Rotary Coolers

A rotary cooler works by passing ambient or chilled air through a horizontal rotating drum. Material is fed into the opposite end of the drum, so the material and air flow in opposing directions (counter-current flow). 

The rotary cooler is set at a slight downward angle to allow gravity to help move material through the unit. As the drum rotates, material lifters, or flights, pick up the material and drop it through the air stream, creating what is known as a “curtain” of material in the cross section of the drum. This maximizes heat transfer between the material and air. Because of this cascading motion, these coolers are sometimes referred to as cascade coolers. 

As continuous industrial coolers, material is continuously fed and discharged.

The difference between direct and indirect rotary coolers lies in the cooling medium. While direct coolers use air in direct contact with the material, indirect coolers reduce the material temperature through contact with the drum shell, which is externally cooled using a chilled water bath (for this reason, indirect coolers may also be referred to as indirect water deluge or water-jacketed coolers). 

Most coolers are of the direct configuration, as this is the most efficient choice, but in some cases, such as when cooling fine materials that would otherwise risk entrainment in an air stream, indirect coolers provide a valuable alternative.

Much like rotary dryer design, engineers must balance a number of different variables to properly size and design a rotary cooler. This includes material characteristics such as bulk density, inlet and outlet temperatures, and more. It also includes factoring in production criteria such as desired throughput, retention time, and cooling medium, to name a few. 

In some cases, flight design and pattern, both of which are highly customizable, may require testing in a flight simulator to establish the most suitable arrangement based on the material’s unique characteristics.

Rotary coolers are high-capacity machines and can process material at a throughput anywhere from one to 200 tons per hour.

Engineers require a number of data points to design a rotary cooler. For basic cooler design, they typically require:

• Material fragility/Potential for attrition
• Material (physical and chemical composition)
• Bulk density of material
• Inlet and outlet temperatures
• Inlet and outlet moisture content
• Desired cooling air temperature
• Specific heat of material
• Particle size distribution
• Desired throughput

Depending on the requirements of the material and process, rotary coolers may be constructed from any of the following metals:

• Carbon steel
• Stainless steel
• Specialty alloys
• Explosion-bonded steel
• Abrasion-resistant (AR) steel

In addition to feeding and off-take conveyors, rotary coolers require an exhaust air and particulate handling system. Most often, this consists of a baghouse, or scrubber, in combination with an ID fan. If excessive carryover is expected, the addition of a cyclone may benefit the process as well. 

In cases where the incoming material is especially hot, water may be directly added at the inlet and refractory lining may be necessary on the inlet end of the unit to protect the drum shell until the material has a chance to cool slightly. 

A waste heat recovery system may also be beneficial to recover hot air generated during the cooling process for reuse in the combustion process of the preceding dryer or kiln. This can help to reduce fuel and energy costs in the drying or thermal treatment stage of the process.

Depending on the inlet material temperature and the desired outlet temperature, rotary coolers may operate at temperatures anywhere between 150°F to over 1000°F.

Both of these industrial coolers provide an effective option for reducing the temperature of bulk solids in a process setting. The choice between the two types typically comes down to several factors, including available spatial footprint, feedstock uniformity, and oftentimes, industry preference. Historically fluid bed coolers have been preferred in applications such as fine chemicals, pharmaceuticals, and food, while rotary coolers have been preferred for more industrial applications, such as in processing minerals and fertilizers. For more information, see our article: Choosing an Industrial Cooler: Rotary or Fluid Bed.

Yes, rotary coolers can be fitted with a simple automation system to streamline start-up and shutdown, or a more complex system that can track and trend historical data, and even alert operators when KPIs fall out of spec.

Rotary coolers offer a number of opportunities to customize the system according to process and material goals. 

This includes various materials of construction, different internals for optimizing efficiency, options in drive assembly type, and more.

Rotary cooler performance is often situation-specific. Apart from maintaining a consistent feedstock, keeping up on cooler maintenance, and operating the unit within original design specifications, FEECO recommends those struggling with cooler efficiency have an expert conduct a process audit. 

A process audit, as with other inspections, can reveal hidden inefficiencies and opportunities for improvement.

Rotary coolers are available in diameters ranging from 3’ to 15’.

Rotary coolers are diverse in their processing capabilities and can be used for cooling any type of material in the form of a bulk solid. They are frequently used in cooling materials coming off of a dryer, such as fertilizers and soil amendments, animal feeds, and chemical products, as well as those that have been processed in a rotary kiln, such as catalysts, phosphate ores, and roofing granules.

Unlike rotary dryers, which can be either co-current or counter-current (referring to the material flow in relation to the air flow), rotary coolers should only be designed with a counter-current configuration. This is because the counter-current flow takes advantage of the thermodynamics and improved heat transfer of material and air moving in opposing directions, making the cooling process more efficient.

There are a number of ways to chill the air for use in a rotary cooler. This includes ammonia chillers, refrigeration, swamp coolers, and more.