Design and Simulation of Condensing Heat Exchanger

Abstract

The conventional heat recovery techniques recover only sensible component of the flue gas heat and a significant portion of the flue gas heat is lost to the atmosphere. Condensing heat exchangers are designed to retrieve both sensible and latent heat components of the heat present in flue gas by promoting condensation of water vapour present in the flue gas.  It has several challenges due to non-availability of the low-temperature heat sink. The paper explains numerical simulations based on analytical modeling of the heat and mass transfer processes involved in condensation of moisture in the flue gas in a cross-counter flow tubular heat exchanger- of size 1.752 m × 1.274 m × 2.230 m which comprises of 78 tube passes and 6 tubes with cooling water flowing inside and flue gas over the tubes. The performance of the system is exhibited as a function of different parameters. A mathematical model using MS-Excel and VB Macros as a tool is developed to study the effect of operating parameters on the performance. The governing equations and correlations based on mass and energy balances for the system are used to compute variables such as flue gas exit temperature, cooling water outlet temperature, mole fraction and condensation rates of moisture. The equations are solved using an iterative solution technique with calculations of heat and mass transfer coefficients and physical properties. The paper focuses on the maximization of heat recovery and the various parameters affecting the heat and moisture recovery and condensation. The effect of operating parameters like inlet temperatures of both the fluid streams and flow ratio is studied in order to achieve the maximum heat and moisture recovery. The condensation and heat transfer rates increase significantly by increasing the flow ratio and decreasing the inlet cooling water temperature.
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