The core components of the paddle dryer are hollow shafts (which can be divided into single shaft, double shaft, and four shafts) and hollow agitating blades welded to the shaft. Generally, the double shaft is selected in the sludge drying process. The shape of the blade is a wedge-shaped hollow semicircle, and the inside can be heated into the heating medium. In addition to stirring, it is also the main heat transfer body of the device. The two main heat transfer sides of the blade are beveled. When the material is in contact with the inclined surface, as the blade rotates, the particles quickly slide away from the inclined surface, so that the heat transfer surface is continuously updated.
The paddle dryer is a continuous operation device. In the case of two spindle configurations, the direction of rotation is reversed, the spindle speed is lower, and the line speed is less than 2 m/s. The main shaft, the blades and the W-shaped grooves are hollow, and the middle can be heated by hot fluid.
The upper dome of the dryer is not heated and is used to open inspection windows and connect air ducts, pipelines, etc. An air suction port is provided in the middle of the top cover to extract the evaporated water vapor in a micro-negative pressure manner. The heat exchange mode is heat conduction, and only a small amount of ambient air flows when the negative pressure is extracted, and the moving direction of the gas and the material is a cross flow. The residence time of the material in the dryer is long, the process loop is open loop, and the treated exhaust gas is no longer returned.
Since the main shaft is both a rotating part and a main heat exchange surface, considering the equipment sealing and mechanical deformation, the process requires that the working temperature should not exceed 200 degrees. The heat transfer oil only releases sensible heat due to the latent heat released by the steam, and the steam working medium is generally selected in the use process. At this time, the hot runner for transporting the hot fluid is small and easy to arrange. Typical saturated steam temperatures are 150-200 degrees, pressures 5-7 bar, and up to 14 bar.
Design and analysis of paddle dryer
1. Dryer tilting arrangement
The paddle dryer is arranged horizontally, generally with a certain angle of inclination. It feeds from one side and discharges on the other side. The material moves forward in the dryer mainly by gravity. This is due to the blade itself. The inclined surface does not have an axial pushing action, and the squeegee at the top end of the blade is arranged at 90 degrees with the blade, and only serves as a radial copying and stirring, and does not constitute an axial advancement, so the material needs to be advanced. It is done by the angle of inclination of the dryer.
The improved dryer can adjust the angle of the blade bevel to generate the axial driving force, realize the material displacement, and adjust the drying time by using the rotation speed.
2. The setting of the overflow weir
Due to the barrier effect of the blade, the movement of the material in the dryer from the feeding port to the discharge port is in the form of a plug flow, and the distribution of the residence time is narrow, so that the product can be processed for a sufficient time and the heat exchange surface is fully obtained. Use, the material must be filled with the dryer, that is, the material level should be “immersed” the upper edge height of the blade. At the start-up operation, the outlet at the end of the dryer must be closed to achieve the “water storage” effect. At the same time, an overflow weir that blocks the material and maintains a high level is required. In theory, the overflow should be slightly higher than the material. Blade height. The weir is located at the end of the dryer, above the dry mud discharge, and it should have a mechanical structure similar to the “lifting gate” to maintain the required level of the process.
3. Heating shaft type
The heating medium of the equipment can be either steam or heat transfer oil or hot water, but the heat carrier has different phase and the hollow shaft structure is different. When heated by steam, the diameter of the hot shaft is small, and the structure is relatively simple; when heated with hot water or heat transfer oil, the structure of the hot shaft is more complicated, and the liquid flow rate in the tube needs to be considered; the thicker the pipe diameter, the more difficult the rotary joint and the seal are. Big.
The heat is supplied to the hollow blade. If the steam medium is used, the diameter of the inlet and outlet pipes is small due to the release of latent heat. If a heat transfer oil is used, the diameter of these lines may become larger in order to obtain a sufficient heat flow rate, which may reduce the structural strength of the spindle. Since the spindle itself has multiple functions (supporting the blade, transporting the hot fluid, transferring heat transfer, etc.), it needs to overcome the viscous force of the material, the friction between the material and the blade, and the wear of the material itself on the surface of the spindle. The stresses that need to be overcome are large. When designing, it is necessary to ensure its mechanical strength, but also to ensure its heat transfer performance, while also taking into account the material strength, etc. These contradictory conditions will make the design complicated, such as increasing the heat exchange area, the blade needs to be increased. Quantity and diameter, but this will result in an increase in the stress of the spindle. If you want to increase the spindle strength, you need to increase the spindle diameter, but this will reduce the heat transfer area of the blade.
For different heating media, the choice of spindle type and structure are different. For example, if the steamer is used as the working medium, the heat flow channel will be completely different when the heat transfer oil is used as the heat medium, and the heat transfer capacity is also very good. Great changes, so you can’t simply copy and use the original process parameters.
4. Stay time
Theoretically, the residence time of sludge drying can be adjusted by feeding rate, rotation speed, storage amount, etc., and can be arbitrarily selected between several hours and several hours, wherein the overflow weir is to adjust the sludge retention in the dryer. The main means.
In order to make full use of the heat exchange area, the sludge retention in the dryer is required to be high, and the material level should exceed the upper edge height of the blade, that is, the so-called “effective volume” needs to be utilized 100%. If the volume of the entire dryer barrel is calculated according to the steam cover, the effective volume may account for 70-80% of the total volume of the dryer. The high retention rate of the material in the dryer will make the actual residence time of the sludge in the dryer correspondingly longer, up to 3-7 hours.
5. Dry mud back mixing
In theory, since the hollow blades on the main shaft mesh with each other and have self-cleaning effect, the hollow blade dryer can perform semi-drying and full-drying operation of the sludge without performing dry mud back mixing. However, in fact, the self-cleaning of the material shear formed by the mutual engagement of the blades still requires certain preconditions, that is, the meshing precision in the device is sufficiently high, the mechanical gap is small enough, and the shear force between the materials is sufficient to overcome the product. Adhesion on the heat exchange surface. When we analyze the internal structure of the hollow blade dryer, it is not difficult to notice that there is a large gap between the mechanical structures. It is impossible to completely clean the dead zone by mechanical occlusion, which means that the hot surface of the hollow blade is truly realized. The means of self-cleaning and renewal is the mutual friction between the materials, ie the shearing force between the metal surface and the material and between the material and the material. To achieve mutual friction between materials, the method of increasing the packing density of materials can be used to maintain the height of the material level, which can improve the mutual contact chance between the materials, and the self-cleaning of some heat exchange surfaces can be realized by the extrusion of the blade blades.
Due to the nature of the wet mud, there is a tendency to agglomerate, form a ball and bridge during the drying process. It is impossible to overcome the pure material level because the shearing force between the wet mud particles may cause the wet mud to be incapable. The updated gap is “compacted” without causing looseness and fluidity between the particles. Only the dry mud has a characteristic of poor rehydration in a short time due to the complete loss of water on the surface of the particles, and the particle gap is large. When it encounters mechanical shearing force, it has the possibility of sliding off the metal surface. Therefore, in actual engineering, dry slurry back-mixing is considered for the drying of hollow blades. The practice is to screen the dry mud, and the fine dry sludge is pre-mixed with the wet mud.
From the perspective of heat transfer efficiency, dry mud backmixing should be one of the necessary means. According to the water loss condition of the sludge, the evaporation rate of the hollow blade dryer has obvious peak-to-valley changes. When the solid content is less than 25%, the sludge has obvious liquid properties under heating, and the heat transfer condition is better, but the material is easy to form an adhesion layer, which leads to a decrease in evaporation strength, and the sludge is polymerized. The effect is that it has a tendency to form agglomerates, and the contact rate with the heat exchange surface is reduced. When the solid content is between 25% and 75%, the sludge may have surface viscosity, obvious tendency to agglomerate, and poor heat exchange effect. . When the solid content is greater than 75%, the evaporation rate rises because the fine and loose particles regain good contact with the heat exchange surface. The hollow blade drying process generally adopts the method of returning partially dried sludge (dry mud back mixing) according to the purpose of drying, so that the dry mud plays a certain “lubrication” role, obtains better fluidity, avoids adhesion, and its return flow only A small part of the dry mud for export. The proportion of the hollow blade dryer back to the dry mud is not high, generally about 40% (far less than the general requirement of 65%, such as the drum machine), the presence of this dry mud powder is enough to be on the hot surface To the role of “lubrication” and “cleaning”.
6. The dryer does not empty
Any sludge drying process that requires dry mud back mixing has strict requirements for the feeding of wet mud. Before the wet mud is fed, it must be carried out under the condition that the dryer has a large amount of dry “bed material”. Avoid the wet mud as soon as it gets in, it will stick to the heat exchange surface and cause scaling. Therefore, the typical practice is to maintain the returning system to continue working when the drying system is stopped, stop the feeding device, and the dry product is fully refilled. At the same time, the system is cooled. When the system temperature is lower than 60 °C, the whole line is stopped. No clearing is carried out inside, and the material is started directly when driving, which means that the dryer is filled with dry mud when the machine is stopped. In the shutdown process and the boot process, there is always the characteristics of high dust and low humidity, but at this time, it is necessary to pay attention to the problems of dryness and safety.
7.The blade top scraper
Any machine has tolerance gaps, and the hollow blade dryer with spindle engagement is no exception. Wet mud has a viscosity at a certain solid content, and may cause a sticky wall between these gaps, and any adhesion on the hot surface will reduce the heat exchange efficiency. In order to avoid the thickening of the sludge scale layer, mechanical scraping is often used, and it is necessary to use the scraper at the tip of the blade to scrape the sludge. From the action of the squeegee, it is known that with long-term operation, the squeegee pair acts only on the material falling between the blade and the W-shaped groove heat exchange surface, and also on the material attached to the W-shaped groove wall. There is a scraping action, whether it is scraping or scraping, because the moving speed of the scraper is about 2 to 5 m / s, the moving direction of the sludge at the outer edge of the scraper at this speed during the process of picking up or scraping There are two, one is outward extrusion (grinding W-shaped groove) and the other is backward movement (grinding plate).
8. Metal surface hardening treatment
Wear can be one of the important challenges facing hollow blade dryers. The sludge contains abrasive particles. The hollow blade dryer is a typical conductive contact heat transfer. Repeated and long-term contact between the metal and the abrasive particles is inevitable. Coating and hardening can reduce the speed of abrasion, but it is limited by the surface of the abraded metal and also the heat exchange surface (such as W-shaped groove, blade, spindle, etc.), and there are not many hardening measures (spraying silicon carbide, etc.) ), under the heating condition, the adhesion of the wear layer, the actual hardness, etc. are not ideal, and can only play a role in slowing the abrasion.
Because the dry mud particles have a prominent effect on the metal surface, the latter half (15~25%) of the blades are usually heat treated, but for the dry mud back mixing process, the abrasion is the whole process. The existence of the abrasive tendency will undoubtedly affect the material selection of the dryer.
In the heat exchange metal surface of the paddle dryer, the W-shaped groove has a small pressing gap with the scraper, and has a significant pressing effect during the hot surface renewal process. When such a squeeze gap is present, the general abrasion is strongly a relatively “soft” metal surface, which means that it is necessary to protect the W-shaped groove as a heat exchange surface, the scraper can be hardened, but not hardened. The life of the scraper will also be limited.
9. Mechanical dead angle
Mechanical dead angle is one of the design challenges that must be solved by the paddle dryer. It can be divided into three categories:
1) the outer edge of the metal without surface mechanical cleaning;
2) There is surface cleaning but there are inaccessible tolerances;
3) Increased inaccessible tolerance due to abrasion.
The direction of rotation of the wedge-shaped blade itself is constant, that is, both spindles rotate inward, when the narrow side of the wedge-shaped blade is in front, the blade is behind, and the blade has no mechanical action from the narrow and wide heat-exchange surface. Cleaning up needs to be updated by the shear force of the material itself. The portion of the squeegee that is larger than the wedge-shaped portion of the wide heat exchange surface will always scrape the sludge and form a squeezing on the W-shaped groove. In addition, the squeegee and the main shaft only have a “tangent intersection” at a certain point (ie, the center of the scalloped notch) (actually close, the cleaning effect is minimal), and the spindle has no mechanical cleaning on the surface in most cases. All of the above are metal outer edges without surface mechanical cleaning, which account for 70-80% of the total heat exchange area.
There are mechanically cleaned heat exchange surfaces. According to the arrangement rules of the wedge-shaped blades, there are the following dead angles due to inaccessible tolerances: the gap between the scraper of the first row and the last row of blades and the outside of the heating spindle, between the dryers Between the paddle and the spindle packing seal. The gap between the axial squeegees, which is clearly visible.
Due to the aforementioned abrasion problem, especially the increase of the radial blade gap, that is, the blade is thinned by abrasion, and the inaccessible tolerance of the heat exchange surface of the blade and the W-shaped groove is increased. At this time, the scraping action of the scraper is reduced, and when the shear force between the materials is insufficient to overcome the adhesion of the wet mud on the heat exchange surface, the stock and scale on the heat exchange surface are generated. When a certain thickness is formed, it will cause shaft jump, vibration, noise, and the like.
Heat exchange surfaces that cannot be cleaned can be called “mechanical dead angles”. Based on the above factors, the part where the hollow blade dryer cannot be mechanically cleaned accounts for most of the heat exchange area. Therefore, the core problem for this process is how to avoid the stickiness of the product.
10. Heat transfer coefficient
Since the blade is perpendicular to the main shaft, the blade is parallel to the main shaft, and the heat exchange surfaces at both ends of the blade are not pushed but only heat exchange, the radial mixing of the material is sufficient, and the contact frequency between the material and the heat exchange surface is high. , long residence time, theoretically should achieve better heat transfer, the overall heat transfer coefficient should be between 80 ~ 300W / m2.K. In the application of sludge drying, due to the different viscosity of different sludges, the solid content of the dried product also affects the process (such as whether low dryness and semi-drying can be carried out), and the heat transfer coefficient given in the actual project. It may be a big difference.
11. Heat transfer area
The wedge-shaped blades and the main shaft on the hot shaft of the paddle dryer are the main heating surfaces, and the heat exchange area accounts for more than 70% of the total heat exchange area. The design has high requirements on its manufacturing precision, spindle type and hot runner layout. It is generally considered that the dryer is “complex in structure and difficult to process”, and the design of large dryers is more difficult. In foreign countries, a hollow blade dryer with a heat exchange area of 1.5 to 295 square meters and a theoretical evaporation capacity of 12 tons per hour has been manufactured. In the field of sludge drying, the heat exchange area of the large installed machine is about 300 square meters. The capacity is less than 5000 kg / hour. The current series design in China is 110 square meters high. The heat exchange area of the commonly used hollow blade dryer is 25-100 square meters, and the height is 160 square meters.
12. The amount of air purged
The blade dryer is a typical conduction dryer. The heat transfer and evaporation are realized by the hot wall instead of the gas convection. In practical applications, the water vapor generated by the drying process needs to be discharged into the dryer in time, and the sludge is dried. In order to prevent odor from overflowing into the environment, it is generally necessary to use a micro-negative pressure operation mode, and there is a necessity to use “purge air”. The extraction of negative pressure will inevitably cause ambient air to enter the circuit from the gap between the dryer and the circuit (shaft joint, wet mud inlet, dry mud outlet, overflow weir seal, etc.), in order to prevent the purge air from causing water vapor in the dryer. Condensation, this gas needs to be heated. The amount of purge air is related to the process itself, measured by the amount of ambient dry air required for water evaporation, generally between 0.1 and 1.2 kg/kg. H2O. The height of this value has an important influence on the net heat consumption of the drying system. A typical hollow blade dryer generally considers the amount of dry air around 0.5 kg/kg.H2O.
13. Evaporation intensity
The evaporation capacity of a conductive dryer is generally measured in terms of evaporation per square meter and hour, and it theoretically achieves an evaporation of 10 to 60 kg/m2.h. However, in the practice of sludge drying, the value of the design value is generally between 6 and 24 kg/m2.h, and the value of 14 to 18 kg/m2.h is mostly. Refer to other conductive drying (such as disc players, discs), the typical values are between 8 ~ 14 kg / m2.h, consider the heat transfer conditions of the hollow blade dryer is very similar to other conductive dryers The more reliable actual evaporation intensity should be between 8 and 14 kg/m2.h.
14. Product outlet temperature
Since the sludge stays in the dryer for a long time, the outlet temperature of the sludge when leaving the dryer is relatively high, and should be about 90 to 100 °C.
