The Ins and Outs of Heat Exchangers

Capturing and reusing waste heat in biomass plants is paramount to efficient, effective operations.
By Ron Kotrba | October 21, 2016

Heat exchangers play a vital role in the operational efficiency and effectiveness at every processing facility in which heat is essential, whether it is an oil refinery or a biomass power plant. As energy efficiency importance grows, the role of heat exchangers will become even greater, their technologies more advanced. 

The idea is simple, but the system designs and inner workings of heat exchange are magnificently brilliant. Heat is generated for and in various industrial processes, and were it not for heat exchangers, massive amounts of thermal energy would be wasted. For instance, boilers combust fuels such as biomass to generate heat to produce steam for powering a turbine in producing electricity. Once that steam does its job, it still has a lot of thermal energy, some of which can be transferred via a vapor-to-liquid shell-and-tube heat exchanger to raise the boiler feedwater temperature. This serves multiple purposes, including cooling down the steam to avoid thermal shock on the boiler’s steam drum, and reducing the heat load needed to raise the temperature of the boiler feedwater. This is one of many heat exchanger applications at Koda Energy, a biomass combined-heat-and-power (CHP) plant in Shakopee, Minnesota, says Stacy Cook, general manager.

Koda Energy, a partnership between Rahr Malting Co. and Shakopee Mdewakanton Sioux Community, began commercial operations in 2009 and produces 500 megawatt-hours of electricity and 2,500 Btu of thermal energy daily from agriculture waste and wood chips. The plant has only the world’s second power boiler to use wall-mounted burners to combust biomass in suspension, meaning airborne burning of biomass. About 25 percent of Koda Energy’s total electrical output is provided to Rahr Malting, slightly more than 50 percent is sold to Xcel Energy (i.e., the grid), and the remainder sustains operations at the CHP plant. And 100 percent of its thermal output is provided to Rahr Malting. The electricity and energy is used to malt barley for beer at the largest site producer of malted barley in the world.

Common sources of waste heat in industry, according to Kevin McGinnis, sales director for heavy industries and mining at Kelvion—the successor to Germany-based GEA Heat Exchangers Group—include turbines, dryers, kilns, incinerators, boilers and flue gas from heaters or burners, to name a few. “In general, waste heat can be captured with various types of heat exchangers and used for various applications,” McGinnis says, “including combustion air preheating; heating a facility, plant, building or home; boiler feedwater preheating; heating a process fluid for another application; or even for the ORC process.” ORC stands for organic Rankine cycle, the thermodynamic cycle using water as a working fluid that provides 85 percent of the world’s electricity production, according to Turboden.

Numerous heat exchanger companies exist, many specializing in particular types. Kelvion offers its customers one of the world’s largest product portfolios in the field of heat exchangers, McGinnis says. “It includes individual solutions for practically all conceivable applications and complex environmental conditions, including plate, shell-and-tube, finned-tube and refrigeration heat exchangers, and modular cooling tower systems.”

Another big name in heat exchange is Alfa Laval. Wes Crozier, product manager, has been with the Sweden-based company for nearly 30 years in various roles. “In the area of heat transfer, we have gasketed plate heat exchangers for liquid-to-liquid and vapor-to-liquid, air coolers for air-to-liquid, fully welded heat exchangers for high pressures and temperatures, a specialty spiral heat exchanger for extreme fouling services, as well as more niche products,” Crozier says.

Netherlands-based HeatMatrix developed what Paul van Dillen, director of global sales and marketing, calls a new generation air-preheater that enables heat recovery from corrosive or fouling gas streams from biomass boilers, refinery furnaces, heaters, ovens or dryers. “This innovative heat exchanger consists of lightweight, corrosion resistant polymer modules, which can be applied at high temperatures and is ideally suited for waste heat recovery from flue gases that are corrosive or have fouling in it,” van Dillen says. HeatMatrix only designs and supplies air-to-air heat exchangers, often used to preheat air from waste heat flue gas streams.

At Koda Energy, another shell-and-tube heat exchanger, called a surface condenser, condenses steam flow on the turbine exhaust, Cook says. “The exhaust enters the condenser and puts a vacuum on the exhaust to allow the turbine to run more efficiently,” he says. In addition, a boiler economizer uses combustion gases leaving the boiler to further elevate the feedwater temperature before entering the steam drum on the boiler. “In general, the economizer adds 10 percent efficiency,” Cook says. “We’re taking every opportunity to capture heat.”

Kelvion manufactures a welded plate heat exchanger that can be used as a small vacuum condenser or traditional fin/fan technology to serve as an air-cooled condenser. “With our global footprint, we manufacture steam surface condensers and feedwater heaters, both of which are used in the steam cycle,” McGinnis says. “With waste heat generating steam for a steam turbine, Kelvion has all the exchangers in the steam process.”  

Alfa Laval’s Niagara Wet Surface Air Cooler can also be used to condense the vacuum exhaust steam from the turbine. “The ability to efficiently condense the turbine exhaust steam at low absolute pressures allows for more power generation,” Crozier says. “This is accomplished by a single approach to the ambient wet bulb temperature for cooling.”

The prime heat mover at Koda is a very large plate-and-frame heat exchanger. This uses propylene glycol as thermal fluid to transfer heat via piping to Rahr Malting. Then, a similar but smaller unit takes even more leftover Btu from condensate from the steamed glycol heater to gain another 3 percent efficiency. “This was sized to capture the available Btu that were being wasted,” Cook says. “We’re capturing those and putting them back into glycol and useful sales.”

Crozier says plate heat exchangers can have a heat transfer coefficient up to four times greater than shell and tube. This, along with its construction, means the footprint can be just 20 percent needed for a shell and tube. “The size advantage is achieved through embossing plates with carefully designed patterns, many of which are patented,” Crozier says. “This embossing creates channels for the fluids to ensure maximum turbulence. This results in maximum efficiency in transferring heat from one medium to the other. In addition, many of our designs are fully counter current, which allow temperature crosses where the outlet temperature of the hot side can be cooler than the outlet temperature of the cold side—very difficult to do with other types of heat exchangers.”

Higher temperatures and pressures are being achieved in compact plate heat exchangers, McGinnis says, through new welding technology Kelvion uses, offering alternate designs to traditional shell-and-tube solutions. “Efficiency gains are realized through more effective channel geometries with built-in enhancements,” McGinnis says.

Additional plate-and-frame heat exchangers are utilized at Koda for cooling applications, mostly for equipment health. Pumps handle high-temperature liquids, and a portion of these liquids is used in a mechanical shaft seal, so heat is shed to the cooling tower before reaching the pumps so as to not burn up the seals. Another application is to cool the turbine lube oil using circulating water from the cooling tower. “This keeps the turbine lube oil at an acceptable temperature,” Cook says. “We’re running 900-pound steam through the turbine and we need to keep the shaft lubed. If the oil heats up to 900 degrees, it will decompose.” McGinnis says Kelvion has a specialty for cooling the rotating equipment associated with waste heat to energy, manufacturing a range of extended surface shell and tubes and compact plate heat exchangers.

Koda also employs a liquid-to-air, radiant air heater using glycol heat to preheat combustion air to avoid acid dew-point corrosion. “If the air temperature drops too low prior to the tubular air heater, it will condense out acid gases on the flue gas side,” Cook says. The plant also uses an air-to-air heat exchanger, called a tubular air heater, which captures waste heat in flue gas. Van Dillen says the heat transfer properties of HeatMatrix’s air-to-air polymer technology, used to capture flue gas heat, is very good in comparison to other, more conventional technologies because of its compactness, thin tube wall, strong honeycomb structure, and 100 percent counter flow.

Kelvion’s finned-tube heat exchanger options for air-to-air or air-to-liquid applications include two major types—compact/plate fin coils and spiral-finned tubes—but also finned elliptical tubes. “A specific difference we offer on our spiral fins is our patented ‘groovy’ fin for our spiral-finned tubes,” McGinnis says. “The ‘groovy’ fin enhances heat transfer by turbulating the air more than a regular spiral fin can, which results in better heat transfer and a smaller coil size, making it ideal for replacement coils or in situations where a smaller footprint is needed.”

Just like the medium flowing through heat exchangers, technology advancements in the field continue to move the needle. Van Dillen says he believes HeatMatrix’s polymer bundle technology will be a game changer in the industry. While the company currently only manufactures air-to-air exchangers, he says air-to-liquid polymer designs are now under development. “This will be the next focus for HeatMatrix in this field,” van Dillen says.

Alfa Laval’s latest patented design, the CurveFlow, is for asymmetrical ports and features a new fluid distribution pattern that Crozier says lowers pressure drop, or increases efficiency with the same pressure drop. “We have also introduced a new method to secure gaskets to plates without glue being required,” he says. The company continues to develop new patterns used on various plates to increase efficiency vs. pressure drop, reduction of material thickness while maintaining pressure capabilities, applying new construction materials for plates and gaskets, as well as special unit types for specific applications. “Unlike the traditional shell-and-tube ‘one design type fits all,’ the Alfa Laval frames have been drastically improved,” Crozier says, “with better ways to open and close the units, studded ports that are in-place rather than nozzles, and bearing boxes that reduce resistance during tightening of bolts.”

McGinnis says in finned-tube technology, Kelvion continues to produce advancements in dry-air cooling in both fin- and tube-side augmentation, fan efficiency increases, and reduced noise footprint. “The future of dry-air-cooled heat exchange is small, incremental improvements that benefit everyone involved,” McGinnis says. “The future of heat exchange is using greater thermal efficiencies through more efficient use of pressure drop, flow distribution, and effective use of proper channel hydraulic diameters based upon application suitability.”

Author: Ron Kotrba
Senior Editor, Biomass Magazine