Explosion-proof exploration of drying oven
The reason is that some users have proposed explosion-proof requirements for these drying ovens. Some users are drying parts that are cleaned or impregnated with gasoline and organic solvents.
Ordinary drying ovens and ovens are completely different devices in terms of safety regulations. Study the explosion-proof problem of ordinary drying ovens. These gasoline, organic solvents and paint volatiles are flammable. In other units, it is also possible to extract a heavy fraction in a small amount by an oven using a fractionation method, and if the separated light fraction is a flammable substance, there is also an explosion-proof problem. The only content explored here is to correctly understand the safety problems of ordinary drying ovens and to improve their safe use.
The physical explanation of the explosion is that the volume of the gas expands rapidly in a short period of time. The moment of the expansion occurs, the high pressure exceeds the strength limit of the container, and the explosion usually occurs with light, fire, gas waves, container fragments or ray impact. Thus constitutes the danger of the explosion to the environment. In order to prevent explosions, it is unrealistic to make a fuss about strengthening the strength of the casing of the drying oven. The article describes various explosion-proof types of electrical products. This article wants to use the composition and nature of flammable substances as an entry point, and then put forward opinions on the safety measures and use of ordinary dry boxes.
2. Analysis of flammability characteristics of organic compounds
Whether it is gasoline, alcohol, toluene or paint, it is an organic compound. The difference is that the composition of alcohol and toluene is mainly a hydrocarbon, and the physical properties are relatively simple. Gasoline and paint are often composed of a variety of hydrocarbon normals, isomers and other non-hydrocarbon impurities, and the physical properties are relatively complex, but it has the following commonalities:
1 Flammable.
2 can dissolve other related organic matter.
3 Volatile.
4 Most organic compounds volatilize vapor specific gravity greater than air specific gravity.
Gasoline and paint are mixtures of various hydrocarbons. Before the dry distillation point (temperature), as the temperature increases, the components with lower evaporation temperature will first be evaporated into gas until the dry distillation point. Therefore, for such organic compounds, there is always a dangerous atmosphere in the tank before the oven temperature reaches the dry distillation point. Unlike pure alcohol and toluene, once it reaches the evaporation temperature, it can be evaporated as long as it is kept at a constant temperature.
The flammability characteristics of organic compounds can be explained from the following three points:
1 Self-ignition. In the absence of open flame, the ambient temperature rises to the autoignition temperature of the organic compound, and these organic compounds spontaneously burn.
2 flash point is low. When the oven temperature is above the flash point, the organic compound vapor can cause burning even if it encounters a little spark. The flash point of organic compounds is generally very low, even below zero. It is indicated that under normal temperature conditions, the flash of these organic compounds can be flashed when exposed to an open flame, and the organic matter is burned by the flash fire.
3 The explosion limit is low. The explosion limit is given in the form of volume percent. Taking toluene as an example, the volume of toluene vapor in the vessel is as high as 1.2%, which is necessary for the explosion. The explosion limit of gasoline is only 1.4%
Third, the explosion-proof measures of the drying box and additional operational advice
From the above analysis, it can be seen that the explosion-proof measures of the drying box need to obey the following rules:
1 It is strictly forbidden to use the open flame heating element to heat the box to prevent explosion caused by flash fire. When the outer surface of the heating pipe flows through the organic steam, the high temperature of the external surface of the furnace must not exceed the auto-ignition temperature of the organic steam.
The electrical control system must not have components that cause flashover. If it has to be used, it must be placed outside the box and positively ventilate the local appliance.
The temperature control and over temperature control system of the 2 cabinets must be absolutely reliable, ensuring that the temperature inside the tank does not exceed the autoignition temperature of the organic steam. Chain design, dual independent over-temperature maintenance and over-temperature spray cooling system should be used when necessary.
3 box structure must have energy release parts (such as safety doors, etc.) This is a dialectical method to control the environmental damage caused by the explosion. As long as the structural design permits, the opening pressure should be as small as possible and the opening area should be as large as possible.
4 Design the ventilation ventilation circulation system according to the volume percentage of the lower limit of the organic vapor explosion limit and the evaporation amount to prevent the accumulation of organic vapor in the tank from exceeding the lower limit of the explosion limit. Zui can measure the concentration of organic vapor in the working chamber) so that the equipment can make maintenance response in time. However, there is currently no such sensor that can be applied to various organic vapor concentration measurements.
The position of the object to be baked on the shelf should be specified so that the object to be heated is evenly heated and the evaporated organic vapor is carried away from the surface of the object in time.
5 If the specific gravity of organic steam is greater than the specific gravity of air, the air duct design should effectively prevent organic steam from sinking in the bottom or dead corner of the oven. Therefore, the inner wall of the box should be smooth and easy to clean. When designing a heated local location, it is also necessary to consider the effect that thermal radiation may have on the object being baked.
6 The mechanical part of the activity shall take measures to prevent the friction of Mars. The parts where high-speed running parts are likely to be rubbed should be designed with a safety gap and a soft metal structure. Grounding problems should be considered in areas where static electricity may accumulate.
The drying box using the above measures shall be subject to the following requirements in the operation sequence:
1 First turn on the fan, and put the air in the box into the object to be baked.
2 After closing the door, first circulate the air (4 times the working volume of the studio) and then turn on the heating switch, so that a large amount of organic steam evaporated at room temperature will be discharged out of the box.
3 The heating power is gradually increased to prevent the temperature overshoot from causing the temperature of the heating tube or the temperature inside the box to exceed the autoignition temperature of the organic vapor. Taking gasoline as an example, the heating power is divided into at least four steps according to the distillation temperature of different organic substances. * The file is heated at a high temperature of 65 °C for a period of time, so that the low-evaporation temperature organic vapor is discharged outside the box (about 10% of the total amount). The second gear is heated at 130 °C for the Zui high point, so that the cumulative total is 50%. The organic steam is discharged outside the tank, and the third gear is heated at a temperature of 200 °C, so that a cumulative total of 90% of the organic steam is discharged outside the tank. The fourth gear is heated at 205 ° C to allow all organic steam to exit the tank. The temperature switching between each gear should be kept at a small heating rate, and should be kept constant for a certain constant temperature according to the shape and arrangement density of the object to be baked, so that all organic vapors are evaporated in the respective evaporation temperature regions.
The above is only taking gasoline as an example. For various organic compounds with different properties, the temperature-increasing method should be established on the basis of sufficient understanding of its physical properties.
4 When setting the heating temperature, the user should set the upper limit of the temperature uniformity index of the cabinet and the upper limit of the temperature fluctuation. Because the actual temperature of some parts of the box is likely to be higher than the set nominal temperature due to the positive and negative amplitude range of the above two indicators. At the same time, due to the difference of the position of the sensor device and the position of the object to be baked, the temperature reflected by the meter does not necessarily coincide with the actual temperature of the center point of the studio. These need to be considered and corrected according to the actual situation when setting the heating temperature. When purchasing and using the drying oven, the user must also pay attention to these technical parameters and structural features of the equipment.
5 When the tray of the baked object is pushed into the shelf, the speed should not be too fast. No liquid solvent should remain in the tray. It is strictly forbidden to drip the liquid solvent to the heat transfer part at the bottom of the working chamber.
6 When the drying box device is positioned, the distance from the wall to the oven can be calculated by considering the following formula. The spacing less than 3m 3 is not less than 0.6m.
s = 0.6 v/3 1/3
In the middle
s spacing, m
v Drying tank volume, m 3
The air inlet of the drying box ensures that there are absolutely no other obstacles, and the discharged organic steam should be reliably led out of the room.
The equipment around the drying box maintains the railing to prevent other objects from being stacked around the drying box and affecting the operation of the door.
Fourth, the control system
The design of the control system for such drying ovens has special requirements compared to conventional drying ovens.
First of all, the control accuracy and reliability of the instrument should be high and the drift should be small. Large temperature overshoots are never allowed. Temperature sensing components must not be exposed to bare glass platinum, and must be packaged in armor or metal sleeves. The set initial heating rate should not be too large, at least the stepwise heating method should be considered, so as to avoid the evaporation temperature reaching the explosion limit due to rapid evaporation. Under the premise of reliable control, it is also necessary to consider an alarm system with a large safety factor.
The control system must ensure that there is a pre-venting procedure for a certain period of time before heating. In the course of operation, if the ventilation system fails, the control system should be able to issue an alarm signal in time.
The control system should detect the pressure. Pressure is a direct factor in the explosion. When the pressure is abnormal, the control system should be able to issue an alarm signal in time.
The purpose of setting up the alarm system is to delay the evaporation rate of the object to be baked and prevent the surge of pressure and concentration in the tank. Therefore, whether the temperature, pressure, concentration, ventilation system has a single or combined critical signal, the alarm system of the drying box must automatically cut off the heating heat source, open the vent or safety door, and adhere to the circulating air state of the fan.
Although a safe maintenance system is envisaged, it is important to note that in the event of a failure of the maintenance system, it will be more dangerous than no maintenance, for which the self-test and automatic safety of the fault should be considered.
The explosion-proof drying oven has a control system that is by no means a single temperature control. Due to the flammable and explosive characteristics of various organic vapors, the evaporation temperature and the alarm range of pressure and concentration, and the pre-venting time, etc., many factors are combined together, and it is difficult to achieve with general control laws. New control algorithms such as weighting are worth exploring.
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