Process control of a spray dryer that is usually used as the last step of production is very crucial in obtaining a quality standard product. To this end, predicting the effect of various operating and environmental parameters on product properties is essential. Modeling was done in microscopic and macroscopic scales by modifying the mass and heat transfer equations used in investigating the drying mechanism. However, as far as momentum transfer is concerned, the equations are the same as the ones used in the available literature models. Unlike conventional models that take only the two streams of hot air and sprayed droplets into account, a third stream is considered in our proposed model and this modification improved the results significantly. The model equations were solved numerically and a new simulation software was developed using Visual C++ 6.0. The model results are in good agreement with the industrial data obtained from a detergent producing plant.