This article takes an in-depth look at types of dryers.
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A dryer is a machine that reduces the moisture content of bulk solids, manufactured parts, and other valuable materials. It is a complex system consisting of a drying chamber, heaters, materials handling equipment, forced convection equipment (if applicable), and others.
Drying is an essential material processing procedure. It can be technically defined as the vaporization of absorbed moisture from wet materials. It is a simultaneous heat and mass transfer process that may be accomplished by several mechanisms. Heat is transferred to the material, which causes the absorbed water to vaporize. The evaporated water is then transferred to and carried by the surrounding air or inert gas. The humidity or moisture content of the surrounding air increases after the process. The total weight of the material decreases after the process, and the moisture-free weight is constant throughout. Drying is carried out due to one (or more) of the following reasons:
Dryers are widely used in food, pharmaceutical, agricultural, sugar, pulp and paper, textiles, wood, metallurgical, metal fabrication, and automotive industries, just to name a few.
Dryers are differentiated based on their mechanism in removing a material’s water content. They come in different sizes and capacities: from small-scale dryers found in houses and laboratories, which draw a few grams of moisture, to industrial scale-dryers, which handle tons of wet feed per hour. Therefore, choosing a dryer with the appropriate type, size, and specifications for a given application is essential. Different dryers can have varying effects on the final product and material properties.
Dryers may be categorized based on the heat transfer mechanism:
Direct dryers transfer heat to the material by convection. The heat transfer medium is forced to have direct contact with the material to be dried. A stream of hot air or inert gas serves as the heat transfer medium, which is blown or circulated around and within the material. This hot air carries and supplies latent heat to vaporize the absorbed moisture. The evaporated moisture is then picked up by the blown air.
Due to their efficiency, direct and continuous dryers are the most common configuration of dryers in industrial practice. Their heat transfer efficiencies are higher compared to indirect dryers. However, they have higher operating costs and may have increased energy losses.
Indirect dryers transfer heat to the material by conduction. The heat transfer medium is separated from the material by a conductive partition. The material to be dried is contained inside a drying chamber and is heated from the chamber walls. The latent heat required to vaporize the absorbed moisture is transferred through the wall. The evaporated moisture is then carried up by the surrounding air. The surrounding or carrier air required by indirect dryers is much lower compared to direct dryers.
Indirect dryers are used in drying sensitive products, such as pharmaceutical products and combustible materials since the flow of air circulating the product is relatively low. These dryers prevent the risk of contamination during the process. They are also used in batch drying of relatively low production volumes.
Radiant dryers transfer heat by directly emitting electromagnetic waves to the material being dried. These electromagnetic waves carry thermal energy or heat and deeply penetrate into the material. The heat absorbed by the material heat is conducted to the moisture causing it to evaporate and combine with the surrounding air. Little heat is lost to the surrounding air. This makes the drying process contactless and low risk for contamination. Like indirect dryers, radiant dryers do not require large airflow and may only need a small blower. Moreover, radiant dryers are cleaner and more environmentally friendly compared to convective and conductive dryers.
Radiant dryers are commonly used in adhesives, paints, and coatings on part surfaces.
The types of radiant dryers are the following:
Microwave dryers utilize electromagnetic waves that fall within the microwave region (300 MHz to 300 GHz). Microwave radiation can penetrate beyond the surface of the material. Hence, these dryers are used to dry the material from within (i.e., volumetric drying). The moisture or solvent evaporates from the pores of the material. Microwave dryers are used to process food and agricultural products.
Infrared dryers utilize electromagnetic waves that fall within the infrared region (300 GHz to 400 THz). This region falls just below the visible light spectrum. These dryers emit energy to heat the material's outer surface, and heat is conducted from the outer surface to the inner portion of the material. Infrared dryers are usually equipped with forced convection equipment to increase efficiency.
Industrial dryers operate on either a batch or continuous mode:
Batch dryers process a fixed volume of material at a particular drying duration. A fixed number or quantity of materials is placed in the drying chamber; this quantity is limited by the holding capacity of the dryer. The dried materials are unloaded from the chamber, and the drying of the next batch of materials may be performed. Batch dryers are used in relatively low production volumes.
Continuous dryers accommodate a continuous, uninterrupted flow of materials throughout their operation. Wet materials are continuously fed and transported across the drying chamber. As the material crosses the chamber, it loses its moisture content. The dried products may be gathered by a buffer tank or directly fed to the succeeding equipment. Continuous dryers are used if a large quantity of products needs to be dried.
The types of industrial batch dryers are the following:
Tray dryers consist of several trays, which contain the materials to be dried, stacked inside the drying chamber. Air is used as a heat transfer medium that is forced to move inside the chamber. Blowers facilitate air movement. The air gains thermal energy when it contacts the heating coils. The heated air is blown on top of the material, causing the moisture to diffuse and vaporize. A portion of the moisture-carrying air leaves through the ventilation duct, while the rest is recirculated in the chamber together with a stream of fresh, dry air. The temperature and velocity of the air may be adjusted depending on a given product and operational requirements.
Tray dryers are the simplest type of industrial dryers; they have inexpensive initial costs and consume less space. They are easy to operate and maintain. However, the drying time is typically long. They may not be suitable for powdered solids.
Vacuum dryers operate at a low (vacuum) pressure to hasten the vaporization of moisture or solvent absorbed by the material. The decrease in operating pressure depresses the boiling point of the absorbed liquid; hence, these dryers operate at a lower temperature than other industrial dryers.
The materials to be dried are loaded in trays. The dryer is equipped with a vacuum pump to reduce the pressure inside the chamber. Heat is transferred from the chamber walls to the material by conduction or radiation. Some vacuum dryers have a steam jacket. The vacuum pump draws the vapor; the vaporized solvent may be recovered by condensation and reused if necessary.
Vacuum dryers are used for heat-sensitive, hygroscopic, combustible, and granular products. They are used in food and pharmaceutical products to keep nutrients from degrading in high temperatures. They remove large quantities of moisture efficiently and have fast drying times.
Pan dryers are equipped with an agitator or a mill that stirs wet materials inside a jacketed vessel. The agitator or mill revolves slowly to increase drying rate (i.e., moisture removed per unit time per unit area), ensure uniform heat distribution, and avoid products sticking on the vessel walls. Pan dryers may be operated in an atmospheric or vacuum pressure.
Pan dryers are used in drying heat-sensitive, viscous, and sticky products.
Freeze-drying (or lyophilization) involves freezing the material to be dried and subjecting it under vacuum pressure. It is employed for products that drastically degrade or change at high temperatures. It preserves the material’s biological activity and chemical properties. It is used in food and pharmaceutical products to preserve their nutrient content and active ingredients. It is also used in biological specimens such as blood, tissues, and proteins. However, freeze-drying takes a long time to complete.
Freeze dryers consist of refrigeration equipment, a drying chamber, holding trays, and a vacuum pump. The sample is first frozen to at least its melting temperature to convert the moisture and solvent within the material into a solid phase. The vacuum pump is then activated to decrease the pressure inside the chamber. Heat transfer under low pressure causes the solidified moisture or solvent to transform into vapor. The vacuum pump draws the vapor sublimed off the material. The freeze dryer will continue to run to drive off the moisture or solvent bound to the material.
Bin dryers consist of a holding tank or bin with a perforated plate at its bottom. A fan blows ambient or preheated air through the perforated plate vertically upward. Air surrounds the moist material, causing it to dry. The material may be arranged on shelves, with the wettest material at the bottom. The moisture-carrying air leaves at the top of the bin. Bin dryers are used in drying agricultural and forest products such as wood, fruits, vegetables, grains, bamboo, and others. However, bin dryers may lack drying controls and be used as a pre-dryer.
In tunnel dryers, the materials to be dried are loaded in trays attached to a trolley. The trolleys enter at one and move slowly through the drying tunnel, wherein a stream of hot air or gas is blown. The material dries as it moves through the tunnel. The trays may move in either of the following directions with the air stream:
The settings of the heater zones and the trolley speed may be optimized according to the type and quantity of materials being loaded and final moisture or solvent content.
Tunnel dryers are considered as an enhancement to tray dryers. Materials continuously enter and exit throughout their operation. The material is steady on the trolley as it traverses the tunnel. Hence, these dryers are suitable for products sensitive to damage or deformation, such as automotive components, electronic parts, construction materials, glass materials, and large fruits, vegetables, and food products. Tunnel dryers are operationally efficient in processing materials requiring long drying times.
Rotary dryers consist of a slightly inclined metal cylinder with internal flights. The material is fed from the upper end of the cylinder. The cylinder slowly rotates to lift and cascade the product. Hot air or gas enters in a concurrent or countercurrent direction with the material. The agitation caused by the rotation and internal tumbling action results in rapid and uniform drying. The product is collected at the lower end of the cylinder.
Rotary dryers may be classified as direct-fired or indirect-fired. In direct-fired rotary dryers, the hot gas directly contacts the material inside the cylinder. High-temperature air may be used to roast food products. In indirect-fired rotary dryers, heat is conducted from the cylinder walls to the material being dried.
Rotary dryers are suitable for drying granular and non-sticky materials. They are commonly used in drying products from a crystallization process. They are suitable for drying salts, minerals, grains, nuts, and animal feeds.
In fluidized bed dryers, the wet material is fed to a vessel by a screw feeder. A stream of preheated air flows from the bottom of the vessel to keep the particles suspended or fluidized. This stream flows perpendicular to the direction of material travel. A horizontal air stream may be supplied to help in conveying the particles. The extensive contact of the particles and air results in a faster drying process. Moisture-carrying air exits at the top of the vessel. The particles carried over by this stream are separated by a cyclone separator or a bag filter. The dried products are collected at a point lower than the material inlet.
In vibrating fluidized bed dryers, vibration is added to enhance the fluidization of large-sized particles and sticky and sluggish feeds.
Fluidized bed dryers are used in powdered and granular products such as grains and cereals. Some fluidized bed dryers are designed and configured to enable batch drying.
Agitated vacuum dryers consist of a jacketed vessel with an agitator. The vessel is operated at low pressure and moderate temperature. The agitator stirs the material in order to increase the drying rate and ensure uniform distribution of heat. The vacuum pump draws the vapor. A cyclone filter prevents particles from being combined with the condensate.
Agitator vacuum dryers are also used in drying heat and air-sensitive products. They are usually placed next to filtration equipment.
Spray drying is a quick and efficient process of directly transforming a slurry, suspension, or paste into fine dried powder form. The feed usually contains a high amount of moisture and must be transportable by a pump. It is used to produce soaps, detergents, milk, coffee, tea, salts, polymer resins, flavorings, extracts, pharmaceutical products, and others.
Spray drying involves the following stages:
Drying takes place as quickly as a few seconds. Hence, spray dryers are used in drying thermally sensitive products which degrade when exposed at a substantial length of time.
Flash dryers dehydrate wet granular materials by conveying them across a drying duct. A high velocity hot gas stream flows through the duct that keeps the particles suspended. The heat from the gas stream removes the moisture or solvent from the product. After passing through the duct, the particles are separated from the gaseous stream by a cyclone and a bag filter. A portion of the exhaust gas is recirculated to the duct in order to increase energy efficiency. Flash dryers are used in drying minerals, salts, starches, dyes, and other granular products.
Drum dryers or roller dryers dehydrate liquid feed in order to produce powders, flakes, or granular materials. They consist of one or two internally heated drums that rotate about their axis. Steam is flowing inside the drum, and heat is conducted through the walls. The feed is sprayed onto the surface of the rotating drum or between the two rotating drums. A thin film of the liquid feed sticks and dries on the hot surface. The product's initial and final moisture contents determine its residence time on the drum surface. A scraper removes the dried particles once the desired moisture content is obtained.
In vacuum drum dryers, the drums are placed in a chamber with vacuum pressure. The low pressure inside the chamber decreases the boiling point of the moisture or solvent, causing it to evaporate at a lower temperature. Hence, vacuum drum dryers are suitable for drying heat-sensitive products.
Disc dryers consist of multiple vertical discs mounted on a shaft, which are all contained in a horizontal cylinder. The shaft is internally heated by steam or hot oil; heat is conducted from the shaft to the mounted discs. The discs provide the area for indirect drying. The wet feed in the form of slurry or sludge enters on one side of the cylinder. Moisture or solvent from the feed evaporates as it touches the discs. The materials are transported axially throughout the cylinder. Scrapers are installed inside the cylinder to ensure uniform mixing and prevent the materials from sticking onto the discs. Finally, the products are collected at the bottom of the other cylinder end.
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