Polymeric Heat Exchangers with Hollow Fibres

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 fiber heat exchangers was opened in 2012 as a part of NETME Centre, now is the base for theoretical and experimental research. 

Introduction

Polymeric hollow fiber heat exchangers are devices consisting of small polymeric tubes - "hollow fibers". Hollow fibers 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), etc. Mechanical and chemical properties of fibers differ and depend on type of polymer used. Hollow fibers, potted together in epoxy resin can create heat exchange element. A lot of fibers (hundreds and even thousands) should be connected into one unit to create heat transfer area. Length, fiber 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.         

 

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

    

   

 Polymeric Hollow Fibers Advantages

  • Dramatically reduced weight
  • Excellent chemical resistance
  • 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 fiber 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 fiber material.

  

Material:

PP

PA11

PC

Fiber Diameter

0.4-0.8 mm

Operating Pressure

4-10 bar, depending on temperature

Maximal Temperature

90°C

140°C

120°C

        


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 fibers have a very thin wall (0.05-0.1 mm in thickness) so heat is transferred through the fiber wall effectively. Moreover, hollow fiber’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 fibers 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.

                              

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Outside convective heat transfer coefficient vs. hollow outer fiber diameter for water flowing across circular tube
Inside convective heat transfer coefficient vs. hollow outer fiber diameter for water flowing through circular tube

   

An prototype of immersed-type heat exchange bundle consists of 500 PP fibers with an effective fiber length of 1300 mm. The inner and outer fiber 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 fibers with a flow rate of up to 400 l/hr to cool water, which had been heated to 56°C, flowing across the bundle fibers with a flow rate of up to 700 l/hr and a velocity of 0.01-0.05 m/s. The fiber 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.

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Prototype of immersed type heat transfer bundle         Prototype of liquid-to-gas automotive radiator           

Prototype of liquid-to-gas heat exchanger consists of 7 layers of PP fibers . The total amount of fibers is 728, the inner/outer fiber 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-ethyleneglycol solution. Brine heated to 40°C was pumped through the fibers 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.      


Additional Information

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