For both existing and new process lines, a review of the process requirements is the starting point in optimizing energy use. With this information, minimum exhaust rates can be determined. Most dryers/ovens supplied today incorporate exhaust gas recirculation systems, which allow the recirculation or reuse of exhaust gases back to the supply side of the dryer. The recirculation of heated process-exhaust gas back to the supply provides an immediate reduction in operating costs of both the dryer and oxidizer.
Exhaust rates for solvent-based applications will be dictated by the need to maintain a safe LFL concentration in the dryer. The use of lower flammable limit monitors can allow high dryer-exhaust recirculation rates that minimize the overall exhaust from the system, reducing energy demand. However, one has to balance production needs and safety with operating cost savings.
For many applications, overall emission control requirements of 95% to 98% are standard by regulation. In order to reach these efficiencies, permanent total enclosures or temporary total enclosures are used. Using the exhaust air from the enclosure as make-up air for the dryer should be considered. This can significantly decrease the amount of air going to the oxidizer and the amount of air needed from the plant make-up air system. Capturing the fumes closer to the source should also be considered.
Oxidation systems used for control of volatile organic compounds (VOCs) in exhaust air streams are available in a wide variety of designs and configurations. It is important to remember that the end result expected from all of these systems is a 98% or higher destruction of the VOC from the process. The design variations available to the marketplace exist to address differences in process conditions, operating schedules, installation space requirements and operating costs.
Oxidizers on the market today can provide up to 97% primary heat recovery. However, that is not the most cost effective approach for all applications. The process parameters need to be reviewed and from them the best oxidizer choice can be made.
SidebarMany options are available for secondary heat recovery from an oxidizer stack. The best options incorporate using the heat back at the process. This is done by using hot oil, steam, water or air-to-air heat exchangers.
Secondary Heat Recovery
The process conditions need to be considered before the best choice can be determined. Building heat, cooling, wash-water heating, electrical production and steam are some other considerations for secondary heat recovery.
Another option is to bring in the equipment manufacturer to do an engineering study to determine the best approach for your particular process. In many cases, the equipment manufacturer has a lot of past experience in your industry and can provide valuable information in regards to the best approach to heat recovery and overall process savings.