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Choosing the right drying technology

Datetime:2015-8-2 14:20:17   Hits:1000

  Dry material for plastics processors for each are inevitable. Meanwhile, in order to produce high quality products, this process is also very important. Choose a reasonable drying technology helps to save costs, reduce energy consumption, while the correct assessment and cost-drying technology has important significance for the selection of suitable drying equipment.

  Increase the water content of the material will gradually reduce shear viscosity. In the process, due to changes in melt flow properties, the quality of products and a range of processing parameters will also change accordingly. For example, dead time is too long causes residual moisture content is too low resulting in increased viscosity, which will cause insufficient mold filling, but also cause yellowing of the material. In addition, changes in certain properties and can not be observed directly with the naked eye to, but only by the material related tests can be found, such as changes in the mechanical properties and dielectric strength.

  When selecting the drying process, the drying properties of the material identification of critical importance. Material can be divided into two kinds of absorbent and non-absorbent. Hygroscopic materials can absorb moisture from the surrounding environment, non-hygroscopic materials can not absorb moisture from the environment. For non-hygroscopic material, any water present in the environment are retained on the surface, a "water surface" and easily cleared. However, non-hygroscopic particles made of materials may also be due to the action of additives or fillers become hygroscopic.

  In addition, a calculation of the energy consumption of the drying process, and may be the complexity of the processing operations and other factors, so the values presented here for informational purposes only.

Convection drying

  For non-hygroscopic material, it can be dried using a hot air dryer. Because the water is just water interfacial tension release material scattered constrained, easily removed. The principle of such machines is to use a fan to absorb ambient air drying and heated to a particular material desired temperature, the heated air passes through the drying hopper, and by convection from heated material to remove moisture.

  Drying of hygroscopic materials are generally divided into three drying sections: The first section is a drying material surface moisture evaporate; drying section will focus on the second evaporator is placed inside the material, then dried slowly lowered, the dry material temperature began to rise; in the final stage, the material reaches the drying gas moisture balance. At this stage, the temperature difference between the inside and outside will be eliminated. At the end of the third paragraph, if the dry material is no longer releasing moisture, which does not mean it does not contain water, but only that the balance has been established between the particles and the surrounding environment.

  In the drying apparatus, the air dew point temperature is a very important parameter. The so-called dew point temperature is maintained at a moisture content of moist air remains unchanged, its temperature drops, when the relative humidity reaches 100% of the corresponding temperature. It indicates the air reaches a temperature corresponding to the condensation of moisture. Typically, the dew point of the air used for drying the lower, the lower the amount of water the obtained residue, the lower the drying speed.

Currently, the production of dry air is to use the most common method of drying the gas generator. The device consists of two molecular sieve in the adsorption dryer is a core, where moisture in the air is absorbed. In the dry state, air flows through the molecular sieve, the molecular sieve to absorb moisture from the gas, the gas is dried to provide dehumidification. In the regeneration state, the molecular sieve is heated to a regeneration temperature of hot air. Gas through a molecular sieve to collect moisture is removed, and will take them to the surrounding environment. Another method is to generate a dry gas to reduce the pressure of the compressed gas. The advantage of this approach is that the compressed gas supply network has a lower pressure dew point. After the pressure is reduced, the dew point of about 0 ℃. If you need to lower the dew point, you can use a membrane or adsorption dryer before the compressed air pressure is reduced to further reduce the dew point of the air. (Flash dryer)

  In the dehumidified air drying to produce the energy required for the drying gas must perform additional calculations. In the adsorption dryer, the molecular sieve must reproduction state from the dry state temperature (about 60 deg.] C) is heated to a regeneration temperature (about 200 ℃). For this reason, the usual practice is to be heated gas through the molecular sieve is heated to a regeneration temperature continuously until it reaches a certain temperature when leaving the molecular sieve. Theoretically renewable energy by the heating necessary for its internal molecular sieve adsorption energy of water needed to overcome the adhesion energy of molecular sieve water needed to form several parts of energy evaporation of the water and steam heating required.

  In general, the molecular sieve adsorption resulting dew point temperature and moisture carrying volume. Typically, less than or equal to a dew point of 30 deg.] C, molecular sieves can reach 10% moisture carrying capacity. To prepare the drying gas, the energy obtained from the calculated theoretical value of the energy requirement is 0.004kWh / m3. However, in practice this value must be higher, because the calculation did not take into account the fan or heat loss. By contrast, the specific energy consumption of different types of drying gas generator can be determined. In general, the dehumidification drying gas consumption between 0.04kWh / kg ~ 0.12kWh / kg, which depends on the material and the initial moisture content varies. In practice, it may reach 0.25kWh / kg or higher.

Drying energy required micelle consists of two parts, a part of which the material is heated from room temperature to the temperature of the drying energy need, the other is the energy required to evaporate moisture. In determining the amount of material needed for the gas, the temperature of the drying gas usually enters or leaves the drying hopper basis. Certain temperature dry air by convection heat delivered to the micelle is also a convection drying process.

  In actual production, actual energy consumption value may be much higher than the theoretical value. For example, the material may be in the drying hopper residence time is too long, the completion of a large amount of gas consumed by drying, molecular sieve or adsorption capacity is not sufficiently exerted and the like. ? Reduce the drying gas demand thus feasible ways to cut energy costs is to use a two-step drying hopper. In this drying apparatus, the drying hopper is only half of the material has not been heated and dried, it is possible to exhaust ambient air or heat drying process to complete. Using this method, often only needs to supply the drying hopper drying gas is typically 1/4 of the quantity? 1/3, thus reducing energy costs. Another way to improve gas dehumidification drying efficiency is controlled by a thermocouple and the dew point of regeneration, while the German Motan companies use natural gas as fuel to reduce energy costs.

Vacuum drying

  Currently, vacuum drying were also entered into the field of plastics processing, for example, U.S. Maguire company developed vacuum drying equipment is already being applied to plastics processing. This type of continuous operation by a machine mounted on the carousel composed of three cavities. In the first place cavity, when the particles are filled, the access is heated to the temperature of the drying gas to heat the particles. In the gas outlet, when the material reaches the drying temperature will be moved to a second evacuated chamber. Since vacuum reduces the boiling point of water, so the water is more easily evaporated into vapor, therefore, the moisture diffusion process is accelerated. Due to the presence of a vacuum, resulting in particles between the interior and the surrounding air to produce a greater pressure differential. Under normal circumstances, the material in the second cavity of the residence time of 20min? 40min, and for some hygroscopic materials, the need to stay up to 60min. Finally, the material is fed to the third chamber, and is thereby removed from the dryer. (Flash dryer)

  Dehumidification drying and vacuum drying gas, the heating energy consumption of plastic are the same, because both methods is carried out at the same temperature. However, vacuum drying, drying gas itself does not need to consume energy, but the energy needed to create a vacuum, creating a vacuum energy required and the amount of dry material and the water content related.

Infrared drying

  Another method of drying particles is infrared drying process. In convection heating, between gas and particles, particles and between particles and particles inside the thermal conductivity is very low, so the heat conduction is greatly limited. While using infrared drying, since the molecules by infrared radiation, the absorbed energy is converted into thermal vibrations directly, which means that the material is heated more quickly than in a convection drying. Compared with convection heating, in the drying process, in addition to the ambient air and the partial pressure of the water particles outside the differential, infrared drying and a reverse temperature gradient. Generally, the temperature of the drying gas and the particles heat difference between the larger, faster the drying process. Infrared drying time is generally 5min ~ 15min. Currently, infrared drying process has been designed to Gatling model, that has taken a turn along the inner wall of a pipe threads, particles are transported and recycled in the center section of the Gatling several infrared heaters. In the infrared drying, the power device can be referred to 0.035kWh / kg? 0.105kWh / kg standard selection.

  As mentioned above, the water content of the different materials will lead to differences in process parameters. In general, the residual moisture content may be different because of the different rate of flow of different materials, so the drying process is interrupted or start the machine, outages can cause different residence times. In the gas flow rate is fixed, the general performance of different materials in circulation for the exhaust temperature curve changes and temperature changes. Dryer manufacturers measure in different ways, and the drying gas flow and the amount of material to be dried match, and then adjust the temperature profile of the drying hopper, so that the dried particles at a temperature experienced stable residence time.

  In addition, different initial moisture content of materials can also lead to instability of residual moisture content. Since the residence time is fixed, significant changes in initial moisture content will inevitably lead to a residual moisture content occur equally significant changes. If desired residual moisture content of stable, you have to measure the initial or residual moisture content. Due to the low correlation of residual moisture content, online measurement is not easy to carry out, and the material in the longer residence time in the drying system, the residual moisture content as an output signal can cause problems controlled system, so the dryer manufacturers have developed a kind of new control concept can achieve a stable residual moisture content of this target. This control concept to maintain a steady component containing residual water for the purpose of the initial water content of the plastic, and the dew point of the effluent gas into the gas flow and the amount of particles in circulation rate and other parameters as input variables, so that the drying system can be based on these different variables timely adjustments to maintain a stable residual moisture content.

  Infrared drying and vacuum drying is a new plastics processing technology, these new technologies have greatly shortened the residence time of materials and reduced energy consumption. However, the innovative drying process its price is relatively high. Thus, in recent years, people are trying to improve the efficiency of traditional dehumidification drying gas. Therefore, when making investment decisions, it should be accurate cost assessment should not only consider the purchase cost, but also consider the pipeline, energy, space and maintenance, etc., to make a minimum investment to get the greatest return.

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