Furnace Brazing Conditions
The recommended furnace atmosphere conditions necessary for good brazing are as follows:
- Dew point: ≤ -40°C
- Oxygen: < 100 ppm
- Inert gas: nitrogen
The most common source of nitrogen is that generated from liquid nitrogen storage tanks. A typical nitrogen gas specification from a liquid source indicates that the moisture content is <1.5 ppm (dew point = -73°C) and an oxygen level of <3 ppm. In brazing furnaces however, the normal atmospheric operating conditions almost always exceed incoming nitrogen contaminant levels. This is due to water and oxygen dragged into the furnace by the incoming products, by the stainless steel mesh belt and by the potential back-streaming of factory atmosphere through the entrance and exit of the furnace. The latter will occur when the exhaust and incoming nitrogen are not properly balanced.
Many furnaces are equipped with dew point and oxygen measurement devices. It is important that the measurements are taken in the critical brazing zone of the furnace because this is where these impurities will reach their lowest concentrations. Measuring dew point or oxygen levels anywhere else in the furnace may be of academic interest, but will not represent actual brazing conditions.
Dew Point Measurement
Measuring the moisture content in the critical brazing zone of the furnace has always been a key indicator of the quality of the brazing atmosphere. Moisture can substantially influence the quality and appearance of the brazed heat exchanger as well as the first time through braze quality (% rejects).
Chilled Mirror Technology
One of the more common principles of measuring dew point is using chilled mirror technology. The measurement of the water vapor content of a gas by the dew point technique involves chilling a surface, usually a metallic mirror, to the temperature at which water on the mirror surface is in equilibrium with the water vapor pressure in the gas sample above the surface. At this temperature, the mass of water on the surface is neither increasing (too cold a surface) nor decreasing (too warm a surface).
In the chilled-mirror technique, a mirror is constructed from a material with good thermal conductivity such as silver or copper, and properly plated with an inert metal such as iridium, rubidium, nickel, or gold to prevent tarnishing and oxidation. The mirror is chilled using a thermoelectric cooler until dew just begins to form. The temperature at which dew is formed on the mirror is displayed as the dew point.
The advantage of the chilled mirror dew point meter is that it is an absolute measurement with high precision. However, this measurement technique is sensitive to pollutants and corrosive contaminants which, in the brazing process, include KAlF4 condensation and trace amounts of HF gas. Consequently, the mirror requires frequent maintenance and replacement. “Dirty” mirrors can lead to false readings.
Coulometric Measurement Principle
The principle of operation for measuring is that an electrolyte is formed by absorption of water on a highly hygroscopic surface (e.g. P2O5) and the current level obtained to electrolyze the surface is proportional to the water content. The advantage of this principle of operation is that it is insensitive to aggressive media. The disadvantage is that the precision is not as high as chilled mirror technology. Some heat exchanger manufacturers have reported good success using this measurement principle in their CAB furnaces.
Relationship between dew point and moisture content
The relationship between dew point and moisture content is not linear. It is important to note that small changes in dew point will result in large changes in actual moisture content. This is evident from the graph shown below.
how do you determine the nitrogen feed for the furnace to ensure a correct atmosphere, is there a rule or a guide to do it?
In a continuous furnace the balance of the nitrogen flow is of primary importance. The major feeding point should be located near the end of the heated muffle section and from there nitrogen should flow towards both furnace ends. Ideal situation is when about 2/3 of the flow is directed backwards and 1/3 towards the furnace outlet. The back flow of nitrogen is required for flashing out the oxygen from the part and from the belt. The forward flow prevents sucking of air through the furnace outlet.
Normally I would say that about 80% of the flow should be directed to the end of the muffle about 15% in the cooling zones and the furnace outlet to compensate for the temperature drop and in some cases small injection in about 2/3 of the muffle length.
Having in mind this general principles for each furnace an individual setting must be optimize.
What is the main source which is distributing nitrogen flow (2/3 towards backward and 1/3 towards furnace outlet), whether it is the power of blower motor on either side or the location of nitrogen inlet points?
can we change the percentage dynamically?
Furnace atmosphere proper distribution is very important to achieve clean atmosphere for brazing. The basic solution used in majority of the furnaces world wide are mechanical curtains installed at both ends of the atmosphere part of the system. With the main atmosphere inlet at the brazing point and several additional inlets in the heating and cooling zones and similar pressure drop on each curtain the proper distribution of atmosphere is achieved. Many working furnaces are equipped with a kind of nitrogen inlet pipes located horizontally inside the muffle with adjustable angle of outlet holes directing the flow of nitrogen in desired direction. By proper adjusting of nozzles position the distribution of the nitrogen supposed to be adjusted precisely. However according to our recent experience most of the furnace users stopped using of this system and relay on not directed atmosphere inlets taking care of keeping the curtains in good condition. This way a high purity of atmosphere with low nitrogen consumption may be achieved as well.
which postion have nitrogen nozzle?
start of preheat,preheat rotation fan,end of muffle,static cooling section?
There are a lot of different furnace designs in terms of nitrogen inlets positions, types of nozzles and nozzle position and direction settings. Furnaces with a large number of nitrogen inlets give more flexibility to set properly atmosphere distribution in order to provide highest atmosphere quality by lowest nitrogen consumption.