Heat Exchanger Retrofit Cuts Costs
March 4, 2009
Four oxidizers that treated emissions from coating lines at a plant making insulation films for the electronics industry appeared deficient, raising concern that the process would not meet regulatory requirements. The plant needed an experienced engineering company to evaluate the problem and make effective repairs.
Anguil Environmental Systems Inc., Milwaukee, was hired to recommend ways to improve the oxidizers' performance. The company conducted a rigorous 75-point preventive maintenance evaluation and discovered minor problems with three of the units and a major problem with the largest oxidizer, a 13,500 scfm catalytic unit. Many of Anguil's cost-saving suggestions for the three smaller oxidizers were performed easily by the plant's in-house maintenance staff. The larger oxidizer, however, required more intensive application engineering and a comprehensive air pollution control solution.
The big unit uses two heat exchangers. The primary heat exchanger recovers energy from the combustion chamber's exhaust and transfers it to the incoming process airstream to reduce fuel consumption. The secondary heat exchanger recovers additional energy from the primary heat exchanger outlet flow and uses it to preheat an air source used in the process. In this case, the secondary heat exchanger is integral to the plant's coating process because it heats the tower dryer of the coating machine.
The plant engineers had seen a steady decline in available heat from the oxidizer and secondary heat exchanger. The oxidizer took up to six hours to sufficiently heat the coater before production could begin, burning natural gas estimated at $266 per startup.
Anguil's inspection revealed damage to the heat exchanger. A third of the front tubes had separated from the bottom plenum and deformed into the second row of tubes, effectively blocking about 25 percent of flow through the remainder of the exchanger. In several areas, airflow bypassed the exchanger, traveling in the gaps between the exchanger and the ceramic insulation and passing through tears in the tubesheet and top plenum. The tubesheet tears and bypass areas allowed solvent vapors to bypass the combustion chamber, reducing the oxidizer's destruction efficiency. Testing when the coated web used 100 percent toluene as a solvent showed that the oxidizer was achieving only 93.6 percent destruction efficiency and did not meet regulatory emission requirements.
Replacing the oxidizer would have drastically interrupted the customer's manufacturing schedule and resulted in capital cost increases that were unacceptable. Anguil eliminated production disruptions by performing repairs and design enhancements over several planned process downtimes.
The process air temperature required for the coating machine now is achieved in one hour. New efficiencies resulted in savings of approximately $230 per startup and up to $30,000 annually. The improved system has reduced plant downtime and increased worker production, as well as improved the oxidizers' destruction efficiency.