Basic research into the potential of hollow fibres for use in heat transfer started in the Heat Transfer and Fluid Flow Laboratory in 2007. Theoretical and experimental study revealed the enormous advantages in 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 have completed small-scale laboratory tests in 2007 alongside development of a theoretical/numerical model describing heat transfer. Problems with fibre space separation in heat transfer media was 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 great potential of the polymer hollow fiber heat exchangers in HVAC technologies.

A new technology of producing the polymer hollow fibres heat exchangers was developed during the 2017 and 2018. The method is based on winding one fibre around the rod and that forms the heat transfer insert to liquid-liquid heat exchanger.

The laboratory has been investigating polymer hollow fibre heat exchangers for ten years. During that time, it has become one of the leading experts in this field, 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 0.4-1 mm and 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 type of polymer used. Hollow fibres, potted together in epoxy resin can create heat exchange element. A lot of fibres (hundreds and even thousands) should be connected into one unit to create heat transfer area. Length, fibre diameter and material can be chosen depending on particular application. Moreover different fillers can be added to plastic material to modify its properties. Our aim is to study and design environmentally friendly polymeric heating and cooling systems with special features and high performance basing on innovative technological solutions.

Extruder head with PP fibres moving down
Detail of PP fibres connected by epoxy resin in one tube desk

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 wide range of application. Hollow fibre heat exchangers can be used in chemical industry, biotechnology, food processing industries, in automotive industry, buildings, and HVAC-R as well. 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 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 transfer insert to shell-and-tube heat exchanger (left) and potting of the bundle of fibres in the aluminium shell (right)

The heat exchangers can be any shape or size.

Large heat exchanger for Air-Liquid applications and (left), small immersed heat exchanger for Liquid-Liquid applications –can be also used as 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 gram (left) and immersed-type liquid-to-liquid heat exchanger made of 10 bundles (3000 PP fibres total, heat transfer surface 7.2 m2) (right)

Thermal Performance

It is a well-known fact that 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 transferred through the fibre wall effectively. Moreover, hollow fibre’s small outer and inner diameter ensures superior convective heat transfer. The graphs show that 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 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 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 air conditioner. The water at 10°C was used as cooling medium inside the fibres with the flow rate 500 l/hr. The cooled medium was air at 27°C and 50% relative humidity. The heat exchanger was tested at the air velocity 1 m/s, respectively 3 m/s. The heat exchanger reached the thermal power 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 in aluminium shell (left) and heat transfer insert with PC fibres in stainless steel shell (right) during the measurement

The inner diameter of the aluminium, respectively stainless steel shell is 80 mm, respectively 40 mm. The larger heat exchanger achieved the thermal power of 36.3 kW with the water flow rate in the shell 1000 l/hr at 80 °C and water flow rate in the fibres 1500 l/hr at 25 °C. The smaller heat exchanger reached the thermal power 23.5 kW with the water flow rate in the shell 1000 l/hr at 80 °C and water flow rate in the fibres 1500 l/hr at 20 °C.

Additional Information

can be found in Publications of the Heat Transfer and Fluid Flow Laboratory.