in the same direction, developing equipment for larger windrows and also equipment that reduces aisle space. For instance, compared to ten years ago, more tractor-assisted turners are single-pass straddle style, as opposed to double pass units. While the double-pass types allow wider windrows, single-pass machines require a driving aisle only on one side of a windrow. Aisle space is eliminated, or nearly so, by self-powered straddle-style turners and especially elevating face turners and front-mounted auger-style turners. For this reason, many manufacturers have added self-driven units.

To manage on difficult sites, more manufacturers offer four-wheel drive and the option of wheels or crawler tracks (or a combination). In keeping with the space (and labor) efficiency trend, self-driven turners have been designed to have a short turning radius. Some models essentially pivot rather than turn.

A number of innovations have taken place concerning the mechanics of turning. For example, several manufacturers have changed the paddle design to achieve a specific effect, e.g., to introduce more air, pulverize the feedstocks less, require less power, reduce paddle wear, increase throughput or exchange windrow contents from the inside out. Other innovations have made turning more thorough or improved the operator’s control. For example, Allu, Backhus, Midwest Bio-Systems and Sandberger have side plows or scoops that direct compost at the edge of the windrow into the turning unit. Komptech’s Topturn model includes a scraper bar that directs compost at the base of the windrow up into the drum (Figure 9). Double-T’s equipment includes a pile gauge to help operators build windrows to the desired width. Scarab has an option for a GPS guidance system. Innovations in drum operation include variable and low drum speed (rpm), adjustable height, reversible rotation, and the ability to lift the drum out of the windrow at any point.

Many innovations address the mechanical components of the machine. For example, several manufacturers have converted the drum power transmission on some or all of their models from hydraulic to belt drives (as the power requirement increases, hydraulic power becomes less efficient). Other modifications include better radiators to handle the dust and debris, automatic reversing fans, digital load controllers (adjusts speed based on drum pressure), enclosed hydraulic system, automatic tensioning system for belt drive, improved couplings at the gear box and PTO connection, improved bushings, stainless steel hose ends and connectors, heavier chains, and enclosed drum bearings. Many manufacturers now use off-the-shelf components that can be replaced easily.

Maintenance is a huge issue for facility operators. The wear and tear on turner components is brutal. Many modifications cited in the previous paragraph were driven by maintenance concerns. Other maintenance improvements are more structural in nature. For example, machine parts, particularly drums and paddles, have been strengthened, either by increasing their thickness or by switching to stronger and/or more durable materials. Reducing the time and cost of maintenance operations also has been an objective. Paddle designs have been changed on some machines to allow faster removal. For example, the paddle on the Brown Bear machine is a two-piece assembly. When worn, only the top piece needs to be replaced. Similarly, for ease of repair, drum shafts are replaceable on models offered by Scarab.

TURNING OUT MACHINES THAT DO MORE

Perhaps the most significant development with windrow turners is that they have taken on added responsibilities. This multi-tasking is probably best exemplified by their recently acquired ability to add water to windrows while turning. Although watering systems have long been rigged onto turners, only in the last ten years have they been integrated into the turner design. Nearly all turner manufacturers now offer water spray nozzles and associated plumbing on their turning equipment, with water supplied via a tank or a hose reel. Tanks (towed behind) are more common on smaller turners (Figure 10). Larger turners are more likely to accommodate a hose reel that unwinds and winds as the turner travels. The watering system can be used to suppress dust generated during turning when a full dose of water isn’t needed. They also can be used to add inoculants to the windrows.

Manufacturers have started to take advantage of the materials handling potential of windrow turners. Some types of turners (auger and elevating face) have always been able to move materials, as well as turn them. However, by adding conveyors and other devices, the materials handling capabilities of all types of turners have been enhanced (Figure 11). Depending on the model and the arrangement of conveyors, a turner can place compost to either side or behind itself and discharge at various heights.

A growing interest in sharing equipment among several sites has created a niche for machines that are transported easily. The concurrent trend toward large equipment makes this particularly challenging (especially if the operator wants to avoid an “oversized” vehicle permit). Nevertheless, most turner manufacturers have filled this niche well, improving the transportability of their existing equipment, or developing new equipment with frequent transportation in mind. For example, Frontier makes a self-propelled straddle-type turner that can be transported without a separate trailer. The wheels pivot 90&Mac251; so the turner can be towed sideways by a truck (Figure 12). Similarly, several smaller, tractor-assisted, turner models made by Doppstadt, HCL, Midwest Bio-Systems and Sandberger are “self-trailering;” the drum frame swings up into a vertical position, leaving a narrow base that can be towed on the road (Figure 13). The final height of the trailer may limit its road status, however.

FUTURE TRENDS

All of the manufacturers surveyed have equipment operating at municipal and commercial sites composting yard trimmings, biosolids, and/or food residuals. All service large-scale agricultural composting facilities and all but two vendors (Doppstadt and Double-T) have equipment for small-scale on-farm composting operations. In addition, every manufacturer indicated that their equipment is being used at bioremediation sites.

As far as the future goes, some manufacturers see a trend toward composters making maximum use out of the sites they have to reduce the overall costs of composting. Some predict this will be achieved through site designs that decrease required aisle space and maximize the amount of material on a site (perhaps by building larger and larger piles and thus requiring larger turners). Several manufacturers see a growing demand for better fuel efficiency.

A few companies expect to see windrow turning technologies developed for indoor facilities that will compete with present day in-vessel systems. Innovations mentioned include turner attachments that take measurements during turning (before and after the drum), attachments for combining shrunken windrows and loading finished windrows into trucks, and a trend toward belt driven drums rather than hydraulic driven drums. Finally, wrote one manufacturer, “the companies that will be successful are the ones that are systems consultants that cover all the bases, and not just heavy metal salespeople.”

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