Numerical simulations and an experimental study for optimal design of a 1500 kW water-tube condensing boiler

Abstract

The present study is focused on investigating the condensation process and numerical simulation for optimal design of a 1500 kW water-tube condensing boiler. The specific geometry of the heat exchanger of this boiler involves compact arrangement of spiral coils to enhance heat transfer efficiency and achieve high capacity. Experimental validation is conducted on a 580 kW condensing boiler. Numerical simulations of the flow for gas and water sides of the boiler are performed in parallel using the ANSYS-FLUENT software with the k-omega (SST) turbulence model. Twelve models with different combustion chamber dimensions, coil numbers and revolutions are considered. Important design criteria include flue gas temperature, combustion chamber pressure and water pressure drop in the coils. The results indicate that increasing effective heating surface area and reducing coils gap decrease flue gas temperature to the dew point. Increasing coil revolutions has a greater impact on flue gas temperature and boiler weight compared to increasing coil numbers. With the same number of coils, by adding an extra revolution the weight is increased approximately 17 % and the effective heating surface area about 21%. On the other hand, for the same number of revolutions, each additional spiral coil increases both of the weight and the effective heating surface area approximately 4 %. Moreover, with an increase in the number of the coils and a decrease in the number of revolutions, the pressure within the combustion chamber is decreased. By reducing the gap size, the flue gas temperature is reduced approximately 2C and the combustion chamber pressure is increased about 18 %. Base on the design criteria, the model six with 27 spiral coils and 7–8 revolutions, is identified as the optimal design. This progress and capability allows the design and manufacture of high capacity boilers for residential sector as well as for industrial applications.