Printed Circuit Heat Exchangers
Introduction
The heat exchangers we are familiar with are usually massive equipment that transfers heat from a hot fluid to a cold fluid, along with a good amount of energy lost to the environment as heat. But what if I told you we could shrink down heat exchangers into palm-sized chips while at the same time improving heat transfer characteristics and pressure handling capabilities? Printed Circuit Heat Exchangers (or PHCEs) are all about heat transfer through a network of microchannels and are currently seeing increased research attention due to their favourable characteristics. Let us learn about them in this short article:
Construction
Printed Circuit Heat Exchangers consist of multiple layers of plates with fluid flow channels etched onto them by chemical deposition methods. These are then stacked up by bonding, usually using high-pressure diffusion processes. Hot and Cold fluids flow in every alternate plate, thus transferring heat, as shown below:
Such an arrangement enables the capability to handle high-pressure fluids (due to the incredible strength of the inter-layer bonds) and a larger surface area to volume ratio favourable to heat transfer.
Types of PCHEs
A good variety of design variations in PCHEs have been ideated, and research is being carried out on their performance in different flow conditions. These can be broadly classified on the following bases:
Based on flow arrangement
The relative direction of the hot and cold fluid flows results in three types of arrangements, which are described schematically above.
Based on channel geometry
The geometric design of the flow channel in each plate is commonly studied for different configurations. Some of the standard designs are shown above.
Applications
Printed Circuit Heat Exchangers are mainly applied in the following systems:
Brayton Cycle with sCO2 as working fluid
Supercritical carbon dioxide (sCO2) is CO2 above its critical point (304.13 K and 7.3773 MPa). In this state, it has good characteristics such as high density, low viscosity, and high thermal conductivity. These enable it to perform an energy cycle with a small amount of compression work and are thus often used in compact Brayton cycle power plants. Subsequently, PCHEs are applied in these plants due to their compact nature.
High temperature gas-cooled reactor (HTGRs)
HTGRs are nuclear power reactors wherein gases like Helium are used to extract energy from nuclear reactions and transferred as process heat for various requirements. The emergence of new concepts for HTGRs with a closed-cycle gas turbine are exciting candidates for future power generation. The principle of HTGRs is shown below:
This novel system can produce higher efficiencies than that of traditional steam cycles. Still, it needs very high-efficiency components, such as the helium/helium recuperator, and PCHEs with their high efficiency are being researched for this purpose.
Future Scope
Printed Circuit Heat Exchangers present an interesting area of research in conjunction with the ongoing race to negate the global energy crisis.
At present, there is a requirement to develop universal correlations for the design of PCHEs, incorporating their geometric structure and operating conditions.
The usage of PCHEs in floating production and offloading systems in Liquefied Natural Gas (LNG) fields is also experiencing colossal interest worldwide. But this calls for more research into the performance of PCHEs in multiphase flow and under the influence of sloshing effects that are possible in LNG fields. PCHEs with a compact structure, good reliability and thermal efficiency on the order of 98% have a good scope in various application prospects.
