Printed circuit heat exchangers (PCHE) are a relatively recent option for process heat transfer. They get their name from the chemical milling procedure used in their manufacture, which is the same process employed for printed circuit boards. Introduced into the market beginning in the late 1980s, these compact exchangers feature flat metal sheets with flow channels chemically etched into them. The plates then are stacked and diffusion bonded to form a solid metal block containing precisely engineered flow passages. The channels typically are 2-mm semicircular in cross section. The small channel size fosters exchanger compactness.
Fluids can move in parallel-, counter-, cross-flow, or a combination of these to suit the requirements of the particular service. The operating temperature and pressure drop constraints for the given duty govern the required configuration of the channels on the plates for each fluid set. The channels can be of unlimited variety and complexity.
PCHEs can operate at a wide range of temperatures from cryogenic (say, -196°C) to above 900°C. They can handle a broad variety of clean fluids and find use in many different processing applications requiring high efficiency and performance.
PCHEs boast thermal efficiency on the order of 98%. Their high heat-transfer surface area per unit volume results in reduced weight, space and supporting structure. As a rough indication, PCHEs are four to six times smaller and lighter than conventional exchangers such as shell-and-tube units. In some cases, the reduction in size and weight compared to a shell-and-tube exchanger has exceeded 70%. For instance, a PCHE weighing 11 metric tons replaced a shell-and-tube exchanger weighing more than 70 metric tons.
The lighter and more compact exchanger greatly decreases costs for foundations, support structures, piping works, installation, operation and maintenance. In addition, the PCHEs’ far lower fluid inventory than conventional exchangers can offer many safety and reliability advantages. For instance, it enables substantially smaller sizes of pressure relief devices and relief discharge piping compared to those of an equivalent shell-and-tube exchanger with far larger inventory of fluids.
The channels of PCHEs are optimized for the flow pattern and thermal duty. This results in a very efficient and high-performance heat transfer. Another advantage is the reduction in energy waste. PCHEs can be used for gases, liquids and two-phase flows. Approach temperatures as close as 2°C typically are achievable within a single counter-flow exchanger, thereby avoiding the need for multiple exchangers or shells.
The fabrication technique — diffusion welding, which also is called diffusion bonding — used for PCHEs gives a high integrity homogenous heat exchanger. Diffusion welding involves the migration of atoms across the joint due to concentration gradients. The two materials are pressed together at an elevated temperature usually between 50% and 75% of the melting point. The pressure is used to relieve the void that may occur due to the different surface topographies. In other words, this special welding process promotes grain growth between the surfaces. Under carefully controlled conditions, diffusion-bonded joints reach parent metal strength. Stacks of plates are converted into solid blocks containing the fluid flow passages. No gaskets or sealing are needed inside a PCHE. The risk of leaks is approximately two orders of magnitude lower than for any other conventional heat exchanger. This is a great operational, reliability and maintenance advantage.
PCHEs can be made from many materials, including different grades of stainless steels, alloy steels, titanium alloys and austenitic nickel-chromium-based superalloys, etc.; some of these alloys are oxidation- and corrosion-resistant materials well suited for service in extreme environments subjected to high pressure and extreme temperature. The compactness of a PCHE allows effective use of expensive materials and special alloys. (As with any exchanger, material selection must consider thermal, mechanical and physical properties, corrosion and environmental resistance, fabricability, availability and cost.)
Disadvantages And Limitations
PCHEs sometimes are more expensive than other options such as shell-and-tube units, mainly due to their need for all-stainless-steel construction at a minimum — unlike shell-and-tube exchangers, which often can extensively use carbon steel. In addition, while the exchangers can handle corrosive and exotic fluids, these fluids must be extremely clean and free of any debris, solids, etc.
A well-known limitation is the pressure drop developed in a PCHE for low-pressure-drop, large-volumetric-flowrate applications. Pressure drop is roughly inversely proportional to the channel diameter. For high-pressure applications, pressure drop might not be a constraint but it can be a barrier for low- or moderate-pressure applications. Each flow channel acts like a small pipe with many bends; swirl flows, reversed flows and eddies occur around a bend corner. The reduction of hydraulic diameter, complexity of routes and other effects can lead to a channel pressure drop that’s unacceptably large.
PCHEs should be engineered and fabricated to be immune to flow-induced vibration and other dynamic effects. In fact, properly designed PCHEs had no failure due to these issues. However, some low-cost PCHEs that were badly engineered and perhaps poorly manufactured have experienced flow-induced vibration in certain situations — despite PCHEs having been proven to be very resistant to pressure fluctuation by their inventor. So, the design and fabrication of PCHEs should take this into account.
Blockages can occur easily due to the fine channels. The high risk of blockage posed in many applications requires installation of fine filters or strainers at the inlet of their PCHEs. Units have been operated for many years in numerous so-called clean fluid services without issues. Including a working filter or strainer as part of the exchanger set-up is mandatory for commissioning and strongly recommended for normal operation in non-clean duties. Of course, these filters demand regular cleaning, which adds to operating and maintenance costs.
It is far better to order the filter or strainer as a part of the PCHE package from its manufacturer rather than to get it independently. (Some manufacturers offer a range of high-integrity inline conical strainers.) Standard off-the-shelf filters or strainers might not suffice for such a special application. The recommended maximum strainer aperture is approximately one third of the minor dimension of the flow passages. As a very rough guide, a PCHE needs a fine filter or strainer of 200, 250 or 320 microns. The filter or strainer should be installed as close to the exchanger inlet nozzle as possible. After commissioning, the strainer should be cleaned, checked for integrity and then reinstalled for the normal operation. The pressure drop should be measured across the strainer element independently of the heat exchanger. This allows monitoring of any particulate build-up and scheduling of cleaning before any damage occurs.
Any blockage that occurs will call for various cleaning methods ranging from high-pressure jetting to advanced and expensive chemical cleaning. This can be difficult in some installations — so, cleaning nozzles and access must be considered to facilitate these operations. Every PCHE system should provide for the possibility of such cleaning methods as part of good operational practices. Galvanic compatibility with the piping material and others has caused some difficulties; an insulation kit or coated spool piece may be needed for on-site installation.
Engineering And Operation
PCHEs can handle extreme temperature and difficult services needing high integrity and effectiveness across different operating conditions. Optimizing a unit involves finding the best geometric variables and operating parameters for the particular application. This requires consideration of both thermal and hydraulic performance.
Many modern PCHEs feature zigzag flow channels. These zigzag flow paths do not allow boundary layer growth and encourage turbulent flow. By enhancing heat transfer area and increasing local flow velocity at channel bending points, the zigzag channel shape improves heat transfer performance compared to exchangers with straight or other simple pattern channels. Wavy geometries such as zigzag ones provide little advantage at low Reynolds numbers; maximum advantage is at transitional Reynolds numbers. For higher Reynolds numbers, the free shear layer becomes unstable; vortices roll up, thus enhancing heat transfer. At high Reynolds numbers, periodic shedding of transverse vortices and other effects raises the Nusselt number with a considerable increase in the friction factor. Some experts have suggested double-faced configuration. However, this is very difficult to implement due to plate alignment, especially when considering large plates on a tall stack — particularly if the flow paths are complex.
S-shaped patterns offer another option. The Nusselt number of PCHEs with zigzag patterns is 25–35% higher than those with S-shaped patterns — but the pressure-drop is about 2.5–5 times larger, depending on Reynolds number. Properly optimized S-shaped models theoretically decrease pressure drop to 25–35% of that of conventional zigzag models while reducing heat transfer performance only slightly. They particularly would suit applications with pressure-drop constraints. However, S-shaped PCHEs need much more expensive etching and challenges exist for proper diffusion bonds.
Headers usually are half cylinders that enable fluid distribution between the nozzles and the channels in the exchanger core. This allows formation of fluid distribution areas inside the exchanger core. Headers and nozzles are welded to the core to direct the fluids to the appropriate sets of passages. There are a variety of methods for supporting PCHEs. Most commonly, end-type supports welded to the exchanger core are used. Alternatives include saddle supports fitted to the lower header in the vertical plane.
Commissioning of PCHEs demands great care. More attention than usual must be paid to the cleanliness of associated piping and equipment. Thorough cleaning is essential — as is flushing and draining of the whole system to ensure removal of any scale, corrosion products, debris, etc.
Comparison To Plate Heat Exchangers
Plate heat exchangers (PHEs) are popular compact alternatives to shell-and-tube units. They come in many different types and constructions. The most common type consists of a series of channel plates pressed together to form a plate pack. Gaskets used in such PHEs impose some limitations and lead to high maintenance costs and even some operational problems. Gaskets, which must be compatible with operating fluids, often restrict the operating temperature. On the other hand, the gasketed construction allows dismantling for maintenance or even adding more plates if needed. PHEs suit a wide range of clean and quasi-clean services (for instance, cooling water applications).
The diffusion-welded construction of PCHEs, while avoiding the limitations of gasketed PHEs, is far more expensive — and enables them to handle extreme operating temperatures. However, their solid structure, while eliminating any maintenance issues posed by gaskets, prevents dismantling like conventional gasketed PHEs. So, PCHEs only make sense for extremely clean services that pose absolutely no risk of dirt, debris, scale, etc.
Because PCHEs typically are more expensive, they cannot replace conventional heat exchangers in all services (even not all clean services). Two typical areas of application are:
1. Where space and weight matter. Light and compact PCHEs can offer great advantages, e.g., in many revamp, expansion and renovation projects.
2. Where process requirements dictate the application of PCHEs. This can include services with small temperature differences between flows or demanding higher efficiency and performance in heat transfer.