Increasing Heat Transfer Efficiencies
Two stainless steel heat exchanger coils are used in the economizers for the boiler upgrades at Enwave’s district heating plant in Toronto. The horizontal manifold in front, with its input/output pipes, distributes water through the tubes and cooling fins behind the manifold.
Industrial and commercial boilers are one of the largest categories of equipment in this country - and one that needs to become more efficient and use less energy. They consume a whopping 37 percent of our energy, according to a May 2010 report from the International Energy Agency (IEA), an autonomous organization, headquartered in Paris, that provides research and energy development programs to 28 member countries.
The IEA also reports that 76 percent of the boilers are more than 30 years old, and about 50 percent have been converted to gas. Other industry experts say many boilers still in use date back to the 1960s and earlier. Even though conversion to natural gas has helped to make these aging boilers run cleaner and more efficiently, more must be done.
For instance, the installed base of industrial and commercial boilers can be made to operate even more efficiently by retrofitting them with better economizers and advanced control systems. Simply stated, an economizer is a heat exchanger coil that uses exhaust gases from the boiler to preheat the incoming cold water before it is fed to the boiler itself, thereby reducing energy consumption.
Unfortunately, many of those aging boilers use standard economizers that were appropriate when the equipment burned high sulfur, thick No. 6 diesel oil. However, standard economizers are not all that efficient. They were built with large-diameter tubes - so water temperatures would never reach less than 270°F (132°C) - to minimize condensation that creates sulfuric acid when mixed with sulfur dioxide from burning diesel fuel. They also needed large fin spacing to ease cleaning that was needed to prevent clogging from ash. In addition, standard economizers weigh in at 25,000 to 30,000 lb and stand 6' tall or more.
Smaller But Mighty
Workers install ductwork and pipes for the housing to contain the economizer, condensing coils and other components above one of the eight boilers at the Enwave plant.
Today’s more efficient economizer heat exchangers use smaller-diameter tubes, small and tightly spaced fins, lighter weight (coming in around 4,000 lb) and an overall smaller size (14" high). Because the economizers take up take up less space, older boilers can be retrofit with new economizers without moving walls or raising roofs.
Economizers like these are a critical part of advanced control systems such as those designed for boilers by Benz Air Engineering. The control system makes the boilers more energy efficient, lowers operating costs and reduces emissions, often without the prolonged downtime and expense of replacing the old burner. In recent years, these kinds of modifications have significantly improved boiler operations for several major California food processors, including Del Monte Foods, Pacific Coast Producers and Seneca Foods. (For more on Seneca Foods, see “Meeting Mandates,” Process Heating, January 2011, or search for it on www.process-heating.com.)
These same technologies also were used last year to upgrade eight massive boilers in a district heating plant operated by Enwave Energy Corp. in Toronto. Each boiler measures 45 by 12 by 14' and weighs about 65 tons. All employ a watertube design and have steam-generating capacity of 100,000 lb/hr at 390°F (199°C) and 200 psig. Combustion temperature of the flame inside the firebox is about 2,200°F (1,204°C) and flue gas leaving the boilers is 450 to 550°F (232 to 288°C).
Two of the boilers that were re-commissioned in February showed a remarkable 79 to 96 percent increase in combustion efficiency. This correlates to an increased boiler output from 100,000 to 120,000 lb/hr and saves 11.5 billion BTUs per boiler, if operated for a full 170-day heating season. The exhaust temperature from the boilers was reduced from 550 to 95°F (228 to 35°C) with no pre-cooling, and the plume from the plant’s 300' stack completely disappeared. Early results also showed NOX emissions fell from 130 ppm to less than 3 ppm and CO2 was reduced by an estimated 2,000 metric tons.
A round coil is used in a vacuum at G-M Enterprises to rapidly quench heat from the unit and the products inside.
Quenching Another Hot Problem
At another installation, an unusual, custom-designed round heat exchanger coil was used. Designed with 90 cooling fins/ft rather than the standard 50 cooling fins/ft, the exchanger was a breakthrough for G-M Enterprises, which manufactures high-tech vacuum furnaces at its plant in Corona, Calif.
The furnaces are used by companies that produce cell phone cases, eyeglass hinges, parts for the aerospace industry and other products, all from powdered exotic metals such as titanium, columbium and super alloys. Vacuum furnaces do not use oxygen because heating metals in a vacuum maintains the purity of the metal and prevents oxidation and imperfections. Instead, heat is transmitted to the metals by injecting an inert gas such as nitrogen, argon, helium or hydrogen into the chamber. Inert gases do not interact with powdered or solid metals.
The round coil fits inside the heating chamber of the company’s vacuum furnaces, which resemble a horizontal cylinder about the size of a large pickup truck. The furnace heats up to 3,000°F (1,649°C), followed by rapid super cooling, called “super quenching.” During quenching, water is forced through the round coil at 150 to 600 gal/min to rapidly cool down the oven and the products inside faster than conventional coils mounted outside the oven. The process, called sintering, causes powdered metal to form a coherent mass and harden without melting. Also, quenching is done in an energy-efficient, closed-loop system.
The closeup view shows the 90 fins/ft that enables the furnace to cool down quickly.
As the brief case histories demonstrate, economizers can help processors reuse what would otherwise be waste heat. Capturing this waste heat allows processors to squeeze as much out of every BTU they buy and run processes that are optimized for energy efficiency. PH