The rooms containing electrical and electronic control equipment are ventilated with temperature and humidity conditioned air to achieve the environmental conditions required by the warranty conditions imposed by original equipment manufacturers. 

The supply or make-up air will be taken from an external location and the outdoor air may be heavily contaminated with acidic gases. If these gases are not removed before the air enters the room, then the control equipment will be susceptible to damage and, ultimately, failure due to corrosion. Molecular filtration can remove these gases. 

The gas concentration in the inlet air may be high – perhaps tens of parts per million (ppm). However, the manufacturer’s warranty typically requires in-room gas concentrations in the very low parts per billion (ppb) range to comply with corrosion class G1 under ISA 71.04-2013. A molecular filter installed in the outdoor supply air system must operate with very high efficiency on a single-pass basis. Importantly, this air, which may account for up to 40% of the total room ventilation rate, is used to positively pressurise the control room to prevent the ingress of fugitive gases. The level of pressurisation can be compromised by unnecessary opening of doors and windows, leaks due to construction defects and leaks at cable entry points. 

To provide additional protection from fugitive gases, additional molecular filtration can be installed in the return/recirculation air system.



Concentrations of gases are much lower in the return air and a filter will operate on a multi-pass basis. A lighter duty filtration solution than that used in the outdoor air system makes a good choice. Sometimes industrial air cleaners make a good choice for both room pressurisation and as recirculation systems.