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FLEXO Magazine : March 2009
TECHNOLOGIES & TECHNIQUES AIR TO FLUID Air-to-fluid heat exchangers are the second most common ener- gy recovery product for oxidizer stacks. As the name implies, heat is transferred from the hot oxidizer exhaust air (again the hot side air stream) to a circulating fluid (the cold side stream). This is typically accomplished by passing the hot air over a coil containing the fluid to be heated. As with air-to-air heat recovery, there are a variety of low-backpressure designs that can allow installation into an oxi- dizer exhaust stack without adversely affecting the oxidizer system. Because piping is less expensive than ducting, air-to-fluid heat recovery has a definite advantage over air-to-air heat recovery when considering Challenge NO.2. However, unless the heated fluid is used directly back in the process that the oxidizer is con- nected to, Challenge NO.3, employing the recovered energy ef- fectively inside the plant facility, can be more difficult to address with air-to-fluid heat recovery. Meeting this challenge requires a detailed analysis of the demands for energy in the fluid system verses the availability of waste heat in the oxidizer stack. For ex- ample, in some plants the biggest hot water demands come in shutdown situations when the oxidizer is not running. For typical flexographic printing applications, fluid circulation between a coil in the oxidizer exhaust stack and a coil in the sup- ply air ductwork for press between color and overhead tunnel dryers is a setup that deserves consideration. AIR TO STEAM When the solvent laden air sent to an oxidizer system is suf- ficiently rich, the oxidizer's internal heat recovery component may need to be partially bypassed or forgone completely. This leads to higher than normal oxidizer stack temperatures and allows for additional options in heat recovery equipment. One such option is a waste heat recovery boiler to recover oxidizer exhaust stack heat and produce steam. Waste heat recovery boilers are custom sized for a particular exhaust gas capacity as well as required steam pres- sure. A variety of systems are available in vertical, horizontal, single or multipass configurations. Oxidizers in the flexographic printing industry rarely have the solvent loading-and corresponding ex- haust stack temperatures-necessary to sustain this option. HEAT TO POWER Sometimes referred to as cogeneration, heat-to-power is an emerging technology that is capable of sending kilowatts directly back into a facility for electrical power. The concept has been implemented on different applications throughout the world but is only now being integrated with combustion devices such as oxidiz- ers. Heat-to-power systems can currently generate up to 100kw per hour from a modest heat source. However, the payback is normally greater than three years, the value most companies use for accept- able capital investment. As electricity costs increase and greater efficiencies are achieved with the technology it will be a very attrac- tive option in the near future. Today, heat-to-power is not necessar- ilya cost-reduction strategy but rather a green initiative that could be used to promote a company as a leader in energy conservation. For the flexographic printing industry, oxidizer stacks represent a significant opportunity for the reclamation of energy. This ap- plies to all oxidizer systems, including both the aging catalytic oxi- dizers popular in the industry years ago as well as the newer, high- - How air flows and heat is transferred through a regenerative thermal oxidizer. Oxidizer with secondary heat recovery system. efficiency regenerative thermal oxidizers (RTOs) being supplied today. Achieving a cost -effective installation of energy recovery equipment with an attractive payback is not without challenges, but those challenges are being met today in a variety of ways. . ABOUT THE AUTHOR: Mike Scholz is a senior applications engineer at Anguil Environmental Systems Inc., Milwaukee, WI, a company that designs and installs new oxidizer systems, services and upgrades existing oxidizer systems, and offers a .fùllline of energy re- covery products and retrofits. For more information from Anguil, call 414-365-6400 or contact Scholz at email@example.com. MARCH 2009 www.flexography.org FLEXO
Sustainable Winter 2009