Featured Story – Freedom! 3D printing will revolutionize heat exchanger design and manufacture

Heat exchanger designs

Until recently, heat exchanger manufacture followed a relatively predictable pattern. The two basic designs are shell-and-tube (S&T) and plate (plate-and-frame, or PHE); in addition, there are multiple subtypes of each, and also hybrid designs. S&T is by far the most common design: a tube or tube bundle is enclosed within a closed, cylindrical pressure vessel (shell). One fluid flows through the tube, the other around and between the tubes, within the shell. This classic design is the tried-and-tested solution for most applications in the energy and process industries, especially where high pressure prevails. Common applications and industries include oil refining, preheating, oil cooking/ cooling, heat recovery and steam generation.

PHEs consist of a bundle of thin, corrugated plates divided into pairs that are bolted, brazed or welded together. Each pair creates a channel through which one fluid flows, and between the pairs a second passage is created through which the other fluid flows. One important variant is the plate fin HE, where fins or spacers are inserted between the plates to allow more flow configurations and the flow of more than two fluid streams.

This compact design is ideal when space is limited. It is better suited to low and medium pressures, where the counter-current flow can lead to energy savings. Common applications are cryogenic, furnace, and food and chemical processing. In recent years the market share of PHEs has increased compared with the S&T design.

Improvements

Manufacturers have improved or modified these basic designs to make them more efficient. The S&T type underwent considerable modification in helical coil and spiral HEs, which allow economies of space. For example, the air-cooled HE (ACHE – also known as the fin fan HE) is much more complex than the S&T type. It was developed to allow ambient air, rather than water, to be used as the coolant. Concerns about water shortages have led to increased demand for this type. They are common in refining and chemical processing and in power generation, especially in the dry cooling towers of concentrated solar power stations. They consist, at the very least, of several tube bundles, usually made of aluminium, an air-moving device such as a fan and a plenum (enclosed space) to separate the bundles from the fan. Typically they are installed above ground so as to leave room for air flow and to save space. An especially important development is a hybrid combining features of the plate and shell-and-tube designs: the plate-and-shell HE (PSHX). In this design a plate pack is enclosed in an outer shell, so that the two flow paths are within both the plate pack and the shell. They are more leak-proof than other designs and are built to withstand higher pressures, higher heat transfer and higher operating temperatures. By dispensing with gaskets, baffles and supports the PSHX cuts down on failure rates and combines the best features of both designs: the compactness of PHEs and the high performance of S&Ts.

3D printing

Whatever improvements have been made in recent years, they are small compared with the giant leap forward in manufacturing technique that is unfolding thanks to 3D printing. By eliminating time-consuming, leak-prone designs dependent on welding and brazing, manufacturers can come up with much stronger, lighter products, and at the same time develop non-conventional designs tailor-made for the process in question.

When HiETA was founded in the UK in 2011, little work had been done on the AM manufacture of HEs. Since then the company has been partnering with Renishaw to 3D-print components on the Renishaw AM250 system. The company produced a cuboid heat exchanger (recuperator) for electric vehicles, then one in annular form that could be wrapped around other components to make the overall system more compact. Later the addition of the Renishaw RenAM 500M, a laser powder-bed fusion AM system, cut manufacturing times and production costs dramatically.

GE, which has already made great strides with 3D printing of aircraft parts, has 3D-printed an ultra-efficient, low-emission heat exchanger for power generation equipment, the fruit of a collaboration with the University of Maryland and Oak Ridge National Laboratory (ORNL). The HE, made of a nickel superalloy designed by GE specifically for AM, is intended to operate at 250°C degrees higher than today’s HEs: at 900°C and at pressures up to 250 bar. This is merely a first step in coming up with more intricate geometries that will deliver higher efficiency and lower emissions.

Ohio State University is using AM to develop new HEs for supercritical power cycles in power generation. It is adopting two techniques which together should have the cost of fabrication: one technique will multiply deposition rates 100 times, while another enables crack-free additive manufacturing of an advanced nickel-based superalloy. These innovations will enable HEs to operate above 800°C and 80 bar.

In aerospace, 3D printing is being used to develop HEs that can evacuate heat from today’s ever-hotter aircraft engines. United Technologies is using AM to build HEs that are more compact and efficient. Departing from the traditional rectilinear or tube-shell designs, the manufacturer came up with a design that, in the words of Venkat Vedula, is ‘truly organic’, ‘conformal’ (i.e. its angles are preserved when the shape is bent) and able to fit into tight spaces (3). Ni-based alloys (Inconel 635 and 718) and aluminium alloys were used in a powerbed fusion process that permitted the manufacture of ultra-thin fins without using support structures.

CEEE has announced that, in collaboration with Oak Ridge National Laboratory, it has used AM to develop a HE that is 20% more efficient, in a shorter time, with significantly lower weight. The new HE will be used in HVAC and refrigeration applications. 3D Systems’ direct metal printing service was used. This enabled CEEE to prototype a design with non-conventional, variable shapes that are not possible to manufacture using traditional forming techniques such as extrusion or stamping. This allowed droplet-shaped tubes and ultra-thin titanium walls allowing more efficient heat transfer without compromising strength. The HE was manufactured in one piece, obviating the need for time-consuming procedures such as brazing or welding.

Conflux is another company (based in Australia) that has developed a HE design that can be made only with AM. Using equipment made by EOS, as well as sophisticated flow visualization software, Conflux designed and manufactured a HE with internal geometries that radically increased the surface area in a given volume, thereby tripling the heat rejection. Pressure drop was also reduced and volume and weight were reduced.

Conclusion

These are just some of the ways that HE manufacturers are – to quote Vedula again – ‘throwing out the rule book’. In the years to come we may look forward to more creativity in the development of HE configurations that are even more efficient and sustainable.