Heat Recovery 101: What’s the difference between a direct-contact and an indirect-contact economizer?

Condensing heat recovery can be applied in two ways: direct-contact and indirect-contact systems. An indirect-contact system is simply a standard heat exchanger: water passes through tubes and the hot exhaust passes outside the tubes. Sensible heat and latent heat are transferred through the tube walls from the hot side to the cold side. By contrast, direct contact systems bring the cold water medium in direct contact with the hot gas in an open spray tower or packed spray tower to recover the heat directly by heating the water. 

Neither of these technologies is a "one size fits all" solution," so it’s best to take a look at the pros and cons of each type before making the right choice for your facility.

Some advantages and disadvantages of the two types of systems include:

Direct Contact Economizer

• Longer life (20 to 30 years) due to neutralization of slightly acidic condensate formed within the spray tower;
• Can produce higher water temperatures with more humid exhausts;
• Can accept almost any kind of exhaust as a heat source, including oil boilers and dirty process exhausts, in addition to natural gas;
• Can deliver more savings when there is lots of heat sink (user) with relatively low (140° to 170°F) temperature setpoints;
• Significant pollution control benefit (effectively a wet scrubber, over 90% reduction of acid gases and over 50% removal of particulate matter at end of pipe), in addition to reduction of fuel use; and,
• 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 Economizer

• 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 temperature water with hotter, dryer exhausts;
• Best applied to clean, non-acidic exhausts as heat sources (natural gas exhaust);
• Can deliver more savings when there is less heat sink (user) with relatively high (170° to 200°F) 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 either direct contact or indirect contact is dependent upon the specific process heating application, both on the characteristics of the heat source and the heat sink. 

For more on this topic see our white paper entitled Condensing Heat Recovery – The Final Step towards the 95% Efficient Boiler Plant, as published in Process Heating, Volume 22, Number 2. 

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