Nowadays, energy efficiency is the crucial element in industry in terms of improving operating conditions steam boilers. Generally, feed-water economizers and air preheaters are used for this aim in steam boilers to recover heat from the flue gases. The measurements of volumetric percentages O2 (5.7%) and CO (0%) in flue gas, temperatures of flue gas (100, 150, 168, 218 degrees C), ambient (23.8 degrees C), surface (38.3, 40.8, 65.5, 132.5, 166.65 oC), feed-water (108 degrees C) preheated feed water (133 degrees C) and steam (166.65 degrees C), consumption of natural gas (149.4 Nm(3)/15 mins) and steam (2000 kg/15 mins), conductivity of feed-water (134.25 ppm) and boiler water (3500 ppm), operating pressures (6.3 bar), absolute pressure (7.3 bar) are performed experimentally. Various case studies such as steam boiler operation without the use of economizer or air preheater, economizer addition to the steam boiler, addition of air preheater to steam boiler, addition of both economizer and air preheater to steam boiler are carried out to discuss the effects of the use of either feed-water economizer or air preheater separately and together. Consequently, total heat loss and efficiency values, fuel consumption and saving amounts, investment and operation-maintenance costs, some economic analyses using the methods of net present value and internal rate of ratio are calculated using measured values for each case study separately and compared with each other considering the case studies. It is expected from this study that investors will benefit from the calculation process used for an example application of steam boiler in a food plant to establish heat recovery systems in plants. As a result of experimental analyses, the heat recovery method from flue gas is found to be one of the effective processes to save energy in a boiler as expectedly.