Heat exchangers called economizers are often seen in boilers. These apparatuses warm substances, most often water, to a temperature that normally does not get beyond boiling. The ability to employ the enthalpy in hot fluid streams—streams that aren't hot enough to be used in a boiler—is what gives economizers their name. They are able to recover more usable enthalpy as a consequence, increasing the boiler's efficiency. In this study, an economizer zone simulation is provided. Examining the fluid's flow patterns as it travels down the economizer's length is made feasible by this simulation. The Economizer Unit's U-bend portions are more prone to erosion due to an increase in flue gas velocity at these bends, as shown by the facts of past failures. It has been shown, nonetheless, that the flue gases' velocity unexpectedly increases at the lower bends compared to the bends higher up. This thesis looks at how heat is transmitted by convection in an economizer when mass flow rates change, utilising CFD and thermal analysis. Alloys 6061 and 7075 made of copper and aluminium are being considered for use in tube production. There are three distinct mass flow rates that will be utilised: 70, 90, and 100 kg/second. A CFD analysis may be performed to determine the rates of heat transfer and temperature distribution by varying the mass flow rates. A heat transfer research is done on the economizer to find out which material is better. ANSYS is utilised for analysis, while CATIA is used for 3D modelling.