At present, there are three types of coolers used in feed companies: vertical, horizontal and countercurrent. Countercurrent cooler with its high degree of automation, small footprint, small suction capacity, low power consumption advantages, quickly replace the other two coolers to become the current mainstream products for the feed business widely used
Countercurrent cooler is the use of counter-current cooling principle of high temperature and humidity of the pellet feed cooling, equipment mainly by the closed feeder, bulk feeder, cooling silos, discharge devices and other components. According to the different structure of the discharge device, the derivative series is more, the current technology is more mature two discharge device is the sliding gate discharge mechanism and the flap discharge mechanism (also known as "rotary valve type discharge mechanism"). Jia Yuan production of counter-current cooler, SKLN series cooler is used in the sliding gate discharge mechanism, SLNF series cooler is used in the flap discharge mechanism.
In the course of use, the countercurrent cooler should consider the following aspects in order to facilitate the equipment in an efficient and economical state of operation.
1.1 airflow state
For countercurrent coolers, the most important air flow parameters are: air volume, wind pressure and wind speed.
The calories and moisture in the pellet feed are carried by the air medium, and there is sufficient air volume to ensure the production of the countercurrent cooler. For the different specifications of the pellet feed its reference cooling air flow in Table 1.
Table 1: Particle feed cooling time and reference air volume
Particle size (mm) Cooling time (min) Reference air volume (m3 / min)
Φ4-φ5 5-6 22.6
Φ6-φ8 6-8 25.5
Φ10 7-8 28.3
Φ12 8-10 31.1
Normally, the SKLN series and SLNF series coolers are equipped with fans according to the reference air volume of φ12 mm.
It is important to determine a reasonable range of wind speed when cooling. Normally, 1.8m / sec is a generally accepted indicator. At the same time, the wind speed in the suction duct should be 13 ~ 16 m / sec. In the conventional sliding gate countercurrent cooler, the wind speed is too large will lead to agglomeration, knot arch and nesting uneven, but also lead to bed "fluidization", resulting in different parts of the material retention time is different, the cooling effect is inconsistent The In addition, the control of wind speed can effectively avoid the "wind tunnel" in the material bed, to prevent air flow short circuit, affecting the cooling effect.
Under normal circumstances, SKLN series and SLNF series cooler material layer thickness between 0.7m ~ 1.1m, the recommended wind pressure should be ≥ 200mmH2O.
1.2 Counterflow cooler discharge port area
The maximum volume of the countercurrent cooler can be determined by considering the cooling air volume of the unit pellet feed and the maximum wind speed. Therefore, in order to obtain higher production, should increase the cooling air volume, at the same time, should increase the corresponding discharge system area. SKLN series and SLNF series cooler discharge mouth are used square or rectangular. This design, compared with the same size of the circular countercass cooler has a better ground space utilization, production capacity increased by 28%. This is also the reason why the SKLN series and SLNF series coolers have greater cooling capacity.
1.3 to prevent knitting and caking
The higher the cooling capacity, the greater the amount of air required. For countercurrent coolers, this means an increase in the probability of knitting and agglomeration. If a larger flow counter-flow cooler is to be obtained, the discharge device must meet certain requirements, that is, the larger air volume required to increase the yield and Under the premise of higher wind pressure, it can still be even and reliable discharge. The smaller size of the sliding grille and the flap countercurrent cooler are not so obvious in this respect, but when the cooling yield is large or the easy-to-arch pellet feed is treated, there is a certain advantage as an improved flap countercurrent cooler. In the nesting, the flap at one end of the lift, play the role of arch, so as to effectively prevent the occurrence of the grid gate conveyor structure of the arch and agglomeration phenomenon.
1.4 Some reference suggestions
Data show that the higher the water content of feed particles, the faster the evaporation of water, the cooling effect is better. Analysis of the reasons, it is easy to see the evaporation of water requires heat, and in the cooler to provide heat is the feed particles themselves, which is the cooling treatment of hope. Therefore, within the allowable range, should consider appropriate to improve feed moisture, which is conducive to improving the cooler production.
Countercurrent coolers can cause problems when starting and clearing: the thickness of the material in the cooling bin is thin and the pressure drop becomes smaller, resulting in a larger air volume. Feed particles are sucked into the duct, and the material bed produces "wind tunnel". Can you discuss setting up an automatic control damper to solve this problem.
2 Drying equipment for feed processing
In recent years, the development of China's feed industry has shown a new trend, that is, extrusion technology with its unique advantages, and gradually for the feed manufacturers are widely accepted. To sheep "century dragon" MY165 extruder as the representative of the large puffing equipment, into the feed processing industry, and soon became the production of high-grade puffed aquatic feed one of the main models.
Under normal circumstances, the extrusion of the product is relatively high moisture, floating fish feed, for example, after the mold is usually in the water 21% to 24%, and puffed aquatic feed safe storage of water is generally required to control about 10% Only ordinary cooling equipment has been unable to meet the needs of water, which requires the production process set up a special drying section, the purchase of specialized drying equipment.
At present, the feed industry used in the drying equipment is divided into vertical dryer and horizontal dryer (also known as "horizontal dryer") two, its characteristics in Table 2. Vertical dryer is now rare, and in recent years the new large and medium-sized aquatic feed mill in the majority of the use of horizontal dryer, the main representative models are: SFGZ series of float dryer and SKGD series belt circulation dryer.
Table 2: Vertical dryer and horizontal dryer characteristics comparison
Horizontal Dryer Horizontal Dryer
Covers an area of small requirements
Plant height requirements level
Mechanical structure and maintenance requirements Simple structure, low maintenance requirements Low structure, high maintenance requirements
The precipitation is too small
Finished moisture uniformity uniformity uniformity is good
To maintain the original grain shape easy to crush, easy to maintain the basic remains
Unit energy consumption is higher and lower
The degree of automation is low
Most horizontal dryers contain single or multi-layer conveyor belts (which can also be perforated steel track) to support the material in the drying room to move. The hot air passes vertically through the material layer and exchanges heat and moisture with the material on the conveyor belt, followed by a special passage. This is the general working principle of horizontal dryers. As the material moves with the conveyor belt in the drying chamber, its movement is slower, the path is longer, the drying time is also increased compared with the vertical dryer, and all the materials have roughly the same external drying conditions, so the horizontal dryer in a During the drying process, both in terms of precipitation or finished moisture uniformity are superior to vertical dryers, which is the drying principle and mechanical structure of the horizontal dryer.
Drying is a more complicated process, subject to many factors, such as drying time, hot air temperature, hot air volume, material properties and geometries of the material components will affect the final drying effect, specifically reflected in the product that is the product moisture and Water heterogeneity of the two main indicators. An excellent and stable drying system not only refers to a powerful dryer, but also should include such as auxiliary machinery, thermal systems, air systems, electrical control systems and other parts. Good drying effect is the result of the combined effect of all the components in the drying system, and should be considered in the selection and operation of the following aspects:
2.1 Spreader
For the horizontal dryer is concerned, the superiority of the spreader directly affect the moisture content of the product is a dry system in a more important part.
Spreaders spread the material evenly on the conveyor belt, making the conveyor belt on the air volume is roughly the same, which is one of the prerequisites for product moisture. Spreaders can be subordinate to the dryer, can also be independent of the formation of a separate device, its form more, such as oscillation, horizontal rake, oscillation conveyor belt and so on. Users can choose according to the actual needs of the appropriate form, do not damage the appearance of the material is in addition to the spread of the other principles.
At present, the more mature is the oscillating spreader, its mechanical structure is simple, reliable transmission, oscillation frequency and amplitude adjustable, spread out better, suitable for most aquatic puffed feed. SKGD series belt circlip dryer on the use of this distributor.
2. 2 system and airway
Should be considered to provide adequate and stable heat dryer, or can not guarantee the stability of the dryer work. Feed dryer can choose steam, heat oil or gas as a heat source, which gas efficiency is higher.
As some of the feed processing process requires steam intervention, such as granulation, puffing, etc., in general, should be preferred steam as a dryer heat source, which can reduce the initial investment in construction, saving daily management costs.
Heat exchangers are the core components of the heating system. From the practical application effect, the performance of the steam heat exchangers of the steel and aluminum composite finned tubes is better, and the finned tubes are not easily deformed.
The traditional horizontal dryer uses a larger heat exchanger, passing the hot air into the drying chambers through a number of branch air ducts. This approach is the problem caused by the drying room air volume distribution is not easy to control, the branch of the air duct are required to do insulation treatment, and take up the dryer outside the larger space, the structure is more complex. SKGD series belt circulation dryer using the drying room independent heating and built-in air, to solve the problem. At the same time, the use of hot air makes SKGD series dryer greatly reduce the energy consumption, the hourly handling capacity of 3.5 tons (φ3mm floating puffed fish), moisture from 24% to 10% of the steam consumption of about 1.5t / h.
2. 3 control system
The control system of the dryer should take into account automatic or partial automatic control, as much as possible to reduce the product differences due to human operation.
The automatic control of the feed drying system is difficult to achieve because there are many factors that affect the drying process and the need for control links. The cost of the entire control system is expensive, which is not economical for feed drying. Then the more realistic method should be on a more important parameter to implement automatic control, the other parameters of the manual adjustment. In the SKGD series dryer, the drying temperature is automatically adjusted, while the air volume and drying time is manually adjusted. The automatic control of the temperature ensures that the dryer is still able to operate in a relatively stable temperature range when the steam fluctuates.
2.4 processing of the material
<P> The composition and physical properties of the material also have a significant effect on the final drying effect:
① If the feed formula contains a high proportion of oil, is detrimental dry.
② When the feed particles are more dense, it is not conducive to the internal water particles along the capillary to the particle surface diffusion.
③ feed particles larger size, the need for longer drying time.
④ all the size of the feed particles should not be too different, otherwise it is difficult to obtain good dry unevenness indicators.
⑤ Under normal circumstances, the nature of feed than the floating feed is difficult to dry.
2.5 Some reference suggestions
① dry need heat, within the allowable limit, the higher temperature, is conducive to improving the efficiency of the dryer. But under high temperature conditions, will affect the nutritional value of feed, such as non-enzymatic browning. Usually the feed drying temperature should not exceed 120 ° C. It is also possible to consider keeping the feed in the high temperature (100 ℃ ~ 200 ℃) environment for several minutes, so that the particles quickly heat up, then reduce the hot air temperature, complete the remaining drying operations.
② should be avoided in the early dry layer of particles caused by rapid water loss, the formation of "water seal", damage the surface of the particles connected to the internal and internal capillary, blocking the core of the final core moisture diffusion.
③ any material drying process will take some time. In a drying process, should not frequently change the operating parameters, should be left to the dryer a certain reaction time, the only way to get the best operating parameters of a material.
④ at the beginning and end of the dry, because the material can not be covered with the entire conveyor belt, most of the hot air will pass from the hole, the product may make the drying effect is poor. You can try to preheat the preheating dryer before the feed, before the end of the appropriate way to reduce the speed of conveyor belt to solve this problem.