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FLEXO Magazine : March 2009
TECHNOLOGIES & TECHNIQUES to 95 percent could save approximately 1.0MMBTU per hour or up to $84,000 per year. A payback of less than one year is possible for this option. DELIVERING THE ENERGY As seen in Challenge NO.1, sizing energy recovery equipment and estimating the overall savings opportunity with oxidizer stack energy recovery are not difficult tasks. To take an opportunity analysis and turn it into an actual payback period however, one has to determine the cost of installing the equipment and provid- ing the infrastructure for delivering captured energy back to the plant. For a cursory analysis, some will take the cost of the energy recovery equipment and double it, calling that the estimated cost of installation (i.e. Total Estimated Cost = One Part Equipment Cost + Two Parts Installation Cost). This can provide for a quick check of whether a particular idea merits additional investigation. To obtain true payback numbers, a site visit by different trades people to estimate the overall cost of energy recovery system installation will be necessary. Fans and/or pumps, control valves, thermocouples, etc. will all need to be both mechanically installed and electrically wired to an existing or new control system. This is often the challenge where the overall project feasibility hangs in the balance. EMPLOYING THE ENERGY The final challenge is also extremely important for optimizing energy recovery project payback. Ideally, the oxidizer end-user should look for ways in which recovered stack energy can be used in the same process that the oxidizer is connected to. This typi- cally provides the best payback because there are energy demands by that process at nearly all times that oxidizer waste heat is avail- able. In contrast, projects focused on recovering oxidizer exhaust stack energy to help heat a facility, for example, may only be use- ful for part of the year. RECOVERY OPTIONS Oxidizer stack heat has been recovered to perform a wide variety of functions in the plant environment. Air-to-air heat ex- changers have been used to provide pre-warmed fresh air back to process ovens, dryers and/or plant make up air units. Air-to-fluid heat exchangers have been used to transfer oxidizer stack heat to boiler feed water, plant makeup water, process water, glycol and other thermal fluid loops. In extreme cases, waste heat boilers have been used with oxidizer stack heat to create steam. And on the horizon, heat-to-power systems are in development for re- claiming oxidizer stack heat and creating electricity. One additional option that has been used sparingly is tak- ing hot oxidizer stack air directly back for use in production processes. This is sometimes referred to as direct heat recovery, while the options mentioned above would be termed indirect heat recovery. Direct heat recovery from oxidizer stacks is gener- ally shied away from due to the risks of introducing products of incomplete combustion back into a plant environment or the risk of oxidizer oven dirt contaminating product, but there are limited cases where this form of oxidizer stack energy recovery has been used effectively. Each of these options for recovering heat from oxidizer exhaust stacks can be considered within the framework of the three chal- lenges discussed previously. AIR TO AIR Probably the most common energy recovery product applied to oxidizer stacks-especially in the flexographic printing industry- is an air-to-air heat exchanger. Be it a shell-and-tube or plate type heat exchanger, there is a cold side air stream (typically fresh air) and a hot side air stream (typically the oxidizer exhaust) that are used for heat transfer. Air-to-air heat exchangers have been integral to oxidizers themselves for decades, so it is a well-known technology for oxi- dizer manufacturers to incorporate into an overall system. The programs for sizing air-to-air heat exchangers are quick and easy to use. There are a wide variety of footprints and physical layouts for ease of installation. There are also many low-backpressure models that work well with existing oxidizer system fans. The limiting factor for air-to-air heat recovery in oxidizer ex- haust stacks is Challenge NO.2, delivering the energy back into the plant facility cost effectively. With air-to-air heat recovery, insulated ductwork is required to transport captured heat back into the facility. Costs for running ductwork in a plant vary widely and can also add up very quickly. The best applications are those with short duct runs for returning heated air. Maximizing internal thermal energy recover. Metal heat exchanger replacement. MARCH 2009 - www.flexography.org FLEXO
Sustainable Winter 2009