Measured performance of a direct-contact heat recovery system installed at a commercial bakery

Many industrial processes can benefit from heat recovery. This paper presents measured results from a direct-contact heat recovery system installed at a large commercial bakery in Brooklyn, NY. The wet-spray, direct-contact system recovers heat from the exhaust stack of a gas-fired commercial baking oven. Heat is transferred to the water via mass and heat exchange in a packed tower (or percotherm). The hot water from the percotherm is used to preheat makeup air supplied to the facility as well as to preheat makeup water for a boiler feedwater system. Detailed energy use and performance data were collected at 15-minute intervals for nearly one year to quantify system performance. The unit's programmable controller was used to collect the monitored data. Sufficient data were collected to predict annual operating trends and extend the analysis to other locations around New York State. Utility rates were also used to extend the economic analysis from this site to other cities around the state. The results showed that the annual value of the recovered heat ranged from $4,500 to $8,000, which was much less than the expected savings. The modest savings occurred because of the unexpectedly light loads on the steam boiler. Parasitic electric use by pumps and fans, which increased costs by $1,800 annually, also diminished the net savings. This result illustrates the uncertainty with projecting production-driven loads on equipment in industrial settings. The cost effectiveness of these systems often depends strongly on future projections of energy use and production capacity.