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Relative humidity inside paint booths: what causes deviations from optimum conditions?

Paint booths are closed, air-conditioned spaces in which the ambient conditions are kept at the ideal values to ensure both a high quality finish of the coated surfaces and safety of the personnel working inside. Temperature, humidity and cleanliness of the air are ensured by an appropriate ventilation system.

The conventional solution is an “all outside air” ventilation system: fresh air from the outside is continuously introduced into the booth, and brought to the optimal temperature by a heating and cooling system. The air that flows across the coated part is blown at a speed such as to carry away contaminants (0.3 - 0.5 m/s) before being discharged directly to the outside, passing through a filtration system.
But what happens to relative humidity in this situation? During the winter, in fact, the outside air is very cold and humid (for example, -5°C - 80%) and is heated to 20-25°C. The relative humidity of this heated air (moisture content as a percentage of the maximum the air can hold, the point where condensation occurs) has been lowered considerably. This has fallen from the initial 80% of the outside air to about 12% in indoor air conditions, without removing any moisture!

A humidification system thus needs to be installed in order to return the humidity level to the correct value. The humidification system must have suitable requirements in terms of capacity, sufficient modulation to quickly respond to sudden variations in outside climatic conditions or compensate for insufficient modulation of the heating systems, which provide heat rapidly.

Another type of ventilation system on paint booths uses (almost complete) recirculation of the process air.  The air that is handled inside the booth is the same in volume as the all outside air solution, however a high portion of the flow is recycled (80-90%) after passing through an exclusively “dry” filtration system, and only a small part is replenished with fresh outside air (10%).
This system keeps the ambient conditions inside the booth much more stable once the system has reached steady operation.


However, relative humidity may still slowly drift away from the optimum conditions. The cooling process to remove the heat generated inside the booth also creates a dehumidification effect via the cooling coil. The moisture content in the air, which is continually recirculated, progressively decreases, bringing the relative humidity to a percentage that is outside the optimum range. 
In this case too, a humidification system is needed to return the system to the ideal conditions. Such systems may also be low-capacity, and need to finely modulate operation so as to avoid creating contrasts between the cooling and humidification functions.

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