JPH01262967A - Apparatus for drying coated matter - Google Patents

Abstract

PURPOSE:To plan a lowering of running cost by installing a loop for circulatory supply of an inert gas to a drying furnace, a cooling device for recovering a paint solvent in a mixed gas and further a preheating device for the inert gas. CONSTITUTION:A metal strip 5 coated by a paint with a coater is introduced into a drying furnace 1 through an inlet opening, discharged out of an outlet opening of the drying furnace 1 after passing through the furnace 1 and led to a cooling process. During that time, the metal strip 5 is heated with an induction heating coil 6, a solvent of the paint applied is evaporated and a drying and curing of the paint is carried out by this heating. Vapor of the solvent remaining in the drying furnace 1 is mixed with and accompanied by an inert gas flowing in a circulation loop 4, led into a piping constituting the circulation loop 4 as an exhaust gas and introduced into a condenser 2, wherein it is cooled and recovered.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a coating drying device for a continuous coating line for metal strips, and in particular to a coating drying device using an induction heating coil as a heating means for baking and drying the coating. Regarding drying equipment.

[Prior Art] A drying device using an induction heating coil as a heating means has been developed for drying the coating of metal slips. this is,
This method takes advantage of the following advantages of induction heating.

In other words, compared to the hot air blowing method, induction heating has advantages such as a faster heat transfer rate, which allows the equipment to be smaller, and a faster response speed to changes in operating conditions.
In the hot air blowing method, the direction of heat transfer is from the painted surface to the inside, so the fixation of the surface layer progresses.This is a drying mechanism in which the internal solvent later comes out to the surface. On the other hand, with the induction heating method, the metal surface of the coating substrate is first heated directly, so the coating layer dries from the inside close to the metal surface to the surface, so the coating has good adhesion and is free from bending, squeezing, etc. Products with good processability can be obtained. As shown in FIG. 4, this apparatus uses a coater 51 to coat one or both sides of a metal strip 50 with paint (
The paint strip 50 coated with an organic paint formed from a synthetic resin, a pigment, and a solvent (where the solvent usually accounts for 40 to 60% by weight) is continuously guided to a drying oven 52; By heating the paint strip 50 with an induced current (induction heating coil 53), the solvent in the paint is evaporated and dried and baked. The evaporated solvent paper is mainly sucked through the opening of the strip introduction part of the drying oven 52. It is discharged from the upper part of the drying oven 52 along with the inflowing atmosphere, but it flows into a hood 55 equipped with a solvent paper liquefaction coil 54 through which cooling water is passed, and a part or a considerable part of the solvent is condensed. It is designed to be liquefied and recovered. The mixed gas of the air discharged from the hood 55 and the remaining solvent that is not condensed is passed through an exhaust pipe 57 by an exhaust fan 56 and is burned and deodorized in a gas incinerator 58 to prevent the emission of pollution. There is. In the figure, 59 is a recovered solvent tank, 60 is a power supply control unit, and 61 and 62 are cooling water inlets and outlets.

[Problems to be Solved by the Invention] In an induction heating device, a metal strip is heated by energizing an induction heating coil, but since the coil itself also generates heat, heating is performed while cooling water is passed inside the coil. There is. However, the problem is that the solvent vapor evaporated from the paint drops below the dew point on the cooled coil surface, condenses and condenses on the induction coil surface and its casing, and this drips onto the strip coating surface, impairing product quality. There is.

Additionally, the solvents that evaporate from paint coatings are highly flammable and, when mixed with air, form explosive mixtures.
Potential fire and explosion hazard. However, in order to avoid this danger, supplying a large amount of air for the purpose of lowering the concentration of solvent in the exhaust gas to the lower explosive limit increases the construction costs and operating costs of exhaust gas treatment equipment such as exhaust gas incinerators, resulting in induction heating. There is a problem in that one of the reasons for adopting this method is lost.

Furthermore, since the induction heating method uses electrical energy as its heat source, it is more expensive per unit of heat than the hot air blowing method that uses liquid or gaseous fuel.Although it has high thermal efficiency, the energy price difference is There is also the problem that it cannot be compensated for.

The present invention was made in view of the above-mentioned circumstances, and its purpose is to prevent organic solvents evaporated from paint from condensing in a drying oven, and to enable safe operation without fire or explosion. An object of the present invention is to provide an induction heating type paint drying device that can reduce costs.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a paint drying oven that uses an induction heating coil as a heating means for baking and drying paint in a metal strip coating line, and a a circulation loop that circulates and supplies an inert gas to the drying oven; and a circulation loop that is provided within this circulation loop and cools a mixed gas of paint solvent vapor and inert gas discharged from the drying furnace to recover the paint solvent in the mixed gas. a cooling means; a preheating means provided in the circulation loop that heats the inert gas that has passed through the cooling means to a temperature higher than the dew point of the paint solvent in the atmospheric gas formed in the drying oven, and supplies the heated inert gas to the drying oven; It is characterized by having the following.

Further, the refrigerant for the induction heating coil is supplied at a temperature higher than the solvent dew point of the inert gas/solvent paper mixed gas formed in the drying oven.

[Function] Since the present invention has the above-described configuration, the paint solvent paper evaporated during the baking and drying process of the paint is mixed with the inert gas as the carrier gas circulating in the circulation loop and is discharged from the furnace. be done. This inert gas is supplied into the drying oven in an amount and at a temperature necessary to maintain the vapor pressure/temperature at which the evaporated paint solvent vapor does not condense within the drying oven.

The paint solvent paper discharged from the drying oven together with the inert gas is cooled by a cooling means, condensed, and recovered as a liquid solvent. A high boiling point portion of the paint solvent is recovered not only by the cooling means but also by a preheating means using a mixed gas of paint solvent paper and inert gas discharged from the drying oven as a heating medium.

Further, as the refrigerant in the induction heating coil, heat transfer oil heated to a temperature higher than the dew point of the solvent paper in the drying oven is supplied, so that condensation of the solvent paper on the coil surface is prevented.

Furthermore, the circulation loop, which includes a drying oven, is filled with inert gas to suppress the oxygen concentration to below a certain value that does not pose a risk of fire or explosion, and also prevents oxidation of paint pigments during the baking drying process. Prevented.

[Examples] The present invention will be specifically described below based on illustrated examples.

Figure 1 shows the first embodiment of the present invention. The coating drying apparatus of this embodiment includes a drying oven 1, a condenser (cooling means) 2, a gas preheater (preheating means) 3, and a circulation system connecting these with piping. It is roughly composed of loop 4.

The metal strip 5 coated in the previous step is continuously introduced into the drying oven 1 to be baked and dried.

The drying oven 1 is equipped with a copper induction heating coil 6 having a rectangular cross section and a width larger than the width of the strip 5 to be treated. The coil is completely filled with a solvent-based, heat-resistant synthetic resin. Therefore, the generation of sparks due to any cause is prevented. The synthetic resin mentioned above is Bakelite (trade name), which has good resistance to many solvents and relatively excellent heat resistance.
However, depending on the conditions, other synthetic resins may be used or a lining of another synthetic resin may be applied.
is converted into a high frequency current of about 3000 to 50°000 H2, and a matching board 9 supplies the voltage and current matched to the load to the coil 6 to heat the metal strip 5 by induction.

Further, the inert gas is supplied from the gas supply means 10 through the valve 11 into the piping that constitutes the circulation loop 4.

Further, the entire drying oven 1 including the coil 6 is covered with a hermetically sealed housing 12 made of a non-magnetic material such as aluminum or heat-resistant synthetic resin.

The circulation loop 4 has piping connected to the inert gas inlet la of the drying oven 1 and the outlet 1b of the mixed gas of the inert gas and paint solvent paper in the oven 1, thereby making the drying oven 1 one element. It becomes a circular loop. Furthermore, an air seal mechanism 13 is provided at the outlet and outlet of the strip 5 of the drying oven 1, through which an inert gas is constantly discharged.

In the figure, 14 is a circulation fan, 15 is a refrigerant for the condenser 2, 16 is a heat medium for the gas preheater 3, 17 is an inert gas for the air seal mechanism 13, and 18 is an atmospheric release valve.

Next, the operation of this embodiment will be explained.

First, when the circulation fan 14 is started, the valve 11 is opened and an inert gas such as nitrogen gas is supplied from the gas supply means 10 to the piping constituting the circulation loop 4 to fill the space inside the circulation loop 4 with the inert gas. Replace with In this case, complete replacement is not required, and the oxygen concentration in the circulating gas only needs to be 5% or less, preferably 2% or less, and when the oxygen concentration reaches a predetermined value or less, the strip inlet of the drying oven l A small amount of inert gas 17 is supplied from the air seal mechanism 13 at the outlet to air seal the opening, and the supply of inert gas from the gas supply means 10 is stopped or the supply amount is reduced. During operation, the oxygen concentration in the inert gas circulation loop including the drying oven 1 is always maintained at 5% or less, preferably 2% or less.

By adjusting the oxygen concentration in this way, the mixture of flammable paint solvent vapor and inert gases such as nitrogen and carbon dioxide becomes non-flammable, reducing the risk of solvent explosion and fire. It can be avoided.

Next, heat medium 16 is placed in the preheater 3, and refrigerant 1 is placed in the condenser 2.
The circulating gas is heated and cooled through each of the steps 5 and 5, and the temperature of each medium is such that the gas temperature after preheating is, for example, 130°C, and the gas temperature after cooling is, for example, -20"C. After that, even during steady operation, the temperature of each heat medium 16 is adjusted by the temperature control device.
, the temperature, pressure, amount, etc. of the refrigerant 15.

Thus, the metal strip 5 coated with paint on both or one side by a coater (not shown) enters the drying oven 1 from the entrance side opening, passes through the class 1, and exits from the exit side opening of the drying oven 1. It exits and is guided to the next cooling step (not shown). During this time, the metal strip 5 is heated in the drying oven 1 by an induction heating coil, and this heating evaporates the solvent of the applied paint, thereby drying and baking the paint. The paint solvent paper that remains in the drying oven 1 is mixed with the inert gas flowing in the circulation loop 4 including the drying oven 1, and is entrained, and is passed through the circulation loop 4 from the outlet 1b of the drying oven 1 as exhaust gas. is guided into the pipes that make up the This exhaust gas is first introduced into the condenser 2 and cooled, and a solvent equivalent to the amount evaporated in the drying oven 1 is condensed and recovered as a liquid solvent 19. The condenser 2 is generally a shell-and-tube surface condenser, but it can also be a plate type or a scrubber condenser in direct contact with a co-liquid of condensate that is cooled and circulated. The refrigerant 15 of the condenser 2 is selected depending on the composition of the solvent used as paint thinner, but solvents with high vapor pressure, such as methanol, MEK, etc., are difficult to condense in the case of thinners with a large composition. Therefore, it is necessary to cool the gas to about 0°C to -20°C, so for example, ethylene glycol-25
°C is used. In addition, capacitor 2 is a two-stage type,
In the first stage, high boiling point solvent papers such as butyl cellosolve, ethyl cellosolve, cellosolve acetate, etc. are condensed and recovered using water at about 25 to 30 °C as a refrigerant, and in the second stage, a low temperature refrigerant is used. It is reasonable to condense the low boiling point solvent to reduce the load on the freezer (not shown).

Condenser 2 for mixed gas of inert gas and solvent paper
The condensation recovery rate of the solvent in can be increased as the temperature of the refrigerant 15 used is lowered, but economically it is less than 90%.
%, and the recovered solvent 19 is reused.

The inert gas containing the remaining solvent that does not liquefy is circulated by the circulation fan 1.
After being pressurized by the gas preheater 3, the gas is heated by the gas preheater 3 and supplied to the drying oven 1. The temperature of the inert gas after preheating is set to a temperature above the dew point so that the solvent evaporated in the drying oven 1 does not condense in the oven. As the heating medium of the preheater 3'C
Low-pressure steam is suitable, but other heating mediums can also be used. In addition, the purpose of the circulating inert carrier gas is to quickly accompany and discharge the solvent vapor evaporated in the drying furnace 1 out of the furnace, but in other words, the inert carrier gas is mixed into the solvent paper, which is an inert dry gas. The objective is to lower the vapor pressure of each constituent solvent and lower the dew point of each solvent. For this purpose, the larger the amount of inert carrier gas is, the lower the dew point will be. The ratio should be 2 to 5 times (weight ratio, or 0.5 to 0.2 in reciprocal).

Figure 3 shows the calculation results of the dew point in the carrier gas of butyl cellosolve, which is one of the commonly used solvents that has the lowest vapor pressure, that is, the highest dew point, in other words, it is one of the ones that easily forms dew condensation. This result shows that nitrogen gas is used as a carrier gas and composition solvent other than butyl cellosolve is used.
Since the calculation is performed assuming a substance with an average molecular weight of 100,
Although it is accurate in the part of 100% Butyl Ceroso J1/B, it requires some modification according to the physical properties of the actual solvent during actual design and execution, but this result is approximately practical and as a parameter. Changes due to solvent concentration H (=solvent amount/dry nitrogen gas amount, kg/kg) are shown. The IHI diagram of cellosolve acetate, which ranks next to butyl cellosolve in terms of vapor pressure, is shown by a broken line in FIG. 3, but many other substances have even lower dew points than these substances. This chart shows that if the amount of carrier gas is kept at twice or more than the amount of evaporated solvent, and the temperature in the drying oven is maintained at 130°C or higher, dew condensation will not occur even when handling solvents containing butyl cellosolve, which are prone to forming. It is shown that.

In addition, the refrigerant for the induction heating coil 6 used in this embodiment is supplied with heat transfer oil that does not boil or deteriorate even at high temperatures instead of cooling water at a temperature higher than the solvent dew point in the atmosphere inside the drying oven 1. The surface temperature of No. 6 (more specifically, the surface temperature of the resin into which the coil is cast) is maintained at a temperature higher than the above dew point to prevent condensation of the solvent. The refrigerant supplied into the induction heating coil 6 is heated by about 20 to 60 degrees Celsius by cooling the coil, and then exits the coil 6. However, it is cooled by a cooler (not shown) outside the coil, and automatic temperature control is performed. It is used cyclically while doing so.

FIG. 2 shows a second embodiment of the invention.

This embodiment differs from the first embodiment described above in the piping system that constitutes the circulation loop. Components having the same configuration as those in the first embodiment are given the same reference numerals and redundant explanations will be omitted.

The piping system of this embodiment includes a heating means 21 that uses the exhaust gas of the drying oven 1 as a heat medium, and a cooling means 22 that cools the exhaust gas that has passed through the heating means 21. In addition to recovering paint solvent from exhaust gas,
A circulation loop is formed in which the exhaust gas that has passed through the cooling means 22 is heated by the heating means 21 and supplied to the drying furnace 1. In this embodiment, butyl cellosolve, ethyl cellosolve, etc. are added to the composition of the evaporative solvent in the drying furnace 1. This can be suitably carried out when a solvent with a low vapor pressure is not included, such as in the case of

In this case, the dew point of the atmospheric gas inside the drying oven 1 becomes high, so the temperature of the inert gas supplied from the circulation loop piping system into the drying oven 1 does not need to be as high as that shown in the left. By simply heating the inert gas with the exhaust gas (heating means 21) discharged from the drying furnace 1, the solvent can be preheated in the drying furnace 1 to a temperature at which no dew condensation occurs.

Further, according to this embodiment, the solvent in the exhaust gas is heated by the heating means 2.
1 performs the first stage recovery, and the cooling means 22 performs the second stage recovery, which is preferable because the load on the refrigerator of the cooling means 22 is reduced.

However, in this embodiment as well, the metal strip 5 coated with paint is heated by induction in the drying oven 1.
The paint is dried and baked.

Furthermore, in the present invention, it is important to prevent outside air from entering through the inlet and outlet of the drying oven 1 in order to keep the ingress of air into the circulation loop to a minimum. As means for preventing the intrusion of outside air, the operating static pressure inside the drying oven 1 is maintained at a slight positive pressure of, for example, about 2 mm underwater, the above-mentioned population and the size of the outlet opening are set to the minimum allowable for operation, and This is possible by providing a suitable form of, for example, known aerodynamic sealing devices such as inert gas curtains.

[Effects of the Invention] Since the present invention is configured as described above, it has the following various features.

■ The solvent evaporated from the paint is diluted with inert gas, lowering the dew point, and the refrigerant in the induction heating coil maintains the coil surface at a temperature above the dew point, preventing dew condensation inside the drying oven. Ru. As a result, the painted surface after drying and baking is not contaminated with dew condensed solvent, and a coated product of good quality can be obtained.

Since the C9 drying furnace forms an inert gas circulation loop as one component, the oxygen concentration in the system gas can be kept at a low concentration, allowing safe operation without fires or explosions caused by solvents. .

Furthermore, since the drying and baking of the paint is carried out in an inert gas atmosphere with a low oxygen concentration, oxidation of paint pigments during this process is suppressed to the utmost, resulting in an excellent painted product.

■ In addition, the expensive solvents contained in the paint are simply burned to prevent pollution, and instead of being released into the atmosphere, they are economically recovered as liquid solvents in the circulation loop, which helps to dissolve the paint. Since it can be used for applications with high added value inherent to other solvents, or as a liquid fuel with a high calorific value, it is possible to reduce operating costs.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of an embodiment of the present invention, Fig. 2 is a system diagram of another embodiment of the same, Fig. 3 is a diagram showing the dew point of a mixed gas containing an organic solvent, and Fig. 4 is a diagram of conventional induction heating. It is a system diagram of coating drying by method. l...Drying oven, 2.22...Condenser (cooling means), 3.21.
... Preheater (heating means), 4... Circulation loop, 5... Metal strip, 6...
・Induction heating coil.

Claims (3)

[Claims] (1) A paint drying oven that uses an induction heating coil as a heating means for baking and drying paint in a metal strip painting line, a circulation loop that circulates and supplies inert gas into this drying oven, and a circulation loop that circulates and supplies inert gas to the drying oven. a cooling means provided in the circulation loop for cooling the mixed gas of paint solvent paper and inert gas discharged from the drying oven and recovering the paint solvent in the mixed gas; A paint drying apparatus comprising: preheating means for heating an inert gas to a temperature higher than the dew point of a paint solvent in an atmospheric gas formed in the drying furnace, and supplying the heated inert gas into the drying furnace. (2) Claim (1) wherein the heating medium of the preheating means is a mixed gas of paint solvent paper and inert gas discharged from a drying oven.
Paint drying equipment as described in section. (3) The paint drying apparatus according to claim (1), wherein the refrigerant temperature of the induction heating coil is equal to or higher than the dew point of a solvent in a mixed gas of an inert gas and solvent paper formed in the drying oven.