An air dryer is a system or piece of equipment that is used to remove moisture present in the air, particularly compressed air. Ambient air typically has a relative humidity of around 30 to 50%. Compressing air packs higher...
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This article takes an in-depth look at types of industrial dryers.
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An industrial dryer is a high powered complex device designed to remove moisture from machinery, products, materials, food products, and mixtures. They are large rugged durable industrial pieces of equipment capable of drying high volumes of product efficiently with exceptional precision and control.
The descriptor, "industrial dryer", is a generic term used to describe a wide assortment of dryers each of which has a different and unique operating system. In the majority cases, industrial dryers are manufactured and configured to meet the needs of a specific product or manufacturing process. The types of dryers include freeze drying machines, chemical drying processes, air bar drying, continuous tray dryers, oven drying, and pan dryers.
Drying can be defined as the vaporization of absorbed moisture from wet materials. It is a heat and mass transfer process that is completed using a variety of mechanisms. The basis of the process is the transfer of heat from the material to be dried, which causes the absorbed water to vaporize that is transferred to and carried by the surrounding air or inert gas.
During the drying process, the humidity or moisture content of the surrounding air increases. The weight of the dried material decreases while the moisture free weight is constant. Some of the purposes of drying are:
Dryers are used in food, pharmaceutical, agricultural, sugar, pulp and paper, textiles, wood, metallurgical, metal fabrication, and automotive industries.
The difference between the various types of dryers is based on the mechanism used to remove moisture as well as the many different sizes and capacities. Small capacity industrial dryers are used for R&D and research laboratories, which remove a few grams of moisture. Large scale industrial dryers handle tons of wet feed per hour.
The many varieties of industrial dryers make choosing the right dryer with the appropriate type, size, and specifications for a given application a process that has to be carefully planned and considered. Each of the types of dryers offers capabilities and features that can affect the quality and performance of the final product.
Conductive heat transfer is a process where materials to be dried are heated through a board, object, or jacket. With convective heat transfer, the drying process is direct without the use of a board or coils. The two drying processes are classified as direct and indirect where conductive heating is indirect and convective heating is direct.
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.
The process of direct drying is further improved and enhanced with lifting flights that move the material over and through the stream of drying gasses. Lifting flights increase heat transfer and radially improve the drying process.
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.
The process environment for direct drying is tightly sealed and controlled, which is an advantage when working with potentially combustible materials. The fuel used to dry materials does not come in contact with the materials keeping dried objects from being covered with the fuel.
Since the fuel for drying is isolated from the materials being dried, lifting flights or circulating mechanisms are not needed by indirect dryers. Furnace exhaust from indirect drying is kept separate from the drying materials and process gasses eliminating the need to treat exiting gas.
Radiant dryers transfer heat by directly emitting electromagnetic waves to the material being dried. These electromagnetic waves carry thermal energy or heat that penetrates deep into the material. The absorbed heat is conducted to the moisture causing it to evaporate and combine with the surrounding air. Little heat is lost to the surrounding air, which 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. They are cleaner and more environmentally friendly compared to convective and conductive dryers. Radiant dryers are used on adhesives, paints, and coatings on part surfaces.
Types of radiant dryers:
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:
It is the process of converting the feed into a mist of fine droplets to ensure fast drying. The feed is pumped continuously to an atomizer installed on top of the drying chamber. There are several techniques in atomizing the feed:
A hot gas stream is introduced either at the top or bottom of the drying vessel. The drying gas for solids in an aqueous solution is air. Nitrogen gas is used for organic or flammable solvents. The moisture or solvent is removed as the atomized feed is dispersed over the gas. The diameter of the drying vessel is large to prevent the particles from sticking on the vessel walls. The particles settle at the conical bottom of the vessel.
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.
A cyclone separator and a filter separate the particles from the drying gas.
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.
Industrial dryers are a central part of the quality of products and need to be carefully chosen to match the needs of an application and product. Like all industrial equipment, industrial dryers are a substantial investment but, when chosen properly, can last for several decades with little need for maintenance.
Manufacturers and engineers have a clear concept of the type of dryer they require, which has been investigated, researched, and analyzed prior to the publishing of a RFP. The six criteria presented below are the factors that engineers consider before developing their RFP.
The size of the particles in the material and their density determines the amount of drying that will be required. Fine powders have different drying cycles than sludge. Large painted or coated parts have further unique drying needs.
The drying process can be further complicated by attempting to dry items of different sizes with different particulates in the same batch. The density of an item determines the speed at which it moves through the dryer and whether special types of conveyance are necessary.
The starting moisture percentage and the ending moisture percentage need to be calculated. This aspect of the assessment process can be completed by testing the weight differences between the wet and dry percentages of a material. The calculated water weight is used to determine the size of the dryer. This part of the process has to be the most precise since the slightest error can cause the water weight to change resulting in the selection of the wrong dryer.
Another distinction between materials is in regard to whether the water is surface water or absorbed, which significantly guides the drying process. Different methods of vaporizing are used for surface water compared to internal absorbed moisture. Whether the water is internal and absorbed or external and surface helps to decide if any pre-drying procedure is required.
Each type of material reacts differently in the presence of heat with some products having a chemical reaction at a certain temperature. Other materials may melt, burn, or explode when exposed to heat over an extended period or when the temperature reaches a certain level. This factor can be controlled by the heat source since heat sources can cause different reactions.
How an item reacts to heat determines if the dryer is large or small, the amount of time of heat exposure, and whether the heating method should be direct or indirect.
Material handling refers to how the substance to be dried will be placed in the dryer. Wet and sticky material may need to be forced through to prevent them from plugging or clogging the dryer. In those instances, nonstick surfaces or paddles can be built into the dryer with clean fittings to remove places for potential buildup.
Many modern products are extremely fragile and have to be handled with extreme care to avoid damage and waste. This type of situation demands a conveyance method that is non-abrasive and gentle. The reverse is true for coarse or gritty materials that can damage the surface of a dryer and shorten its lifespan.
As with every industrial operation, there are production limits to be met with each production run. The size of an industrial dryer has to meet the production amounts that are expected of a process. Included in this aspect of the selection process is whether the dryer will be running continuously as part of an assembly process or work with batches. Under sized dryers can slow down production and be overloaded with inappropriate demands.
Industrial dryers can be a separate mechanism that is used when needed. Conversely, they can be a central part of an assembly process and must be timed to the needs of the production operation. This part of the selection process determines where an industrial dryer will be located in relation to assembly and production needs.
Industrial dryers are a heavy duty piece of equipment that are an important part of production and product quality. As such, they are purchased using the same parameters as any other type of machinery, which includes their initial upfront cost and the expense of operating them.
Although there are industrial dryer options that are less expensive at first, they can be more costly during operation due to maintenance costs, being inappropriately matched to the needs of an operation, being less efficient, and having a shorter life span. These factors make operating a less expensive dryer more costly by increasing operating costs.
During the planning and research stages, engineers carefully examine the total investment in regard to the purchase of an industrial dryer including its efficiency, enhancement of product quality, and the cost of operation, which should match and exceed the needs of the operation. In essence, it is developing a perfect match between a dryer's abilities and production requirements.
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