This paper presents the economic optimization of indirect sewage sludge heat dryer for sewage sludge incineration plants. The objective function based on two-phase heat transfer, and economic relations is provided to demonstrate the optimum size for the minimum investment cost. De-watered sludge is fed into the dryer with a mass flow rate of 165 tons per day and consists of 27% dry matter. After the sludge drying process, the dryness of sludge increases up to 40%. In the indirect sludge dryer unit, thermal oil is used to heat the dryer wall and to prevent heat loss. Thermal oil is circulated in a closed cycle and gathered into an oil tank. Total cost of the sludge dryer unit changes proportional to the dryer area. The optimum dryer area is found as 32.54 m2. The corresponding minimum cost is found as $35,700.

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