Vacuum System of Vacuum Aluminum Brazing Furnace
The development of control system for large vacuum aluminium brazing furnace is introduced. The main functions of the control system are temperature control and logical control, which require a large number of control points, and each controlled object is related to each other. The core of the control system is programmable logic controller. The method of CX-Programmer software programming is used to control and set up various protection functions for each mechanical system. The programmable controller has the advantages of flexibility, convenience and easy maintenance. The final debugging results and the state of the vacuum furnace after putting into use show that the control system can not only achieve the control objectives, but also ensure the long-term fault-free operation of the equipment.
Vacuum aluminium brazing refers to the method of filling matrix metal gap with liquid filler metal with lower melting point than matrix metal under vacuum heating condition to form a solid bonding. The process of filling matrix metal gap is completed by capillary suction. From 1980s to 1990s, vacuum aluminium brazing technology and vacuum aluminium brazing equipment have been greatly developed in our country. The technological rules of mature small aluminium plate-fin heat exchanger and the technical requirements for aluminium brazing equipment are put forward. At the beginning of the 21st century, due to the need of manufacturing process of large-scale aluminium plate-fin heat exchanger, large-scale vacuum aluminium brazing furnace has been successfully developed in China.
With the continuous improvement of industrial automation level and the rapid development of computer technology, the level of automation control is also getting higher and higher. However, as a control device, programmable logic control (PLC) can take the advantages of complete function, flexibility and versatility of computer and the advantages of relay system, such as simplicity, easy operation and low price. Points are integrated and can adapt to the industrial environment. It is considered that the programmable controller has obvious advantages in the electric control system of vacuum aluminium brazing furnace as the control core. Programmable logic controller has been applied in most of the medium and small vacuum aluminium brazing furnaces. In the control system of large vacuum aluminium brazing furnaces, there are many points to be controlled, so complex chain protection needs to be set up, and further development should be made on the basis of the original one.
Taking a large vacuum aluminium brazing furnace as an example, the power point of mechanical system is put forward, the overall idea is put forward, and the hardware and software of the system are designed.
1. Brief Introduction of Mechanical Structure of Vacuum Aluminum Brazing Furnace
In order to better explain the electrical control system, the mechanical structure of this type of vacuum aluminium brazing furnace is introduced. The main body of vacuum aluminium brazing furnace is composed of heating chamber, furnace body, front and rear furnace doors and various functional components. The heating chamber is installed in the furnace body and consists of several heating resistors and insulation layers. The heating resistors are connected by the electrodes and cables outside the furnace body to form 28 heating zones. Each zone is powered by a set of SCR voltage regulating power supply circuit. The furnace body is a double-layer water-cooled jacket structure, on which the necessary functional components of the brazing furnace are welded, and the front and rear furnace doors form a vacuum chamber: the front and rear furnace. The door is also a double-layer water-cooled jacket structure. It is installed on the door car which can move vertically along the axis of the furnace body. The door can also move along the axis of the furnace body driven by the cylinder on the door car. The translation of the two directions of the door enables the door to open and close.
The external system of vacuum brazing furnace includes vacuum system, water cooling system, air charging system, pneumatic system and charging car outside the furnace.
Aluminum alloy brazing requires very high vacuum system. Vacuum system should not only have strong enough vacuum acquisition ability, but also have sufficient capacity to ensure that the vacuum in furnace is kept in the order of 10-3 Pa when a large amount of gas evaporates. This requires that the vacuum system has fast and stable pumping speed. This type of vacuum brazing furnace is equipped with 5 sets of vacuum units, each set of vacuum units configuration: the main pump is a high vacuum oil diffusion pump, oil diffusion pump belongs to a steam jet pump, it has the advantages of high pumping speed, high limit vacuum: the role of mechanical pump unit is pre-pumping vacuum and as the front pump of diffusion pump, it is composed of Roots pump and sliding valve pump: maintenance pump uses rotary vane pump. It can ensure that the vacuum degree in the pump chamber is maintained when the diffusion pump is preheated before the equipment starts to work. It can also ensure that when Roots pump and sliding valve pump are stopped to save energy during the cooling process of the diffusion pump after work is completed, the diffusion pump oil oxidation will not occur. The valves of each vacuum unit include large valves, front valves, pre-pumping valves and unit exhaust valves. Water cooling system is used for cooling brazing furnace wall, diffusion pump, Roots pump, slide valve pump and rotary vane pump. Electric contact water pressure meter is installed in main intake pipe, and alarm will be given when water pressure is insufficient. The water flows through the cooled parts to the backwater tank, which is connected with the main backwater pipe. The equipment is equipped with inert gas charging interface, and equipped with manual ball valve and manual needle valve. According to the need, the replacement of nitrogen-filled gas in the furnace can be realized in the pre-vacuum stage.
The equipment is equipped with pneumatic system, which is composed of gas source triple parts, pipelines and sealing parts. The pneumatic system pipeline can bear pressure of 0.8-1 MPa. It is used for components driven by cylinders on the equipment, including large valves, front valves, pre-pumping valves and front and rear furnace doors. Workpiece truck is used to load and unload workpieces. Its main body is the frame. The size of the frame matches the maximum size of the workpiece after loading, which is strong enough to withstand the total weight of the load. The track fixed on the ground is used to support the frame and guide the movement of the frame. The forward and backward motion of the frame is driven by the deceleration motor fixed in the pit (power is 2.2 kW). The frame moves towards the furnace body to realize the connection between the guide rail on the frame and the guide rail in the heating room. The other group of deceleration motors fixed on the frame (power is 2.2 kW) can drive along the guide rail. The moving workpiece Trailer moves forward and backward, so that the workpiece moving with the roller assembly enters and exits along the guide rail in the furnace and on the frame, thus realizing the loading and unloading process. The movement of the frame and the entry and exit of the workpiece are all limited by the stroke switch, so as to realize the accurate docking of the guide rail and the placement of the workpiece in and out of the furnace.
2. Design and Development of Control System
2.1. Overall Layout of Electric Points and Control Systems for Mechanical Systems
For vacuum brazing process, the most important two properties of brazing furnace are vacuum degree and heating performance. Mechanical system and electrical control system are designed around these two properties. In order to realize the function of brazing furnace, the two systems are inseparable. Electric point of equipment is the point related to electrical control in mechanical system of equipment, that is, the joint point of mechanical system and electrical control system. Table 1 summarizes the key electric point of equipment.
Based on the mechanical system of this equipment, the electrical control system with PLC as the core and the overall layout of the electrical control system are constructed.
Electrical control system can be divided into two parts, temperature control part and logic control part. Temperature control is mainly completed by temperature control meter, logic control part is completed by PLC, and configuration software on PC is the main monitoring interface. The development process of electrical system is divided into hardware system development and software system development. The hardware design mainly includes heating circuit, vacuum pump circuit, running vehicle circuit and instrument selection.