Metal Finishing Guide Book

2013

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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 recovery, 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 media causes heat transfer to occur between the gases and the bed. The heat storage 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 chamber 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 recovery and media regeneration. The nature of an RTOs heat recovery process requires it to have at least two beds of appropriate heat recovery media. In many applications, the additional step of purging a bed before reversing the flow through it from inlet to exhaust is necessary to maintain very high destruction efficiencies. This purge step creates the requirement for an additional (or odd number) chamber making the RTO more complicated and more expensive than a recuperative oxidizer. RTO systems can utilize more than two beds (operating in parallel) in order to be capable of handling larger air volumes. The primary advantage of an RTO is lower operating costs due to high heat recovery and low fuel consumption. Depending on the mass of media included in an RTO, heat recoveries of up to 95% are common. Because of their capability for high heat recovery, RTOs are often operated in an "auto-thermal" or self-sustaining mode, where the heat content of the VOCs being oxidized is enough to sustain the combustion chamber temperature at setpoint, requiring no external fuel input. RTOs are a well-proven technology, but are being called on to become more efficient than ever, to reduce operating costs to even lower levels than have traditionally been seen. That challenge has been met by developing improvements in heat transfer media, alternative oxidation technology and fuel usage optimization techniques. • Heat Transfer Media: Traditionally, the heat transfer beds of an RTO 652

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