Dryers

View A Video on Dryers - A Quick Introduction

The general terms food dryers, parts dryers, and industrial dryers are applied depending on what the dryer is used for, regardless of the method used to accomplish their task. While there are many different kinds, there are three main methods of removing moisture from substances: air dryers, infrared dryers and spray dryers. Air drying brings the product to be dried into direct contact with hot or cold air. It is the most widespread method and is used by flash dryers, freeze dryers, fluid bed dryers and rotary dryers for applications in the food processing, paper production, pharmaceutical, pollution control and agricultural industries. Infrared dryers use energy from the infrared wavelength to remove moisture from the inside out without overheating the substance. These dryers are used in paper production as well as in the apparel industry. Spray dryers use a third method. In spray dryers, after any present solid material is separated from the substance, it is directed into a spray of hot steam that quickly causes moisture to evaporate. Spray drying is used in the pollution control, food processing and pharmaceutical industries. Drying can be done in batches or in a constant stream in continuous dryers, which are frequently also conveyor dryers. Conveyor dryers use a conveyor belt to move the substance through a dryer. Both are commonly used in the air drying process.

Most drying units used in homes are simple air dryers. Air warmed by gas or electric heating elements is dispensed into the machine and interacts with the substances or items inside. The moisture evaporates and the resulting vapor leaves through an exhaust vent. Rotary dryers, sometimes referred to as drum dryers, use hot air to heat a rotating drum. Wet substances and materials are placed inside and may pass through multiple chambers before completing the process. Air may come in direct contact with the articles when they pass through the same compartment, or it may be held in a separate chamber whose radiating heat is adequate to dry the substances. Many powders are dried this way, including laundry detergent, metal powders and industrial chemicals. Flash dryers can be classified as air dryers as long as air is the heat source used to dry the substance or item almost instantaneously. A short burst of high heat is used to dry the articles quickly as it passes through the drying duct. At water treatment plants, cyclones or bag filters separate the dried sludge from the remaining liquid before contaminants can be removed.

Fluid bed dryers use a permeable surface to support the substances or materials. Air or gas is heated and released into the dryer until all particles begin to vibrate slightly. The vibration indicates that the velocity of the air slightly exceeds the velocity of gravity. This kind of drying is often used for certain foods, like cereal, that are mostly uniform in size. Freeze dryers are technically air dryers except that in their case, the air reaches such a low temperature that sublimation occurs. Substances become so cold that all moisture freezes. The solid moisture (ice) is then given just enough heat that it becomes a vapor, skipping the liquid stage altogether and leaving the substance dry. Freeze drying is used to preserve certain items and temperature-sensitive materials like blood plasma or penicillin and to produce instant coffee. Infrared dryers do not use air to dry substances. Instead they harness heat energy from certain wavelengths of electromagnetic radiation. Infrared radiation is absorbed by solid matter rather than the air and so dries from the inside out. By matching the absorption characteristics of the material, the drying process is efficient in terms of time and resources. Infrared dryers are being used increasingly for curing coatings and drying ink, either on apparel or on paper. The third method of drying is done by spray dryers. As the substance enters the dryer, any present solids are separated out. The remaining substance is directed into a stream of hot steam that quickly vaporizes the moisture. Dry solids are left behind and collected. Spray dryers are fast and separate solutions into a dry powder and vapor in one step. They are the dryer most often used for food flavorings, pigments, salts and more.

There is great variety of dryers available for drying in all kinds of contexts. Sludge dryers are dedicated to reducing the weight and volume of sludge and slurry by drying solid matter within the sludge and evaporating as much water as possible. Companies that work with chemicals often use these dryers before sending the remaining water to a waste treatment plant. Steam tube dryers are another variation. Material tumbles inside a rotating cylinder with a series of smaller internal tubes providing increased heat transfer surface area. Besides the wide assortment of dryer types and techniques, industrial dryers are also getting progressively larger. With multiple chambers, compartments and heat sources, these machines are able to dry high capacities in small amounts of time. A company in New Zealand recently installed a dryer that can process 27 metric tons of milk powder an hour. At the same time, however, food dryers are now available for personal use and can easily fit on a countertop. Demand for the best and latest dryers in all aspects of their functions have led to innovations and new technologies. Improvements in materials and manufacturing techniques have had an impact on dryers and their potential because with every new development in metalworking or plastics, dryers become stronger, safer and more effective.

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Dryer Types

• Air dryers are the most common types of industrial dryers. The product to be dried comes into direct contact with heated air dispensed by the machine. 
• Continuous dryers remove moisture from a constant flow of substances or items that enter the dryer, are dried and then exit, all without stopping.
• Conveyor dryers use a belt to move materials through the dryer at a continuous feed. Agricultural products are often dried on a belt or similar conveyor dryer system.
• Compressed air dryers remove water and other contaminants from compressed air and the lines that carry it. This is done via refrigerant or desiccant processes.
• Desiccant dryers remove water and moisture from a product with absorption by way of grainy materials such as silica gel or activated alumina.
  
• Flash dryers are industrial dryers that dry a substance or material during exposure to a short burst of high heat. Water sludge is often dried this way at water treatment plants for the easy removal of contaminants. 
• Fluid bed dryers force heated gas through a permeable surface where it comes into direct contact with a bed of solid materials, causing the moisture inside them to evaporate. 
• Food dryers remove moisture from fruits, vegetables, meats and other edible substances. Dried foods preserve nutrients, color and flavor while reducing weight and volume, making them easy to pack and transport.
• Freeze dryers are used to dry heat sensitive materials such as chemicals and pharmaceuticals in small batches.
• Gas dryers use gas to heat and dry a product as desired. These dryers are quite common. 
• Industrial dryers are used for removing moisture from substances, products or materials in a business or manufacturing capacity.
• Infrared dryers are used most often for the curing of water-based inks and coatings found in the painting and the screenprinting industries. These use infrared radiation to quickly dry or set the material.
• Parts dryers remove moisture from machined components by use of blown air or heat.
• Rotary or drum dryers can be gas, liquid or solid fuel fired and pass material through a heated rotating drum. Sometimes the drums have multiple chambers that allow a longer pass to get through the drum.
  
• Spray dryers take a material, such as wet sludge or wet powder, and spin it centrifugally, then direct a line of spray of the material into a drying chamber, which rapidly dries the material. This type of dryer is also called a dispersion dryer.
  
• Thermal oxidizers are used during a certain method of pollution control. They take air polluted with small particles or combustible solids and liquids and oxidize the organic parts before destroying them.
  
• Vacuum rotary dryers are used to provide low temperature drying of materials in the chemical and pharmaceutical industries.

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Dryer Terms

Agglomeration - Adherence of particles to a small mass due to moisture, static charge or chemical or mechanical binding.
 
Bag Filter - A bag that filters air by catching air-borne particulates.
 
Boiling Point - The temperature at which particles within a liquid move rapidly enough to escape the surface and become gas or vapor.
 
Bulk Density - The weight per unit volume of solid materials, expressed as pounds per cubic foot or grams per cubic centimeter.
 
Conductive Drying - Heat transferred by conduction through a solid (the steel shell) of  industrial dryers, separating the heating medium (steam/oil) and the material being heated. Vapor is usually removed by a combination of a condenser and a vacuum pump.  
 
Contactive Drying - Direct contact between the material and the air heated industrial dryers.
 
Diffusion - The movement of molecules away from the center of concentration of a substance.
 
Dry Gas - Any gas or gas mixture that contains no water vapor and/or in which all of its constituents are substantially above their respective saturated vapor pressures at the existing temperature.
 
Evaporation - The process of extracting moisture.
 
Flash Point - The minimum temperature at which vapor forms at the surface of a liquid.
 
Humidity - The moisture content of air.
 
Latent Heat - The energy required by industrial heaters to change a substance from one state to another.
 
Manometer - An instrument for measuring fluid pressure.
 
Non-Hygroscopic Material - A material that contains only unbound moisture.
 
Oxidation - Increasing the amount of oxygen within a molecule.
 
Quench - The rapid cooling of combustion exhaust with showers of water.
 
Radiation - Heat transfer from industrial dryers by infrared radiation through space (air or vacuum), from a warmer mass to a cooler mass.  
 
Sublimation - The direct vaporization of a solid by heating using industrial heaters, without passing through the liquid state.
 
Throughput - The maximum potential of industrial heaters, devices, or other industrial machines, expressed in amount per time.
 
Vapor - A visible suspension in the air of a substance's particles.
 
Vitrification - A high-temperature combustion process that melts and converts non-combustible inorganic materials.

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