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