Condensing Flue Gas Heat Recovery System Excerpt from Arthur D. Little Report November 1990 Pamela J. Waterman Section 1.4        Heat-Exchanger Operation 1.4.1                    Condensing Heat Exchanger Design Principles A heat-exchanger system is a mechanical unit that uses normally wasted heat to pre-heat water prior to feeding that water to the existing boiler.  The waste heat (in the form of flue gas) is circulated in a closed unit surrounding pipes carrying the water to be heated.  The quantity of heat transferred is limited by either the minimum heat available from the heat source (wasted flue gas) or the heat requirement of the heat sink (makeup water, return condensate, or dearator.)  The heat available from the heat source is a function of flue-gas temperature, quantity, and minimum feasible exhaust temperature; here, we decided to use the makeup water as the heat sink.  We chose to use a condensing heat exchanger (CHE) for this application because it is designed to tolerate the corrosive effects of "cooled' flue gas.  Below a certain temperature, flue gas displays highly acidic properties, corroding the equipment found in conventional metallic economizers.  This limits the extent to which economizers can recover heat. In contrast, the flue-gas side of the selected heat exchanger (from CHX® Corporation) has a Teflon coating and is not susceptible to corrosion.  The system can therefore cool the flue gas below its dew-point, permitting latent heat extraction from the flue-gas water vapor. 1.4.2        Factors Influencing Feasibility The attractiveness of a CHE, a combination of cost savings and other benefits, depends on the following criteria: a) Available heat sinks - the type of heat sink, the amount of energy that can be transferred to the sink, and the variations of sink capability with time are important elements in defining both the technical feasibility and economic justification of condensing heat exchanger projects. b) Available heat sources - the flue gas rate, temperature, and composition define the energy available for recovery and, along with heat sink conditions, are the primary variables influencing condenser heat exchanger design. c) Value of heat recovery - the value of energy being replaced by the heat recovery determines the value of the heat recovery.  The steam being displaced by CHE installation has a direct bearing on the fuel savings.  If 200 psig steam which could be sent to the plant is used to heat makeup water in the dearator, that steam is more valuable than if 5 psig steam from another source were used. d) Arrangement of existing equipment - flue gas duct arrangement and available space for the exchanger and associated equipment will affect both the design and the cost of the condensing heat recovery system.