Copper evaporator coils are highly efficient at transferring heat, but they are vulnerable to micro-leaks over time. The primary cause of evaporator leaks is formicary corrosion, where household organic acids react with moisture on the copper bends, forming microscopic tunnels.
Finding Micro-Leaks: The Nitrogen Method
Soap water testing only finds large leaks. For micro-leaks, follow the Nitrogen Pressure Decay method:
1. **Isolate the Coils:** De-solder the connecting pipes and seal the ends of the evaporator coil using brass flare plugs.
2. **Pressurize with Dry Nitrogen:** Connect a manifold gauge set to a nitrogen cylinder. Slowly inject nitrogen gas up to **350 PSI**. *Never use oxygen or compressed air, as moisture and oil will contaminate the system.*
3. **Monitor Pressure:** Observe the pressure gauge over 12 to 24 hours. A drop of even 2 PSI indicates a micro-leak.
4. **Halogen Bubble Search:** Brush a thick soap-water solution over all U-bends and capillary joints. Bubble expansion indicates a leak spot.
Step-by-Step Copper Brazing Protocol
1. **Prep the Joint:** Clean the leak area using a wire brush until the copper surface is bright and clean.
2. **Nitrogen Purging:** Run a tiny, low-pressure flow of nitrogen through the coil during brazing. This displaces oxygen and prevents black copper oxide scale from forming inside the pipe, which could clog the expansion capillary later.
3. **Torch Setup:** Set your oxy-acetylene torch to a neutral flame.
4. **Heat Control:** Heat the copper joint until it glows cherry red (approx. 700°C).
5. **Apply Rod:** Apply a silver-phosphorus brazing rod (typically 2% to 5% silver content) directly to the hot joint. Let the capillary action pull the molten silver alloy into the leak joint.
6. **Cool and Inspect:** Allow the joint to cool naturally. Re-pressurize with nitrogen to verify the seal before vacuuming and charging.