PERFORMANCE of HEAT RECOVERY SYSTEM FOR A SPRAY DRYER

A factorial experimental design was used to study the effect of exhaust heat recovery, air flow rate and outlet temperature on the energy consumption of a spray dryer. A commercial-scale cocurrent flow dryer with pressure atomization was used to dry 15 wt% sodium sulfate solutions. the inlet air to the dryer was preheated by using a shell-and-tube heat exchanger to recover energy from the exhaust air. Heat recovery reduced the energy consumption of the dryer by 12–28%, with the largest reduction occurring at the high levels of air flow rate (13,000 kg/h) and outlet temperature (104°C). the energy consumption of the dryer was lowest when heat was recovered at the low air flow rate (6500 kg/h) and low outlet temperature (82°C). No problems were encountered with fouling or cleaning of the heat exchanger. the total pressure drop across the heat exchanger varied from 0.05–0.2 kPa depending on the air flow rate. This additional flow restriction resulted in an estimated 8% decrease in air flow rate but had negligible effect on energy consumption. the warm-up time and the space required by the heat recovery unit were insignificant factors for this dryer. the payback period of the heat recovery system was estimated as 4.1–5.4 years with a rate of return of 21–28%.