Brazing Components for Refrigeration
Today, we address correct brazing alloys as well as proper brazing methods for refrigeration components. Our main topics are:
- Alloy selection
- Gas and torch selection
- Brazing method
- Nitrogen purge
- System evacuation and charge
Remember, brazing in the HVAC/R industry is much different from brazing in the plumbing industry. Solder alloy is adequate for plumbing components due to the prevalence of low-pressure water or fluid lines. However, refrigeration cycles and air conditioning systems are high-pressure, high-temperature systems requiring true braze alloys that are stronger than solder alloys.
The HVAC/R industry uses these four common base materials:
From these four base metals, there are five different braze alloys:
- Copper-to-copper - This is the most common joint application. The recommended product is Lucas-Milhaupt's Sil-Fos® alloy, containing a chemical composition range of 5-15% silver. With Sil-Fos, you do not need paste flux, because Sil-Fos alloys contain a fluxing agent. Its phosphorus content acts as a reducing agent to remove the oxides formed during brazing. Copper-to-copper brazing with a Sil-Fos alloy is the only application that does not require a paste flux or using a flux-cored or -coated braze alloy.
- Copper-to-brass - Another common HVAC/R application is copper-to-brass joints. We recommend Lucas-Milhaupt's Sil-Fos 15, Silvaloy® 450, or Silvaloy 560 alloy. Unlike copper-to-copper brazing with a Sil-Fos alloy, copper-to-brass applications do require the addition of a paste flux or the use of a flux-cored or flux-coated brazing rod. Paste flux is required when using solid wire Silvaloy 450 or 560 filler metals.
- Copper-to-steel - The next HVAC/R application is copper-to-steel joints. We recommend either flux-cored or flux-coated Silvaloy 450 or Silvaloy 560 alloy. You can also use solid wire, but you must braze this with a paste flux such as Lucas-Milhaupt Handy® Flux or Ultra Flux®.
- Copper-to-aluminum and aluminum-to-aluminum - In the air conditioning industry, there are two more common joint applications: copper-to-aluminum and aluminum-to-aluminum joints. The alloys we recommend for these joint applications are Handy One AL 802 and Handy One AL 822. Handy One AL 822 is recommended for aluminum repair due to its wider melting range.
Some refrigeration joints may require joining stainless steel valves or tubing to other base materials. When brazing any stainless steel components, the use of Silvaloy 505 with Handy Type B-1 flux or Ultra Black Paste Flux is recommended.
Alternative filler metals are available for brazing in this industry, Please contact your Technical Services representative for more information.
Gas and Torch Selection
After determining which alloy to use for your application, the next step is selecting the correct gas and torch type. There are four main gasses used in the HVAC/R industry for brazing:
- Propane gas - This gas has a flame temperature of roughly 1800°F (982°C). Use this gas primarily for aluminum-to-aluminum joints and aluminum-to-copper joints, as it has the lowest flame temperature.
- MAP Pro gas - MAP Pro gas is a propylene-propane gas mixture with a flame temperature of roughly 2200°F (1204°C). It can be used for aluminum-to-aluminum joints and aluminum-to-copper joints.
- Air-acetylene - This gas has a flame temperature of around 2700°F (1482°C). This type of brazing requires a cylinder of acetylene gas and incorporates atmosphere air to produce the torch flame. Air-acetylene is primarily used for copper-to-copper, copper-to-brass and copper-to-steel joints. It is not recommended for aluminum joints, as the flame temperature is significantly higher than propane or MAPP gas and can melt the aluminum base metals.
- Oxy-acetylene - This gas has a flame temperature of roughly 4700°F (2593°C). Oxy-acetylene brazing uses a cylinder of acetylene gas and a cylinder of oxygen to create the oxy-acetylene mixture. You can use it for brazing copper-to-copper, copper-to-brass and copper-to-steel joints.
Once you have selected the proper gas, choose the correct torch tip before starting to braze. Each torch body manufacturer has a list of recommended torch tips, based on the size of the tubes being brazed. Contact your torch body manufacturer for proper sizes.
After selecting the correct gas and torch type, it is time to braze. Remember these key steps to producing a leak-proof, quality joint:
- Good fit - Make sure the joints fit together well, and the clearance is 0.002-0.005 in. (0.05-0.13 mm).
- Clean metals - Surfaces must be clean and free of contaminants. Clean refrigeration oils and dirt or soot off parts before installing new components or when repairing leaks in installed systems. Also be sure to properly cut and ream tubes to remove any burs.
- Proper flux - Use the correct flux for each application of base metals. Flux for this industry comes in three forms: paste, flux cored and flux coated. Use paste flux with any solid wire Silvaloy product. When brazing with aluminum, you do not need additional flux, as the flux is inside the wire of the alloy.
- Fixturing of Parts - Make sure that all tubes seat against the bottom of valves and fittings.
- Proper heating - When brazing in HVAC/R, use the correct heating method to form leak-proof joints. When heating the joints, apply uniform heat over both the fitting and the tube by constantly moving the torch back and forth over the joint.
- Final cleaning - After brazing with either paste flux or with flux-cored or -coated alloy, the flux residue must be cleaned off the parts to prevent corrosion. Remove residue using hot water and mechanically cleaning the flux off the joints. You do not need to clean flux from flux-cored aluminum products.
When brazing in HVAC/R, keep the inside of the tube free of oxidation by purging the system with nitrogen. Nitrogen acts as a cover gas to prevent surface oxidation inside of the tubes. Note that the industry's move toward polyolester oil (or POE oil), requires purging with nitrogen during brazing. POE oil is very hygroscopic (water loving) and reacts with residual moisture in the system if not purged with nitrogen.
System Evacuation and Charge
Once brazing is complete, and you have removed flux residue from parts, the next step is checking for leaks in the system. To ensure that moisture is vacated, the lines must have an evacuation to 500 microns, measured with a micron gauge. If you cannot evacuate the system to 500 microns, this may indicate a leak. Finally, charge the system with the required refrigerant per customer or industry specification.
In summary, choosing the correct brazing alloy and flux, the correct torch and gas, and then following the proper brazing methods helps you produce leak-proof, quality HVAC/R joints. Following industry-specific practices of purging, then charging the system to the correct pressure, prevents contamination and joint failure.
This article is Part 2 of a two-part series on brazing for refrigeration. See Part 1: Brazing for the Refrigeration Cycle for the basic processes within a refrigeration system. As an additional resource, our video on Brazing Copper to Copper will walk you through the brazing process. For repairing aluminum joints, please see our video on Aluminum HVAC/R repair.
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Questions about brazing? Contact us for further assistance. For more information on HVAC/R brazing, see our blogs and website. For detailed questions regarding specific applications, please call Lucas Milhaupt's Technical Department at 800.558.3856.