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FLEXO Magazine : March 2009
TECHNOLOGIES & TECHNIQUES · Expected hours of operation per year. · Current energy rates for the plant (gas or oil and electric). The first two items are often monitored already on a continu- ous basis in oxidizer data recorders. If that is not the case for a particular system, the most recent EPA (Environmental Protection Agency) stack testing data can be an excellent source for this information. Two other issues for consideration during this phase of an evaluation are: · Constituents in the exhaust gases (and especially their dew points). Any effort to reclaim energy in the exhaust stack of an oxidizer will lower the oxidizer exhaust gas temperature, bringing with it the potential for condensation of acids. Suppliers of energy recovery equipment will typically take care to ensure that final stack temperature is above any acid dew points. Given the typical solvent-laden exhaust from printing presses, this is rarely an issue of concern for oxi- dizer systems in the flexographic printing industry. . Current oxidizer main system fan capacity. Adding energy recovery equipment to an oxidizer exhaust stack will also come with a system back-pressure penalty. The existing oxidizer fan will usually be tasked with pushing or pulling air through the hot side' of the added heat recovery compo- nent. To keep overall project payback attractive, the goal is usually to choose energy recovery equipment that will limit the added system back-pressure to an amount that the exist- - ing oxidizer system fan can handle without major modifica- tion. Therefore, knowing the additional capacity available in the oxidizer system fan will help narrow down which cost- effective options for energy recovery are feasible. Challenge NO.1 for a typical flexographic printing applica- tion may look like this. Consider a flexographic printer with a 10-year old regenerative thermal oxidizer (RTO). The combined exhaust from all dryers and capture hoods routed to the RTO is 20,000SCFM at approximately 1500 F. The average exhaust tem- perature from the RTO is 275 0 F. Implementing Plan C, a 50-percent effective heat exchanger installed in the oxidizer exhaust stack to transfer the waste heat to air or fluid would drop the stack temperature by approximately 12SoF, capturing approximately 2.7MMBTU per hour. If this en- ergy was 100 percent useful inside the plant and the plant oper- ated around the clock, this could lead to a yearly savings of up to $225,000. A payback of one to two years is certainly possible for a proj ect of this nature. By comparison, in Plan A, reducing airflow to the RTO by 10 percent could save approximately 0.3MMBTU per hour, equiva- lent to $25,200 per year. This could likely be accomplished with very little capital investment at all. A payback of less than six months is possible for this option. Alternatively, for the data pre- sented, this RTO is operating with an internal thermal energy recovery (TER) of approximately 92 percent. Plan B would be in- stalling additional ceramic heat recovery media to raise the TER MARCH 2009 www.f I exog ra p hy.o rg FLEXO
Sustainable Winter 2009