During the operation of the tunnel drying chamber. There will be various contradictions. The process of operation is the process of properly handling and resolving these contradictions. In summary, the main contradictions are divided into the following ten pairs.
1 Heating and Cooling The wet green body enters the drying room and is gradually heated from normal temperature, and the water in it is gradually evaporated until it leaves the drying room and has become a dry green body. In this process, the green body is heated up according to the established temperature curve. The required heat is generally provided by the waste heat or smoke heat of the roasting kiln. The purpose of sending hot air is to provide heat to heat up the green body. During the drying process of the green body, under normal circumstances, the established temperature curve should be implemented and should not be changed arbitrarily.
During the heating process, we must attach great importance to how to transfer heat. The hot air releases heat. How to make good use of this heat so that the heat can be well transferred to the green body is learned. If it is not done well, it will cause a lot of waste of heat energy. . In order to improve the heat transfer effect, efforts should be made to adjust the hot air flow rate, wind pressure, the density of the blank stack, and the interval time between the drying car entering the drying room.
The hot dry air enters the drying chamber from the exit end, exchanges heat and moisture with the green body, and when it reaches the entry end, it has become cold and wet air, and is finally discharged outside the drying room by the moisture exhaust fan.
The heating of the green body and the cooling of the wind are the unity of opposites. Only when the heating effect is good can the moisture in the green body be evaporated efficiently, and the cooling of the wind makes sense. If the heated blank does not meet the established requirements, no matter how good the air cooling is, a large amount of waste products may come out of the drying chamber (such as uneven drying, etc.). And the water evaporation effect obtained by heating the green body will be further guaranteed by the cooling of the wind. Without a reasonable cooling process of the wind, the heating results will be wasted.
In short, the heating of the green body will inevitably lead to the cooling of the wind, and the cooling of the wind is to consolidate and develop the heating effect of the green body.
2 positive pressure and negative pressure
Pressure system Whether the pressure system is reasonable or not has a great influence on the drying effect of the green body in the drying chamber. The zero pressure point of the drying chamber is the transition point of positive pressure and negative pressure in the drying chamber. The position of the zero pressure point is mainly determined by the air volume and air pressure of the supply and exhaust fans, the resistance in the drying chamber and the drying properties of the green body. Generally, the positive pressure section of the countercurrent tunnel drying chamber accounts for about 2/3 of the length, and the negative pressure section accounts for about 1/3 of the length. After the position of the zero pressure point is determined, do not change it during normal production, because the change of the zero pressure point marks the change of the temperature, humidity and air volume in the drying room, which will affect the drying quality.
For example, when the zero pressure point moves like the entry end (the negative pressure section shortens and the positive pressure section increases), there are two situations. One is that the exhaust air volume decreases. At this time, the relative humidity of the drying medium at the entry end increases, and the moisture cannot be exhausted. go out. In severe cases, the green body does not dry and cracks occur; the second is that the air supply volume increases. At this time, the temperature of the drying medium at the inlet end increases, and the relative humidity decreases. After the green body enters the drying chamber, the surface is rapidly dehydrated to form a hard shell, resulting in the green body Cracks occur. When the zero pressure point moves to the exit end (the negative pressure section increases and the positive pressure section shortens), there are also two situations. One is that the air supply volume decreases, which reduces the temperature at the entry end of the drying room and increases the relative humidity, resulting in green body The second is that the exhaust air volume increases, which causes the relative humidity of the drying medium at the inlet end to decrease, resulting in splitting of the green body. Therefore, under normal circumstances, the zero pressure point must be kept stable, that is, the established positive pressure point must be maintained. , Negative pressure system.
3 Laminar and turbulent flow
The tunnel drying chamber is an advection drying chamber. The general flow direction of gas in the drying chamber is horizontal and forward, so it is easy to cause laminar flow and gas stratification, resulting in an increase in the cross-sectional temperature difference in the drying chamber. This is one of the weaknesses of tunnel drying chambers. To solve the problem of gas stratification, do everything possible to convert laminar flow into turbulent flow.
Using various aerodynamic measures, disturbing the air flow can promote temperature uniformity. For example, install several circulation fans to make local horizontal circulation of air flow, etc.
In addition, appropriately extending the positive pressure section can also promote the cross-sectional temperature in the drying chamber to become uniform.
4 on and off the car.
The upper part of the kiln car and the lower part of the fairy pond are interpenetrating, restricting and influencing each other. If this contradiction is not handled well, it will bring adverse consequences to the drying of the green body. For example, if the length of a certain drying room is 60m, the distance from the zero pressure point to the vehicle entry end is 20m, which is in a negative pressure state. The zero pressure point is 40m away from the exit end, which is a positive pressure state. In the section where the vehicle is in a negative pressure state, the cooling air under the vehicle is easily sucked into the vehicle, which increases the burden on the exhaust fan, weakens the effective suction of the exhaust fan on the vehicle, and causes moisture to accumulate at the entry end , Saturated, the green body is easy to wet and collapse; in the section where the vehicle is under positive pressure, the hot air on the vehicle can easily rush into the bottom of the vehicle, which not only wastes heat energy, but also reduces the drying efficiency of the green body.
In order to ensure the normal operation of the drying process and ensure a high heat energy utilization rate and a good drying quality of the green body, it is necessary to separate the images of the kiln cars on the rocking car to minimize mutual interference. There are two methods. ①Strengthen the seal; ②Equalize the pressure on and off the car to cause the head on and off the car to fight against the head. In other words, create a pressure curve under the car that is similar to that on the car. Make the upper and lower pressure difference △P={C}{C}{C}{C}{C}{C}－{C}{C}{ C}{C}{C}{C}=0, this is called dynamic balance. After the dynamic balance is implemented, it can effectively prevent the cold air under the negative pressure section from being sucked into the vehicle, and at the same time prevent the hot air from entering the vehicle under the positive pressure section. Practice has proved that as long as the pressure equalization curve is properly controlled, the above adverse consequences can be avoided.
5 Entering and exiting the car
In the contradiction between getting in and out of the car. Entering the car is the main aspect of the contradiction. The speed of entering the car directly affects the quality of the dry body on the kiln car. The speed of entering the car depends on the drying process requirements for the structure of the drying chamber and whether the operation of the drying system is reasonable. Once the entry speed is determined, it must be strictly enforced and must not be accelerated or slowed down arbitrarily. Otherwise, it will cause the disorder of the drying thermal system and affect the quality of the dry body on the vehicle. To shorten the drying cycle and implement rapid drying, conditions should be actively created: take various measures to reduce the temperature difference of each cross-section of the drying room, and do not destroy the drying system at will (such as dragging out several vehicles at the end of the vehicle, etc.).
Most of the drying chambers are provided with a door at the entry end and exit end (some exit end does not have a door), and it is difficult to be tight and airtight after a door is closed. The cold air from the outside is sucked into the car by the loose part of the door. The hot air in the kiln is blown to the outside from the untight part of the door at the exit end (if there is no door, a large amount of hot air blows out), the best way to reduce the air leakage from the door is to install Double doors.
6 Inlet and outlet
How much air volume enters the drying room? How much air volume exits the drying room?
It has to be figured out. Only in this way can the relative balance of the air volume in the Werther drying kiln be managed, so as to achieve the purpose of stabilizing the pressure system and temperature system in the drying chamber.
The air volume and heat entering the drying chamber should be compatible with the evaporated water volume of the green body. If the heat is insufficient, the green body will not be dried, and it is easy to form mesh cracks. If the heat is too much, it will not only increase the energy consumption, but also easily cause the green body to crack or crack. Therefore, the required hot air volume must be calculated according to the water evaporation of the green body. In the air volume out of the drying chamber, in addition to the air volume discharged and entered by the kiln, the water vapor evaporated from the green body must also be discharged.
7 Heat transfer and temperature transfer
The movement of the hot gas (medium) makes it exchange heat and moisture with the bad body, so as to achieve the purpose of dehydration of the green body. The drying process is a process of heat transfer and moisture transfer.
The hot gas transfers heat to the green body to evaporate the water in the green body. When the hot gas cools down, it receives the evaporated water vapor from the green body and increases the humidity.
Theoretically speaking, the higher the temperature of the medium, the faster the drying speed, the lower the relative humidity of the medium, the faster the drying speed: the faster the flow rate of the medium, the faster the drying speed; the larger the contact area between the medium and the green body, the faster the drying speed quick.
However, since the green body has been formed (not a bulk body), it is necessary to safely dehydrate the hot gas (medium) as quickly as possible under the premise of no deformation, no cracking, no wet collapse, and no large internal stress. Various heat transfer technical parameters should be determined according to factors such as the maximum evaporation (humidity transfer) intensity allowed at each stage of green body drying. There is a certain knowledge between heat transfer and moisture transfer.
8 stacked code and single code
The blank stacks in the tunnel drying chamber in my country generally adopt 5 or more layers of stacking. There are also some dry exercise rooms that use a single yard (single floor).
The main advantages of stacking: ①Compared with the same length and single size, it can accommodate several times the number of blanks. Although under the same circumstances, the drying cycle is longer than that of single size, the output is still higher than that of single size; ②The construction funds are less .
The main disadvantages of stacking: ① due to the use of multi-layer stacking. It is difficult to ensure that the green body (especially the green body at the bottom layer) will not be deformed or cracked by compression, and the quality of the dried green body is generally lower than that of a single size, so it is not suitable for drying high-grade green bodies, such as dry wall decorative bricks. ②Due to the high inner height of the drying chamber, it is difficult to obtain uniform drying of each part of the green body.
Compared with stacked stacks, the main advantages of single-code: ① Since the single-layer stacked body has no risk of deformation or cracking, and the quality of the dried body is good, it is suitable for drying high-grade green bodies such as dry wall decorative bricks; ②Due to the low inner height of the drying chamber. All parts of the green body can be dried uniformly. The dried green body is in a relatively uniform gas (medium) environment, so it can be operated at a higher temperature and can be dried quickly. Usually, the multi-layered porous transfer blank needs 12 hours of drying time in the drying room, and it only takes about 4 hours when using a single code.
The main disadvantages of single yard: ①The drying chamber with the same length and width can accommodate a small number of blanks. Under the same circumstances, although the drying cycle is shorter than that of stacking, the output is still lower than that of stacking: ②The construction cost is more.
9 Waste heat and smoke heat
At present, the heat source of the drying room of most brick factories comes from the smoke heat of the roasting kiln. Flue gas is the heat that flows from the Kazakh wind of the roasting kiln into the flue. The extraction direction of the smoke heat is consistent with the airflow direction of the roasting kiln, so the fire speed of the roasting kiln will not be restrained. As long as it is controlled properly. Its heat can meet the needs of the drying room. When using flue gas, attention should be paid to the sulfur content in it. The existence of sulfur oxides not only corrodes drying vehicles, fans and metal pipes, but also contains pollutants in the moisture discharged from the drying room, which needs to be purified. At present, the heat source of the drying room of some brick factories comes from the waste heat of the cooling zone of the roasting kiln. The extraction direction of waste heat is opposite to the airflow direction of the roasting kiln. If the same extracted air volume cannot be replenished in time, the spark speed of the roasting kiln will be restrained. If you do not take other supplementary measures (such as extracting some hot air with higher humidity at the end of the moisturizing belt if it is allowed), the heat will not meet the needs of drying. The waste heat in the cooling zone of the roasting kiln is a kind of clean energy. Under normal circumstances, the moisture discharged from the drying room contains very little pollutants. It is not easy to corrode metal objects, and can be discharged directly without purification treatment.
Whether it is the smoke heat at the head of the roasting kiln or the waste heat at the end of the roasting kiln, it is actually the waste heat of the kiln. How to make full use of these heat is a subject worthy of study.
10 Yield and quality
Green body stacking is beneficial to increase output but the drying quality is often unsatisfactory, while single stacking can ensure high drying quality, but the constant output is often lower than stacking. How to solve this pair of contradictions to make the output high-quality and good, I think, should develop large hollow thin-walled products, and stack the green body in a single layer for rapid drying. The main way to promote rapid drying is to force the hot gas to circulate on each billet, so that the inner surface area of the hole of the hollow body is exposed to the hot gas to the greatest extent. Doing so can greatly shorten the drying cycle. The decoding drying cycle is 12 hours, and the drying cycle of a single code is shortened to 2 hours, which is only 1/6 of the stacked code, so it must exceed the output of 6 layers of stacked code.
Some people attribute cracks in the green body to rapid drying. It should be noted that cracks in the green body are not a direct result of rapid drying. Drying cracks are caused by inconsistent shrinkage of the green body itself, and the difference in shrinkage depends on the distribution of moisture content inside the green body. During the rapid drying process, the surroundings of the green body and the inside of the hole are evenly blown by the hot air, and the resulting humidity difference is very small. Coupled with an appropriate increase in the flow rate of the gas, the phenomenon of gas stratification is effectively overcome.
Especially for the large hollow thin body, it is particularly important to promote the gas circulation in the hole due to the large inner surface area of the hole. This has the concept of circulation and through flow. The so-called circular flow refers to the air flow and velocity around the green body, while the through-flow type only passes through the flow and velocity of the holes in the green body.
The conditions that a successful rapid drying room should have are: ① the gas flow rate and temperature on each cross-section are the same; ② ③ when conditions permit, the gas flow rate should be increased appropriately; Tend to be consistent to ensure that the inner and outer shrinkage of the green body is also consistent.
For each green body, there is an optimal drying curve, and the drying curve used should try to be close to the optimal drying curve. In order to ensure the long-term and stable implementation of the established drying curve to deal with drying, the indoor gas flow rate and flow rate, temperature distribution, checking temperature and checking humidity and temperature are automatically controlled. Only in this way can the dried green body be not only of good quality, but also of high output.