Rotary Dryer FAQs - FEECO International

Rotary drum dryers are one of the most widely used types of industrial dryers on the market today. Their heavy-duty design, high throughput, long-term reliability, and simple operation has lent them to a wide range of applications in a variety of industrial process settings. They are the dryer of choice for processing fertilizers and soil amendments, animal feeds, minerals and ores, and so much more.   

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While rotary dryers have a relatively straight-forward design, many questions often arise in considering a rotary drum dryer for a given application, even when the application is well-established. Below are answers to some of the most frequently asked questions surrounding rotary drum dryers.

How does a rotary dryer work? 

Rotary dryers consist of a large, rotating drum through which wet solids are passed. As the material moves through the dryer, heat and air are used to remove the desired amount of moisture, with the mode of heating dependent on whether the unit is of the direct or indirect configuration. Most dryers are of the direct design, in which the material is in direct contact with the combustion gases, as this is the most efficient approach to moisture removal. 

The material is cascaded in the dryer at carefully controlled temperatures for a predetermined amount of time (referred to as the retention or residence time) to reach the desired final moisture content.

In the case of direct dryers, internal flights, or lifters, are used to pick up material and cascade it through the stream of combustion gases in order to maximize heat transfer efficiency.  

Lifting flights inside a FEECO Rotary Dryer

What’s the difference between a direct dryer and an indirect dryer? 

The principle difference between a direct rotary drum dryer and an indirect one, is how heat is introduced to the material. Direct dryers rely on convective heat transfer from direct contact between the material being processed and the combustion gases to efficiently dry the material. Conversely, indirect dryers avoid contact between the material and combustion gases, instead relying on heat transfer to occur through the drum shell wall via conduction. 

While this approach is less efficient than the direct dryer type, it is an essential processing option when working with materials that must be processed in an inert atmosphere, such as when processing sterile products. The indirect configuration is also chosen when processing ultra-fine materials, as it avoids the risk of solids becoming entrained in the gas flow and ending up in the exhaust gas system. 

What types of fuel can rotary dryers accept?

Rotary dryers can accommodate a variety of fuel types, including:

• Fuel oil
• Natural gas/Propane
• Waste heat
• Biogas
• Electricity
• Steam

What does the rotary dryer design process look like? 

The design process differs across manufacturers and depends on whether or not the manufacturer relies on standardized designs. 

Since rotary dryers are best designed around the specifications of the given application and material to be processed, standardized designs, while less costly and requiring less production time, are inherently not as effective or efficient. 

For a truly high-performance rotary dryer, the design process must take into account a wide range of factors such as inlet and outlet moisture, bulk density, heat transfer properties, material consistency and fragility, and more. 

The rotary dryer manufacturer will gather the necessary data points and use computer dryer sizing calculations to determine the optimal drum size. Depending on the process goals and application, dryer feasibility testing may be necessary to assess the material’s behavior in the dryer and define the process data necessary for scale-up. 

In addition to the process data, the dryer manufacturer will also consider material characteristics and process goals that will influence the design. This might mean selecting different materials of construction, assessing flight design, adding customizations such as knocking systems or trommel screens, and more. 

What capacities can a drum dryer handle?

Rotary dryers are capable of handling a wide range of throughput – typically anywhere from 1 TPH – 200 TPH. Rotary dryer capacity is a function of drum size and allowable air velocity. 

What information is necessary to design a rotary dryer? 

Rotary dryer manufacturers will require the following information to design a rotary dryer:

• Desired throughput
• Inlet and outlet moisture
• Material bulk density
• Inlet and outlet temperature
• Maximum material temperature
• Specific heat
• Particle size (minimum, maximum, and average)
• Desired fuel source
• Higher heating value of fuel source
• Plant elevation
• Ambient air temperature at plant
• Special accommodations (is material explosive, abrasive, etc.)

FEECO Rotary Dryer prepared for shipment

How is a rotary dryer different from a rotary kiln?

Rotary dryers and kilns are similar in design, but are employed for different reasons. Rotary dryers serve to remove moisture from the material, while rotary kilns are intended to cause a chemical reaction or physical change in the material by adding heat. For this reason, rotary kilns are typically operated at higher temperatures than dryers, and therefore, are designed with a higher heat tolerance in mind. 

What materials is the dryer constructed from?

Rotary drum dryers can be constructed from a wide range of materials, the choice of which is dependent on the temperatures employed and the characteristics of the material to be processed. Common materials of construction include:

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

The dryer may also utilize multiple materials of construction to meet the needs of the material being processed as well. For example, the sticky nature of some materials may benefit from using polished stainless steel for the inlet section, and carbon steel for the remainder of the drum shell. The polished stainless steel will discourage sticking near the inlet and give material a chance to dry a bit before it reaches the carbon steel, at which point the potential for sticking will be reduced.

What other equipment is needed to support the rotary dryer?

Rotary dryers are not an isolated device, but rather, require a few pieces of support equipment. A complete dryer system consists of:

• Exhaust gas handling (or off-gas handling), where emissions are treated and any particulates are recovered. This includes scrubbers, cyclones, baghouses, electrostatic precipitators (ESP), and more.
• Induced Draft (ID) fan, which draws air through the system to create a negative pressure
• Burner, to produce the combustion reaction between the fuel source
• Dilution air fan to allow for better control of the on gases
• Combustion chamber (optional) to house the combustion reaction
• Control system to control the drying rate
• Material handling equipment, such as conveyors, to carry material to and away from the system

What temperatures do rotary dryers typically operate at? 

Rotary drum dryers typically operate at temperatures between 200°F and °F (93 to °C).

Which dryer type is best for my application – rotary drum dryer or fluid bed dryer? 

While there is some overlap between these two industrial dryer types, the choice between a rotary dryer and fluid bed dryer is a matter of capabilities. Rotary dryers are best suited for demanding drying applications, such as those found in the mining industry. They are also a better choice when variation in feedstock is common. For more information, see Rotary Dryer or Fluid Bed Dryer.

Can rotary dryers be automated? 

Yes: Rotary dryers are easily automated. Depending on the manufacturer, automation capabilities may differ. FEECO has partnered with Rockwell Automation to offer dryer automation capable of monitoring, collecting, and trending various data points such as current (amps), fan speed, feed rate, fuel usage, gas sampling & analysis, horsepower, system pressures, temperatures, and more. 

FEECO Rotary Drum Dryer installed at customer site

Are rotary dryer specifications customizable?

Yes. In fact, FEECO recommends rotary dryers be customized for the application as much as possible, as this will produce the most efficient and reliable drying solution. 

Several aspects of the rotary dryer can be customized, including materials of construction, flight design and pattern, sizing, support equipment, and more. 

Conclusion

Rotary drum dryers offer efficient, reliable industrial drying; their high capacity, robust build, and flexible design makes them a fit for nearly any application.  

The company is the world’s best Industrial Dryer Supplier(hi,in,vi) supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

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FEECO is the leading provider of custom rotary dryers, with hundreds of installations around the world. Our rotary dryers are built to the highest quality standards and are engineered around the unique characteristics of the material to be processed for an optimal drying solution. For more information on our dryers, contact us today!

Table of contents

In addition to learning about the primary types of air compressors and how to choose between them, it’s also important to learn about what varieties of air dryers are available. Like air compressors, there are many different types of air dryers, and each one offers unique features that make it suited for different purposes.

Why Are Air Dryers Important in a Compressed Air System?

Air dryers are commonly found in manufacturing plants, being used to help with day-to-day operations. They can also be found in many naval applications as well. Air dryers are essential when you have an air compressor, regardless of the type. Air compressors produce a lot of moisture and, while excess moisture can be drained, it can't be entirely removed. Water often sticks around as a vapor or aerosol, which is challenging to remove. Air dryers can help remove the excess moisture in the air from air compressors.

In many cases, any problems with your air compressor are often caused by the accumulation of excess moisture. Excessive water in compressed air systems causes operational problems such as freezing of outdoor lines, fouling of processes and corrosion of equipment. Drying prevents water from forming downstream, where it can cause product spoilage, operating problems and costly repairs.

Compressed air dryers are devices that take compressed air and remove the water vapor from it. By concentrating water vapor, the air compression causes condensation as the air cools downstream. A variety of commercial and industrial facilities use these devices.

A compressed air dryer prevents moisture from condensing by lowering the pressure dew point to a point lower than the temperature to which airlines are exposed. The pressure dew point — evaluated at 100 pounds per square inch gauge — is the temperature at which moisture begins to condense at a given pressure. The lower the dew point, the dryer the air. If the goal is to eliminate corrosion in the air system piping, you should consider a pressure dew point of -20 degrees Fahrenheit or less.

You can tell whether your air compressor system needs an air dryer by looking out for these signs:

• The manufacturer label on your air compressor specifies the need for dry air.
• Rust or corrosion starts to appear on parts of your system, such as the pipelines.
• Water vapor or liquid comes out of the exhaust, hoses or lines.
• You can identify water spots on your system's air tools.

An air dryer will help prevent your compressed air system from taking unnecessary damage that can impact operations and require expensive repairs or replacements. When operations halt due to damaged equipment, this delay can impact your productivity since you'll have to spend time and money getting everything up and running. Because of this, it's essential to place a high value on compressed air dryers to extend the life of your air compressor and increase productivity.

Types of Air Dryers

Compressed air dryers come in multiple different forms, but before you can choose the right air dryer, you need to understand how your air compressor works and what the needs of your business are. Use the following information to help you narrow down your options for an air dryer:

• The intended use of your compressed air system
• The air temperature requirements
• Dew point pressure requirements
• The operating environment of the air compressor
• The frequency that the compressed air will be used

The specifics will vary based on the needs for your operation and the manufacturing information provided with your air compressor. Once you have all of this information, you can make an informed decision about what air dryer you'll need.

Some industries might require multiple types of air compressors, which means they'll need multiple types of air dryers. Even smaller operations can benefit from investing in a compressed air dryer system since the equipment can help increase your production volume.

There are five main types of compressed air dryers you can choose from:

Desiccant Dryers

Regenerative desiccant dryers — also known as regen dryers, twin tower dryers, pressure swing dryers or temperature swing dryers — soak up moisture on non-liquid desiccants like silica gel, molecular sieves or alumina. Moist air passes into the desiccant bed of one tower at the same time as a second tower dries or regenerates, the desiccant.

Desiccant dryers guard control and instrumentation systems, moisture-sensitive operations and laboratory equipment. They’re useful when an application requires the maximum dew point performance — which can vary from -40 to -100 degrees Fahrenheit. This low dew point protects against airline freezing in the most severe weather conditions. Desiccant dryers should also be used for applications requiring flow rates over 100 standard cubic feet per minute, or SCFM.

There are three possible methods for reviving the desiccant bed.

1. Heatless: When renewing the desiccant bed, heatless dryers employ compressed air to expand the atmospheric temperature enough to dry out the desiccant. Heatless dryers are safer because they don’t require high temperatures or voltages. In addition to being safer, they’re the least costly desiccant dryers, which are ideal for systems smaller than 2,000 SCFM. However, they can lead to high costs in bigger systems because the cost of air required is proportional to the size of the system. The desiccant in heatless dryers lasts from 10 to 15 years with an oil-free compressor.
2. Internal heating: Heated desiccant dryers apply electrical resistance to dry out the desiccants, and are primarily used in the 500-3,000 SCFM range. These dryers are more expensive than heatless dryers, due to the number of additional components. The more complicated controls may require additional skilled personnel to maintain them. Heated dryers can accelerate the aging process of absorbent material, requiring new desiccant material every three to five years.
3. External heating: Blower-purge compressed air dryers dry the desiccant bed without the need for compressed air by opting for an outside blower to push heated air into the regenerating tower. Blower units tend to be the most expensive and are used in bigger systems — typically ones that are over 3,000 SCFM. When there’s a larger desiccant bed, it’s technically challenging to conduct heat evenly without using many heaters. It’s easier to heat the desiccants more evenly when using a blower.

Membrane Dryers

With a dew point range of 20 to 60 degrees Fahrenheit, membrane dryers use a dehumidification membrane that removes water vapor from the compressed air. The variance of the compressed air flow rate and pressure controls the drying power of the membrane. Membrane dryers are quiet, have no moving parts, require no electricity to operate and can operate continuously. They require little maintenance, apart from changing the pre-filter cartridge twice a year. Membrane dryers are ideal for applications requiring dew points of 35°F to 52°F and flow rates up to 600 SCFM.

Point-of-Use Dryers

Point-of-use dryers are compact, low-maintenance, plug-and-play dryers that provide clean, dry air where it’s needed most: the point of use. With a dew point range of -40 to -100 degrees Fahrenheit, they can run for up to 12,000 hours between desiccant replacements. Like membrane dryers, they require little maintenance, apart from pre-filter cartridge replacement.

Refrigerated Dryers

Refrigerated dryers are the most commonly used dryers. They operate by condensing water in a heat exchanger, cooling air enough to condense the entrained moisture and separating it from the air supply. The cold, separated air is then reheated and discharged from the system.

Refrigerant dryers provide a constant dew point range between 38 and 50 degrees Fahrenheit, regardless of fluctuations in inlet temperature. They can’t achieve dew points below freezing. When operating such a dryer in dirty environments, it’s recommended to use a water-cooled condenser.

Refrigerated dryers run continuously, which permits immediate response to airflow demands. They’re used for applications that require constant use at full capacity, although enough low usage can cause problems. Cycle-type refrigerated dryers don’t respond as quickly, but they use less power and conserve energy when air consumption is low. Cycling refrigerated dryers are ideal if you don’t plan to consistently use your compressor at full capacity.

Deliquescent Dryers

Deliquescent dryers use tablets to attract moisture and then dissolve. They produce a dew point range of 40 to 80 degrees Fahrenheit, which is directly related to inlet-air temperature, as it reduces dew point by a fixed amount below inlet-air temperature. Operating costs are directly proportional to air use, consuming no desiccant when there’s no air demand. Deliquescent dryers, however, require daily refilling with deliquescent tablets.

How to Choose the Right Air Dryer: Things to Consider When Choosing an Air Dryer

Choosing the right air dryer will increase system efficiency, increase productivity and reduce downtime. When considering the purchase of an air dryer, the following considerations may be useful:

• Flow rate: Choosing the right air dryer depends on the maximum capacity of your air compression system. Capacity is determined in SCFM at 100 PSIG and can also be approximated by multiplying the air compression horsepower times four.
• Operating pressure: The best air dryer for your needs also depends on the minimum and maximum operating pressure of your system. Dryers are rated at 100 PSIG. For every increase from 100 PSIG, capacity is reduced. As pressure increases, moisture load decreases, reducing strain on the compression system.
• Air inlet and dew point temperature: You should also consider the minimum and maximum operating air inlet temperature, based on your system, and then determine the dew point requirements. The best dryer type will meet those requirements, working effectively at a dew point temperature below the lowest ambient temperature to which the compressed air system may be exposed. You can calculate the required dew point temperature by taking the lowest air temperature and lowering it by 20 degrees. While refrigerated dryers are sufficient for most purposes, critical applications requiring low moisture in the air line may require a desiccant dryer.
• Ambient temperature: By determining the minimum and maximum operating ambient air temperature of your system, you should be able to choose between a low-temperature dryer and a high-temperature dryer. Ambient temperature above 100 degrees Fahrenheit may exceed the maximum inlet temperature of your dryer. Consider larger driers or high-temperature driers, which can withstand higher ambient temperatures during summer months. The ideal pressure dew point for your air system should be lower than the lowest ambient temperature experienced at your facility. If not, moisture will condense in the air lines. You should consider whether air lines are exposed to outdoor temperatures in summer and winter or through air-conditioned areas.
• Application and environment: A significant factor in choosing the right type of dryer involves consideration of air usage. Most applications can use a refrigerated dryer, which produces air with 10 to 20 percent relative humidity. Desiccant dryers, on the other hand, produce less than 0.5 percent relative humidity in the outlet air, and are used in higher-quality air instrumentation applications that require a pure stream of air.

Other factors to consider include weather, indoor vs. outdoor use and location. A quality air compression company should be able to help determine the best air compressor and air dryer for your facility.

Choosing the Best Air Dryer Company

When purchasing an air dryer, consider the following questions when choosing a compressor company:

• How long has the company been in business?
• Does the company manufacture machines, or is the company only a reseller?
• Does the company sell accessories for their machines?
• Does the company have a quality service network?
• Does the company have a broad distribution network?
• Does the company have a good reputation for building quality machines?
• Are they knowledgeable about air compressors and air dryers?

With more than 40 years of experience as a market leader in air pressurization and compressed air treatment systems, Fluid-Aire Dynamics offers one of the most comprehensive parts and service networks in the industry. We have a full line of air pressurization equipment for a variety of industries, including telecommunications and electric utility, and we also offer state-of-the-art design, maximizing reliability with reduced operating expense. 

Choose Fluid-Aire Dynamics for Your Air Dryer and Air Compression Needs

Fluid-Aire Dynamics offers a wide selection of air compressors. No matter what your needs are, we have something to suit them. We offer only equipment and products manufactured by companies we know and trust, so you can be sure you’re getting exactly what you’re paying for.

We not only offer a wide variety of air compressors and dryers, but we also offer filters, aluminum piping, receiver tanks, condensate drains, oil/water separators, air monitoring equipment, generators, used equipment and much more.

But our work doesn’t end with simply selling these high-quality products. We also have years of experience in installing and repairing these types of products. We offer a full slate of repair, maintenance, installations and emergency services to ensure our customers have less downtime and increased profitability.

Contact us to discuss your requirements of Tableting Manufacturers. Our experienced sales team can help you identify the options that best suit your needs.