CONCURRENT VS. COUNTERCURRENT DRYERS
There are two basic types of Direct Fired Dryers:
• Concurrent (or parallel flow)
• Countercurrent (or counter flow)
In both types of dryers, the material to be dried comes into direct contact with the hot process gases. These gases are generally provided as products of combustion from an oil, gas or solid fuel fired furnace in temperatures ranging from 250 to 1,000 degrees Celsius.
To obtain greater thermal efficiency or process materials that may be hazardous, exhaust gas recycling may be used in direct fired dryers. A large proportion of the exhaust gas from the dryer can be recycled back to the air heater. The increased humidity makes for a safer environment through the displacement of the majority of the oxygen with water vapor. Recovering heat from the exhaust also saves a significant amount of energy. Further treatment of the humid exhaust gas can be accomplished economically through the use of a wet scrubber, scrubber/condenser or evaporator system.
Concurrent Rotary Dryer
In a concurrent dryer, wet material and the hot process gas enter the dryer on the same end. Thus, dry material and the exhaust vapor exit the dryer on the same end. Concurrent dryers are the most commonly used and are used for drying materials that contain a high moisture content, are heat-sensitive, or tend to stick or cake.
There are two primary advantages to using a concurrent dryer:
1. Since both the material and the gas enter the dryer in the same location, the wettest material comes into contact with the hottest gas. This helps the surface moisture to evaporate very quickly. This configuration also allows the dryer shell to operate at a somewhat cooler temperature than in countercurrent, which prolongs the life of the dryer and reduces the need for maintenance, keeping both the dryer and the material from being overheated.
2.Because the dry material and vapor leave the dryer on the same end, there is a direct relationship between the temperature of the exhaust gas and the dried product’s condition. This enables the system to adequately respond to the exhaust gas temperature, allowing for immediate heat load adjustment to changes in conditions upstream of the dryer. The dried product comes into contact with the gas at its lowest temperature, which enables control of the moisture content, generally through keeping the exhaust gas temperature at a pre-set value.
Concurrent dryers are used for a variety of applications, depending on whether the heat transfer is by convection or by a combination of convection and radiation.
In cases where heat transfer is by convection, the wet material comes into contact with the hottest gases, which are supplied by an external source. This application is used for filter cakes, minerals, fertilizers, clays, coal/coke, phosphates, animal feeds, sludges and stillage.
In cases where heat transfer is by a combination of convection and radiation, the wet material comes into contact with the hottest gases, which are supplied by an internal burner, providing higher thermal efficiency. This application is best suited to heavy ores, sand, refractory materials, crushed rock and stone, slag and limestone/clay separation.
Countercurrent Rotary Dryer
In a countercurrent system, wet material is fed into the dryer on one end, and the hot process gas enters the dryer on the opposite end. Similarly, the dried material exits from the end of the dryer opposite the exhaust vapor.
Countercurrent dryers are well suited for materials that need to be dried to high temperature levels, in cases where the final traces of moisture are difficult to eliminate or where a higher product temperature is required. Countercurrent dryers are also useful as combined dryer/preheaters.
A countercurrent dryer is useful in two specific types of drying processes:
1. The material that will be dried also needs to be heated. Some aggregate and ore processing jobs require that the material be heated as well as dried. In such applications, the wet material is fed into the cold end of the drum and moves toward the hot end. Because the exhaust vapor and the material exit the dryer at opposite ends, there is no correlation between the two temperatures.
2. All materials have some level of internal moisture. Materials with a high percentage of internal moisture need ample time to permit the water to move to the particle’s surface for evaporation.
Countercurrent dryers are used for a variety of applications, depending on whether the heat transfer is by convection or by a combination of convection and radiation.
In cases where heat transfer is by convection, the dried product comes into contact with the hottest gases, which are supplied by an external source. This application is best suited for sugar, chemical salts, silica gel, ores and minerals, pigments and crystalline products.
In cases where heat transfer is by a combination of convection and radiation, the dried products comes into contact with the hottest gases, which are supplied by an internal burner. This application is used for gravel, limestone, crushed rock, sand, roadstone, slate, combined drying/preheating and calcining.
Other Rotary Systems
Direct Fired Calciners are practical for high-temperature applications in which the state of the material needs to be changed. In most calciners, an oil or gas burner fires directly into the discharge end of the system. The material can be heated in three ways: by radiation from the burner flame, conduction from the refractory lining and convection through contact with the hot process gases.
Indirect Heated Systems are used for dusty, fine materials and low-temperature calcining processes. In an indirect heated dryer, the rotary drum is partially enclosed and heated by a series of burners mounted in an insulated jacket.
Rotary Coolers are similar to rotary dryers, except that they always operate in countercurrent flow. In a rotary cooler, instead of hot gases, ambient or chilled air is brought into direct contact with the material. This type of system is generally used in applications with lower temperature, coarse products. For higher temperature applications, an indirect rotary cooler can be used. These units are used for cooling the hot materials discharged from kilns, calciners or ovens.
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