The invention provides a system and method comprising an apparatus for removing contaminants from a gas in a semiconductor processing device, which can include a filter unit having at least two parallel filter stages located therein. The filter stages are designed to remove a least a portion of the contaminants present in the gas flowing through them. The apparatus can also include a flow controller for distributing the gas flow among the filter stages. In one embodiment, the controller may consist of a diffuser plate. The invention also provides a sampling tube orifice for gas flow control in a system or method of the invention. In another embodiment, an apparatus for removing contaminants from a gas in a clean room comprises a filter unit having at least two parallel filter stages, which are used to remove a portion of the contaminants in the gas as it passes through the apparatus.