Consulting-Specifying Engineer - August 2007 - (Page A3)

stream, preheating it and cooling the blowdown water before it is discharged for treatment or disposal. Manufacturers of packaged blowdown heat recovery systems, including Cleaver Brooks, Madden Manufacturing, Penn Separator, Sentry Equipment and Spirax Sarco, offer systems for a wide range of boiler types and sizes. These units are designed to simultaneously accept multiple blowdown streams, cooling them well below the condensation point and recovering a high proportion of the thermal resource. Simpliﬁed diagram of a boiler blowdown heat recovery system. BUILT-UP SYSTEMS FOR LARGE BOILERS Large industrial boiler plants can install a built-up heat recovery system. An example of such a large-scale blowdown heat recovery system is in a newsprint mill operated by Abitibi Consolidated in Augusta, Ga. The mill already was capturing the waste heat from the blowdown ﬂash tank and using it for heating boiler make-up water. The system was expanded by adding heat recovery to the ﬂash tank drain, tripling the amount of energy captured. Together, the two heat recovery systems provide an annual energy savings of $117,000 by raising the temperature of the makeup water by 18°F. Paybacks for blowdown heat recovery are variable, but periods of 18 months to three years are common. SPRAY TOWERS OFFER MAJOR SAVINGS Two methods are commonly used for natural gas boiler exhaust gas heat recovery—spray towers and direct heat exchangers. With spray towers, the ﬂue gas is blown through a water spray that absorbs both sensible and latent heat. The collected spray water from the tower goes through a heat exchanger where it is used to preheat boiler makeup water. This circulating water then returns to the spray tower. The spray tower system is practical only with natural gas boilers because of the clean nature of the exhaust gas. This is the system type that was discussed in the Winter 2005 issue of Gas Technology. In that example, a system designed by Menex Inc. of Mississauga, Ontario, was installed at a Dial Corporation manufacturing plant in Montgomery, Ill. According to the owner, the installation cost about $600,000 and has the potential to save as much as $400,000 per year. CAPTURE HEAT GOING UP THE STACK A second opportunity to make signiﬁcant efﬁciency improvements is through stack heat recovery systems. The typical boiler extracts most of the heat of combustion but still may have exhaust stack discharge temperatures ranging from 350°F to 550°F. This exhaust has useful thermal content, both from sensible heat and the latent heat of water vapor from the combustion process. When fuel costs were low, this heat loss was seen as minor. Today it is a major target for boiler system energy improvement. both sensible and latent heat residual from the combustion process. Such systems are situated downstream from the boiler economizer and have the potential to improve the overall boiler system efﬁciency by as much as 10%. The cooling stream for the heat exchanger is usually boiler makeup water, but could be any other plant water stream where the heat is used beneﬁcially. One manufacturer of such systems is Combustion & Energy Systems Ltd. of Markham, Ontario. According to Cameron Veitch from Combustion & Energy Systems, the condensing heat recovery option is important because typically 11.5% of the total heat released by the fuel is lost to the atmosphere. He indicates that the company’s ConDex heat recovery system captures nearly all of this heat. FINNED TUBES FOR HEAT EXCHANGE The ConDex unit has ﬁnned tubes of a proprietary design on the heat exchanger that capture enough energy from the exhaust gas to condense out the water vapor from the exhaust, releasing its heat of vaporization. The phase change alone recovers approximately 1,000 Btu for each pound of water condensed. Veitch notes that in the ConDex design, the ﬂue gas and the heat exchange liquid are not in contact with USING STACK HEAT EXCHANGER TO CAPTURE HEAT The second type of exhaust gas heat recovery system is the direct condensing system, or condensing economizer. These are engineered systems that direct boiler exhaust gas across a heat exchanger to condense vapor and salvage A3

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - August 2007

Contents M/E Roundtable Giants 100 Pullout Poster Giants Stand Tall: CSE Giants 100 Report Electrical Design for Tall Buildings HVAC and Fire Safety for Elevator Systems How to Beat the Codes GAS Technology Supplement Recovering Waste Heat from Boilers New Applications for Tankless Water Heating Systmes Techniques Improve for Powder Coating MDF New Tools Available for Heat Treating Steel Direct Drive Engines

Consulting-Specifying Engineer - August 2007

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