Industrial dryers: types, faults and how to monitor them

The purpose of drying within an industrial process is to extract as much moisture as possible from certain granular substances. Industrial dryers are used to accomplish this task.

Dryers are mainly used for drying materials in the food, textile, agricultural, pharmaceutical and paper industries, among others. The function of drying is to achieve the ideal standard of product quality, to help preserve products and also to extend their shelf life.

Industrial dryer applications

In the food industry, the drying process has the function of substantially reducing the volume of water in food, increasing the osmotic pressure and thus hindering the proliferation of contaminating microorganisms that contribute to its decomposition. 

In pharmaceutical production, the same process is used to prepare the substances for the next stages, such as encapsulation, and to maintain the quality of the formula. Industrial dryers are also used in the processing of ores and minerals, and in the plastics and petrochemical industries.  

Main types of industrial dryers 

Rotary dryers  

Rotary dryers are assets for continuous use and their operation is relatively simple, with low operating costs, and they also have a wide range of applications for drying various materials. In addition to drying, continuous rotary dryers heat, cool, mix, granulate and calcine. They are therefore ideal for processing granular and fibrous materials. Pharmaceutical, chemical, food and fertilizer industries are some examples of processes that require rotary dryers.

A rotary dryer consists of a rotating cylindrical steel drum, positioned horizontally or inclined, which circulates the flow of solid matter and hot air. This drum is supported by a set of rollers and bearings and expands as the temperature rises. Suspenders inside the drum convey the material to the top of the dryer and throw it into the hot air flow. On the outside, stops act to reduce the build-up of substances on the walls inside the drum.

The direct or chain drive generates the rotational movement of this type of dryer. In short, the operation begins with the material to be processed entering the drum, which flows parallel to the hot air currents (or in the opposite direction).

In turn, the solid material inside the drum interacts with axially arranged suspensors, promoting upward and downward movement. As it descends, the material comes into contact with the hot air, resulting in the drying, heating, mixing and uniformization of the material.

There are subcategories of industrial rotary dryers: indirect steam tube rotary dryer; direct rotary dryer; roto-louvre direct rotary dryer; direct contact rotary dryer.

Vibrofluidized dryers 

Vibrofluidized dryers are devices used in the continuous drying process. They work by using vibrational energy, which is recommended for dispersed materials.

The main function of these dryers is to promote an increase in mass transfer coefficients, which results in more efficient drying. Thus, they become especially useful for drying thermolabile materials, such as pharmaceuticals, cosmetics and biological materials, which require short residence times, high drying rates and low temperatures.

Vibrofluidic dryers are commonly used to dry granular materials, pastes and powders, such as pharmaceutical, biological and food materials. This includes, for example, the drying of enzymes, which are biomolecules that act as catalysts in various chemical reactions.

Tray dryers  

Tray dryers are mainly used for dehydrating foods such as fruit and vegetables. This asset is nothing more than a thermally insulated chamber with heating and ventilation systems that circulate the hot air over the trays.

These assets have centrifugal or axial fans to keep the heated air circulating over the trays. Approximately 10 to 20% of the air is new, and the rest is recirculated. Therefore, during drying processes with dryers of this type, the trays must be covered in order to dry the products more evenly.

Tray dryers are more common in low production applications, offering more versatility in operation. The difficulty in using this type of asset on a large scale is the work required to load and unload each batch.

Atomization dryers 

Spray drying is a continuous process in which a liquid or pasty substance becomes a powder. To do this, the material is sprayed and subjected to a controlled stream of hot air inside the chamber. The solvents then evaporate and separate solids and solubles. The result of this process is the original material in powder form.

The chemical and food industries use this type of dryer the most. Some examples of products that can go through the atomization drying process are milk, juices, coffee, soups and detergents, to name the most common.

Fluidized bed dryers 

Fluid bed dryers dry products that break easily. They work with a high flow of hot air under the product. The basic operating concept is to transport the product through a constant flow of air, making the product sit on an air cushion.

The high pressure at the air inlet transports the product through the air to the outlet. In addition, this type of dryer requires continuous feeding. It is commonly used in the food, pharmaceutical, chemical and mineral industries.

Fluid bed dryers can be classified according to the type of bed: vibrating and static. The vibrating fluid bed dryer is a very successful improved fluid bed dryer and is increasingly becoming the main type of drying equipment. The static fluid bed dryer has no moving parts and the bed remains fixed in position during the drying process.

In addition, fluid bed dryers can be classified into two categories according to their production methods: continuous and batch. A continuous fluidized bed dryer consists of a series of chambers connected horizontally by air chambers. A discontinuous fluidized bed dryer dries materials in cycles.

Tunnel dryers and conveyors 

Tunnel or belt dryers are assets that allow the drying and continuous transportation of materials for dehydration. As such, one of the components of this type of dryer is a belt, made of steel or mesh, which allows hot air to pass through the material placed on it. The drying chamber or tunnel, in turn, has heating, ventilation, recirculation and exhaust systems.

This type of dryer dries on a large scale, so that the load of raw material is sufficient for the asset to operate continuously. They are an essential part of many industrial processes where moisture removal is crucial to the quality and durability of products.

Main components of industrial dryers

Despite the variations, the main components of industrial dryers are as follows:

Exhaust system: Removes the moist air from the drying chamber to the outside of the system. The presence of filters prevents the release of polluting particles into the environment.

Drying Chamber: This is the space where the drying process of the materials takes place. It can be a closed structure (such as a tunnel) or an open chamber with conveyor belts.

Ventilation system: Responsible for circulating the hot air inside the drying chamber. It can include fans to move the air and distributors to ensure even heat distribution.

Heat Source: Provides thermal energy to heat the air circulating in the drying chamber. Gas burners, electric heaters, steam or other means of heating can generate the heat.

Conveyor belts or trays: Used to transport materials through the drying chamber continuously. Belts are designed to withstand high temperatures and varying material loads.

Controls and Instrumentation: These include thermostats, temperature controllers, automation systems and monitoring devices (such as humidity and temperature sensors) to ensure that the drying process is controlled and efficient.

Thermal Insulation: Insulation material around the drying chamber to reduce heat loss and improve the energy efficiency of the system.

Conveying and Feeding System: In some cases, especially in more complex dryers, there may be automatic systems to feed the materials into the dryer and to remove them after the drying process.

Main failure modes in industrial dryers

The most common failure modes are related to the heating system, where problems with the source of the heat, such as burners, electrical resistors or steam systems, can result in inconsistent temperatures inside the drying chamber. This, in addition to compromising the efficiency of the drying process, can also lead to products with lower than expected quality.

In addition, misalignment faults, excessive wear or damage to dryer belts or trays result in production line stoppages and difficulties in the continuous transportation of materials through the dryer. An inadequate flow of materials impacts the efficiency and production capacity of the dryer.

Problems with temperature and humidity control systems can also be serious and damage the process. Faults in sensors, thermostats or controllers can cause uncontrolled variations in operating parameters, directly affecting the quality of the drying process.

Another common fault is related to the ventilation and air circulation system. Blockages in the air ducts, faulty fans or inadequate distribution of hot air inside the drying chamber can lead to uneven drying of materials.

In addition to operational failures, structural problems such as wear, corrosion or damage to thermal insulation can also occur, especially in harsh industrial environments. These issues not only reduce the useful life of the equipment, but also increase maintenance and operating costs.

Solution for monitoring industrial dryers

Dynamox Solution identified a fault in a paper machine’s dryer cylinder, a key asset in the paper industry’s production process.

The analysis of the vibration spectrum confirmed the fault identified by DynaDetect. In the images below, you can see the speed vibration spectrum and the scepter under acceleration. In both, the fault is visible, indicating its severity.

The Dynamox Web Platform allows for different analyses of the data collected. With it, it was possible to follow the distribution of the defect over the bearing using the autocorrelation graph. When it was found that the defect was already spreading outside the bearing’s load zone, the decision to replace it was made.

By detecting the fault, it was possible to schedule the maintenance and carry it out at an opportune time. With the preventive replacement, the machine was down for 6 hours. A corrective action would have taken 15 hours. This prevented the loss of 76.5 tons of paper. Monitoring continued and a reduction in vibration levels was observed, proving that the action was taken at the right time.

Rely on the Dynamox Solution to monitor these assets and avoid failures, downtime and accidents. Get in touch and find out more.