Brazed Copper-Brass Finds Niche in Locomotive Applications
As OEMs around the world embrace brazed copper and brass for a variety of heat-exchanger applications, oneapplication, in which CuproBraze technology is being adopted at a rapid rate, is onboard heat exchangers for locomotives.
Railways enjoy a rich history and remain a major mode of transportation despite being overshadowed in the last century by trucking and airlines. Trains offer major advantages in energy efficiency for the transportation of passengers as well as freight. In 2009, the International Union of Railways (UIC) and the Community of European Railway and Infrastructure Companies (CER) jointly published a 40-page brochure titled “Railways and the Environment: Building on the Railways’ Environmental Strengths,” comparing railways to other modes of transportation such as road, air and waterways. By all measures, rail was shown to be the most energy efficient of the modes.
CuproBraze technology is uniquely suited for cooling systems in locomotives and has demonstrated its success in diesel-engine and all-electric locomotive applications, both of which require large onboard cooling systems. It is making a significant impact on railway operations around the world because heat exchangers built with this process provide excellent cooling efficiency and durability.
Locomotives historically have relied on heat exchangers made with copper plates and round brass tubes. Such plate-fin systems are extremely rugged and easy to clean. Yet serpentine fins, also known as corrugated fins, are much more efficient than plate fins for heat removal. When corrugated copper fins are brazed to brass tubes, the result is a heat exchanger that isnot only efficient but also extremely durable and easy to clean.
There are other advantages to using corrugated fins rather than plate fins. It is less labor intensive to build a heat exchanger with corrugated fins instead of plate fins. Plate-fin heat exchangers are not designed for anufacturability. Inserting tubes through holes in plates is difficult and, ultimately, the finished product lacks the cooling efficiency of a corrugated-fin design. The use of corrugated fins also allows for the possibility of down-gauging the thickness of the fins. In other words, corrugated fins can be thinner than plate fins.
When corrugated fins are used in locomotive radiators, rectangular (i.e., square wave) fins are recommended over ordinary triangular fins because the latter are prone to clogging, which tends to limit fin density, also thereby limiting the thermal performance triangular fins. On the other hand, when square-wave fins are used, the spaces between the fins are large enough for easy maintenance. Rectangular (square-wave) corrugated fins are much easier to clean and so are an excellent choice for locomotive radiators.
Copper fins are recommended over aluminum fins because the copper fins are much harder and consequently are more resistant to sand erosion and impingement by stones. The stiffness and durability of the copper fins facilitate washing by pressurized water. CuproBraze heat exchangers maintain their performance for especially long periods of time because the brazed joints between the copper fins and brass tubes are extremely strong.
CuproBraze technology is a breakthrough for locomotive radiators because it provides the unique combination of durability and performance. CuproBraze radiators with rectangular corrugated fins deliver reliability, cleanliness and long service life essential for locomotive applications.
CuproBraze technology is not only suitable for radiators but also for chargeair coolers, which could play a role in lowering diesel emissions. The U.S. EPA has targeted locomotives for stringent emissions reductions. On March 14, 2008, a regulation was signed to further restrict emissions from locomotives. The EPA Tier 4 standards for locomotives ultimately will apply to model years 2015 and beyond, limiting NOx to 1.3 g/bhp-hr and PM to 0.2 g/bhp-hr.
In addition to stringent Tier 3 and Tier 4 standards, the 2008 EPA legislation calls for tougher standards for remanufactured Tier 0 to Tier 2 locomotives. These new regulations apply to locomotives dating back to the 1973 model year. Since many locomotive diesel engines are remanufactured, the new regulations are having a significant impact on both the railroad industry and the environment.
In Europe, locomotives must adhere to off-road emissions standards. The Stage III/IV standards cover railroad locomotive engines as well as marine engines used for inland waterway vessels. In effect, the EU off-road standards place stringent restrictions on locomotive emissions comparable to the U.S. EPA emissions standards for locomotives.
For locomotive radiators and chargeair coolers, weight is much less of a consideration than durability and volume. Locomotives require huge onboard heat exchangers. In retrofit or upgrade applications, it is often necessary to squeeze more cooling capacity into a smaller space and the heat exchangers may need to operate at temperatures higher than those required in the past.
The increased durability of brazed copper and brass along with the increased cooling capacity of serpentinefin designs are gaining attention for CuproBraze technology. Major OEMs are already using CuproBraze technology in locomotive applications. Railways from India to Africa to North America and Asia are examining this technology or already upgrading their locomotives with CuproBraze heat exchangers.
In many cases, CuproBraze replaces older plate-fin heat exchanger technology. The CuproBraze brazing process provides rugged joints between the fins andtubes. These components are as durable as soldered plate-fin heat exchangers but more efficient and more compact.
Young Touchstone, a Wabtec Co. in Jackson, Tennessee, U.S.A., manufactures mammoth CuproBraze locomotive radiators with core face areas up to 3 m long by 1.5 m wide. According to Sales and Marketing Director Mike Sprenger, “In the past several years, OEMs have shown increasing interest in the benefits of this technology. We have successfully tested cooling systems with our customers and are now experiencing the initial waves of demand for CuproBraze.
CuproBraze technology is used to make radiators and charge-air coolers for the diesel engines, yet other locomotive cooling applications can benefitfrom its durability and cooling efficiency Bombardier, for example, recently installed brazed copper-and-brass oil coolers on refurbished electric locomotives that transport coal in South Africa.
The cooling experts in the Propulsion & Controls section of Bombardier Transportation Laboratory had been studying CuproBraze for some time. An opportunity to test the technology in real-world applications materialized when Bombardier began overhauling 45 Class 11E electric locomotives for South African Railways (SAR). The original design of the heat exchangers was inefficient and SAR had many problems with overheated locomotives. SAR asked during the refurbishment if the heat exchangers could be overhauled or replaced with a more efficient cooler.
“The oil cooler design was one of the weakest components in the locomotives,” said Jan Andersson, a consultant for Bombardier. “These heat exchangers were being made by a number of small private companies at great expense, yet they were not measuring up in terms of durability. The oil coolers frequently had to be overhauled, taking the locomotives out of service due to overheating.”
According to Andersson, the old design was based on round copperalloytubes with aluminum fins. The latter were not rugged enough for the operating environment. “It became a matter of pride with us,” says Andersson. “We did not want our refurbished system to be a problem for SAR. So we studied the problem closely and looked for alternative cooling systems.”
These particular locomotives must haul coal through harsh terrain. A single train may be 3.5 km in length and contain as many as 200 wagons plus four locomotives. Andersson explained that the heat exchangers are subject to three extreme conditions of service: clogging; shock and vibration; and high air humidity, salt water and dirt from the many sugar fields that the trains pass en route to the seaport.
Electric locomotives require large heat exchangers for cooling the electrical transformers and inverters that convert the high-voltage electricity from the transmission lines into the lower voltage, higher mperage electricity necessary to drive the trains. Although the electrical equipment is extremely efficient, a certain percentage of waste heat must continuously be removed. The CuproBraze heat exchanger in this application is rated for a cooling capacity of 38 kW for the converter plus 220 kW for the transformer. By comparison, the locomotive is rated at 3780 kW at the wheel rims.
The cooling system circulates a special type of oil that is used as a coolant. Transformer oil is usually a highly refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. The coolant passes through the electrical equipment and then through the huge oil cooler.
The two CuproBraze heat exchangers are 1.65 m in length, 0.35 m high and 0.76 m wide mounted on the same frame. The oil coolers have seven rows of tubes for converter cooling and fifteen rows of tubes for the transformer cooling mounted in one assembly. They are in close proximity to the transformerand converter, with the main transformer requiring the most cooling.
The new design of oil cooler has corrugated fins with spacing of eight fins per 25.4 mm for easy cleaning. Heat exchangers can be designed with a depth of many rows. Standard 19 mm tubeswith a wall thickness of 0.41 mm are used. Internal fins, or “turbulators,” are inserted inside the tubes to enhance the exchange of heat between the transformer oil and the tube walls.
CuproBraze is ideally suited for diesel and electric locomotive applications. Bombardier overhauled 45 Class E Locomotives for South African Railways, installing CuproBraze heat exchangers for cooling the transformer oil.
A prototype was completed and placed in a locomotive in service for comparison with the old-style oil coolers remaining in the rest of the fleet. Bombardier carefully studied the Cupro- Braze cores in this application for more than one year. The engineers concluded that the durability was clearly superior for the CuproBraze corescompared to the previous style of cores.
Astonishingly, the price quotes obtained for the initial cost of CuproBraze heat exchangers were lower than the initial cost of the previous style of cores. “At that point, the decision to use CuproBraze was easy,” said Andersson. “The cost savings was direct.”
It wasn’t long before oil coolers, plus spares, were ordered for 45 locomotives and three spare units. Bombardier expects to place additional orders for hundreds of locomotives in the next five years.
“We are looking to replace the oil coolers not only in South Africa but also in Sweden, Australia and the United States. Many locomotives are reaching 20 to 30 years of service, which is about the middle of their life cycle. They are ready for refurbishing,” he said.
The cooling experts at Bombardier are looking forward to using CuproBrazetechnology in new designs of original equipment. “We have many new designs on the drawing boards. The experience gained from installations of CuproBraze in the harsh environments of South Africa is good assurance that CuproBraze is durable enough for practically any application,” concluded Andersson.