How To Measure The Suitable Humidity Inside Spray Paint Booth

How To Measure The Suitable Humidity Inside Spray Paint Room / Environmental Spray Paint Room

Abstract: The wet spray booth can be used without any restrictions on the site to reduce the investment in painting and baking. It is suitable for mass production of large workpieces with large spray fog and is a new type of coating equipment. The definition of spray paint room, the difference, characteristics and application conditions of dry and wet spray paint room and the necessity of development of wet spray paint room are introduced. It is stated that the humidity is not exceeded when drying is wet spray paint. The key reason for the room technology. From the theoretical analysis, structural design and operation of the spray booth, the measures to ensure the humidity of the wet spray booth are not exceeded. The results show that the problem of excessive humidity in the wet spray booth can be solved.

1 Introduction

Painting is the last step in the production of many products. The design of the coordinated paint room is more complex than the single-function paint or baking equipment. The bright color and bright paint film can not only make the decoration beautiful, but also make the surface of the workpiece good. Protection. In recent years, domestic companies have improved the painting process and the equipment of the painting workshop.

The spray paint booth is a new type of painting equipment that meets the requirements of large-scale and difficult-to-work workpieces. It is divided into dry spray paint booth and wet spray paint booth according to the method of capturing spray mist. The first spray paint booth is integrated. The design is a dry spray booth, the so-called "dry" means that the capture of the paint mist is carried out using dry filter cotton. Due to the limitation of the dust-retaining rate of the filter cotton, it is necessary to change the filter surface frequently, otherwise the process parameters of the paint and baking paint and the use effect of the equipment will be greatly affected, and the frequent replacement of the filter bottom cotton is time-consuming and expensive to use. In order to solve this problem, the design idea of the wet spray paint booth is proposed, that is, the liquid (common water) is used for the capture of the paint mist, and the liquid (water) has the strong ability to capture the paint mist, and is suitable for the working condition with a large amount of paint spray.

2, the key technology of wet spray paint room

The spray paint booth is a painting equipment that can meet the requirements of the paint process environment and meet the requirements of the baking process environment. The spray booth design needs to provide the clean air required for painting, the appropriate illumination, temperature, humidity and even and reasonable air flow according to the national environmental protection, sanitation and safety production specifications, and timely discharge the polluted air. The paint booth needs to provide the uniform temperature, proper moderation, clean air, exhaust gas discharge and other functions required for paint baking. The design of the spray booth is more complicated than the single function spray period or baking equipment. It is necessary to solve the problem of conversion and control of different process parameters during painting and baking.

When painting, the indoor air is supplied with fresh air, and the humidity and temperature of the air supply are not affected by the water stored at the bottom of the chamber. In the case of baking paint, in order to save energy, hot air is recycled. Since there is water in the lower part of the wet spray booth room grille, if no measures are taken, the volatilized water will be sent back to the room through the circulation system, increasing the humidity of the indoor circulating wind, resulting in exceeding the standard in the humidity chamber. Humidity affects the volatilization speed of the solvent, which in turn affects the leveling and sag performance of the coating. The coating operation is carried out under high humidity. The solvent volatilizes to make the surface temperature of the wet paint film lower than the dew point temperature, and the water vapor is condensed on the surface of the wet paint, causing the paint film. "Whitening." Therefore, solving the indoor humidity problem during baking is a key technology for the development of wet spray booths.

Generally, the humidity in the atmosphere is high in summer, and the humidity in winter is low. In addition, the level of humidity is also closely related to the region. The coating standard stipulates that the relative humidity in the coating equipment should be between 55% and 75%. In the case of high requirements for the quality of the paint, in order to ensure the appropriate humidity, humidification and dehumidification devices can be used in the air supply system, but the air-conditioning device with high air volume has high cost and large space, which increases the operating cost during use. The following discussion is not to design a humidification and dehumidification system separately, to achieve the process parameters of spray baking.

3. Humidity analysis during spray baking process

The physical properties of the moist air are related to the composition of its composition and, in addition, to the state in which it is located. The state of the wet air can usually be expressed by parameters such as pressure, temperature, humidity, specific volume, and helium. The relationship between the state parameters of the wet air can be obtained from the wetting diagram of the wet air [2]. The workpiece to be sprayed has to go through three stages in the spray booth, namely the painting stage, the leveling (flash drying) stage and the baking stage. Different stages correspond to different process changes, and the indoor air also undergoes three stages of changes. This process is illustrated by taking a large wet spray booth in the Qingdao area with high humidity as an example.

(1) Initial parameters The air supply volume of the air blower sent by the spray booth is Q / h, and the whole system volume is V (including the operation room, hot air source, circulation air duct). The average summer temperature in Qingdao is 25.1 °C, the average relative humidity is 85%, the temperature rise level is determined to be 30 °C, the leveling time is t m in, the baking temperature is 60 °C, and the baking time is 1 h.

(2) In the painting stage, according to the initial conditions, the summer environmental conditions do not meet the requirements of the paint specification. According to the characteristics of the wet air, the wetting diagram can be obtained. When the fresh air is heated to > 27. 2 °C, the relative humidity will drop. Less than 75%. This method is simple, easy to control, and cost-effective, but at the expense of ambient temperature.

(3) Leveling stage The purpose of leveling in the wet spray booth is not only the evaporation of the solvent, but more importantly, the residual moisture on the ground (water-repellent board). The amount of water that can be taken away by leveling is related to the leveling time and temperature. The longer the time, the higher the temperature and the greater the amount of water taken away. In order to ensure that the humidity in the leveling meets the requirements, it is necessary to achieve the temperature rise leveling, and in the case of steam volatilization, the relative humidity in the drying room is guaranteed to be < 75%. The “wet air wetting diagram” shows that the moisture content in the wet air at a relative humidity of 85% at 1 °C is 17.1 g / kg, and the moisture content in 75% wet air at 30 °C is 2012 g/kg. From this, it is calculated that the amount of water that the ventilation is Q and the leveling tm in can take away is: 1. 2 Q ·t /60 ×(2012 g/ kg -17. 1 g/kg) = 01062Q ·t ( g 2公斤/m3。 The weight of the air is 1. 2 kg / m3. Assuming Q = 100 000 m3 /h, leveling 10 m in, the maximum amount of water that can be taken off by leveling is W 1 = 62 kg.

(4) Baking stage After the leveling, the spray drying room enters the baking temperature rising stage, and it is assumed to rise from 30 °C to 60 °C, at which time the hot air is circulated. To achieve such an effect during the heating process of baking, the characteristics of temperature rise and relative humidity drop are utilized to ensure that the entire circulation system with volume V is continuously volatilized during the process, and the relative humidity does not increase but also decreases. Up to 75%. The baking temperature rise and the vapour volatilization are decomposed into two processes: first, the isothermal temperature rises, when the absolute humidity of the system is constant, and the relative humidity decreases with the increase of temperature; isothermal humidification, that is, the surface water vapor volatilizes, When the temperature is constant, the volatilized water vapor increases the relative humidity and absolute temperature. 75% wet air moisture at 30 °C is 2012 g / kg, while 75% wet air moisture at 80 °C is 80 g / kg [ 2 ], so the drying temperature is theoretically allowed to volatilize the maximum water vapor The quantity is: 112 × V · (8010 ~ 2012) ( g). Assuming that the system has a volume of 2 000 m 3 , the amount of water vapor that can be volatilized only by drying and warming is: W 2 = 14315 kg.

In addition, during the baking process, in order to prevent the concentration of organic exhaust gas in the drying room from reaching the lower limit of the explosion of the gas, a small amount of exhaust gas is discharged during the hot air circulation and the same amount of fresh air is added at the same time (assuming 3 000 m3) / h), the supplementary fresh air ( 25. 1 °C) can still absorb a certain amount of water when heated to 60 °C. It is known that the moisture content in the humid air at a relative temperature of RH85% at 2 °C is 17.1 g/kg dry air, so the fresh air absorption in 1 h is: W 3 = 3 000 m3 / h × 1. 2 kg/m3 × (80 g/kg -1711 g/kg) = 226 kg / h

That is, the amount of water to be volatilized in the wet spray booth is 369. 5 kg, and the amount of water that can be taken away by the warming leveling is 62 kg, for a total of 431.5 kg of water.

According to the above analysis, if the design is reasonable, the residual moisture on the ground can be taken away during the leveling, and the evaporation of moisture under the water swirler is restricted during baking, so that the problem of excessive baking humidity is not generated.

4. Measures to ensure that the humidity in the wet spray booth is not exceeded when baking

4. 1 Structural design aspects

(1) The circulating air line is separately designed for baking, and the secondary return air is discharged from the pipe on the water-repellent board.

(2) Appropriately increase the slope of the water-repellent board to facilitate the rapid flow of water above the water-repellent board.

(3) Consider using a material that is water-repellent and has low water absorption to make a water-repellent board so that most of the water is drained during the leveling stage.

(4) Design the exhaust air outlet to the bottom of the water swirler to ensure that there is always a top-down wind flow in the water swirler during baking to avoid and reduce the evaporation of water vapor into the room.

(5) Improve the precision of the manufacture and installation of the water spinner and the water-repellent board, ensure the uniformity of the wind deflector, and at the same time facilitate the rapid flow of water above the water-repellent board.

4. 2 Engineering design and production operations

(1) Set the air volume adjusting valve in the baking hot air circulation system, adjust the air volume of the baking circulating air system and the ratio of the circulating air to the exhaust air volume to ensure a positive pressure is formed above the water rotating device, and the water vapor is not suppressed from evaporating into the room.

(2) In the control, it is required to stop the painting, the pump stops immediately, and it should be leveled for 10 min before baking. At this time, the air is heated, the hot air is not circulated, and the air is discharged according to the state of painting, and the residual water on the water-repellent board is discharged.