|Fig. 1 Typical dew-point meter for panel-mounting, with a gas moisture probe, the tip of which is placed in the atmosphere flow pipe. (Photo courtesy of Michell Instruments)
On a warm, moist day, our earth’s atmosphere will contain a significant amount of moisture in it. During the night, when the sun has gone down, this atmosphere will become cooled, and will not be able to hold onto the amount of moisture (water) that it could when it was warm, and so, some of that moisture will condense out onto the grass in the form of “dew”. Then, during the following day, when the sun heats the air up once again, the dew will evaporate from the ground.
It is well known that the warmer the gas, the greater will be the amount of moisture that gas can hold.
At any given point in time, all gases will have what is called a “dewpoint”. The “dewpoint” of any gas is the temperature to which that gas must be cooled to get the first droplet of moisture to condense out of that gas (assumed to be at one standard atmosphere of pressure). The less the amount of moisture in that gas, the cooler must be the temperature to which that gas must be cooled in order to get the first condensation to occur. Based on that fact then, it will be understood that the lower the dewpoint of a gas, the drier (lacking moisture) is that gas.
Thus, the dewpoint of a gas tells you how much moisture (water) is present in a gas.
|Fig. 2 Some dewpoint meters are portable, and can moved from location to location on a cart. (photo courtesy of Shaw Moisture Meters)|
Please note, however, that water (H2O) also represents the presence of oxygen, and oxygen is an element that can react with metals to form oxides on the metal surface being heated up in that furnace. Thus, to prevent oxidation of metals during a furnace brazing process, it is important to keep oxygen away from the part being brazed, which means controlling the amount of moisture present in any gaseous atmosphere being used in a vacuum-furnace during that brazing process. Gaseous atmospheres are indeed introduced into vacuum furnaces for rapid-cooling after brazing, or may be introduced into the vacuum chamber in order to build up a partial-pressure to prevent outgassing of certain elements from the metals being brazed, or from the brazing-filler-metal (BFM) itself. Gases may also be introduced into “multi-bar” vacuum furnaces for purposes of perhaps heat-treating parts during or following a brazing run.
The dewpoint of a gas can be measured fairly easily by the use of a dewpoint-meter (also called a hygrometer), which, as the name suggests, is an instrument that will measure the amount of moisture in a gas and provide that information via readouts, either as part of the furnace control instrumentation (as shown in Fig. 1), or perhaps as a separate, portable dewpoint-meter that can be connected to a special port in the atmosphere-piping system to the vacuum furnace (as shown in Fig. 2).
Three very important things to know about dewpoint control:
1. Dewpoint must be measured at the furnace, not at the gas source.
Receiving a certification from your gas-supplier stating that the atmosphere (gaseous or liquid) they have delivered to you has “such and such” dewpoint, is actually quite meaningless to your brazing operations. What is important, instead, is the dewpoint of that atmosphere when it is inside the furnace chamber itself where the brazing is taking place – after it’s been piped from the outside tanks all the way to the furnace. The piping and pipe-fittings can have a major effect (either negative or positive) on the dewpoint of an atmosphere gas being piped from a liquid tank or other external source of gas. This will be the topic of another article soon.
2. If you are, in fact, putting a gaseous atmosphere into your vacuum chamber, you MUST know its dewpoint.
If a gaseous atmosphere is being used in your vacuum furnace brazing process, perhaps for outgassing-suppression, or for rapid cooling, etc., or for multi-bar HT/brazing, then the dewpoint of that gas MUST be measured – at the furnace — and provided to the furnace operator, or the brazing process is NOT being properly controlled! I am often amazed by brazing shop personnel who do NOT know the dewpoint of the gaseous-atmosphere being used inside their furnace. They merely “assume” its okay. That is a very poor brazing practice.
Remember, dewpoint represents moisture. Moisture represents oxygen. Oxygen at brazing temp causes oxidation of metals. Unfortunately, brazing filler metals (BFMs) don’t like to bond to, or flow over, oxides on metals! Thus, if you don’t accurately monitor, and control, the dewpoint of the gaseous atmosphere you are using in your brazing operations, you can easily ruin your brazing.
Therefore, the dewpoint of any brazing-atmosphere used must be known in order for the brazing process to be “in control”. There are a number of good manufacturers of dewpoint equipment available in the marketplace. Remember, if a gaseous atmosphere touches a brazed component at any time during the entire vacuum-brazing cycle, from loading to unloading, know (and control) the dewpoint of that gas.
3. If the dewpoint is not acceptable, it can be “dried” further.
Okay, so if you’ve now installed, and are using, a good dewpoint-meter, and it’s showing you that the atmosphere is not sufficiently “dry” for use in your vacuum-brazing operations, what can you do about it? The answer may very well be to add a desiccant-drier, placed right next to the furnace, to further dry the atmosphere prior to its entering the furnace. An example of such a gas-drier system is shown in Figure 3.
A regenerative desiccant dryer can automatically dry the atmosphere-gas just prior to its entering the vacuum chamber. The dual drying chambers are alternately cycled through drying and regeneration cycles, so that one chamber is drying the atmosphere-gas at all times.
The compressed atmosphere-gas flows through one drying tower where any residual water vapor is adsorbed into the desiccant (the desiccant is an inert, hygroscopic material that has an enormous surface-to-mass ratio). The off-steam drying tower is regenerated by taking a small amount of the dried process gas (purge) and passing it though the off-stream tower’s desiccant, thus grabbing any moisture molecules trapped in that desiccant. These moisture molecules are released and carried off by the purge flow and exhausted to atmosphere.
Is the atmosphere-gas (hydrogen, helium, nitrogen, and argon) that you are using in your vacuum furnace for partial-pressure, or for rapid cooling, or for multi-bar operations, truly a dry gas of at least -80°F (-65°C) dewpoint or drier when it is introduced into your vacuum furnace? If not, then your brazing process is not fully in control, and you will need to implement some of the recommendations described in this article regarding dewpoint control via dewpoint-meters, perhaps also with some desiccant-driers at the furnace, if needed. Dewpoint control is absolutely essential whenever industrial gases are used as part of anyone’s brazing operations.
And, as mentioned earlier, you cannot depend on certifications from your gas-suppliers that the gas they delivered to you is very dry, since the “dryness” of that gas can change dramatically (for the worse) when you pipe that gas to your furnace, especially if that piping system is leaky. We’ll look at that topic next month.
Dan Kay – Tel: 860-651-5595 860-651-5595: – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
To View a listing of all Dan’s articles please click here
Read Dan Kay’s Biography
© Copyright Dan Kay 2014