A critical heat exchanger is usually the exchanger installed on
the heat source. Depending on its nature, this heat exchanger
must withstand high temperatures and can be subject to fouling
and/or corrosion. In case of waste heat recovery, the heat
exchanger must not interfere with the process, i.e. the pressure
drop should be limited and its dimensions must comply with the
available space. Moreover, in case of flue gases with sulfur
content, the acid dew point should be avoided. This explains
why in most commercial plants the exhaust gases are not cooled
below 120–180 1C, depending on the sulfur content of the gases.
Research is being carried out to design heat exchangers able to
withstand and evacuate acid condensates.