GESCHFTSFELD FAHRZEUGTEILE

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AUTOMOTIVE DIVISION
VDA Alternate Refrigerant Wintermeeting 2002 -
Automotive Air Conditioning and Heat Pump
Systems Flexible metal hoses for the
guidance of CO2 in air-conditioning systems of
passenger cars Dr. Carlo Burkhardt Product and
Processing Development Automotive
Parts Witzenmann GmbH, Pforzheim
(Germany) c.burkhardt_at_witzenmann.com Saalfelde
n (Austria), 31st January 2002
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• The use of CO2 as an alternative refrigerant in
  air-conditioning systems of passenger cars does
  change the requirements for the hose and tube
  materials of the piping.
• While suitable tube materials are ready
  available, the flexible polymer hoses currently
  applied in R134a air-conditioning systems do not
  fulfil the requirements for a safe operation of
  CO2-systems.
• Especially the permeation rate of polymer
  materials for CO2 and the phenomenon of explosive
  decompression demand flexible hose materials that
  build a barrier for CO2 molecules and therefore
  prevent leakage and absorption of CO2.
• These materials additionally have to withstand
  operating temperatures of up to approx. 180-200C
  and must be chemically resistant against the
  fluids and lubricants present inside and outside
  of the system.

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• The described initial demands can be met with an
  annularly corrugated metal hose made of
  austenitic stainless steel, as shown in fig. 1.

• It enables the decoupling of engine movements,
  road excitations and the compensation of mounting
  tolerances in the piping system, while being
  technically gas tight and therefore preventing
  CO2-permeation and explosive decompression.
• However, a metal hose in this configuration does
  not with-stand the system pressure, the pressure
  pulsations and the vibrations induced by the
  internal combustion engine or the
  air-conditioning compressor.

4

• To achieve a high pressure resistance, metal
  hoses usually are equipped with an outer braiding
  that prevents a lengthe-ning and widening of the
  hose (fig.2).

• However, due to the pressure pulsations occurring
  in the CO2 air-conditioning system, mechanical
  wear between the hose and the braiding would
  simply grind through the corrugations and lead to
  leaks.

5

• Therefore an additional layer has to be
  introduced, either as an additional outer polymer
  hose with integrated aramid or steel braiding, as
  illustrated in fig. 3

or as a silicon coating between the hose and an
austenitic steel braiding, as shown in fig. 4
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• Both shown additional layers ensure in
  combination with the braiding
• a high pressure resistance of the hose
• a protection of the flexible hose against forces
  induced by cyclic impulse pressures
• prevention of mechanical wear of the braiding on
  the corrugations of the flexible hose
• mechanical protection of the metal hose
• effective damping of the resonance frequencies of
  the hose, as shown in fig. 5

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Fig. 4 Determination of the resonance
frequencies of HYDRA air-conditioning hoses DN 6
(blue) and DN 8 (green) in 180-bend with an
excitation of 1g and an internal pressure of 130
bar in the z-axis
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A flexible metal hose (Di 6mm,NL 300 mm)
with silicon coating between the corrugations
and brai-ding fulfils following requirements

• Burst Pressure gt 680 bar

• Impulse pressure test 150.000 cycles between
  30-160 bar at 0,25 Hz superimposed by a vibration
  test in the resonance frequency of 88 Hz with
  amplitude of 2mm (z-axis) at a test temperature
  of 180C 18 x 106 cycles

• Torsional test with hose in 90 bend with an
  inner pressure of 100 bar at a testing angle of
  5 gt 10 x 106 cycles

• Torsional test with straight hose and inner
  pressure of 100 bar at a testing angle of 4 gt
  10 x 106 cycles

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• A flexible metal hose for the guidance of CO2 in
  air-conditioning systems of passenger cars has
  been deve-loped. Under prototype conditions on
  the test bench, all the technical requirements
  known to date are fulfilled.
• An improvement of the welding seam between
  connec-tion pipe and corrugated metal hose is
  currently carried out in order to enable the
  decoupling of larger movements of the
  air-conditioning hose.
• A validation of the air-conditioning hose in the
  vehicle with respect to function, life expectance
  and acoustical validation is currently carried
  out.

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• However, air-conditioning hoses based on
  annularly cor-rugated metal hoses exhibit
  significant differences com-pared to polymer
  hoses used to date in R134a systems
• The ageing behaviour is completely different
  Critical for metal hoses are pressure pulsations,
  vibrations, large movements (load changes/bad
  road excitations) and torsional overload. An
  ageing of the polymer, as it is known for the
  currently used polymer hoses, is -at least for
  the solution with the silicon layer- NOT
  critical. Due to these differences, an adapted
  testing programme is necessary (as described
  afore).
• The resonance frequencies of the whole piping
  system will have a significant influence on the
  lifetime of the flexible hose. This has to be
  taken into account when designing the package.

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• The minimum bending radius of metal hoses are
  larger than those of polymer hoses. Due to the
  high operating pressures, the design of
  pre-shaped hoses will not be possible.
• However, one solution could be to shape the
  piping and to have straight, relatively short
  flexible areas. These areas would have to be long
  enough to decouple the occurring movements and
  vibrations and to allow the compensation of
  mounting tolerances. This possibility would have
  to be taken into account when designing the
  package.
• Torsional overload has to be avoided during the
  assem-bly of the piping (e.g. by the use of
  flanges) and during operation in the vehicle.
  This has to be taken into account when designing
  the piping package.

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• The integration of the flexible hose in the
  piping system CANNOT be carried out by
  press-fittings. The hose has to be
  welded/soldered into the system or connected with
  detachable joints (e.g. flanges).
• These differences will require a close
  co-operation between the automotive
  manufacturer/system supplier of the
  air-conditioning system, the piping manufacturer
  AND the supplier of the flexible metal hose even
  at the early stages of the vehicle packaging in
  order to get the best, most cost-efficient
  solution.