This paper presents the experimental investigations of a system as a second treatment means to increase boiler efficiency and heat transfer from combustion gas to heating surfaces in the case of spray combustion. In order to reburn residual combustible components accelerate the burning rate of sprayed fuel droplets, improve the diffusion flame and delay the residence time of the flame, advice with slit type nozzles for spouting preheated supplementary air is used in this study. In the experiment, boiler efficiency and smoke concentration in the exhaust gas at given conditions are measured in both case of installing and not-installing device in the model of combustion chamber which was designed to be equipped with five surfaces. The results obtained in this experiment are as follows ; 1. The optimum values of air rate ${lambda}$ are about 1.3 in both case. 2. The exhaust gas temperature in the case with device increases about $30{sim}70^{circ}C$ above that of the case without the device. 3. Boiler efficiency and reduction effect of smoke emissions are improved considerably.