How does a spray booth work?

Find out how the Autokabina spray booth works!

How are the spray booths built?

Typically, the cabin is made as a self-supporting structure from panels insulated with various materials. As an insulation material, panel manufacturers use: polystyrene, foamed polyurethane, standard mineral wool and pressed mineral wool. Imagine an experience.

Put on the table: a piece of polystyrene, a piece of polyurethane and some mineral wool. Let’s play with the lighter a bit and see what the effect of this game is. Now let’s take some (a teaspoon) of the solvent used in the paint shop, which is one of the paint components, and pour it on each of the prepared materials. After completing this experiment, we will already know what kind of insulation should be used in the construction of spray booths.

Most manufacturers only insulate the walls of the cabin. Reputable manufacturers also insulate the roof of the cabin, and even the entire generator, because this is where air at a temperature of about 60 ° C enters during the drying phase. There are also opinions that say that no insulation is needed and that such uninsulated cabins are delivered to Poland. Imagine it’s late fall and it’s cold outside. In the workshop hall, we manage to maintain the temperature of around 15 ° C. The recently painted car is dried in the spray booth installed in the hall. The temperature inside the cabin is 60 ° C. Thus, the temperature difference reaches 45 ° C. Let us answer the question: in such a situation, if the cabin wall is only steel sheet with a thickness of approx. 1 mm, does heat transfer occur or not? Another issue that affects the efficiency of the entire device is the method and size of the floor grates, under which there are paint-stop floor filters. There are several solutions, which we will try to present in the order related to their efficiency, from the poorest to the most efficient. Narrow grille under the car – a somewhat archaic solution, found in the cheapest versions of cabins with small ventilation systems. The surface of the car is slightly blocking the access of air to the drain channel. Two narrow grilles located under the wheels of the vehicle – a more efficient solution due to the fact that usually a part of the grille protrudes beyond the side contour of the car, which allows for smoother penetration into the drainage channel.
Two narrow grates located under the wheels of the vehicle connected by additional grates at the beginning and end of the canal in the so-called “RING” system – a solution similar to the previous one, but giving effective air flow also at the front and rear of the vehicle, which significantly increases the effect of air fall in these parts and enables good painting of these elements.

The above solutions require precise central positioning of the vehicle so that the air fall effect is as even as possible. Such solutions are dominant in devices equipped with low-efficiency fans (often single-fan systems), inter alia, so that thanks to the small surface it is easier to achieve overpressure in the cabin without the use of regulating systems. The easier and faster this effect is achieved, the smaller the surface of the floor filters, which become contaminated with solid particles of varnishes. And we must remember, as we have already mentioned, that regardless of the surface of the filters, the same amount of varnish material is released during varnishing and therefore the amount of particles is also constant, so a filter with a small surface will be “clogged” very quickly. The consequence of contamination of the floor filter is a rapid increase (especially in single-fan cabins) of overpressure in the spray booth, which in turn leads to a decrease in the air fall velocity.

If this value drops below 0.16 m / sec, the conditions that can be achieved in the proverbial garage with the door open begin to prevail in the spray booth.

So is it worth spending that amount of money? A large-area grated floor, usually 5 x 3 m – a very efficient solution, devoid of the above-mentioned defects. 100% grating floor – the best solution, but also the most expensive due to the large number of platform gratings and the necessity to make support systems on which the grates are suspended. NOTE – some manufacturers use the last two solutions, but still place metal sheets under the floor gratings and filters, which reveal most of the floor grating surface, leaving only two narrow stripes located under the wheels of the vehicle, as described in the previous three cases. This solution is most often caused by the use of low-efficiency fans. No comment. Entrance DOOR to the spray booth – also here, as it turns out, there may be differences in the way it is made. The purpose of such doors is to enable easy entry and, when closed, such a seal that no air containing harmful and dangerous substances can escape outside. There are two basic ways of making these doors: the first is a double-leaf door, the second is a three-leaf door. Three-leaf doors, due to their narrower wings, occupy less space in front of the cabin when opening, and in addition, one of the leaves acts as a service door, which must be equipped with safety locks that enable the door to open under pressure from the inside in the event of an emergency. In the case of double-leaf doors, due to their large width and the related handling nuisance, it is necessary to install separate service doors. Of course, all structures should include peripheral seals and a locking system, e.g. of a bolt, in order to ensure precise pressing against the frame. It would also be good if the door leaves were also thermally insulated. The directly noticeable advantage is the maximum height and width, as it ensures trouble-free entry to the spray booth.

Are we aware of how a spray booth works?

There are two main phases of work: the painting phase and the drying phase. In the painting phase, the principle of operation of all spray booths, due to the need to maintain safety and effective ventilation, should be the same. The blower fan takes clean air from the outside, directs it to the heat exchanger where the air is heated to the set temperature, and then this air is directed to the cabin filtration plenum where, after passing through the ceiling filters, this air flows vertically from top to bottom and depending on of the cabin equipment, or it is “forced” through the grates and floor filters and pushed to the exhaust ventilation duct exposed above the roof of the room (single-fan cabins – blowing) or “sucked” by the exhaust fan and ejected with great force through the exhaust ventilation duct above the roof of the room ( two-fan cabins – blowing and exhaust). As this air is heavily polluted, it cannot, contrary to some opinions, be used to heat the hall.

During the drying phase, however, individual devices work differently. This is due to the necessity to obtain a much higher temperature, oscillating around 60-70oC. As we remember from the fragment of this study concerning the ΔT thermal jump, it will not be possible to obtain a temperature increase of 70-80oC with the use of a heating system with standard heating capacity. What can we do then? One solution would be to install a heating module with enormous power, but as we have already explained to ourselves, it would be completely unjustified economically. Therefore, some solutions were invented, which, using standard capacities of heating systems, allow to achieve the desired temperature. At the moment, three basic solutions to this problem are known. Just as a convertible is a great car in sunny California or on the Cote d’Azur, in Poland its full operation is possible for two months a year, and in the remaining months, to put it mildly, it does not give the full and expected satisfaction. The same applies to technical solutions for heating in the drying phase in spray booths. The simplest solution from the technical point of view consists in creating such a situation that only about 20-25% of the value of the initial fan efficiency flows through the heat exchanger. It is realized either by mechanically limiting the inlet cross-section or by reversing the fan rotation direction, which results in the same effect of reducing the efficiency to about 20-25% of the initial efficiency. Such a small amount of air flowing through the exchanger can be heated to the desired temperature without any problem. The disadvantage of this solution, due to the small amount of hot air supplied to the spray booth, is very uneven drying of the vehicle. The vertical parts of the bodywork take much longer to dry than the horizontal parts and the drying process of the vehicle can be relatively long. In small plants with low throughput, however, this is not a problem at all. In this solution, the entire amount of hot air after passing through the chamber is ejected outside into the atmosphere.

It is a bit reminiscent of an open-circuit central heating system, where the water, when heated to the appropriate temperature and overflowing through the heating system, would be poured out to sewage. Another solution is to mechanically reduce the cross-section of the intake duct, which in turn reduces the efficiency of the fans to about 50%. In addition, a flap opens between the intake duct and the heat exchanger chamber, which allows a certain amount of hot air to be sucked in by the air stream sucked in through the inlets. Thus, the air supplied to the heat exchanger is mixed with hot air, therefore its temperature is much higher, which means that the air flowing through the heat exchanger can obtain an appropriate temperature increase. People involved in the production of cabins refer to this solution as “by pass”. Its advantage is providing much more hot air, which greatly improves and speeds up the drying process. As in the previous solution, hot air is released into the atmosphere.

The next solution is a system that causes 90% of the air mass to circulate in a closed circuit. This is due to the system of flaps that automatically close the air intake and open the connection of the blower fan with the moat, which causes the fan to draw air from the spray booth, i.e. the air already heated, and direct it back to the spray booth through, of course, the heat exchanger where it is refilled there are temperature drops. The flap closing the inlet is specially leaky to ensure the share of 10% of fresh air which is necessary for the drying process. Until recently, during this phase of operation, the exhaust fan was turned off and 10% of the excess air was expelled by gravity, but the EU directives in this regard have changed and now, to support the removal of this 10% of air, the exhaust fan must also operate in the drying phase.

Which solution is best for our climate? We will use a pictorial example again. Please imagine that we are driving four people. It’s winter, it’s frosty outside. We are driving a few kilometers and the engine has reached the correct temperature. The driver turned on the heating and the fan turned to the lowest speed. Will rear seat occupants feel warm? After what time? What happens when the fan is turned on at full speed? Will the heat not reach the passengers much faster?

More technically advanced paint booths, equipped with electronic control systems, may have additional functions: – ventilation phase, which follows the painting phase, cooling phase, which turns on automatically after the drying process is completed, a special system that saves heat energy, which automatically switches the spray booth into the recirculation phase if we interrupt the painting process, the damage visualization system given in the form of codes on the controller, the possibility of installing a fully automatic regulation of overpressure prevailing in the cabin, i.e. ensuring the same speed of air fall.

Finally, let’s return to the heart of the spray booth, i.e. a ventilation and heating unit, commonly known as a generator or an aggregate. Manufacturers, as usual, use various solutions, most often caused by technical possibilities, and most of all by production costs. In most solutions, such a generator is built as a self-supporting structure made of steel sheet bent appropriately to obtain spatial profiles, connected on site with rivets or self-drilling sheet metal screws, filled with covers made of sheet metal. Individual elements of the structure connected with each other create a supporting structure for a heat exchanger, fan or fans and additional equipment such as a recirculation flap (if such a system exists), pneumatic actuators, servomotors, support for air intake and exhaust air ducts and other elements that may be present in different constructions.

Such a solution is quick in mass production, but its durability may largely depend on the reliability of the assembly. The fans are quite heavy and spin at high speed. Inaccurate, just any assembly can quickly take revenge.

Manufacturers of highly reputable devices are usually companies with many years of tradition that do not specialize in creating mass products and as such cannot afford the risk associated with the lack of assembly control and correct operation, therefore, the generators of these companies are built on the basis of reliable steel structures, capable of carrying heavy loads, delivered to the recipient in the form of ready-to-work modules containing all the necessary elements inside, such as fans, motors, flaps, heat exchanger, etc. It causes some problems related to transport and, above all, unloading such devices, but it is it is assumed that it is transported and set up at the workplace only once, so you may be tempted to overcome these inconveniences in order to be guaranteed the highest durability of workmanship.

The most important elements of the generator are fans and a heat exchanger. Here, too, significant differences in the approach to the product can hide. The heat exchanger, as the name suggests, serves to “mediate” between the burner flame and the air supplied to the paint chamber. It works in such a way that the burner suspended on the wall of the exchanger has a flame directed towards the inside of the exchanger, towards the combustion chamber. The flue gas is discharged into the flue gas chimney through the flue gas ducts. The air supplied by the blower “washes” the exchanger and heats up due to its hot elements. The combustion chamber heats up to a significant temperature, but also the flue gas ducts discharge the fumes to the outside.

Most manufacturers produce exchangers of a fairly simple structure with flame tubes made in the shape of vertical slots placed next to each other in a row above the combustion chamber. In such an exchanger, air flows between these slots in an unimpeded, linear motion.

Other constructions place flame tubes with a circular cross section in several alternating rows above the combustion chamber. In such exchangers, the flowing air must make a “snake” movement, which makes the exchange surface very large. These constructions additionally use special “reduction gears” installed in flame tubes in order to maximize the use of heat from exhaust gases. The effect of this design is very high efficiency of the heat exchanger. Depending on the availability of media, oil or gas burners are used in the heating systems of spray booths. It is good if they are burners from a well-known manufacturer who was able to perform appropriate tests on its products. The burner power must be adjusted to the required power of the heating system.

Often the question is asked about the fuel consumption of such a burner. This value depends of course on the power of the burner. We must be aware that this consumption also depends on the time (intensity) of the burner operation. In practice, this means that if it is cold outside, the burner must work longer for the air flowing through the heat exchanger to reach the set temperature. If it works longer, it consumes more fuel. That’s what it’s installed for. Reliably, you can only provide information about the maximum fuel consumption per hour of work. The following oil and gas burners are used in the heating systems of spray booths:

One-stage – the most common, but burdened with a certain inconvenience, in practice, the occurrence of significant plus or minus differences, even 3-4oC during the painting phase, which means that the painter (after setting the temperature on the controller to 20oC) paints a fragment of the car body at a temperature of 17oC and for a while, the second fragment at a temperature of 23oC, and yet we know that the varnish will have a different viscosity under these conditions and may apply differently.

Two-stage – if they have an appropriate (e.g. spraytronic) control element installed, they are almost devoid of the above-mentioned disadvantages. Such a burner then operates in such a way that, depending on the heat demand, the first or both stages are operated. Gas burners can also have so-called modulated flame system – it is a very expensive solution, but it ensures stable maintenance of the set temperature.

Fans used in spray booths are roughly divided into:

single-stream embedded directly on the motor axis,
double stream driven by belt gears.

However, they are always centrifugal fans. Single-stream fans mounted on the motor axis are a very good and cheap solution. Their operating characteristics are such that they withstand high contamination of ceiling filters relatively well. They do not require adjustment, and in fact they do not have the possibility of adjusting the rotational speed and thus the efficiency, because it is the same as the engine revolutions.

Double-stream fans are driven by a belt transmission which allows, by changing the size of the pulleys, to “fine-tune” the fan efficiency, eg due to the high height of the cabin installation above sea level, which even happens in Poland. These fans have a much higher efficiency per 1 kW of engine power. Due to the much larger number of blades, such fans are not prone to pulsation of air.

It seems to us that this information will allow you to orientate yourself in the multitude of offered offers and to systematize these studies. We must be aware that most manufacturers probably know how a spray booth with high utility values should be constructed, and the fact that they produce devices with such and not other parameters and using such and not other components and elements is most often due to well-conducted marketing and costing.

A separate issue here is the knowledge and reliability of traders. Most of them are hard-working people with extensive professional knowledge who deserve deep respect, but unfortunately, as in any environment, there are also people for whom the most important thing is quick profit without considering your needs. Ask for technical details, and it will turn out who takes their customers seriously

Do we realize how a spray booth works?

There are two main phases of operation: the painting phase and the drying phase.
In the painting phase, the principle of operation of all spray booths, due to the need for safety and effective ventilation, should be the same. The supply fan draws in clean air from the outside, directs it to the heat exchanger where the air is heated to the set temperature, and then this air is directed to the booth’s filtration plenum. After passing through the ceiling filters, the air moves vertically from top to bottom and, depending on the booth’s equipment, is either “pushed” through floor grates and filters and expelled into the exhaust ventilation duct above the roof of the room (single-fan booths – supply) or “sucked” through the exhaust fan and forcefully expelled through the exhaust ventilation duct above the roof of the room (dual-fan booths – supply and exhaust). Since this air is heavily contaminated, contrary to some opinions, it cannot be used to heat the hall.

In the drying phase, however, different devices operate in various ways. This is due to the need to achieve a significantly higher temperature, oscillating around 60-70°C. As we recall from the section of this study concerning the thermal jump ΔT, it will not be possible to achieve a temperature increase of 70-80°C using a heating system with standard heating capacity. So, what can we do?

One solution would be to install a heating module with enormous power, but as we have already explained, this would be completely unjustified economically. Therefore, certain solutions have been devised that allow the intended temperature to be reached using standard heating system capacities.

Currently, there are three main solutions to this problem. Just as a convertible is an excellent car in sunny California or the French Riviera, in Poland its full use is possible for only two months a year, and during the remaining months, it does not provide full and expected satisfaction, to put it mildly. Similarly, with the technical solutions for heating in the drying phase in spray booths. The simplest technical solution involves creating a situation where only about 20-25% of the initial capacity of the fan flows through the heat exchanger. This is achieved either by mechanically restricting the intake cross-section or by reversing the fan’s direction of rotation, which results in the same effect of reducing the capacity to about 20-25% of the initial capacity. This small amount of air flowing through the heat exchanger can be easily heated to the desired temperature. The drawback of this solution, due to the small amount of hot air delivered to the spray booth, is very uneven drying of the vehicle. Vertical parts of the body dry much longer than horizontal parts, and the drying process of the vehicle can be relatively long. However, in small workshops with low turnover, this is not a problem. In this solution, all the hot air is exhausted outside into the atmosphere after passing through the chamber.

This solution is somewhat reminiscent of an open-circuit central heating system, where water, after being heated to the appropriate temperature and flowing through the heating system, would be discharged into the sewers. The next solution involves mechanically restricting the intake duct cross-section, effectively reducing the fan capacity to about 50%. Additionally, a damper between the intake duct and the heat exchanger chamber opens, allowing some hot air to be drawn in by the air stream from the intake. Thus, the air supplied to the heat exchanger is mixed with hot air, resulting in a significantly higher initial temperature, which allows the air passing through the heat exchanger to achieve the desired temperature increase. People involved in spray booth production refer to this solution as a “bypass.” Its advantage is that it provides a significantly larger amount of hot air, which greatly improves and accelerates the drying process. As with the previous solution, the hot air is expelled into the atmosphere.

The next solution involves a system that causes 90% of the air mass to circulate in a closed loop. This is achieved through a system of dampers that automatically close the intake and open the connection between the supply fan and the pit. This allows the fan to draw air from the spray booth, which is already pre-heated, and re-circulate it back into the booth through the heat exchanger, where temperature losses are compensated. The damper closing the intake is intentionally designed to be slightly leaky to ensure the intake of 10% fresh air, which is necessary for the drying process.

Until recently, during this phase of operation, the exhaust fan remained off, and the 10% excess air was expelled by gravity. However, EU directives have changed in this regard, and now to assist in the expulsion of this 10% of air, the exhaust fan must also operate during the drying phase.

Which solution is best for our climate? Let’s use another illustrative example. Imagine we are driving a car with four people. It’s winter, and it’s freezing outside. We’ve been driving for a few kilometers, and the engine has reached the correct temperature. The driver has turned on the heating and set the fan to the lowest speed. Will the people sitting in the back be warm? After how long? And what will happen if the fan is set to the highest speed? Won’t the warmth reach the passengers much faster?

More technologically advanced spray booths, equipped with electronic control systems, can have additional operating functions:

Ventilation phase: This occurs after the painting phase.
Cooling phase: This activates automatically after the drying process is completed.
Energy-saving system: This automatically switches the spray booth to the recirculation phase if the painting process is interrupted, conserving thermal energy.
Damage visualization system: Displays errors in the form of codes on the controller.
Automatic overpressure regulation: Ensures consistent air drop speed by maintaining a constant overpressure in the booth.

These advanced features enhance the efficiency, safety, and user-friendliness of the spray booth, providing better control over the painting and drying processes.

Let’s return to the heart of the spray booth, namely the ventilation and heating unit commonly referred to as the generator or aggregate.

Manufacturers typically use various solutions driven primarily by technical capabilities and production costs. In most designs, such a generator is built as a self-supporting structure made from steel sheets, which are bent appropriately to create spatial profiles. These are assembled on-site using rivets or self-drilling screws and covered with steel sheet panels. The individual components of the structure combine to form a supporting framework for the heat exchanger, fan(s), and additional equipment such as a recirculation damper (if such a system is present), pneumatic actuators, servomotors, supports for the intake and exhaust ducts, and other elements that may be included in different designs.

Such a solution is efficient in mass production, but its durability can largely depend on the reliability of the assembly. Fans have a considerable mass and rotate at high speeds. Inaccurate or poor assembly can quickly lead to problems.

Renowned equipment manufacturers are typically companies with a long-standing tradition. They do not specialize in mass production and, as such, cannot afford the risk associated with lack of assembly control and operational reliability. Therefore, generators from these companies are built based on solid steel constructions capable of bearing heavy loads. They are delivered to the customer as ready-to-use modules containing all the necessary internal components, such as fans, motors, dampers, heat exchangers, etc.

This approach results in certain challenges related to the transport and unloading of such equipment. However, it is assumed that the equipment will be transported and set up at the workplace essentially only once. Therefore, overcoming these inconveniences is worthwhile to ensure the highest durability and quality of construction.

The most critical components of the generator are the fans and the heat exchanger. This is where significant differences in approach to product design can be found.

As the name suggests, the heat exchanger serves as an intermediary between the burner flame and the air supplied to the spray booth. It works in such a way that the burner, mounted on the heat exchanger wall, directs its flame into the interior of the exchanger, into the combustion chamber. The exhaust gases are then directed to the chimney through flame tubes. The air supplied by the blower fan flows over the heat exchanger and heats up from its hot elements. The combustion chamber reaches a high temperature, as do the flame tubes that channel the exhaust gases outside.

Most manufacturers produce heat exchangers with a relatively simple design, featuring flame tubes shaped as vertical slots placed side by side in a single row above the combustion chamber. In such an exchanger, the air flows between these slots in an unobstructed, linear motion.

Other designs place round flame tubes above the combustion chamber in several alternating rows. In these heat exchangers, the passing air must follow a “serpentine” path, which results in a very large exchange surface area. Additionally, these designs use special “baffles” installed in the flame tubes to maximize the utilization of the heat from the exhaust gases. This construction results in a very high efficiency of the heat exchanger.

In the heating systems of spray booths, oil or gas burners are used depending on the availability of resources. It is beneficial when these burners come from a reputable manufacturer who has conducted appropriate testing of their products. The burner’s power must be matched to the required power of the heating system.

A common question is about the fuel consumption of such a burner. This value naturally depends on the burner’s power. We must understand that fuel consumption is also related to the duration (intensity) of the burner’s operation. In practice, this means that if it is cold outside, the burner must operate longer to heat the air passing through the heat exchanger to the desired temperature. The longer it operates, the more fuel it consumes. This is precisely why it is installed. The only reliable information that can be provided is the maximum fuel consumption per hour of operation.

The following oil and gas burners are commonly found in the heating systems of spray booths:

Oil Burners: Examples include models from manufacturers like Riello, Weishaupt, and Bentone, known for their reliability and efficiency.
Gas Burners: Examples include models from manufacturers such as Maxon, Eclipse, and Dungs, which are well-regarded for their performance and safety features.

These burners are selected based on their ability to provide the necessary heat output while maintaining efficient fuel consumption and reliable operation.

Single-stage burners – These are the most commonly encountered but come with a certain drawback. In practice, they cause significant temperature fluctuations of plus or minus 3-4°C during the painting phase. This means that a painter, after setting the controller to 20°C, might paint one part of the car body at 17°C and another part at 23°C shortly afterward. We know that the paint will have different viscosities under these conditions, which can affect its application.

Two-stage burners – If equipped with the appropriate control element (such as Spraytronic), they are almost entirely free from the aforementioned issue. Such a burner operates in a way that, depending on the heat demand, either the first stage or both stages are activated.

Modulating flame gas burners – These burners can also feature a modulating flame system. This is a very expensive solution but ensures stable maintenance of the set temperature.

These different types of burners address the need for precise temperature control in the spray booth, ensuring consistent paint application and quality.

Fans used in spray booths can generally be categorized into:

Single-inlet fans mounted directly on the motor shaft.
Double-inlet fans driven by belt drives.

However, these are always centrifugal fans.

Single-inlet fans mounted on the motor shaft are a very good and inexpensive solution. Their operating characteristics allow them to tolerate high levels of ceiling filter contamination relatively well. They do not require adjustment and essentially have no possibility for speed regulation; thus, their capacity is the same as the motor’s speed.

Double-inlet fans are driven by a belt drive, which allows for “tuning” the fan’s capacity by changing the size of the pulleys, for example, due to the high installation altitude of the booth above sea level, which can even occur in Poland. These fans have a much higher efficiency per kilowatt of motor power. Due to the significantly larger number of blades, these fans do not have a tendency to cause air pulsation.

The choice between these types of fans depends on various factors, including the specific requirements of the spray booth, the level of air cleanliness needed, and the operational conditions such as altitude and desired airflow control.

We believe that this information will help you navigate the multitude of offers and systematize these studies. We must understand that most manufacturers likely know how to construct a high-performance spray booth. The fact that they produce devices with specific parameters and use particular components and elements is usually the result of well-conducted marketing and cost calculations.

Manufacturers aim to balance performance, cost-effectiveness, and market demands to deliver products that meet industry standards and customer expectations. By being aware of the various components and their functionalities, you can make more informed decisions when evaluating different spray booth options.

Another important aspect is the knowledge and integrity of the salespeople. Most of them are hardworking individuals with extensive professional knowledge who deserve deep respect. However, as in any field, there are also those who prioritize quick profits over your needs. Ask for technical details, and you will quickly see who treats their customers seriously.

How does a spray booth work?

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