The widespread use of low solvent, water-borne base coats and paints means that humidity control is essential in spray booths. Relative humidity (RH) levels as high as 50-55% are often used for water-based paints, delivering consistent spray quality by preventing aerosol evaporation and pigment agglomeration, consistent deposition rates and preventing premature drying. Spray or evaporative humidifiers are commonly used for their economy, with steam also used in smaller applications.
High RH is undesirable in the drying process, of course, but when metal components leave the oven the local environment in sanding, repair and inspection areas can be hot and dry. This leads to worker discomfort and electrostatic build-up, drawing dust to the component.
Electrostatic painting and powder coating booths
Economic and environmental imperatives such as waste reduction, overspray control, particulate recovery and solvent reduction, mean that electrostatic paint and powder coating is now very widely used, particularly in large industrial applications in robotic, manual and conveyor booths. The process is dependent on maintaining a differential charge between the powder and earthed item, giving good coat uniformity directly and indirectly, minimal overspray and reduced waste.
If the RH is too high, then the effects of electrostatic charge are disrupted, coat adhesion is affected and higher flow rates required for the same finish. In low relative humidity, the charging characteristics of the corona are affected so the powder does not become properly charged, and there is lower transfer efficiency between the powder and items being coated, leading to inadequate film thickness and Faraday cage effects. Costs rise and waste increases. Low RH also provides the potential for arcing.
For electrostatic paint and powder coating, a relative humidity between 45-60% RH is ideal, at 20-25°C.
Dipping and electroplating
In multistage coating and surface treatment processes, metal items are moved from tank to tank. In hot, dry climates the lack of moisture in the atmosphere can cause a job to dry prematurely and unevenly, particularly if there is a delay and work cannot be moved on to the next stage. This leads to uneven drying at the surface, causing staining and affecting the surface in subsequent process tanks. Stripping and reworking the piece is time-consuming and wasteful. Maintaining a relative humidity in the air of 50-60%RH prevents rapid surface drying.
Controlling the painting and coating environment
Commonly, control over air temperature and relative humidity will be provided by an air handling system, fitted with a humidifier, delivering conditioned air directly to one or more spray booths. This might be supplied by the booth manufacturer or designed separately and supplied by an air handling unit (AHU) manufacturer.
For individual spray booths within a premises, an associated AHU may be incorporated or a positive pressure containment room in which materials and equipment can also be kept is built around the booth, conditioned to provide the appropriate temperature, RH and air filtration.
The relative humidity of ships’ holds is always particularly high at sea: It amounts to up to 95 per cent. Hygroscopic cargo such as coffee, tea, tobacco, paper and powder are particularly affected by humidity during global transit. As sea temperatures fall, air in the cargo hold cools causing a rise in the internal relative humidity – often to the point where condensation is formed.
There is only one remedy for mould, microbial contamination and corrosion on cargo and ship: to supply constantly dry air with humidity suitable for the cargo into the ship’s hold. To achieve reliable results, the dehumidification system must have been planned professionally.
Target values for the inside of ships: 40 to 60 per cent humidity depending on the cargo
For storable goods, relative humidity must be 65 per cent,
Non protected steel components of the ships’ holds as well as points where the protective layer has been damaged corrode at 65 per cent humidity and above.