The invention offers an improvement in a process for start-up in the occasion of producing acrylic acid by catalytically oxidizing acrolein at vapor phase under high load conditions, the start-up meaning the step of increasing the acrolein supply rate from the non-reacting condition to the prescribed reaction conditions. The housing can be installed on a narrow bathtub rim yet also accommodates substantial substrate thickness of bathtub decks where required. A register set records each change on a track-by-track basis. An electronic controller, in communication with an operator display and the at least one device.