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About paint booths and drying systems
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About paint booths and drying systems

1. INTRODUCTION

Depending on the type of product, the amount of production, and the identity of the paint booth product, large-capacity flow facility may be used. They can be treated with products of different dimensions or with the same high-quality products. Such facilities today can be fully automated and robotic, and humans only take control functions. Objects of this type are needed and justified only in the largest production facilities and require huge investments.

Much more common is the need for flexible paints in which it is possible to treat the entire product range of a company or to carry out the service of varnishing various products.

No matter what type of paint shop they are, they must satisfy the basic requirements:

  • ensure the highest possible quality of varnishing (surface protection);

  • ensure the statutory working conditions;

  • to provide for the conditions prescribed by law for the protection of personnel and the environment (applicable human and environmental laws);

  • ensure safe operation with respect to fire and explosion protection (applicable PP and PEX protection law).

Also, no matter what painting facility it is, technologically they consist of:

  • parts (rooms) for the preparation of varnishing – cleaning, puttying and grinding;

  • paint application rooms;

  • dying rooms.

In practice, the whole process of treating a subject is cyclically repeated. The number of cycles depends on the type of medium and the level of quality required.

1.1. WORK DESCRIPTION OF PAINT BOOTH

The main elements of each paint booth are the suction and supply air system, filters, control sensors and safety sensors.

The exhaust and supply systems are made up of fans and piping that stream the air that is being injected or ejected from the paint booth.

Picture 1.1.1 shows the parts of the paint shop and the airflow through it. There are filters at the entrance to filter the polluted atmospheric air to prevent dust from entering the paint booth. At the outlet are filters that filter the polluted air generated by the varnishing process to the point where it is no longer harmful to health and the environment and is released as such into the atmosphere. The type and fineness of air filtration filters at the outlet of the paint both depends primarily on the means used in the paintwork process. Also, depending on the painting process, the outlet filtration is carried out in several stages (multiple filtration – from 2 to 4 filtration stages).

The proper operation of the paint booth requires the maintenance of the minimum amount of air required. The criteria for calculating the amount of air required for ventilation to be correct are:

  • effective removal of overspray particles from the workspace that do not end on the subject being varnished;

  • maintaining dangerous concentrations below 10% of the lower explosive limit of the agent (s) to be handled (25% per ATEX);

  • size of paint booth.

The specified flow rate is achieved by controlling the exhaust fan. The flow rate during operation is primarily affected by the clogging of the output filter.

For the lacquering process it is necessary to keep the entire lacquer room in a certain overpressure or underpressure (50 – 200 Pa) depending on the type of process (type of lacquers used). If it is dangerous to create vapor around the outside of the paint shop, the paint shop is kept under pressure (a rarer requirement), and if it is necessary to ensure that no dust particles enter the outside area, the paint shop is kept pressurized (a more frequent requirement). The pressure, that is, the overpressure, is achieved by regulating the supply fan. The amount of supply air also depends on the blockage of the filter, this time the inlet.

Picture 1.1.1. Cross section of paint booth

In addition to maintaining the flow rate and overpressure, maintaining the operating temperature is also important for the process. It should not be below 10 oC, so in winter, when temperatures are regularly below the set point, it is necessary to ensure that the inlet air is heated. Temperature control is by acting on control valves which essentially change the power of the heater (a larger or smaller amount of heated medium passes through it).

In addition to these requirements, each paint shop contains certain safety features. Primarily, these include the door and window enclosure control sensors to prevent the spread of polluted air into the surrounding area or the entry of dust particles from the surrounding area. Also, the process of lacquering leads to the formation of highly flammable and explosive concentrations of gases (primarily due to evaporation of solvents in lacquers), therefore it is necessary to pay attention to the regulations and requirements of the system for work in explosive atmospheres (applies to the use of certified devices) and to control the correctness of the same. , and adherence to the default workflow.

In order to distribute the injected air evenly throughout the space on the ceiling of the paint shop, there are filter panels for air distribution. They can be designed in different ways, and the essential requirement is to reduce the inlet air velocity to less than 1 m / s.

Depending on the method of separation of the varnish from the polluted air, the varnishes can be divided into two main groups, namely:

  • dry and
  • wet

In dry air it is filtered by passing through cardboard, inertial and mineral wool filters, while in wet water it is a medium which takes over the polluted particles.

1.2. PAINT BOOTH WITH DRY FILTRATION

Dry filtration plants use multiple filtration to purify lightly saturated air. In more recent embodiments, a solution is preferred in which air first passes through a cardboard filter where larger particles of lacquer are filtered. This is followed by a finer filtration in which the air passes through the stop lacquer filters, and finally, as the third stage of filtration, the air passes through absolute filters, after which it is completely clean and without any odors, and as such is released into the atmosphere. The varnishes used and the lacquering process, as well as the regulations, determine how many filtering stages will be used and which filters will be used, so in some cases it is possible to skip the third filtering stage (absolute filters).

Picture 1.2.1. Dry suction wall

1.2.1 Types of filter materials

Cardboard pre-filters:

With the cardboard filter, Figure 1.2.1, the separation of solids transmitted by the air flow is due to the effect of two changes of direction affecting the air. The effect is good for particles larger than 10 μm in diameter.

Picture 1.2.1. Cardboard pre-filters

Mineral wool filters

In the mineral fiber filters shown in Picture 1.2.2, the separation takes place at a much finer level, both because of the inertia effect and the strength of the electrostatic attraction between the particles and the outer surface of the fibers. Larger particles are retained due to a sieve-like effect. Mineral fiber filters are common post-filters for cardboard filters. A feature of these filters (stop lacquer filters) is the ability to absorb lacquers up to 5 kg / m2.

Picture 1.2.2. Mineral wool filters

Multi-layer inertial filters made of fire-resistant cardboard

In the case of multilayer filters made of fire-resistant cardboard, Picture 1.2.3 filtration takes place by inertia (for larger particles). The separation of fine particles takes place on a synthetic post-filter (sieve effect, collision, inertia and scattering). There are different combinations of multilayer filters depending on the parameters and requirements of the lacquering process itself.

These filters successfully replace the cardboard pre-filter and the stop light filter, however, due to cost, their application is still limited

Picture 1.2.3. Multilayer filters made of fire-resistant cardboard

Table 1.2.1. Comparison of filter materials

 

FILTER ADVANTAGES DISATVANTAGES
PREFILTERS – selfsupporting, simplifies cabin design

– long working life

– almost always is required a post-filter

– unique model

MINERAL WOOL FILTERS

(PAINT STOP)

– progressive accumulation

– gamma of different models

– shorter working life, prefilter is required

– required the supporting structure

– classification R38-40

– precautions S36-37

MULTILAYER INJECTION FILTERS – gamma of manny models with measurable efficiency, with varnishing products, from 50 to 99.99%

– progressive efficiency

– high accumulation

– easy maintenance

Gamma multilayer filters made of moisture resistant cardboard

– required supporting grid
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Picture 1.2.4. Five cardboard models, with a minimesh finish. Filtration efficiency from 50% to 98%.
Picture 1.2.5 Five models with synthetic fine-fiber post-filter (20 μm). Filtration efficiency from 95% to 99.99%.
Picture 1.2.6. Filter unit with active carbon

1.3. PAINT BOOTH WITH WET PROCESS OF FILTERING

A requirement for a wet filtration plant is to mix contaminated air with water which takes over the varnish particles and thus filters the air.

Depending on where the air comes in contact with water, there are two types of wet process lacquer filtering, which are:

– water paint booths;

– fountain walls.

Picture 1.3.1. Wet suction wall

1.3.1. Water paint booths

For bulk rocks, the water rises from the pool and slides evenly across the back of the cabin back into the pool (depending on the requirement, versions are possible in which the water slides on the sides of the cabin as well). Between the pool and the edge of the rear of the cab is a passage that can be adjusted at will through which the exhaust air passes. Most of the saturated air by inertia is mixed by striking the water curtain while clearing the varnish, while complete cleaning takes place by passing air through the turbulent water curtain at the bottom of the cabin. The further procedure is similar to that of water paints. Before venting, the air is additionally dried by passing through a droplet eliminator and possibly an additional dry channel filter.

1.3.2. Fountain rocks

The impression rock consists of metal slats on the back (and / or sides) of the paint booth. On each side of the cabin, there are usually two impression ducts whose openings are located above the surface of the pool with water.

A pool of water is usually when lowered into the base of the floor in front of the impression wall, covered with floor grilles on which the lacquer is placed. The connection between the pool and the water filled by the water pump through the water pump and the suction ducts is the narrow gap through which all the suction and lightly saturated air flows. The width of the span is regulated by the position of the ‘saw blade’, a device which changes the speed of the passage of air, which simultaneously strikes the surface of the water and causes it to swirl. The consequence is that ‘vortex chambers’ are created, that is, an area of ​​dense concentration of water droplets that ‘flushes out’ saturated air from varnish (paint). There are cascade barriers (droplet eliminators) in the exhaust duct in front of the fan that prevent water from entering and allow only clean air to be released into the environment. To achieve an even higher filtration rate, an additional dry filter (quartz) is often installed before the fan itself in the exhaust duct.

1.4. APPLICATION

In practice, due to the extreme depreciation of filter materials, ease of construction, environmental friendliness and minimal maintenance, dry filtration paints are nowadays prevalent. The use of water filtration booths is recommended for service paints or plants where the need for varnishing is 8 hours or more per day.

2. CABIN FOR VARNISHING

The varnishing and / or drying cabin can be a building (masonry) building or a cabin composed of insulating panels and associated structures. It can be located inside or outside the building. When choosing a paint shop location, it is important to agree on all production processes and to select the location of the paint shop / dryer in relation to them, it is important to pay attention to fire protection.

2.1 Example of paint booths and drying rooms
2.1 Example of paint booths and drying rooms

2.1. VENTILATION SYSTEM IN THE PAINTING AND DRYING CABIN

The ventilation system in the paint booth can be vertical: suction – floor and supply – ceiling or side: suction – side, and supply side or ceiling.

The number of air changes in the paint shop is 100-250 i / h, which is satisfactory for all branches of the industry (more air changes are used for the automotive and wood industry – high gloss, fewer air changes in the metal industry).

Depending on the industry, two types of ventilation are most commonly used in pressure paint and pressure ventilation systems. The pressure ventilation system is used in the branches of the industry without special requirements on the quality of varnishing, and this way it is achieved that the danger zone does not expand beyond the varnish (suction ventilation greater than 5-10% compared to supply ventilation). The overpressure ventilation system is used in industries with high quality requirements for the varnished surface (automotive, timber, etc.). This kind of ventilation system prevents dust from entering the surrounding area (suction ventilation is less than 5-10% compared to supply ventilation).

2.1.1. Exhaust ventilation

Exhaust ventilation is installed sideways or underfloor. The choice of side or floor suction depends on the workpieces to which the paint is applied. Thus, in the automotive and metal industries where large pieces are processed, the installation of a floor suction unit is recommended, while in the branches of the industry where smaller pieces are processed, the lateral suction walls are predominantly used. In addition, a combination of floor and / or side suction is possible, and through the same consumers (the distribution of ventilation inside the cabin depending on the size and shape of the work pieces – switching mode is achieved by automatic or manual flaps).

There are two basic ways of filtering suction air from a paint shop: a dry filtration process and a wet filtration process. Dry suction rocks have built-in filters (pre-filter and post-filter) that retain lacquer particles and prevent environmental pollution, while in wet filtration in addition to water, a filter integrated into the rock itself to filter the waste air.

The suction rock (lateral or floor / dry or wet) is connected by a galvanized pipeline to a suction fan, which is mounted inside the cabin or outside to a wall or concrete base. The design of the fans and electric motors can be standard or explosion proof. The choice of fans and motors is influenced by the colors and varnishes used.

If water-based paints and varnishes are used, fan and motor performance may be standard. They should not be used for the screening of paints and varnishes that form potentially explosive mixtures.

For this purpose a blower and an electric motor must be used in explosion-proof version (the choice of explosion-proof protection depends on the class of colors and varnishes used). Exhaust ventilation must be fitted with ventilation control switches and filter fill gauges.

2.1.1. Exhaust walls
2.1.2 Exhaust fan

2.1.2. Supply ventilation

Supply ventilation consists of an air supply unit, a conduit and a flow ceiling in which an inlet air filtration filter is integrated. The choice of the unit for compensation of air depends on the type of heating medium (water, steam, natural gas, LPG, fuel oil), and accordingly the unit itself is selected. The air intake unit consists of a supply fan, a heat exchanger, a pre-filter (pre-filtration of the inlet air) and a control blind (manual or engine). Supply ventilation is equipped with PTC probes to control the desired temperature. The air supply equipment is connected by a galvanized pipeline to the discharge ceiling. A fire damper is provided at the entrance to the cabin, which closes the air supply and switches off the ventilation if the temperature rises above 70 ° C.

Depending on the wishes of the Investor, in addition to the heating system, air humidification and cooling systems are often installed in the paint shops. They achieve the desired microclimatic conditions in the area of ​​the paint shop, which are a prerequisite for achieving a superior product in terms of surface protection, ie the application of paint and varnish.

2.1.3. Air supply filter units - Hotwater platoon unit
2.1.4. Air supply unit - Gas thermogen
2.1.5. Drainage ceiling

3. ELEKTROINSTALATIONS

Electrical installations include complete automatic process control via PLC and frequency converters. Visualization, control and alarming of the process is done via HMI (touch display).

This kind of electrocardis cabinet allows different processes of varnishing and drying depending on the individual wishes of the customer.

There is a possibility to design multiple processes of varnishing or drying. In addition to these processes, the control cabinet can only have a process of reduced ventilation, in which the preparation work is carried out, without the varnishing and drying process. During this process, the complete ventilation equipment is not used but the same because it has at least twice the capacity.

An integral part of the wiring is various digital and analog sensors that enable ventilation control and regulation of the paint shop, or dryer, in order to ensure safe working conditions and optimum operation.

The cabinet of automation and power supply for all consumers according to the conditions of Ex Agency contains:

– full automatic control of the process of varnishing / drying,

– pre-ventilation,

– control of ventilation,

– control of PP protection,

– control of door closure,

– control of filter fill,

– gun lock for varnishing,

– postventilation,

– regulation of varnishing / drying temperature,

– regulation of pressure / pressure,

– moisture control,

The testing of complete wiring with the issuance of the test protocol necessary for Ex Agency findings consists of:

– timing of the drying process.

– Dielectric testing tensile strength,

-Testing of insulation resistance,

-Testing of continuity of protective conductor and equipotential bonding conductor,

-Testing of grounding resistance,

-Testing of fault loop impedance,

-Testing of floor and wall resistance.