A Breakthrough in Air-Cooled Steam Condensers

By Jean-Pierre Libert, Vice President – Power, Product Development, EVAPCO, Inc., Westminster, U.S.A.

Developing an air-cooled steam condenser for thermoelectric power plants is a balancing act: The goal is to maximize heat transfer, minimize pressure drop, and constrain energy consumption and costs. Leading manufacturer EVAPCO, Inc., used ANSYS Fluent to design new heat exchanger fins for its air-cooled condenser — and their success led to the development of an entirely new heat exchanger configuration.

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air-cooled steam condenser

"ANSYS Fluent simulation delivered a 15% increase in thermal capacity and decreased cost significantly."

EVAPCO’s Arrowhead™ fin

EVAPCO’s Arrowhead™ fin

Nearly all the electricity produced in the United States comes from thermoelectric plants. Historically, these facilities have relied on a water-intensive process: Water is boiled to create steam, the steam spins the turbines to generate electricity, and cold water cools the steam back to its liquid form in a condenser for reuse. Most power plants originally sourced water from rivers or lakes for the bulk of their cooling, but as environmental regulations expanded, power plants added evaporative cooling towers, first to reduce the temperature of the water returned to the source, and later in closed loops to further reduce the environmental impact. However, even by switching to evaporative cooling, significant amounts of water are still consumed.

As global water demand grew over the past several decades, legislators responded with water conservation laws. As a result, the number of power plants that employ air instead of water as the coolant is increasing. These facilities condense steam using ambient air, meaning no water is consumed. Although dry cooling systems do not eliminate the use of water in power plants, they come close: It is estimated that they decrease total consumption by more than 98%.

But water savings come with a trade-off. Dry cooling systems require a larger upfront capital investment and typically require higher auxiliary power to operate.

To maximize the heat transfer capacity in its air-cooled steam condensers (ACC) while improving energy efficiency, EVAPCO, Inc., a leading manufacturer of evaporative and dry cooling products, used ANSYS Fluent to identify areas where it could improve the fin technology used in the heat exchanger tube bundles.

Modeling enabled a breakthrough that led to the development of the patent-pending Arrowhead™ fin design. The Arrowhead fin significantly improves heat transfer compared to current technology; it also limits the amount of energy needed to overcome pressure drop as air is forced through the heat exchanger.

A VORTEX MOVES AIR BETTER, WITHOUT INCREASING PRESSURE DROP

ACCs route turbine exhaust steam to an array of finned tube air exchangers where ambient air serves as a cooling fluid. Air is forced at high velocity between the fins to condense the fast-moving steam, which may be traveling at speeds of up to 125 meters per second inside the tubes.

The purpose of the fins is to extend the heat transfer surface to create more surface area for cooling. Fin designs that include ripples, corrugations and dimples further increase heat transfer but at the expense of greater pressure drop. While it is possible to compensate for pressure drop by increasing the fan motor power to push more air through the heat exchanger, this adds to the parasitic energy and operating costs of the installation.

ACC heat exchanger tubes and air-cooled condenser heat exchanger tubes

EVAPCO applied the successful concept of microchannels to the ACC heat exchanger tubes.

Engineers designing air-cooled condensers are faced with the challenge of maximizing heat transfer capacity, minimizing air-side pressure drop, and limiting energy consumption and capital costs. To achieve this balance, EVAPCO modeled hundreds of fin geometries in ANSYS Fluent to characterize heat transfer and pressure drop properties. Simulations focused on improving vortex generation: Vortices enhance heat transfer by promoting air mixing between the fins while minimizing increases in air-side pressure drop. After EVAPCO engineers discovered that a certain type of vortex moved the air more efficiently without increasing pressure drop, they used this knowledge to develop the Arrowhead fin technology.

Using ANSYS Fluent computational fluid dynamics software saved the company the time and expense of building and testing what could have been as many as 100 prototypes. More importantly, even if EVAPCO had done that much prototyping, without the knowledge gained from utilizing ANSYS Fluent its engineers still might not have had the same insight into air vortices or how to make the fin design more efficient.

"Employing ANSYS Fluent to maximize heat transfer while minimizing pressure drop, EVAPCO created a leading-edge heat transfer product for the global power generation industry."

SIGNIFICANT IMPROVEMENTS IN THERMAL CAPACITY, PLUS COST SAVINGS

After EVAPCO determined an efficient fin design, the company maximized its benefits by improving the entire heat exchanger system. Engineers applied the successful concept of microchannels from other heat transfer industries to the ACC heat exchanger tubes. By studying steam flow inside the tubes with ANSYS Fluent, EVAPCO engineers were able to develop modified tube geometries to optimize steam-side pressure drop.

Fluent allowed engineers to investigate internal steam flow fields inside the piping and header of the heat exchanger and provided new knowledge about inlet pressure drop losses, which varied in every tube. Fluent also enabled EVAPCO engineers to investigate flow-accelerated erosion and create internal structures to minimize its potential. That included ensuring good flow inside the steam header and avoiding choking the flow in the tubes. The information gained from this analysis was instrumental in creating the final header design.

The combination of the new Arrowhead fin technology with the improved tube geometry resulted in the nuCore™ heat exchanger. nuCore greatly improves heat transfer across a wide range of operating conditions to minimize the ACC surface area requirements and reduce material costs.

Using Fluent simulation as part of EVAPCO’s fin research and development and redesigning the heat exchanger configuration with the new fin and tube technology resulted in a significant increase in thermal capacity of at least 15%.

air-cooled condensers

Air-cooled steam condensers

UNPARALLELED SUPPORT LEADS TO A BETTER HEAT TRANSFER PRODUCT

Prior to choosing ANSYS Fluent, EVAPCO engineers had used a simpler computational fluid dynamics software, but they found it too limited to tackle the work of improving their heat exchanger tube fins. Going into this project, they evaluated three products, including ANSYS Fluent. In addition to looking at each simulation system’s technical strengths, they considered tech support, training and whether they could buy or rent the product — and concluded that nothing could compare to ANSYS’ support capability, including personalized training that helped them create some of their initial models with the highest accuracy and speed of calculation.

By employing ANSYS Fluent to maximize heat transfer while minimizing pressure drop, EVAPCO created a leading-edge heat transfer product for the global power generation industry — one that is more efficient, more compact and less costly to operate. It also helps meet the needs and requirements of a water-conscious world.

air-cooled steam condenser simulation

Simulations focused on improving vortex generation

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