Results are presented which show that the water formed during the combustion of hydrocarbon fuel is significant in relation to the evaporation potential of the fuel when used in a direct gas‐fired drier. As a result, the outlet air humidity of direct‐fired driers is higher than that of indirect‐fired plant operating at the same outlet and inlet air temperatures. The powder moisture increases with increasing humidity at constant temperature. To compensate for this increase in moisture content, either the drier must be run at a lower inlet temperature throughout, or the outlet temperature of the drier must be increased. However, with heat sensitive materials such as milk a serious loss in powder solubility can occur if the latter action is chosen. These results suggest that the difference in overall efficiency of direct and indirect forms of heating in spray driers is less than has previously been as