Heat exchangers

Recuperative and regenerative are terms used to describe the heat exchangers in oxidation systems. This terminology is consistent with both the thermal and catalytic systems. Recuperative heat-recovery systems typically use a metal shell and tube or metal plate-type heat exchanger to recover energy used in the oxidation process. Regenerative type heat-recovery systems typically use ceramic media to collect and store energy. The ceramic media is contained in multiple towers or canisters, which are interconnected, by ducting and a valve system.
The valve system in regenerative designs directs the incoming exhaust stream between the various canisters of ceramic. By switching from one tower of ceramic to another (called 'cycling'), one ceramic bed will release its energy while the ceramic bed in the other tower absorbs energy. For both types of heat recovery systems, the recovered energy is used to preheat the process exhaust as it enters the oxidation system.
Oxidation systems using recuperative-type heat exchangers typically recover 40-80% of the oxidation process energy. Most system designs fall into the range of 60-70% recovery. Several factors impact the successful use of metal recuperative heat exchangers. These factors include process-exhaust temperatures, system operating-temperature requirements, temperature stratification within the unit (which relates to flow turndown), type and concentration of the VOCs treated, and the process operating cycle. All these factors affect the efficiency and life of the unit. Temperature limitations of the metals in heat exchangers along with stresses induced by changing process conditions can severely reduce unit life.
Regenerative heat-recovery systems are capable of recovering up to 97% of the energy used in the oxidation process. Most operate in an energy-recovery range of 85-95%. The ceramic media used in these systems are typically capable of continuous operating temperatures of 1,800-1,900 deg F.
High-temperature capabilities, along with the use of hot-gas bypass systems allow today's regenerative systems to operate effectively over a wide range of airflows with VOC concentrations of nearly zero to 25% of the LFL (lower flammability limit for the VOC).

By Fred HornCourtesy of MEGTEC Systems Originally published 2003