Classification of Paint Booths

1. According to the nature of coating production, spray booths can be broadly categorized into two types: batch production and continuous production.

Intermittent-production spray booths are commonly used for coating single pieces or small-batch workpieces, and can also be employed for large-scale coating of small workpieces. Depending on the way workpieces are placed, these booths come in three main types: trolley-type, suspension-type, and mobile (including telescopic) type. Intermittent-production spray booths are typically open-style.

Continuous-flow spray booths are used for coating large batches of workpieces. Typically, they are through-type and employ conveying mechanisms such as suspended conveyor systems, electric rail carts, or floor conveyors to transport the workpieces. Continuous-flow spray booths can be integrated with pre-treatment equipment prior to painting, film-curing equipment, and conveying machinery to form an automated coating production line. Such spray booths may also be equipped with paint-preparation rooms and drying chambers at the inlet and outlet of the booth.

II. According to the air supply and exhaust method, spray booths can be classified into open-type and closed-type (air-supply type) spray booths.

The open-type system is equipped only with an exhaust ventilation system and lacks a separate supply-air unit; it directly extracts air from within the workshop and is suitable for general painting applications. The closed-type system, on the other hand, features a dedicated supply-and-exhaust ventilation system that draws fresh air from outside the facility without interfering with the workshop’s ventilation or heating systems, making it ideal for decorative painting applications. The supply-and-exhaust airflow patterns include a vertical laminar flow configuration (i.e., top supply and bottom extraction) and a horizontal laminar flow configuration (i.e., side supply and side or side-bottom corner extraction).

3. According to the method of treating paint mist, spray booths can be classified into dry-type and wet-type.

The dry method involves direct capture, in which paint mist is collected and treated using filtering materials or equipment such as baffle plates and filter layers (bags). The wet method is an indirect capture technique that uses a circulating water system to clean the exhaust air from the painting booth, thereby capturing the paint mist. The circulating water contains a paint coagulant that causes the paint mist to lose its stickiness.

The commonly used wet spray booths include three types: water-spray cyclone, Venturi-type, and water curtain. For large workpieces, such as car bodies, Venturi-type or water-spray cyclone spray booths are often preferred; for smaller workpieces, such as car wheels, water curtain spray booths are more commonly used.

Working Principles of Several Common Spray Booths

I. Dry Paint Booth

　Advantages of dry spray booths: simple structure, uniform air volume and pressure, minimal paint wastage, and high coating efficiency. Since no water is used, there’s no need for wastewater treatment, resulting in low operating costs. The spray booth is completely free of water and oil contamination.

　The dry spray booth is currently recognized internationally as one of the most technically *comprehensive* spray booths available. It has addressed the shortcomings of older-style spray booths, such as severe contamination in the work area, poor paint mist treatment capabilities, low safety standards, unstable performance, and failure to meet environmental protection requirements.

　　Dry spray booths offer advantages such as advanced design, reasonable structure, attractive appearance, high efficiency in handling paint mist, stable performance, safety and reliability, excellent rigidity and strength, and shadow-free lighting. A dry spray booth mainly consists of components including the booth body system, passive air intake filtration system, paint mist filtration system, organic waste gas treatment system, exhaust ventilation system, lighting system, and control system.

II. Water Curtain Painting Booth

The water curtain spray booth is a wet-processing device. The front of the equipment features a water curtain panel, above which is an overflow trough. Behind the water curtain panel lies a multi-stage water curtain filter. During painting, the paint mist entering the spray booth first comes into contact with the water curtain and is washed down into the water tank. The remaining paint mist is completely trapped in the water as it passes through the multi-stage water curtain filter. The water in the tank is pumped up to the overflow trough at the top of the water curtain and the multi-stage water curtain filter, where it overflows onto the water curtain panel, forming a continuous water curtain.

The spray booth’s water curtain is equipped with stainless steel water curtain panels. The structural design of these panels is advanced and well-optimized, ensuring appropriate air flow velocity within the booth, enhancing paint application efficiency, and improving the capture rate of residual paint. Moreover, the water curtain layer remains uniform, continuous, reliable, and free from interruptions or splashing. The vortex device and the adjustable, modular installation of the air-water separation baffles enable thorough washing and separation of residual paint and mist, effectively purifying the environment and providing a convenient and efficient solution for cleaning and maintaining the painted surfaces—making maintenance and upkeep particularly easy.

　Water curtain spray booth: Composed of a booth body, water tank, stainless steel water curtain panels, a water circulation system, and an exhaust and filtration system, this booth effectively prevents waste paint from being discharged and polluting the environment while ensuring the smooth completion of workpiece painting.

Working principle of the water curtain spray booth: Workpieces are conveyed into the spray booth via a suspended conveyor system or placed on a workbench turntable. Operators use either a handheld electrostatic spray gun or a fixed rotary cup spray gun to apply paint to the workpieces. When using an electrostatic spray gun, the paint mist is effectively captured. The scattered overspray is drawn into the water curtain by airflow, undergoes further purification through spray washing, and then passes through a gas-water separation device before being discharged outdoors as clean air. The paint mist captured by the water curtain flows down with the water into a collection tank, where it is pumped up and filtered. Paint residues float on the water surface. Next, a paint coagulant is added to the tank, causing the paint residues to aggregate into loose clumps. These clumps are then scooped out using a container for centralized disposal, ensuring that the water remains clean and achieving the purpose of paint mist purification.

Application scope of water curtain spray booths: Painting of workpieces in industries such as machinery, automotive parts, metal products, decorative components, home appliances, and furniture.

The water curtain spray booth is suitable for single-sided spraying; it can also be configured in multiple units to enable double-sided spraying.

III. Pumpless Water Curtain Painting Booth

The pump-free spray booth uses air-induced water lift to create a circulating water curtain.

　After the air containing paint mist collides with the water curtain, it passes through the water screen into the gas-water channel, where it undergoes intense mixing with the water inside the channel. Once it enters the gas collection box, its flow velocity suddenly decreases, causing the gas and water to separate. The air then flows past a water-deflecting baffle and is discharged into the atmosphere by an exhaust fan. Meanwhile, the separated water collects in the gas collection box and flows into an overflow trough. From the overflow trough, the water overflows onto the splash plate, forming a new water curtain that returns to the water tank. A special flocculant is added to the circulating water to alter the adhesion properties of the paint mist, causing it to coagulate into sludge clumps that are easy to remove.

Principle: By leveraging the high velocity of exhaust air to induce water lifting, the exhaust system and the water-lifting system are integrated into one, creating a pump-free water circulation system. This eliminates the entire pump-based water supply system, simplifying the structure and reducing the floor space required. After achieving pump-free water circulation, the system overcomes the critical drawback of existing pump-based water supply systems—easy clogging by paint sludge—and offers easy maintenance, reliable equipment performance, reduced likelihood of malfunctions, and convenient operation.

　Due to the air containing paint mist, after passing through the water curtain, water screen, and a gas-water channel where it undergoes intense mixing and agitation with water mist, a multi-stage purification process is achieved, thereby enhancing purification efficiency.

4. Water-spray painting booth

　　The water-spray painting booth employs an upward-supply, downward-exhaust ventilation system, with a paint mist collection device located at the bottom of the booth. It is suitable for medium- and large-sized workpieces. This paint mist treatment equipment uses water as the medium and adopts an upward-supply, downward-exhaust ventilation approach to ensure thorough mixing between the paint mist and water in the lower section of the booth.

　The water-spray painting booth can be designed with either a large or small floor plan, making it suitable for **spray coating** of medium- and large-sized workpieces.

　　Application scope of water-spray painting booths: household appliances, bicycles, residential and commercial steel furniture, automobiles, motorcycle parts and surface coatings, instrument and meter housings, electrical cabinets, agricultural machinery, sports equipment, and other applications.

　　Principle of the Water-Spray Painting Booth: Fresh air is delivered into the equal-pressure chamber at the top of the water-spray painting booth via an air-conditioning blower unit. After passing through a flow-distribution regulator and a filtration layer, the air is evenly introduced into the booth interior at a face velocity of 0.45 m/s, flowing from top to bottom. This ensures that workpieces are immersed in a uniformly flowing air layer with a specified wind speed, causing any splashed paint mist to be drawn into the water-spray device within the booth. Under the impact of the high-speed airflow, the water is atomized and thoroughly mixed with the paint mist, thereby drawing the mist into the water and carrying it away. The air, now containing moisture, undergoes gas-water separation, after which the clean air is discharged into the atmosphere via the exhaust system, achieving a paint-mist removal efficiency of ≥98%. Meanwhile, the water containing paint mist flows into a circulating water tank, where it is purified through coagulation (by regularly adding a specialized coagulant to the water). The treated water is then pumped back into the painting booth for reuse via a circulation pump. Any floating paint sludge is periodically skimmed off and subsequently disposed of by deep burial or incineration.

Introduction to Exhaust Gas Treatment in Paint Booths

During the spray coating production process, organic solvents are used; these organic solvents are highly volatile. Among them, low-boiling-point, highly volatile solvents such as xylene, toluene, ethyl acetate, and methyl ethyl ketone contain aromatic hydrocarbons that are both toxic and flammable.

I. Process Comparison and Selection

There are many types of treatment methods for organic waste gases, each with its own unique characteristics. Commonly used methods include water scrubbing, condensation, absorption, combustion, catalytic oxidation, and adsorption.

1. Water Spraying Method: The water-spraying process is widely used in the treatment of atmospheric pollutants and is also employed in coating operations—for example, water curtain booths. The principle behind this method is to spray water onto exhaust gases, causing water-soluble or large-particle components in the exhaust to settle out, thereby separating pollutants from clean gas. Its advantages include easy access to water resources and the ability to reuse the water after filtration and sedimentation, thus minimizing water waste to the greatest extent possible. Water spraying demonstrates remarkably high efficiency in treating large-particle components and is often used as a pre-treatment step in exhaust-gas purification.

2. Condensation and Recovery Method: In this method, exhaust gases are either directly condensed or adsorbed and concentrated before being condensed. The condensate is then separated to recover valuable organic compounds. This method is suitable for treating exhaust gases with high concentrations, low temperatures, and small air volumes. However, it requires substantial capital investment, consumes a great deal of energy, and incurs high operating costs. Therefore, unless there are specific requirements, this method is generally not employed.

3. Absorption Method: This method can be divided into chemical absorption and physical absorption. However, since “three-benzene” exhaust gases have low chemical reactivity, chemical absorption is generally not employed. Physical absorption involves selecting a liquid absorbent with low volatility and high affinity for the component to be absorbed. After the absorbent becomes saturated, it is heated to release the absorbed components, then cooled and reused. This method is suitable for exhaust gases with large volumes, low temperatures, and low concentrations. The equipment is complex and requires substantial investment; selecting an appropriate absorbent is relatively challenging, and there is a risk of secondary pollution.

4. Direct Combustion Method: This method uses the heat generated from the combustion of auxiliary fuels such as gas or oil to heat the mixed gas to a specific temperature (700–800℃), where it is held for a certain period, enabling the combustion of combustible harmful gases. The process is simple and requires relatively low equipment investment; however, it consumes significant energy and incurs high operating costs.

5. Catalytic Combustion Method: This method involves heating exhaust gases to 200–300°C and passing them through a catalytic bed for combustion, thereby achieving the goal of purification. The method features low energy consumption, high purification efficiency, no secondary pollution, and a simple process that is easy to operate. It is suitable for treating high-temperature, high-concentration organic exhaust gases but is not appropriate for treating low-concentration, large-volume organic exhaust gases.

6. Adsorption method:

(1) Direct Adsorption Method: Organic gases are directly passed through activated carbon, achieving a purification rate of up to 95%. This method features simple equipment, low investment, and easy operation. However, the activated carbon needs to be replaced frequently and is suitable for applications involving low concentrations of pollutants where the recovered contaminants are not required.

(2) Adsorption and Recovery Method: Organic gases are adsorbed onto activated carbon, and once the activated carbon becomes saturated, it is desorbed and regenerated using hot air.

II. Design and Process Description

To minimize initial investment, the design incorporates a single exhaust gas treatment system. Paint spray exhaust gases first enter a water-based mist spray booth via pipelines for dust removal, then pass through a pre-treatment unit before being sent to an activated carbon adsorption bed for purification and emission. Once the activated carbon becomes saturated, it undergoes desorption using hot air followed by catalytic combustion, converting the organic exhaust gases into CO2 and H2O before they are discharged in compliance with emission standards.

III. Equipment Instructions

1. Paint mist purification room

Based on the analysis of the on-site exhaust ducts and drawing on our company’s experience with similar enterprises, we’ve determined that the current paint booth’s dust removal efficiency fails to meet the requirements of the exhaust gas treatment system. Therefore, we’ve designed and added a paint mist purification room to protect the subsequent exhaust gas treatment system. Once the exhaust gas enters the paint booth, the cross-sectional area suddenly increases, causing the air velocity to decrease. As a result, larger dust particles (particulate matter) settle out under the influence of gravity. During its passage through the booth, the dusty gas comes into contact with water droplets (mist) sprayed from nozzles; the particulate matter becomes enveloped by these liquid droplets and subsequently settles down. Thus, the particulate matter is separated from the gas. This process demonstrates high efficiency in particulate separation, reducing the impact of dusty gas on subsequent treatment stages and extending the service life of downstream equipment. Meanwhile, the wastewater generated in the paint booth—after undergoing processes such as filtration, sedimentation, and biochemical treatment—can be recycled and reused.

2. Preprocessor (paint mist filter)

To prevent secondary pollution and protect the activated carbon, it is necessary to use dry filtration materials with high purification efficiency and no secondary pollution to remove paint mist and moisture from exhaust gases. These dry filtration materials have been specially developed to meet the unique requirements of paint-mist purification. They are composed of multiple layers of flame-retardant fiberglass composites, with density gradually increasing as thickness increases. A layer made of a different material is added at the end to provide structural support. During filtration, the multi-layered fibers trap, collide with, diffuse, and absorb paint-mist particles, effectively containing them within the material. The paint-mist purification filters manufactured by our company utilize specialized paint-mist filtration materials imported from the United States. These filters boast advantages such as high combined purification efficiency, large paint-mist capacity (3 kg–8 kg/m²), ease of cleaning, low operating costs, and zero secondary pollution.

⑴ The frame and support are made of metal mesh, with filtering material sandwiched inside. The filter is installed within a metal housing and should be replaced periodically.

⑵ The filter material is made of synthetic fiber nonwoven fabric and aluminum composite, formed into pleats. It features high air flow rate, low resistance, and large dust-holding capacity.

3. Activated Carbon Adsorption Bed

Utilizing the adsorption properties of activated carbon—its numerous micropores—to remove organic waste gases is a highly effective industrial treatment method. The activated carbon adsorption bed employs a new type of activated carbon that boasts a large specific surface area and high porosity, resulting in strong adsorption capacity as well as excellent mechanical strength, chemical stability, and thermal stability. Its purification efficiency can reach as high as 95%. As the organic waste gases pass through the adsorption bed and come into contact with the activated carbon, the organic pollutants in the gases are adsorbed onto the surface of the activated carbon, thereby being removed from the gas stream and achieving the desired purification effect. The gas stream exiting the activated carbon adsorption bed already meets emission standards, allowing the air to be directly released into the atmosphere.

4. Catalytic combustion bed

Before the organic waste gas is introduced into the catalytic combustion unit, it is first preheated by a preheater. Then, an electric heater inside the catalytic combustion bed further heats the waste gas to a temperature of around 280°C. Under the action of the catalyst, the heated gas undergoes a thermal reaction, producing harmless H₂O and CO₂. At this point, no additional electric heating is required; the system relies on its own thermal equilibrium to efficiently treat high-concentration organic waste gases. The combustion process releases substantial heat, which can be recovered and utilized via a heat exchanger, thereby reducing the energy consumption associated with preheating. The entire process can be fully automated and controlled by a PLC control cabinet.

IV. Features of the Organic Waste Gas Purification Device

① This equipment features advanced design principles, unique materials, stable performance, and is simple to operate. It is safe, reliable, and produces no secondary pollution. The equipment occupies a small footprint and is lightweight. The adsorption bed adopts a drawer-type structure, making it convenient to load and easy to replace.

② A new type of activated carbon adsorbent—honeycomb-shaped activated carbon—is employed. Compared to granular (or rod-shaped) activated carbon, it boasts superior thermodynamic performance, including low resistance, low energy consumption, and high adsorption efficiency, making it particularly well-suited for applications involving large air volumes.

③ The catalytic combustion chamber uses a precious-metal catalyst supported by a ceramic honeycomb structure, which offers low resistance and can operate smoothly even with a low-pressure fan. This not only reduces power consumption but also minimizes noise levels.

④ Based on the concentration of this waste gas, the air volume of the catalytic combustion device is one-tenth of that of the waste gas source. Meanwhile, the heating power is maintained for 1 hour, thereby saving energy.

⑤ The activated carbon bed used for adsorbing organic waste gases can be regenerated by desorption using the heat generated from catalytic combustion of the waste gases. The gas released during desorption is then sent to a catalytic combustion chamber for further purification. This process requires no external energy input, resulting in low operating costs and significant energy savings.