Efficient paint booth operations depend on meticulously controlling air conditions to safely remove contaminants and regulate humidity. Two primary humidification methods come into play: isothermal, which introduces water vapor at high temperatures to maintain a stable air temperature while enhancing moisture content, and adiabatic, where water evaporates at room temperature, naturally cooling the air and adjusting its humidity. 

This delicate balance is critical in systems using 100% outside air (with their high heating and cooling demands) and in systems with partial recirculation, where maintaining optimal conditions is achieved through controlled humidification and dehumidification processes.

Paint booths with 100% outside air

These paint booths use only fresh air taken in from the outside to remove the solvents and paint particles suspended in the inside air, to ensure a safe environment for workers and avoid the risk of flammability and explosion. The solvent-rich exhaust air is then conveyed through filtration systems (wet or dry) before being discharged back outside.

Paint booth with 100% outside air

Since this type of booth uses 100% outside air, the heating, cooling and humidification needs will be highly dependent on outdoor conditions. It is therefore necessary to use humidification systems capable of compensating for variations in relative humidity (and temperature) due to outside climatic phenomena, in order to ensure that the set point is reached without the excessive swings typical of ON/OFF systems. This happens especially in winter, when the outside air is cold and humid. In winter, the heating needed to reach the set point can cause the relative humidity to drop to very low levels, even below 10% RH. This occurs because the air, as it is heated, increases its capacity to hold water vapour and consequently the relative humidity decreases. Bringing this back to optimal values (50-70% RH) requires high humidification loads, on the one hand due to the large differentials in terms of relative humidity (between inside and outside) and on the other to the configuration of the booth (operating on 100% outside air).

For paint booths using 100% outside air, it is therefore advisable to take into consideration the following characteristics, which can have a significant impact on the design of the humidification system:

• high heating loads
• high cooling loads
• high variability in outside conditions

Paint booths with partial recirculation

Over the last few years, thanks to the use of dry separation systems and the introduction of robotic paint sprayers, it has become possible to use ventilation systems that recirculate the process air.

A ventilation system with a high level of recirculation can mitigate the influence of outside climatic conditions on the air conditions inside the paint booth, significantly reducing the humidification load and making the process very stable.

To make the most of the savings achieved by recirculating up to 80-90% of the total air flow (10-20% fresh outside air), the paint booth must be equipped with robotic systems that can operate in highly contaminated air (solvent-rich), and all the components inside the booth and in the air handling unit, including the humidifiers, must be explosion-proof (ATEX marking in the EU).

Paint booth with partial recirculation

The main purposes of an air conditioning system with a high degree of recirculation are to bring the small amount of outside air introduced into the booth to the desired temperature-humidity conditions, and offset the sensible heat generated by the process.

It is therefore necessary to humidify both in winter, when the outside air is dry, and in summer due to dehumidification.

The cooling needed to maintain the correct temperature in the paint booth leads to systematic dehumidification of the air stream, due to partial condensation of the humid air recirculated across the cooling coil, given that the surface temperature is below the dew point (usually between 7 and 12 °C).

It is therefore important to adopt a humidification system capable of correcting any deviations in relative humidity and of precisely modulating the amount of humidity produced, to avoid swings that can have an impact on control and maintaining the set point.

Air transformation in a paint booth with partial recirculation

Key:
A: set point conditions
B: air heated by sensible loads in the paint booth
C: cooled and dehumidified air
D: adiabatically humidified air

Combined painting and drying booths

Some types of paint booths can be combined with curing ovens, for example, for car repairs or in heavy industry, where it is preferable to move the products only after the process is completed. During the curing phase, the temperature usually varies from 40°C to 60°C inside the booth, while it reaches even higher temperatures inside the air handling unit heating section: here the system can reach temperatures above 90 °C near the gas-fired burner, exactly where the humidification system is usually installed.

The humidifier, positioned downstream of the heating section for efficiency reasons, therefore has to withstand very high temperatures. A steam humidifier is naturally able to withstand temperatures of around 100°C, i.e. the temperature of steam, while adiabatic humidifiers need to be designed especially to withstand these temperature levels.

A combined booth can benefit from the use of an adiabatic humidifier for several reasons. Adiabatic humidification is particularly useful for lowering the temperature of the paint booth following the curing phase, thanks to the natural cooling effect it produces. The humidifier must be highly responsive, i.e. able to quickly control the amount of water introduced into the air stream, as well as interrupt the supply of humidity into the booth when the process switches to the next phase. On the contrary, were it to have high inertia, the humidifier would produce uncontrolled humidity, with swings between very low and very high value.

Paint booth operating in painting mode with 100% outside air (on the left) and in oven mode with full recirculation (on the right)

A robust air management strategy is vital for ensuring that paint booth operations run safely and efficiently. Whether utilizing 100% outside air or embracing partial recirculation, the choice of humidification method (be it isothermal or adiabatic) plays a critical role in maintaining the desired environmental conditions. 

By carefully addressing the challenges of fluctuating outdoor climates, high heating and cooling loads, and the specific demands of the painting process, operators can create a stable environment that minimizes risks and maximizes performance. 

Ultimately, a deep understanding of these systems enables the design of solutions that not only protect worker safety but also enhance overall operational effectiveness.

The topics covered in this blog post about humidity and temperature in painting booths are explored in detail
in the white paper “Highly-efficient solutions for painting – Humidity, temperature and heat recovery control”.

Download the white paper