Key points
Although they are required far less than their drying counterparts, rotary coolers are a crucial piece of equipment in a range of different facilities working with bulk solids, allowing products to be handled, stored, and bagged after processing, without the risk of injury to personnel, damage to equipment, or spoiled product.
As a key capital investment, ensuring the rotary cooler in any bulk solids plant is properly maintained is essential to not only minimising downtime, but also to prolonging the life of the equipment.
And while this is true of most types of process equipment, it has special importance when considering the constant movement, temperature changes, and static and dynamic loads characteristic of rotary cooler operation, as this exacerbates problems at a quicker pace, making preventative maintenance even more critical.
Consequences of Poor Maintenance of Rotary Coolers
Understanding the importance behind proper rotary cooler maintenance is the first step to adopting the right procedures and ensuring they are carried out in a timely manner.
A poorly maintained rotary cooler incurs costs far beyond the cost of proper maintenance: lost production time, spoiled product, increased labor, and more extensive repairs. In cases where maintenance has been severely neglected, premature failure is not uncommon.
Other issues associated with overlooked rotary cooler maintenance include:
- Holes or thin spots in the drum shell
- Damage to gears, or chain & sprocket
- Bearing failure
- Trunnion shaft failure
- Damage to inlet and/or discharge breechings
Despite its importance, rotary cooler maintenance is often neglected. What follows are the indispensable practices behind maintaining a rotary cooler to maximise its service life while minimising unnecessary downtime and maintenance costs.
Essential Rotary Cooler Maintenance Practices
As with any type of rotary equipment, there are a few basic procedures that are central to upholding the integrity of the rotary drum and its many components:
Alignment
The single most fundamental aspect of maintaining a rotary cooler is ensuring proper alignment. The alignment of a rotary cooler, or any rotary drum for that matter, refers to the structural balance of the unit, adjusted by positioning the trunnion wheels in such a way that the load of the drum is evenly distributed.
Alignment affects almost all components of the rotary cooler: the shell, the drive components, the breechings, and more. As a result of the dynamic loads and heat stresses a rotary cooler undergoes during normal operation, the rotary drum naturally falls out of alignment over time. As a drum falls out of alignment, the entire load becomes increasingly unbalanced, putting added strain on all components.
As such, operators should be trained in spotting early signs of misalignment and making minor adjustments. A routine realignment should also be incorporated into the unit’s preventative maintenance plan.
The frequency of realignment required is different for all drums and depends on the unique specifications of the drum and its operating conditions.
Many factors, including foundation, drum loading, humidity, dust, operating environment, climate, and more, can influence cooler alignment. Some coolers may run for years without requiring realignment, while others may require realignment every few months.
A laser alignment system syncs with special software so FEECO Customer Service Engineers can see a digital rendering of the drum’s alignment and where adjustment is needed.
Tire & Trunnion Grinding
The tires and trunnions serve as the load-bearing surfaces on which the cooler rests. In addition to supporting the drum, they also contribute to its alignment and ensure smooth rotation. As such, it is important to maintain the surface condition of tires and trunnions.
The irregular wear shown here on both the tire and trunnion indicates that the drum is likely out of alignment. This type of wear should be resurfaced through grinding.
When tires or trunnions show signs of wear, whether from misalignment, fugitive material, or otherwise, they should be resurfaced as soon as possible. This is done through a process known as grinding, in which a specialised machine grinds down the worn surface to a like-new condition.
Since the base conditions have changed, the drum should be realigned after grinding has been conducted. Further, as the diameter of the wheels becomes smaller, whether through wear or grinding, the drum will lower slightly. At a certain point, this will cause the seal rings to drag on the drum, requiring the wheel bearings to be adjusted such that they are in line with the drum, seal rings, and inlet hopper.
Tire grinding (in progress) restores this tire to like-new condition.
Drum Training
Drum training refers to the relationship of the drum to the thrust rollers. Thrust rollers are situated around each tire and prevent the drum from drifting uphill or downhill by providing a place for the drum to rest against.
Thrust loading is best kept to a minimum, because it not only contributes to the overall alignment of the cooler, but it also indicates that the drum is not drifting longitudinally. Rotary drums are positioned so they “float” between thrust rollers, meaning they are not resting too hard on either end.
Like alignment, achieving optimal float ensures that the drum is properly supported and not moving in a way that it should not be. Improperly floating drums put added stress on all components and are likely to damage the thrust rollers and tires of the drum.
Damaged thrust roller from a drum that has been out of proper float for a long time, allowing the drum to ride too hard against one of the thrust rollers.
Because the drum is constantly rotating on a slope under static and dynamic loads, however, float, like alignment, also falls out of balance over time and requires regular monitoring and adjustment.
Adjusting the float of a drum is referred to as “training” the drum. This is done by skewing the trunnion bearings in small increments using shims until the proper float is achieved, a process which can take time, because the effects of an adjustment are not always immediately obvious.
Regular Cleanout
Regularly cleaning out the cooler is one of the easiest and most cost-effective ways to maximise cooler life, particularly when working with corrosive materials.
When the cooler is processing corrosive materials such as potash or urea, regular cleanout significantly reduces the potential for corrosion to occur. Were material allowed to sit in the unit for extended periods of time, corrosive wear could be considerable.
It is generally considered good practice to regularly clean out the cooler at the end of a production run, as starting the cooler under full load could damage the drive components, but cleaning out the cooler regularly has other benefits as well.
During cleanout, operators and maintenance personnel have the opportunity to do a quick inspection of the unit’s internals. Personnel should pay close attention to any wear areas in the unit, looking for signs of wear caused by corrosion, abrasion, or damage from dislodged clumps of material. Identifying and resolving these issues early negates the need for more extensive repairs and unexpected shutdowns down the line.
By regularly cleaning out the cooler, plant managers can be confident in both the drive components of their cooler, as well as the internals. Bearing exterior should also be kept clean, as this helps prevent foreign material from getting inside the bearings.
Annual Inspections
Many consider an annual inspection as an unnecessary expense. On the contrary, annual inspections are the most underutilised, yet most effective tool in maintaining a bulk solids cooler.
The dynamic nature of a rotary cooler means that any problem left untreated has the potential to evolve into something severe as the forces acting upon it continue to fall further out of spec. This makes catching the onset of problems critical to avoiding more extensive damage.
“A lot of the issues we see on a regular basis could easily have been caught and prevented with a simple annual inspection,” says Chad Wiza, FEECO Customer Service Engineer. “Instead, the customer ends up having to rush service technicians out to the site to get them up and running, and unfortunately, it’s not usually an easy fix.”
Annual inspections can be carried out by the original equipment manufacturer, or through a third-party service provider, and typically include a comprehensive review of the unit’s mechanical components, as well as recommendations on areas in need of attention.
Monthly Dynamic Inspections
Likewise, maintenance personnel and operators should be trained in conducting monthly dynamic inspections, which will also help in catching problems at their onset. Monthly dynamic inspections include a visual walk-through of the cooler by trained personnel. Gear or chain alignment (backlash) and tension should also be checked, as should the following components:
- V-belt
- Reducer
- Couplings
- Bearings for trunnion wheel and pinion drive (if applicable)
- Thrust assembly
- Hammers (where applicable)
- Drum shell
- Graphite blocks for tire lubrication
Routine Maintenance
Replace items as they become worn. Wear on bearings, gears, seals, and more will cause the drum to become misaligned. Further, some areas, such as the seals and shell, have the potential to impact the effectiveness of the cooler if they are allowed to become worn.
When such wear is present, cooler efficiency goes down because chilled air can leave the drum, while ambient air enters the process, reducing the cooler’s ability to decrease material temperature.
Flights (material lifters) are also a significant contributor to the drum’s ability to efficiently cool materials. As they are in constant contact with the material, they are also the first component to show wear. Fortunately, many manufacturers make flight replacement simple with a bolt-in design.
Ensuring adequate lubrication of mechanical components such as bearings, speed reducer, and drive motor will help to minimise wear potential as well. Lubrication guidelines are typically found in the original equipment manual.
Conclusion
Properly maintaining a bulk solids rotary cooler is vital to promoting equipment longevity and avoiding unnecessary downtime and repairs. While it’s easy to overlook maintenance, regular cooler realignment, tire and trunnion wheel grinding, drum training, and cleanout, in combination with keeping up on routine maintenance and annual inspections, are the foundation of a reliable rotary cooler.
