Most formaldehyde is produced from methanol. The remaining part is produced by the partial oxidation of hydrocarbon gases with air or oxygen. At this stage, while preheating the methanol, the plate heat exchanger suitable for the process is selected by considering the small temperature difference and the large heat transfer rate. In formaldehyde cooling, the produced formaldehyde solution, e.g. 37% and pre-cooled during heat recovery in the methanol preheater. Then, final cooling is performed in this unit. Coolant salt water, glycol, etc. can.