Condensing heat recovery can be applied in two ways: direct-contact and indirect-contact systems.  Essentially, we are talking about heat exchangers, which will use the hot exhaust stream to heat a cold medium (almost always water, sometimes glycol). 

The advantages and disadvantages of direct-contact and indirect-contact systems are compared and contrasted.

Direct Contact

  • Longer life (20 to 30 years) due to neutralization of slightly acidic condensate forming within the spray tower.
  • Can produce higher temperatures with more humid exhausts.
  • Can deliver more savings when there is lots of heat sink (user) with relatively low (140 to 170°F or 60 to 76°C) temperature setpoints.
  • Significant pollution control benefit (effectively a wet scrubber, over 90 percent reduction of acid gases and over 50 percent removal of particulate matter at end of pipe), in addition to reduction of fuel use.
  • Less simple process, typically requires a new pump (due to loss of liquid pressure in the spray tower) and often requires a hot water recirculating loop.

Indirect Contact

  • Shorter life (5 to 15 years) due to formation of acid droplets on the outside of tube materials, resulting in potential spot corrosion due to the acidity of the droplet and the dielectric effect.
  • Can produce higher temperatures with hotter, dryer exhausts.
  • Can deliver more savings when there is less of heat sink (user) with relatively high (170 to 200°F or 76 to 93°C) temperature setpoints.
  • Limited pollution control benefit in addition to reduction of fuel use.
  • Simpler process, typically no need for new pump or hot water recirculating loop.

Clearly the selection of direct contact vs. indirect contact is dependent upon the specific process heating application, both on the characteristics of the heat source and the heat sink.

See the related feature article, "Condensing Heat Recovery for Industrial Process Applications." Capturing heat that would otherwise escape the process through the stack, waste heat recovery allows industrial thermal processing operations to recover and reuse process heating.