At present, there are three main types of coolers used by feed enterprises: vertical, horizontal and countercurrent. The counterflow cooler has quickly replaced the other two types of coolers as the current mainstream products due to its high degree of automation, small floor space, small suction volume and low power consumption. It is widely used by feed enterprises. Gauze Balls,Cotton Gauze Balls,Absorbent Gauze Balls,Disposible Gauze Ball Zhende Medical Co.,Ltd , https://www.zdmedicalproduct.com
The counter-flow cooler uses the principle of counter-current cooling to cool the high-temperature and high-humidity pellet feed. The equipment is mainly composed of a closed-air feeder, a bulker, a cooling bin, and a discharge device. According to the different structural forms of the unloading device, there are many derivative series. The two unloading devices with mature technology are the sliding gate discharging mechanism and the flap discharging mechanism (also called “rotating valve type discharging mechanismâ€). Among the counterflow coolers produced by the Shepherd Group, the SKLN series coolers use a sliding gate discharge mechanism, and the SLNF series coolers use a flap discharge mechanism.
During use, the counterflow cooler should consider the following aspects in order to facilitate the operation of the equipment in an efficient and economical state.
1.1 Airflow Status For counterflow coolers, the important airflow parameters are: air volume, wind pressure and wind speed.
The heat and moisture in the pellet feed are carried away through the air medium, and there is sufficient air volume to ensure the output of the countercurrent cooler. The reference cooling air volume for different sizes of pellet feed is shown in Table 1.
Table 1: Cooling time of pellet feed and reference air volume Particle size (mm) Cooling time (min) Reference air volume (m 3 /min)
Φ4-φ5 5-6 22.6
Φ6-φ8 6-8 25.5
Φ10 7-8 28.3
Φ12 8-10 31.1
Normally, SKLN series and SLNF series coolers are equipped with fans with a reference air volume of φ12 mm.
When cooling, it is important to determine a reasonable range of wind speeds. Typically, 1.8m/sec is a generally accepted indicator. At the same time, the wind speed in the suction pipe should be 13 to 16 m/sec. In the conventional sliding grid counterflow cooler, excessive wind speed causes agglomeration, arching and uneven discharge, which also causes the fluidized bed to “fluidizeâ€, resulting in different retention times due to different materials retention time. . In addition, controlling the wind speed can effectively avoid creating a "wind tunnel" on the bed, preventing the airflow from short-circuiting and affecting the cooling effect.
Normally, the thickness of the SKLN series and SLNF series coolers is between 0.7m and 1.1m, and the recommended wind pressure should be ≥200mmH 2 O.
1.2 The discharge area of ​​the counter-flow cooler combines the cooling air volume of the unit pellet feed with the large wind speed of the Zui to determine the large yield of a certain type of counter-flow cooler. Therefore, in order to obtain higher production, the cooling air volume should be increased, and at the same time, the area of ​​the discharge system should be increased accordingly. The discharge ports of the SKLN series and SLNF series coolers are square or rectangular. This design has a better floor space utilization than the circular shape counterflow cooler of the same size, and the production capacity is increased by 28%. This is why the SKLN series and SLNF series coolers have greater cooling capacity.
1.3 The cooling operation to prevent the higher the arching and agglomeration yield, the greater the required air volume. For counter-flow coolers, this means an increase in the probability of arching and agglomeration. If a counterflow cooler is to be obtained with a larger output, the unloading device must meet certain requirements, ie the large air volume required to increase the output and Under the premise of higher wind pressure, it can still discharge evenly and reliably. Smaller size grate and vane counterflow coolers are less distinct in this respect, but have a certain advantage as an improved flap counterflow cooler when the cooling output is large or the easy to pellet pellet feed is processed. At the time of discharging, one end of the flap is lifted up to break the arch, thereby effectively preventing arching and agglomeration which may occur in the discharge mechanism of the sliding gate.
1.4 Some reference suggestions have data showing that the higher the water content of the feed pellets, the faster the water evaporation rate and the better the cooling effect. Analysis of the reason, it is not difficult to see that the evaporation of water requires heat, and it is the feed pellet itself that provides heat in the cooler, which is what cooling treatment hopes. Therefore, within the allowable range, consideration should be given to appropriately increasing the feed moisture, which is beneficial to increase the cooler output.
When the counter-flow cooler is started and emptied, there may be a problem that the pressure drop becomes small due to the thinner thickness of the material layer in the cooling chamber, resulting in a larger air volume of the fan. Feed pellets are drawn into the duct and a "wind tunnel" is created on the bed. Is it possible to explore the setting of an automatic control damper to solve this problem.
2 Drying equipment in feed processing In recent years, the development of China's feed industry has shown a new trend, that is, the extrusion technology has been widely accepted by feed manufacturers due to its unique superiority. After entering the feed processing industry, the large-scale puffing equipment represented by the shepherd "Century Dragon" MY165 extrusion extruder has quickly become one of the main models for producing high-grade puffed aquatic feed.
Under normal circumstances, the moisture content of the extruded product is relatively high. Taking the floating fish feed as an example, the moisture content after the mold is usually 21% to 24%, and the safe storage moisture of the expanded aquatic feed is generally controlled to be about 10%. It is impossible to meet the need for water removal by ordinary cooling equipment. This requires a special drying treatment section and special drying equipment in the production process.
At present, the drying equipment used in the feed industry is mainly divided into vertical dryers and horizontal dryers (also known as "horizontal dryers"). The characteristics are shown in Table 2. Vertical dryers are rare nowadays, and most of the large and medium-sized aquatic feed mills newly built in recent years use horizontal dryers. The main representative models are: SFGZ series float dryers and SKGD series belt loop dryers.
Table 2: Vertical Dryer and Horizontal Dryer Features Comparison Model Vertical Dryer Horizontal Dryer Floor Space Requirements Small and large plant height requirements High and low mechanical structure and maintenance requirements Simple structure, low maintenance requirements, complex structure, maintenance requirements High precipitation amplitude is small, large product uniformity of water uniformity is poor, uniformity is good, the original particle shape is easy to be crushed, easy to damage, basically maintain the original unit, higher energy consumption, lower automation, lower level, most horizontal dryers, built-in single A layer or multi-layer conveyor belt (which may also be a perforated steel track) is used to support the movement of the material within the drying chamber. The hot air passes vertically through the layer, where heat and moisture are exchanged with the material on the conveyor belt and subsequently discharged through a dedicated passage. This is the general working principle of a horizontal dryer. As the material moves with the conveyor belt in the drying chamber, its movement speed is slower, the path is longer, the drying time is also increased compared with the vertical dryer, and all materials have approximately the same external drying conditions, so the horizontal dryer is in one In the drying process, it is superior to the vertical dryer in terms of precipitation amplitude or product moisture uniformity, which is determined by the drying principle and mechanical structure of the horizontal dryer.
Drying is a relatively complicated process and is subject to many factors, such as drying time, hot air temperature, hot air volume, physical properties and geometry of the material components, etc., which will affect the drying effect of the final product, which is reflected in the product moisture. And two main indicators of water unevenness. An excellent and stable drying system not only refers to a powerful dryer, but also includes parts such as auxiliary machinery, thermal systems, air duct systems, electrical control systems. Good drying results are the result of the combined action of all components in the drying system and should be considered in the following aspects during selection and operation:
2.1 Spreader For the horizontal dryer, the advantages and disadvantages of the spreader directly affect the uniformity of the product moisture, which is an important part of the drying system.
The spreader spreads the material evenly on the conveyor belt, so that the air volume at each point on the conveyor belt is approximately the same, which is one of the prerequisites for uniform moisture of the product. The spreader can be subordinate to the dryer, or it can be independently formed into a single device in many forms, such as an oscillating type, a horizontal cymbal, an oscillating conveyor belt, and the like. The user can select the appropriate form according to the actual needs, and does not damage the appearance of the material is another principle other than the spreading effect.
At present, the more mature one is the oscillating spreader, which has simple mechanical structure, reliable transmission, adjustable oscillation frequency and amplitude, and good spreading effect, and is suitable for most aquatic products. This spreader is used in the SKGD series belt circulation dryer.
2. 2 The system and the air path should consider providing sufficient and stable heat to the dryer, otherwise the stability of the dryer operation cannot be guaranteed. The feed dryer can be selected from steam, heat transfer oil or gas as a heat source, wherein the gas efficiency is high.
Since some aspects of the feed processing process require steam intervention, such as granulation, puffing, etc., in general, steam should be the preferred source of heat for the dryer, which can reduce the initial investment in the plant and save on daily management costs.
The heat exchanger is the core component of the heating system. From the practical application effect, the steam heat exchanger of the steel-aluminum composite finned tube has better performance, and the finned tube is not easily deformed and accumulated.
Conventional horizontal dryers use a larger heat exchanger to deliver hot air to each drying chamber through multiple branch air paths. The problem brought by this method is that the air volume distribution of each drying room is not easy to control, and the branch air path needs to be insulated, and takes up a large space outside the dryer, and the structure is relatively complicated. The SKGD series belt-type circulation dryer solves the above problems by using separate heating and drying channels in each drying chamber. At the same time, the hot air recycling makes the energy consumption of the SKGD series dryer greatly reduced, the hourly processing capacity is 3.5 tons (φ3mm floating puffed fish), and the steam consumption from 24% to 10% is about 1.5t/h.
2. 3 Control system dryer control system should consider automatic or partial automatic control to minimize product differences caused by human operation.
The automatic control of the feed drying system is difficult to achieve, because there are many factors affecting the drying process, and there are many links that need to be controlled. The cost of the entire control system is expensive, which is uneconomical for feed drying. Then the more realistic method should be to automatically control a certain important parameter and manually adjust other parameters. On the SKGD series dryers, the drying temperature is automatically adjusted, while the air volume and drying time are manually adjusted. The automatic control of the temperature ensures that the dryer can still operate in a relatively stable temperature range when the steam is fluctuating.
2.4 The composition and physical properties of the material being processed also have a great influence on the drying effect of the final:
1 If the feed formula contains a high proportion of oils and fats, it is unfavorable to dry.
2 When the feed particles are relatively dense, it is not conducive to the diffusion of moisture inside the particles along the capillary to the surface layer of the particles.
3 When the feed pellet size is large, a longer drying time is required.
4 All feed particle sizes should not be too different, otherwise it is difficult to obtain good indicators of dry unevenness.
5 Under normal circumstances, sinking feed is more difficult to dry than floating feed.
2.5 Some references suggest that 1 drying requires heat, and within the allowable limits, higher temperatures help to increase dryer efficiency. However, under high temperature conditions, it will affect the nutritional value of the feed, such as non-enzymatic browning. Usually the feed drying temperature should not exceed 120 °C. It is also conceivable to let the feed stay in the high temperature (100 ° C ~ 200 ° C) environment for a few minutes, so that the particles quickly heat up, after which the hot air temperature should be reduced to complete the remaining drying operations.
2 should avoid the rapid loss of water in the surface layer of the particles in the early stage of drying, forming a "water seal", destroying the capillary layer connecting the surface layer of the particles and the inside, preventing the particle core moisture from spreading outward.
3 The drying process of any material takes a certain amount of time. In a drying process, the operating parameters should not be changed frequently, and should be left to the dryer for a certain reaction time. Only in this way can the good operating parameters of a certain material be obtained.
4 At the beginning and end of the drying, most of the hot air will pass through the cavity due to the fact that the material cannot cover the entire conveyor belt, which may make the drying effect of the product worse. It is possible to try to solve the problem by preheating the dryer at a high temperature before feeding and appropriately reducing the speed of the belt before the end.
Application of cooling and drying equipment in feed processing