Leseprobe_Berichte_363

The semi-continuous system can be provided as a fully integrated production configuration including Thermal
Degreaser with Afterburner, Fluxer Unit, unloading table after Fluxer unit and the Drying Oven in the front of Active
Only® CAB furnace. The Thermal Degreaser and Fluxer operate in continuous mode to assure work safety of the
degreaser and stability of the fluxing process parameters (identical to the continuous CAB equipment). Every process
step before the Dryer, starting with the CAB line loading table, works in semi-continuous mode. A common
mesh belt for the Dryer and Active Only® furnace indexes at a time interval which is based on data feedback from
the control system of the furnace.
After the drying/debinding process, the load is conveyed into an insulated entrance purging chamber. The entrance
chamber maintains the load temperature and removes oxygen from the chamber and product by purging it with nitrogen.
This ensures that the atmosphere in the brazing chamber will not be contaminated with oxygen once the load
is moved into it. In the brazing chamber the load is heated to the pre-determined recipe brazing temperature.
Total brazing time in brazing chamber consists of the time for heating the parts to brazing temperature and soak time
at brazing temperature (also defined in recipe).
The brazing chamber utilizes a set of thermocouples. An overtemperature thermocouple is located outside the muffle to
prevent the heating chamber from exceeding a preset temperature setpoint. A process control thermocouple is placed
directly inside the muffle, in the nitrogen stream between the muffle and recirculation baffle. This thermocouple provides
setpoint temperature control data for the ACCUBRAZE® computer control system. Load thermocouples – one directly
above the load and one beneath the load – read the nitrogen temperatures near the load area. When the load is moved
into the brazing chamber, the bottom load thermocouple senses the temperature of the atmosphere before the product.
The top thermocouple senses the temperature of the recirculated atmosphere after passing through the load. At the
beginning of the heating process the top thermocouple records a lower temperature than the bottom one.
Fig. 5.
Convection brazing chamber cross section
The greater the load mass, the greater will be the temperature deviation between top and bottom load thermocouples.
When the average temperature recorded by load thermocouples reaches the brazing temperature, the control
system starts counting the soak time. Reaching the brazing temperature indicated by the load thermocouples means
that the load is not absorbing the energy from the circulating nitrogen anymore. It is a signal that the load has
reached the brazing temperature.
When the brazing temperature is achieved and the preset soak time has elapsed, the load is moved into an atmosphere
cooling chamber to solidify the braze joints.
The functionality of the load thermocouples as described above allows the operator to reduce the main recipe parameters
in the Active Only® CAB furnace to only the two mentioned before: brazing temperature and soak time.
Regardless of the type, mass, or design of the brazed heat exchangers, the time needed to heat up the part is determined
by its energy absorption. It is one of the main advantages compared with any type of continuous CAB line.
Considering the time required to heat up the parts, an Active Only® production rate of two to five loads per hour can
be achieved, depending on the mass and type of heat exchangers.
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DVS 363