Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 238 of 331

lution control device is 50 PPM by volume calculated as carbon with no dilution. • The owner/operator demonstrates that the system collects at least 90% by weight of the emissions generated by the sources of the emissions. Other examples of minimum required or allowable VOC and HAP collection and destruction efficiencies can be found in various federal, state, and local reg- ulations. In many cases the most advantageous type of oxidizer can be selected based on the following general guidelines. In other cases two or more oxidizer types may be practical and a detailed economic analysis based upon your specific costs of fuel and electricity will be required to determine the best selection. Recuperative Oxidizers A recuperative oxidizer is a direct-fired unit that employs integral primary heat recovery. To minimize the energy consumption of the oxidizer, the hot air exit- ing the combustion chamber is passed over an air-to-air heat exchanger. The heat recovered is used to preheat the incoming pollutant laden air. The prima- ry heat exchangers are usually supplied as either a plate-type or a shell and tube type heat exchanger. These heat exchangers can be designed for various heat transfer efficiencies, but the nominal maximum is 70%. Thus by the addition of a heat exchanger, the net heat load on the burner can be reduced by up to 70% of that required in a DFTO. The addition of the heat exchanger, because it is made of heat corrosion resistant alloy, substantially increases the cost of the oxidizer system. Also, the fan for moving the polluted gas through the oxidizer must be more powerful to overcome the additional pressure drop of the heat exchanger. In most cases, the savings in fuel will more than offset the additional up-front cost within the first two years of operation, however, even with 70% heat recov- ery, recuperative oxidizers can be expensive to operate, especially if the airflow is large and has dilute concentration levels, unless additional secondary heat recovery can be applied to the customer's process. Regenerative Thermal Oxidizers (RTOS) A regenerative oxidizer is also a direct-fired oxidizer that employs integral primary heat recovery. However, the RTO operates is periodic, repetitive cycle rather than a steady state mode. Instead of conventional heat exchangers which indirectly transfer heat from hot side to cold side across the exchanger walls, RTOs use a store and release mechanism. The hot gases exiting the combustion chamber of an RTO are made to pass over a bed of inert and temperature tolerant media with a high heat capacity. The temperature difference between the gas and the me- dia causes heat transfer to occur between the gases and the bed. The heat stor- age media is either a granular or structured form of heat resistant ceramic. Once the bed has been saturated with heat, the air flow is reversed and redirected by a valve mechanism. Reversed flow allows the cooler process air to pass over the hot bed, and hence become preheated before entering the combustion cham- ber where the remaining heat is provided by a burner. The hot gas is redirected to a cold bed (one that just completed being an inlet bed) and "regenerates the bed, making it hot and ready for the next pre-heat cycle. In other words, one bed (or chamber) is used as a heat source and one is used as a heat sink. The flow through an RTO must be frequently reversed in order to maximize heat 237

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