Direct vs Indirect
Heat Recovery Vessels
Open vs Closed

Direct Contact LLC (DCLLC) Heat Recovery Systems bring heated exhaust or vent gases into direct contact with a cooling liquid. This differs significantly from systems that employ coils or plates to provide the heat transfer surface and keep the streams separated (indirect).

The heat transfer coefficient of DCLLCs unique direct contact method (500-1000 BTU/hr-ft2-F) is many times larger than that of the typical indirect method (approximately 10 BTU/hr-ft2-F), providing a much more efficient method of recovering heat and allowing for smaller units that perform better.

The DCLLC system also efficiently drops the temperature of the gas stream to below its condensation temperature (Wet-bulb temperature (view graph),) forcing water condensation from the waste stream. This capability generates additional utility savings by recovering water that is otherwise lost and using it to significantly lower makeup water demands.

 

Condensing Economizers - Direct

The Direct Contact Condensing Economizer contacts water with flue gas.


  • The water adiabatically cools the flue gas while the flue gas warms the water
  • The flue gas cools to below its dew-point temperature and condenses water vapor (converting latent heat to sensible heat)
  • Overall heat transfer coefficient ranges between 500-1000 BTU /hr-ft2-oF.
The heated contact water can be used:
  • Directly as makeup water to the deaerator, or
  • Indirectly to heat process fluids with conventional heat exchangers

Condensing Economizers - Indirect

The Indirect Contact Condensing Economizer is similar to an Economizer, transferring captured heat through an extended surface coil.

Such a unit will:

  • Lower gas temperature to below its dew point
  • Have a low overall heat transfer coefficient (~10 BTU/hr-ft2 oF)
This design has:
  • High first costs
  • Relatively high pressure drops
  • Relatively high operating costs