about 65 u00b0 f to 85 u00b0 f after exiting scrubber direct heatexchanger 9 where it was also subjected to a further cooling the heatexchanger 7 has a corrosion resistant coating or is made of low cost corrosion resistant materials with heating surface provides frequent low pressure direction changes of the moisture laden hot flue gas thus effectively removing droplets from the stream the drainage 13 is provided for the said acidic moisture removal the energy consumption by the main induced draft fan 11 is also reduces thanks to significant volume change of the flue gas when passing the invention 39 s disclosed low pressure waste heat recovery system as shown since flue gas exiting heatexchanger 7 gains back its temperature reaching 150 u00b0 f to 180 u00b0 f and maintains its natural draft ability while avoiding substantial moisture carry out typical for the previous art it is also desired to claim it as the novel feature of the disclosed whrs the stream of water and or ambient air is supplied to scrubber 9 or inertia separation device for instance cyclone downstream of heatexchanger 7 the quantities of said stream of water and or air is sufficient to providing further cooling of flow to accomplish an additional moisture condensation in 9 the rest of the cold and slightly wetted flue gas is evacuated by the main induced draft fan 11 that passes through a low pressure drop fiber filter 10 to remove small liquid droplets including particles of one micron size the drainage lines from 9 and 10 deliver liquid acidic stream for additional treatment before disposal to a sewage or for reuse in the whrs thus in conjunction with whrs not only the efficiency of fuel utilization is improved but favorable physical and economically beneficial conditions for.