Steel drum far infrared drying technology
At present, many domestic barrel manufacturers have adopted far infrared drying paint technology. It is a steel drum that is dried by radiation from a far-infrared radiator. When the far-infrared rays match the intrinsic motion frequency of the basic mass point on the surface of the heated steel drum, the radiant energy can be well absorbed and the intense resonance is generated, so that the steel drum can achieve rapid and uniform heating. This technology has the characteristics of energy saving, high production efficiency, good heating quality, small equipment footprint, low investment, quick effect and easy automation.
At present, most of the barrel manufacturers focus their attention on the selection of radiating elements when applying this technology. Once the savings are not obvious, they are blindly blamed for the low quality of the components or the mismatch with the heated steel drum. In fact, the quality of the radiating components is only one of the factors affecting the heating effect. It should also be seen by other factors such as heating method, furnace structure, radiation distance, temperature and so on. Due to the imperfect design of the heating furnace, accidents such as explosions and fires often occur.
In order to make steel drum production enterprises do a good job in the design of steel drum paint drying oven, the following questions about the design of the drying furnace are as follows:
1. What are the original materials needed for the design of the oven?
A: When designing a steel drum film drying oven, the following raw materials must first be collected:
1 Material, weight and dimensions of the heated steel drum.
2 steel barrel paint film drying required temperature.
3 heating time (including heating, holding, cooling time).
4 Production capacity of the drying furnace - the weight (kg) of the steel drum once entered or the weight of the steel drum (kg/h) passing through the furnace.
5 The quantity, weight and external dimensions of the auxiliary equipment (such as trolley, hanging conveyor chain, hangers, etc.) when entering the furnace at the same time as the heated steel drum.
6 Process arrangement of heating steel drum in the furnace.
7 shape of the drying oven, internal dimensions and door opening dimensions.
2. What factors should be paid attention to when heating the steel drum drying furnace and selecting the furnace type?
A: The thermal energy transfer in the far-infrared oven is only a choice between radiant heating and radiative convection hybrid heating. In the so-called radiant vacuum drying furnace, in addition to vacuuming the furnace, natural convection usually occurs, but radiation is the main way of heating. Radiant convection hybrid, in addition to the far-infrared heating steel drum radiated by the radiator, there is hot air as a medium, artificially utilizing and promoting the heat transfer of convection (mostly forced convection) to accelerate the heating of the steel drum.
The general requirement for the drying furnace is that it should have the shortest heating time. The temperature distribution in the furnace satisfies the process requirements, it is convenient to adjust the temperature, the energy consumption is small and the heating quality can be guaranteed. Whether it is radiant or convection heating, the steel drum can be heated faster than the air temperature, that is, the steel drum temperature can be higher than the ambient air temperature. The radiation convection type uses forced convection to make the temperature distribution in the furnace uniform than the radiant type, and the larger air flow is beneficial to the escape of the film film volatiles, and also brings the problem that it is difficult to ensure the heating environment is clean.
The choice of furnace type mainly refers to whether the relative position of the heated body and the radiator changes, that is, whether it is fixed or through. This mainly depends on the batch size of the drying steel drum. It is not economical to use the through-type drying furnace for small-volume production. For large-volume or medium-volume production, there is a through-type drying furnace for those who have expanded remanufacturing space. The heating quality of the tunnel furnace is generally better than the fixed type. In order to improve the heating quality of the fixed furnace, an improved fixed furnace (or semi-fixed furnace) can be used. It installs a transmission mechanism in the stationary drying oven, which can rotate or move the heated steel drum during heating, thereby changing the heating quality.
3. How to determine the length of the drum drying oven?
Answer: The length of the fixed drying oven depends mainly on the total length of the steel drum process arrangement in the furnace, or the batch size determined by heating the steel drum once. The more the steel drum is heated once, the longer the furnace body.
Automatic wire pass furnace length:
Where: t - heating time; v - steel drum moving speed.
The heating time t can be determined experimentally in a laboratory furnace by using a steel drum or a bucket. It should be noted that the heating time t is related to many factors such as radiation intensity, absorption coefficient, irradiation distance, and ambient temperature. If the moving speed V of the steel drum can be determined by the speed of the automatic line, then L is easy to determine.
When the heating rate cannot be determined first, the length of the furnace body can also be determined by the heating capacity. Although there is relative movement between the workpiece and the radiator of the through-type drying furnace, it can be seen from a little analysis that the through-type drying furnace is essentially the same as the fixed furnace. The time required for each steel drum in the tunnel to pass should be the same as the heating time in the same fixed furnace. The steel drums are arranged in sequence through the furnace. The longer the furnace body, the greater the processing capacity per unit time. Conversely, the length of the furnace body can be determined according to the required production capacity. Of course, the final determination of the length of the furnace should also take into account the impact of various factors such as the production site, shifting mechanism, heating time.
Considering the gap between design and practicality, the selected transmission structure has a certain speed regulation capability, so that the heating time can be changed by the speed adjustment to achieve the best heating effect.
4. How to determine the shape and shape of the drying oven?
A: The shape of the drying oven is varied. Depending on the shape of the steel drum, it can generally be made rectangular. As for the shape of the entire far-infrared tunnel furnace, since its length is limited by the production site, it is not necessarily all straight, and it can also be made into a U-shaped, S-shaped or three-dimensional multi-layer S type to reduce the floor space. In order to facilitate the insulation and discharge of flue gas, the through furnace body is often arched.
The outer dimensions of the oven are generally equal to the internal space size plus the thickness of the furnace wall.
Under the premise of ensuring the heating quality, the fixed drying furnace should have the smallest possible furnace body. When the furnace body is too large, although the distance between the radiator and the workpiece can be adjusted to ensure the heating temperature, the heating time is prolonged due to the absorption of heat by the air and the excessive furnace body, and it is difficult to ensure the heating quality. The size of the internal space of the drying furnace should be equal to the external dimensions of the workpiece arrangement plus the distance between the radiator and the workpiece, plus an appropriate amount of adjustment margin. Since the height of the drying furnace has a great influence on the temperature difference between the upper and lower sides of the furnace, it should not be too high.
Pass-through ovens should also have as small a width as possible and a suitable height.
In addition, in order to facilitate the discharge of moisture or organic volatiles during heating, the cross-sectional shape of the furnace body is often not rectangular, but the top is triangular or curved. In this way, the formation of a dead zone of volatiles can be avoided, which affects the heating quality.
5. How to determine the electric power of the drying oven?
A: The power of the far-infrared drying furnace of the steel drum film can only be approximated. The calculation methods of the power of three far infrared drying ovens are described below for reference.
(1) Heat balance method
Set Q1 - the heat required for the steel drum base; Q2 - the heat required for the paint on the steel drum base; Q3 - the heat required for the steel drum conveying equipment (car, catenary, hanger, etc.); Q4 - through the furnace Wall, door joints, inlets and outlets and heat lost from flue gas.
According to the law of conservation of energy:
Q total = Q1 + Q2 + Q3 + Q4
The heating power is:
W=1.2Q total / 860Î·
Where: 1.2 - Consider the fold coefficient of unforeseen heat loss. Î· - drying furnace efficiency. For the sealed furnace, it can be between 0.6 and 0.85; for the pass furnace, it can be between 0.5 and 0.6; for the open furnace, it should be between 0.25 and 0-35.
Regarding the specific calculation of the heat of Q1, Q2, Q3 and Q4, we will discuss it later.
(2) Estimation method
Electric power consumed by simply heating the steel drum
Where: W - power consumption (kwh); p - "total weight of heated steel drum (kg or kg / h); c - specific heat of steel drum material (kcal / kg / degree); t-heating Temperature difference before and after (Â°C); Î·â€”â€”drying furnace efficiency
Electric power consumed by drying paint
Where: W - power consumption (kwh); p1 - treatment weight of the paint (kg / h); p2 - weight of volatiles (kg / h); p3 - weight of the material (kg / h); t - temperature difference before and after heating (Â°C); q - heat of evaporation of volatiles (4.1868 * 103J / h); Î· - drying furnace efficiency
(3) Radiated power density method
The far infrared drying furnace consumes electric power
Where: E - radiation power density (kw / m2). Generally take between 3 ~ 8kw / m2, when the steel drum capacity is large, take a large value. Fâ€”â€”heating area per unit time (m2/h)
6. What is the design and sealing of the drying oven?
Answer: From the principle of far infrared heating technology, the heated steel drum absorbs far infrared rays and generates self-heating effect. Therefore, there is no need to seal and heat the heating furnace, but in open heating equipment, it is often taken away by air flow. A large amount of heat increases the heating time and reduces the efficiency. After adopting the sealed furnace body and the heat preservation measures, the heating time and the holding time of the heated steel drum can be greatly shortened due to the reduction of the heat loss, that is, the heating efficiency is improved, so the fixed furnace body is mostly sealed and outside the furnace body. Apply a layer of insulation. Commonly used insulation materials are mica slag, strontium, slag cotton, glass fiber cotton, expanded perlite and the like. The thickness of the insulation layer varies depending on the structure of the furnace body and the insulation material, and may be determined as appropriate, generally in the range of 75 to 300 mm. The ideal insulation effect is that when the drying furnace is working normally, the temperature of the outer wall of the furnace is not higher than 50 Â°C. For the passing furnace body, the cross section of the inlet and outlet should be minimized, and the thickness of the insulation layer around the inlet and outlet should be appropriately increased or Add curtain insulation. In addition, arching the furnace rail can also reduce heat loss and facilitate heat preservation.
7. How to solve the upper and lower temperature difference in the drying furnace?
Answer: In actual use, the internal air of the drying furnace is naturally convected, causing the upper atmosphere temperature in the furnace to be higher than the lower part, which will also cause a temperature difference between the upper and lower parts of the heated steel drum, which will inevitably affect the drying quality. If the steel drum paint coating often occurs, the upper part has been dried, but the lower part is not 1000; or the lower part is dried, and the upper part has been â€œbaked yellowâ€ and the drying is uneven. To this end, it is necessary to control the temperature difference between the upper and lower parts of the furnace. Generally, the temperature difference between the upper and lower sides of the furnace is preferably less than 5 Â° C, and the maximum is not more than 10 Â° C. In order to ensure this, when arranging the radiating elements in the furnace, if the components of the same power specification are used, the part near the bottom of the furnace may be appropriately dense, and the upper part is relatively sparse. The attendant problem is that the heating quality is affected by the uneven distribution of the irradiance. In order to balance the uniformity of the irradiance and the temperature difference between the upper and lower sides, radiating elements of different power specifications can be used, and the components in the furnace can still be evenly arranged, but the power is gradually reduced from bottom to top. If it is difficult to use different power radiating elements, the same power components can be evenly arranged, but the same effect can be obtained by grouping the electric power of the control elements with the voltage adjusting device. In addition, proper forced convection of the air in the furnace can also reduce the temperature difference between the upper and lower.
The difference between the upper and lower radiant energy of the steel drum caused by the natural convection in the furnace can be determined by the following formula:
Where: Eâ€”â€”radiation energy density (w/cm2) in the upper part of the steel barrel; Eâ€²â€”â€”radiation energy density (w/cm2) in the lower part of the steel barrel; Hâ€”â€”steel height (m)
The above answers the basic problems in the design of steel drum paint drying oven. In addition, there are several important issues that are more complex and we will discuss them in the future. These few questions are:
1 Calculation and design of ventilation system for drying oven
2 control of drying furnace temperature
8 Far infrared radiator selection and process layout.
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