Basic research into the potential of hollow fibres for heat transfer started in the Heat Transfer and Fluid Flow Laboratory in 2007. Theoretical and experimental studies revealed the enormous advantages in big heat transfer area and commercial potential. Prototypes of heat transfer surfaces were built, and theoretical findings were verified by laboratory measurements.
Presently, the laboratory partners completed small-scale laboratory tests in 2007 alongside developing a theoretical/numerical model describing heat transfer. Problems with fibre space separation in heat transfer media were overcome at laboratory scale in 2010. Research continued by heat transfer studies of large heat exchangers using two principles, chaotic and regular space distribution. A problem of potting was resolved in 2011.
Full-scale prototype units tested in 2012 in the certified laboratory were found to have competitive performance against classical metal units currently available. The new laboratory of fibre heat exchangers was opened in 2012 as a part of NETME Centre.
In 2015, shell and tube heat exchangers were developed. Prototypes of the air-conditioning unit were made and successfully tested in 2017. The research shows the great potential of the polymer hollow fibre heat exchangers in HVAC technologies.
New technology for producing the polymer hollow fibres heat exchangers was developed during 2017 and 2018. The method is based on winding one fibre around the rod, which forms the heat transfer insert to a liquid-liquid heat exchanger.
In 2019, a supercompact cooling system of lithium-ion batteries was developed, especially for thermal management in electrical vehicles. Hollow fibres were embedded into the polydicyclopentadiene. The whole housing copies the surface of inserted batteries, so the heat transfer area is maximized.
Another leap in automotive occurred in 2020. The very first automotive radiator from the polymeric hollow fibres was tested in the car. A similar heat exchanger was used for the cooling of flue gas with a temperature of more than 200 °C in 2021.
The laboratory has been investigating polymer hollow fibre heat exchangers for ten years. It has become one of the leading experts in this field during that time, acting as an advisory board for industrial companies. Most of the Heat Transfer and Fluid Flow Laboratory’s projects are experimentally oriented.
Introduction
Polymeric hollow fibre heat exchangers are devices consisting of small polymeric tubes – “hollow fibres”. Hollow fibres have an outer diameter of 0.7–1.5 mm. They can be produced by the extrusion process from a wide range of polymers such as PP (poly-propylene), PC (poly-carbonate), PA (poly-amide), PEEK (poly-ether ether ketone), etc. Mechanical and chemical properties of fibres differ and depend on the used polymer. Hollow fibres potted together in the epoxy resin can create a heat exchange element. Many fibres (hundreds and even thousands) should be connected into one unit to create a heat transfer area. Length, fibre diameter and material can be chosen depending on a particular application. Moreover, different fillers can be added to plastic material to modify its properties. We aim to study and design environmentally friendly polymeric heating and cooling systems with unique features and high performance based on innovative technological solutions.
Polymeric Hollow Fibres Advantages
- Large heat transfer area with respect to the volume
- Great heat transfer coefficients
- Dramatically reduced weight
- Excellent chemical and corrosion resistance
- Easy shaped
- Reduced fouling
- Superior thermal performance
- Flexibility
- Environmentally friendly and recyclable
- Material can be chosen depending on application
- Low cost
Application
The Heat Transfer and Fluid Flow Laboratory is able to prepare liquid-to-liquid and liquid-to-gas prototypes of heat exchangers for a wide range of applications. Hollow fibre heat exchangers can be used in the chemical industry, biotechnology, food processing industries, automotive industry, buildings, and HVAC-R. Its low weight and compact design make transportation and installation simple. Elements can be connected in parallel to obtain the heat transfer capacity needed. Operating temperature and pressure depend on the type of fibre material.
| Material: | PP | PA | PC |
|---|---|---|---|
| Fibre Diameter | 0.7–1.5 mm | ||
| Operating Pressure | 4–10 bar, depending on temperature | ||
| Maximal Temperature | 90 °C | 140 °C | 120 °C |
Possible application
- Liquid-Liquid heat exchanger
- Liquid-Gas heat exchanger
- Liquid-Steam heat exchanger
Prototypes overview
The heat exchangers can be any shape or size.
Large heat exchanger for Air-Liquid applications (left) and small immersed heat exchanger for Liquid-Liquid applications. It can also be used as a heat transfer insert to form the shell and tube heat exchanger (right).
The heat exchanger consists of 10 heat transfer bundles with PA fibres, a total empty weight of 488 g (left) and an immersed-type liquid-to-liquid heat exchanger made of 10 bundles (3000 PP fibres total, heat transfer surface 7.2 m2) (right).
The car radiator (left) can lower CO2 production of each vehicle by roughly 2 g per 100 km, which can also save 190 EUR on emission fines. A heat exchanger for flue gas cooling (right) can be operated at flue gas cooling temperatures up to 205 °C, the maximum possible flue gas temperature of conventional combustion boilers.
The first automotive radiator from the polymeric hollow fibres was tested in Škoda Octavia in 2020. Moreover, the car equipped with the polymeric hollow fibre heat exchanger passed the Grossglockner test at low speed and with a trailer.
Ultra-small heat exchanger for thermal management of electronics.
PP fibres embedded into a polydicyclopentadine housing represent a modular system for battery thermal management of li-ion 18650 cells. The modularity can be advantageously utilized to construct batteries with arbitrary size to power e-vehicles or store energy.
Vibration of Hollow Fibres
In the case of gas-to-liquid heat exchangers, we can observe considerable vibration on the fibres. How does this affect the pressure drop on the air side?
The investigation compares a measured and calculated pressure drop of a plastic heat exchanger in the cross-air flow. The heat exchanger, shown in the following photo, is made of one thousand hollow polypropylene fibres with an outer diameter of 1 mm. The core size is 300 x 200 mm. Two staggered configurations are considered, namely 2 x 2 and 2 x 3 mm. The fibers are either empty inside or contain hot water (60 °C) or rigid steel wires. The fibers can be prestressed by twin hydraulic cylinders in a range of the total tension between 0 and 10 kN. The tension in fibers is controlled by force transducers S9M and C9C.
Polymeric hollow fibre heat exchanger inside a support frame with hydraulic cylinders.
The following figures show the dependency of pressure drop on the total tension for both heat exchangers 2 x 2 mm and 2 x 3 mm, respectively.
The vibration effect on the pressure drop of novel plastic heat exchangers is a subject of ongoing fundamental research, which mainly relies on national funding resources.
The images have been reprinted with the kind courtesy of Tereza Kroulíková – a PhD student at Heatlab.
Thermal Performance
It is a well-known fact that a polymer’s thermal conductivity is very low (a hundred times lower than metal’s) and significantly limits heat transfer. Hollow fibres have a very thin wall (0.05–0.1 mm in thickness), so heat is effectively transferred through the fibre wall. Moreover, hollow fibre’s small outer and inner diameter ensures superior convective heat transfer. The convective heat transfer coefficients rise when the tube diameter decreases. Tests with hollow fibres confirmed high values of heat transfer coefficient: values up to 450 W/(m2K) were observed for liquid-to-gas and up to 2100 W/(m2K) for liquid-to-liquid applications, respectively.
A prototype of an immersed-type heat exchange bundle consists of 500 PP fibres with an effective fibre length of 1300 mm. The inner and outer fibre diameters are 0.45 mm and 0.55 mm, respectively, and the total heat transfer area is 0.96 m2. This bundle was tested as an immersed-type water cooler. Cooling water at a temperature of 20°C was pumped through the bundle fibres with a flow rate of up to 400 l/hr to cool water, which had been heated to 56 °C, flowing across the bundle fibres with a flow rate of up to 700 l/hr and a velocity of 0.01–0.05 m/s. The fibre bundle transfers 6–13 kW of heat depending on the flow regime and temperature differences. These data correspond to the overall heat transfer coefficient of up to 1450 W/(m2K).
Prototype of immersed type heat transfer bundle (left) and prototype of the liquid-to-gas automotive radiator (right)
Prototype of liquid-to-gas heat exchanger consists of 7 layers of PP fibres. The total amount of fibres is 728, the inner/outer fibre diameter is 0.6/0.7 mm, and the total heat transfer area is 0.22 m2. This heat exchanger was tested as a car radiator to cool the water-ethylene glycol solution. Brine heated to 40 °C was pumped through the fibres with a flow rate of up to 600 l/hr and cool air at a temperature of 20 °C (with velocity in the cross-section of 4–10 m/s) was used as the cooler. The heat exchanger transfers up to 2.1 kW of heat depending on the flow regime and temperature differences. These data correspond to an overall heat transfer coefficient of up to 450 W/(m2K).
Prototype of liquid-to-gas heat exchanger consists of 6 layers of PA fibres. The total amount of fibres is 798, the inner/outer fibre diameter is 0.64/0.8 mm, and the total heat transfer area is 0.38 m2. This heat exchanger was tested as an alternative for an air conditioner. The water at 10 °C was used as a cooling medium inside the fibres with a flow rate of 500 l/hr. The cooled medium was air at 27 °C and 50 % relative humidity. The heat exchanger was tested at the air velocity of 1 m/s, respectively 3 m/s. The heat exchanger reached the thermal power of 0.8 kW, respective 1.2 kW.
Prototype of liquid-to-gas heat exchanger consists of 6 layers of PA fibres
Example of condensation on the outer surface of PP fibre (left) and PA fibre (left) under the same condition – air speed 3 m/s and relative humidity 50%, horizontal position of the fibres
Heat transfer insert made of PA fibres (left) placed in a PA shell (right).
The heat exchanger achieved the thermal power of 38 kW with the water flow rate in the shell 2000 l/hr at 11 °C and the water flow rate in the fibres 800 l/hr at 70 °C. The maximum heat transfer coefficient of 1170 W/(m2K) was achieved at the same conditions.
Additional Information
can be found in Publications of the Heat Transfer and Fluid Flow Laboratory.