Overview of Low Temperature Vacuum Belt Drying Machine Equipment
Vacuum Belt Drying (VBD) is a low-temperature drying method that continuously applies liquid raw materials onto a conveyor belt under vacuum conditions. The materials move along the conveyor belt through a heating zone and are then cooled and brittle. During the entire drying process, the traditional Chinese medicine extract is in a vacuum and sealed environment, with a mild drying process (working temperature of 40.60 ℃), which can maximize its physical properties and obtain high-quality final products. Due to the direct entry of the material into high vacuum and gradual drying over a period of time, the particles obtained after drying have a certain degree of crystallization effect, and there are micro pores inside from the microstructure. After being directly crushed to the desired particle size, the flowability of the particles is very good, and they can be directly compressed or encapsulated. At the same time, due to the micro porous structure of the particles, they have excellent solubility.
The vacuum belt dryer has a wide range of applications and can be used for the vast majority of traditional Chinese medicine extracts. Especially for the materials with high viscosity, easy agglomeration, thermoplasticity and heat sensitivity, the use of vacuum belt drying can overcome the shortcomings of too fine and dense spray drying powder and too high temperature. In addition, a series of parameters such as the operating speed of the conveyor belt, material thickness, temperature, and environmental vacuum degree of vacuum belt drying can be adjusted to meet the process requirements of different products. In this regard, the belt vacuum dryer is far superior to the spray dryer.
The vacuum belt dryer adopts multiple different heating temperatures to control the foaming degree of the paste and avoid material overheating during the deceleration drying stage. Using a cooling system to cool and dry products can increase their brittleness and improve their quality. The dried traditional Chinese medicine extract is heated through a heating partition under vacuum or low pressure to evaporate the water in the extract and obtain granular products. The entire drying process keeps the extract at a lower temperature, which can minimize the loss of effective ingredients in traditional Chinese medicine extract.

Advantages of Low Temperature Vacuum Belt Drying Machine Technology
The common vacuum drying method is intermittent production, which has good product quality but relatively low efficiency. The belt vacuum dryer not only maintains the advantages of vacuum drying, but also enables continuous production and is suitable for drying various materials (liquid, powdery, and solid), making it another choice for processing high-quality products.
The vacuum belt dryer has made great progress in large-scale production and shortening drying time, and can be applied in multiple production fields, such as drying seasonings, yeast extracts, malt, maltose, and so on. According to reports, there is a continuous vacuum belt dryer abroad that can produce approximately 2000T of drying products annually.
The main advantages of vacuum belt drying are: ① low-temperature drying, suitable for heat sensitive materials; ② Dilute oxygen drying, suitable for easily oxidizable materials; ③ Can achieve continuous production and facilitate large-scale engineering production; ④ Sealed work, good hygiene conditions, easy to ensure product quality and safety; ⑤ Especially suitable for drying materials with high viscosity and particles; ⑥ The product is porous and has good solubility.
This type of equipment is mainly used in the following industries: ① Food industry (pure fruit juice powder, instant coffee, sweeteners, seasonings, etc.); ② Medical and health industry (bioactive substances such as proteins, enzymes, and herbal extracts); ③ Chemical industry (dyes, metal oxides, and explosives).

Main components of vacuum belt dryer
The vacuum belt dryer mainly consists of a vacuum system, a belt dryer, a heating system, and a control and measurement system.
1. Vacuum system
A vacuum system is a device that provides mass transfer power for the drying process. There are currently two main methods for removing water vapor in vacuum drying. One is the use of a water vapor jet pump and a water ring pump in the pumping system, commonly including Roots vacuum pump, water ring vacuum pump, water vapor jet vacuum pump, and water jet vacuum pump system, which directly extracts a large amount of water vapor; Another method is to use a water trap with a vacuum pump. First, the water vapor is captured by the water trap, and then the remaining gas is removed by a vacuum pump such as a rotary vane pump.
2. Belt dryer
It is the core of vacuum belt drying equipment, mainly composed of drying chamber, feeding mechanism, transmission device and its dragging mechanism, heating plate and cooling plate, shearing and discharging mechanism, etc.
The drying chamber body is a stainless steel container that requires strict sealing. When in use, the air inside is evacuated to form a vacuum, and its shape is generally cylindrical or square box shaped. The feeding mechanism generally consists of raw material tanks, valves, conveying pumps (which can be peristaltic pumps or metering pumps), hoses, nozzles, and nozzle drag mechanisms. Liquid and slurry materials can be transported to the drying chamber by a metering pump, while solid and sheet-like materials require dedicated vacuum feeding/suction equipment to complete the feeding process. Liquid and slurry materials should generally undergo vacuum concentration before feeding to increase their solid content to about 50% before vacuum belt drying, in order to save energy and achieve higher drying efficiency. A conveyor belt is a device that carries and transports materials, and its material should meet the requirements of vacuum drying technology. The number of conveyor belts in the drying room can be one or multiple. The heating plate provides the heat required for the evaporation of moisture from the dried material, while the cooling plate cools the dried product, making it easier to cut and discharge. The cutting mechanism is used to cut materials into small pieces for easy crushing and discharging. When discharging, the situation is to open the gate valve on the finished product warehouse after reaching a certain amount of dry products (the finished product warehouse has been evacuated before opening), put the crushed product particles into the finished product warehouse, and then close the gate valve. Repeatedly store the dried and crushed products in batches in the finished product warehouse. When the finished product reaches a certain amount or the drying process is completed, open the gate valve under the finished product warehouse to discharge the finished product.
3. Heating system
A heating system is a device that provides the heat required for the evaporation of moisture from materials. The indirect heating system usually consists of a heat exchanger, a hot water pump, a gas water separation tank, a thermometer, pipelines, and the advantages of stable heating temperature, uniform material heating, and easy control. However, this heating system is relatively complex and requires the installation of a heat medium circulation pump and heat source equipment, resulting in relatively high costs. Heating systems generally do not use electric heating and direct heat conduction methods because the equipment often needs to be cleaned with water after the material is dried, which can easily cause malfunctions. Generally speaking, the heating zone can be divided into several sub heating zones. In different drying stages, suitable heating temperatures are provided according to the properties of the material to achieve energy conservation and ensure product quality.
4. Control and measurement system
The control of vacuum belt drying equipment mainly includes the start and stop of vacuum pumps, refrigerators, and circulation pumps, the start and stop of conveyor belts and nozzle drag mechanism motors and their speed regulation, the control of heating temperature, safety protection and alarm devices, and various sensing devices. Equipped with vacuum gauge measuring device and temperature measuring device. The main function of the control and measurement system is to facilitate operation and measure the vacuum degree in the drying chamber, the temperature of heating plates and materials in each section online during operation, in order to achieve real-time monitoring.

Process flowchart of low-temperature vacuum belt dryer

Design highlights of low-temperature vacuum belt dryer

The main components of a vacuum belt dryer include: dryer cylinder, fabric mechanism, nozzle, track conveyor, spiral conveyor, PTFE conveyor belt, tensioning device, correction device, crushing mechanism, roller frame, shearing mechanism, discharge mechanism, pneumatic knife gate valve, rotary cleaning ball, testing instrument, electric control cabinet, etc.
(1) The fabric should be uniform
It consists of a high shear emulsifier, screw pump, valve, hose, connecting pipe, nozzle, and bracket. When feeding materials, first open the valve and then start the screw pump. The liquid material flows through the hose to the nozzle and is evenly sprayed onto the track. The nozzle is connected to the bracket, which rotates with the drive shaft to make the nozzle swing around the drive shaft, achieving uniform distribution. The nozzle holder is driven by a reduction motor to rotate the rotating wheel, which has a sliding groove. The slider and connecting rod are fixed on the rotating wheel, and the other end of the connecting rod is connected to the rack. The rack can slide left and right in a straight line on the fixed frame. The rotational motion of the motor is converted into the left and right linear motion of the rack. Adjusting the distance between the slider and the center of the rotating wheel can adjust the stroke of the rack. If the distance is small, the stroke of the rack is small; otherwise, it is large. The left and right movement of the rack drives the gear to rotate forward and backward, and the gear is fixed on the transmission shaft, thus achieving the purpose of fabric distribution. Generally, the swing angle is controlled to be ≤ 60 °, and the width of the material layer is controlled to be 90% to 95% of the bandwidth. The swing frequency is 10-15 times/min, and the swing frequency can be adjusted by frequency conversion.
The vertical distance between the nozzle and the conveyor belt is crucial for the uniformity of the placement of the dried material on the conveyor belt. Here, this distance is denoted as Hv. In the feeding experiment of the material, it was found that when Hv is large, the material does not immediately fall onto the conveyor belt from the nozzle mouth, but has a trend of gradually increasing volume downwards, attached to the nozzle mouth, growing in a funnel shape, and then falling onto the conveyor belt in clusters. Firstly, the material needs to go through a period of time from leaving the nozzle to the conveyor belt, and during this time interval, the conveyor belt is moving, so the material is not evenly spread on the conveyor belt, which cannot effectively utilize the conveying area. Secondly, due to the accumulation and growth of a large amount of materials, forming a larger cluster, the thickness of the materials increases significantly, requiring a longer time to dry. In addition, while the material passes through the nozzle, it is still in liquid form. Once it enters the vacuum environment, a large amount of water evaporates and there is a phenomenon of bubble explosion, which causes many small particles to splash around and adsorb on the walls of the drying chamber, heating plate surfaces, and other component surfaces. This increases the difficulty of cleaning and also causes material loss. When Hv is small, the material comes into contact with the conveyor belt as soon as it exits the nozzle, and adheres tightly to the conveyor belt. The material does not adhere to the nozzle mouth, which ensures the uniformity and continuity of the material in the transverse direction of the conveyor belt. In addition, when Hv is small, the phenomenon of bubble explosion causing contamination of the chamber wall will be greatly reduced. In the experiment, take Hv=3mm.
(2) The track conveyor and correction device should operate reliably
It consists of a frame, driving wheel, driven wheel, conveyor belt, spring tensioning mechanism, supporting wheel, nose wheel and other components, and is divided into 5 layers, each layer is equipped with an independent transmission mechanism. Both the driving wheel and the driven wheel adopt the expired Swiss patent "Crown Wheel" structure, which enables the conveyor belt to automatically correct deviation during operation. There are guide limit slots on both sides of the "Crown Wheel" to prevent the conveyor belt from deviating easily; The function of the nose wheel is to correct the deviation of the conveyor belt by adjusting the angle of the nose wheel; The spring tensioning mechanism is installed on both sides of the driven shaft, with spherical ball bearings with slider seats at both ends of the shaft. When the spring is compressed, the bearing seats slide, thereby achieving belt tensioning. The purpose of tensioning is to increase friction and prevent belt slippage; The function of the idler is to prevent the conveyor belt from sagging due to the weight of the conveyor belt itself under heating conditions, which is beneficial for extending the service life of the conveyor belt; The drive belt is a component that carries and transports materials. It is made of fabric coated with PTFE on both sides, wrapped with edges and guide strips, seamlessly overlapped at an angle to ensure no leakage of materials, and completes the entire process of heating, drying, cooling, and peeling.
The correction problem is the core issue of vacuum belt dryers. How to ensure that the conveyor belt does not deviate, and that the track does not deviate under vacuum conditions, load conditions, and uneven fabric conditions? Once the track deviates, it can automatically correct it. To this end, six correction measures can be taken:
I. Install guide bars on the tracks, and set guide bar grooves on the driving and driven wheels to allow the guide bars to move in the grooves. The guide bar material is PTFE, and under normal working conditions, the guide bar can play a certain guiding role;
II. The driving and driven wheels adopt a "crown wheel" structure, with a certain slope controlled at both ends and a certain distance and length controlled. From the force analysis, it is ensured that the upward climbing force is greater than the downward sliding force. Its principle is somewhat similar to the commonly used "drum" on large conveyors;
III. Use a tensioning device for correction, adjust the tightening nut with a torque wrench to ensure even force distribution. Under the action of the compression spring, the spring can compensate and correct the uneven force on the track to a certain extent;
IV. Install a "nose wheel" at the bottom of the track, which serves to guide and correct any changes in the circumference of the track at both ends. Attention: No adhesive is allowed on the nose wheel. Once adhesive is applied, it will lose its effectiveness, so anti adhesive measures should be taken;
V. Adopting seamless tracks with reinforced edging, controlling the circumference deviation of the two ends of the tracks to be ≤ 5mm and the diagonal error to be ± 3mm, ensuring machining accuracy. The surface should be smooth and even;
VI. Set up a stop wheel. In case of deviation, the stop wheel will limit its position and prevent the track from running out of the track. The stop wheel will use rolling friction with the edge of the belt to avoid damaging the track and causing the system to stop or shut down completely.
(3) Flexible movement of cutting, peeling and discharging mechanism
The cutting mechanism is composed of components such as a cylinder, sealing ring, guide block, cutter, and cutter bar. The knife rod, also known as the commonly used piston rod, drives the cutting blade up and down under the action of the cylinder's intake and exhaust. Its stroke is adjustable and is generally suitable for cutting dry materials on the conveyor belt. The cutting blade speed can also be pre-set, and a knife is installed on each layer. The length of the cutting blade is slightly larger than the width of the material's fabric. Due to the brittleness of the material itself, the cutting blade does not need a blade. In fact, the shearing mechanism is a set of lifting mechanisms that perform up and down reciprocating motion. Its main function is to break, open, and disperse the foam layer of large, dry and brittle materials, prevent material bridging, and avoid affecting normal crushing and discharge.
The discharge mechanism consists of a crusher, a screw conveyor, an upper gate valve, an intermediate tank, a lower gate valve, and a receiving bucket. The crusher adopts a combination structure of a moving knife and a static knife, which crushes the sheared block or strip materials and enters the screw conveyor. When the material in the intermediate tank accumulates to a certain amount, the upper gate valve is closed and the three-way valve is placed in the air filling position. After the intermediate valve is filled with air, the intermediate tank returns to normal pressure. Then, the lower gate valve is opened to release the finished material, and the lower gate valve is closed to evacuate the intermediate tank. When the vacuum degree of the intermediate tank is basically balanced with that of the cylinder, the upper gate valve is opened, and this process is repeated and cycled. The crushing speed and spiral conveying speed are controlled by frequency conversion, and the knife gate valve is controlled by pneumatic control. The opening and closing speed and time of the valve can be set arbitrarily, so as to adjust the optimal operating cycle.
(4) The efficiency of heating and cooling systems should be high
There are three heating methods: conduction, radiation, and microwave. The heating medium can be hot water, hot oil, steam or direct electric heating.
I. Conductive heating using hot oil or steam as the heating medium. The heating plate is placed below the conveyor belt and is divided into three heating zones and one cooling zone. It is cooled by circulating cooling water with a cold water pump.
II. A typical heating and cooling system uses a flat plate heat exchanger, which is divided into three heating zones. The first zone is steam heating, and the steam pressure is controlled at 0.1-0.15 MPa. Because the steam pressure is too high, the flat plate heat exchanger is prone to deformation, the material is prone to denaturation, and the color of the material will deepen. Only saturated steam can be used, and the use of superheated steam is strictly prohibited; 2、 Three zones are heated with hot water, with a controlled heating temperature of 50-60 ℃ to ensure the invariance of thermosensitive materials. The cooling system, also known as the cooling zone, uses industrial chillers to automatically produce cold water. The temperature of the cold water is controlled at 15-20 ℃ and cooled by plate heat exchangers. The temperature is automatically controlled to make the material brittle during rapid cooling, which is beneficial for material crushing and transportation. The temperature of cold water should not be too low to prevent condensation and moisture regain. The time for passing cold water should not be too early, and the dew point temperature should be strictly controlled. The main function of the system is to facilitate operation and measure the vacuum degree in the drying chamber, the temperature of heating plates and materials in each section online during operation, in order to achieve real-time monitoring.