During the last three decades, CAB (Controlled Atmosphere Brazing) has become the basic process for manufacturing automotive heat exchangers with many other applications in HVAC, power generation, household equipment, etc. There are however, areas where VAB (Vacuum Aluminium Brazing) is still used. One of the reasons for investing in this premium option is to improve the quality and appearance of the cores brazed in the controlled atmosphere process. By adapting the vacuum purging operation in the CAB process, the brazing quality may be significantly improved as well as reducing operational utility costs. In some cases, vacuum purging enables the CAB process in applications where VAB was the preferred method. This option can provide valuable savings. For more information, visit www.secowarwick.com, and select CAB-Controlled Atmosphere Brazing.

1. Vacuum purging:
Progressive Innovation for
CAB equipment and processes
P. Skarbiński, A. Bielewicz, SECO/WARWICK, Poland

2. 2
Typical Processes for Aluminum Brazing
/ Controlled Atmosphere Brazing (CAB) In this process, the role of the flux is to dissolve the
oxide film on the Al surfaces to be joined and prevent re-oxidation during brazing.
Typical brazing conditions:
- flux load : 2 - 5 g/m2
- fast heat-up rates (> 20°C/min)
- nitrogen atmosphere oxygen < 100 ppm; dew point < -40°C
- average demanding deoiling – (assembled heat exchangers cleaned in TD)
- max. joint gaps ≤ 0.15 mm
/ Vacuum Aluminum Brazing (VAB) When an aluminum alloy is heated, the oxide layer
cracks, due to differential expansion. In vacuum conditions the presence of Mg vapor
enhances filler metal flow and wetting of aluminum alloy surfaces.
Typical brazing conditions:
- alloys containing magnesium are required
- pressure (vacuum) < 1x10-4
mbar
- very demanding deoiling – very clean surface required! (single parts cleaned before
assembly or washed in special cleaners with solvents)
- good fit-up (0.05 to 0.10 mm)
http://www.alueurope.eu/wp-content/uploads/2012/01/AAM-Joining-4-Brazing.pdf

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Why use vacuum purging in the CAB process?
A Vacuum Purging cycle provides:
/High brazing quality of products – products will be braze in perfect brazing atmosphere
level of 10ppm of O2.
/A perfect brazing atmosphere within the internal parts of the heat exchanger.
/Shiny silver surface after brazing – results comparable with vacuum brazing.
with vacuum purging standard process
/Saves nitrogen – only a single backfilling is required.
/Saves time – 8 cycles/volumes of standard purging compare to vacuum purging - some
vacuum purging systems can reduce purge time to 4 minutes.

4. 4
Vacuum – what is it?
/ The definition of vacuum is:
- “the state of a gas at which its pressure in a vessel and therefore its particle density is
lower than that of the ambient surrounding atmosphere or in which the pressure
of the gas is lower than 300 mbar,“.
/ There are several levels of vacuum defined:
Pressure range in vacuum technology - Pfeiffer Vacuum 2013 [1]
VAB
CAB

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Vacuum purging for CAB process
/ Vacuum pumping level at 0,25 mbar (25 Pa) will allow the
system to
achieve 50 ppm after refilling with pure nitrogen (5 ppm of
Oxygen).
/ A Typical (properly sized) Medium Vacuum (MV) system
guarantees a short purging time of 7 minutes – the time
required to get to 0,25 mbar vacuum (50 ppm of Oxygen) for a
chamber volume of 5 m3
.
Data from Oerlikon Vacuum technology, 2007 [2]

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Conventional chamber purging
/ Purity at level of 50ppm of Oxygen required ~ 8 purging chamber volumes changes of
nitrogen.

7. 7
Pumping Systems for VAB
/ For a Vacuum Aluminum Brazing furnace (High Vacuum – HV), a high vacuum system
is required. An HV system is more demanding and expensive compared with a standard
MV system.

8. 8
Pumping Systems for CAB
/ For a CAB system with the vacuum purging option (to prepare the brazing atmosphere)
a lower pressure range system also known as Medium Vacuum – MV is required.

9. 9
Pumping Systems
/ The differences between MV and HV pumping systems:
High Vacuum – HV
(HV system includes MV system)
Medium Vacuum – MV

10. 10
CAB System Configurations available with vacuum purging
/ Batch – one chamber
/ Batch – two chamber
/ Semi-continuous Active Only 10

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CAB batch furnace with vacuum purging
Medium Vacuum
System
Brazing
chamber
Vacuum
Purging/Cooling
chamber
Thermal
door
Vacuum
door
Integrated single body design
Load area size 1100 x 1100 x 1500 mm. Load mass 250 kg. Output – 1 cycle per hour.

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CAB Line with vacuum purged batch furnace
Batch Brazing
Furnace
Dryer
Debinder
Loading
table
Transfer
System
Cooling - unloading
System
12

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Vacuum Purging / Cooling chamber
Heat exchanger
Recirculation fan
Nitrogen
inlet
Medium Vacuum
System

14. 14
Brazing chamber
Heating Elements
Scrubber
Recirculation
fan
Nitrogen
inlet

15. Vacuum purge option on a semi-continuous Active Only furnace
Loading
table
Dryer
Debinder
Vacuum
Purging
Brazing
Chamber
Cooling
With
Vacuum
purging
Air
Cooling
Unloading
table
Load area size 1500 x 500 x 3000 mm. Load mass 250kg. Output – 3 cycles per hour.

16. 16
Dryer / Debinder
Exhaust Fan
Gas
burner
Recirculation fan

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Vacuum purge chamber with preheating
Medium
Vacuum
System
Heating Elements

18. Brazing chamber
Heating Elements
Recirculation fan
18

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Cooling chamber with vacuum purging
Medium
Vacuum
System
Recirculation fan
Heat
Exchanger

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Load area size 250 x 2100 x 1350 mm, Load mass 400 kg. Output – 4 cycles per hour.
Vacuum purge option on a semi-continuous furnace for vertical core position

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/ Temperature survey for plate and Bar from the system done on maximal size load - 160mm
thickness.
Brazing point with +/- 3 ◦C uniformity was reached in the brazing chamber within 10 minutes.
CAB Line with vacuum purge option on a semi-continuous furnace
for vertical core position

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/ Working area size of 900 x 1900 x 2000 mm; Load mass - 100 kg. Output - 1 cycle per hour.
Single chamber vacuum purged Batch CAB Furnace for vertical core position

23. 23
/ Single Chamber furnace with working area size 500 x 300 x 500 mm. Load
mass 2 kg. Laboratory furnace with CAB process simulation system.
Laboratory Batch vacuum purged furnace

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Summary / Conclusions
/ By utilizing the vacuum purging option to the CAB furnace system savings in utility consumption
as well as an improvement in the brazing quality can be achieved.
/ For semi-continuous and batch brazing operations a single Nitrogen backfill instead of
approx. 8 exchanges of the purging chambers is required.
/ While reducing the Nitrogen consumption, excellent brazing atmosphere purity is achieved
in the whole preheating, brazing and initial cooling phases of the process, usually on the
level of 10 ppm of oxygen. This provides the highest quality of brazing atmosphere for
forming the joints and excellent appearance of the cores after brazing.
/ The internal volume of the core is purged equally as well as the external surfaces of the
core.
/ CAB process with Vacuum purging, in some cases, may be considered as a lower cost
alternative to VAB. Savings may be achieved due to less demanding tolerances of the heat
exchanger components, simpler cleaning process, and elimination of magnesium containing
alloys..

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This presentation was developed by SECO/WARWICK Europe for the
International Aluminium Brazing Congress organized by the DVS – German
Welding Society on 3 – 5 June, 2014 in Düsseldorf/Germany.
To learn more about the CAB Brazing process & equipment, visit
www.secowarwick.com