For the isolation test, you will need four line access Schraeder valves with pig-tails, a drum of nitrogen, and a nitrogen regulator. One feature about the inert gas nitrogen is the fact it is not affected by temperature changes as are refrigerants. The advantage of this in testing is that the pressure placed on the system will be the same when it is checked a day or two later, regardless of what the temperature is. In Fig. 25-1 the system is divided into four segments. Each segment is sealed with an access service valve hard soldered to it.
Nitrogran is then administered to each of the four segments with the use of your service manifold and gauges. Place the exact amount of pressure required in each segment. As stated before, do not exceed the specified pressure testing amounts that the manufacturer had placed on the unit. In an R-22 system, 350 psi pressure is sufficient; for R-12 system, 250 psi pressure is sufficient. The important thing to remember about this test is that the pressure amounts should be the same in the four segments. The nitrogen should be left in the system for several days, the longer the better.
When you return to the unit, place your manifold gauges on each segment and check the pressure. The unit with the leak will naturally have the least pressure. This isolation test is a costly one, but it definitely confirms which section of the system has the leak. Many times, this method reveals the leak to be in the condensing unit that you checked in the beginning. This test eliminates tearing things apart for nothing. When separating the system for this test, remember you have to put it together again, so don’t destroy piping where joints will be needed to place the system back together. If the leak is found to be in one of the lines connecting the condensing unit with the evaporator unit, a new line might be run instead of trying to repair the leak in the original one. This test gives you that option by eliminating any doubt as to where the leak is.
Tests like this one will detect a leak in an evaporator coil or condensing coil. You might then remove it from the unit and seal its ends so that it may be dipped in water, just as an inner tube is tested to find a leak. A leak in a coil might be located directly under one of the fins that is attached to the tubing. Water bubbles will show exactly where the leak was. Then the fins can be cut out of the way to make the repair.
You will be dealing with copper, steel, or aluminum when making your repair. You should be familiar with the three materials. In addition to silver solder and flux, it is advisable to carry a couple of brazing rods. A can of flux or pre-fluxed rods can be used if you store them in a dry place. Many steel parts are being used in the
field such as condenser coils, receivers, accumulators, oil separators, to name a few. A rust hole or small crack can be repaired easily
with the use of a brass brazing roa. The secret in any molten metal repair is to have the surfaces of the piece to be repaired clean and dry. Many coils are being epoxy coated at the factory to extend their service life. This must be sanded from the area to be repaired very thoroughly. Aluminum solder is used extensively in the field today. It is a special solder that needs very low heat. It is expensive, yet in comparison to the cost of a evaporator coil or condensing coil, the price of the solder is cheap. The use of this solder takes practice and is difficult to teach with a written word. Practice on an old coil first. Remember, the surface must be very clean, fluxed, and low heat must be moved constantly over the repair area. The use of the oxygen-acetylene torch is a must in this industry, and you must practice to be proficient at it.
Other means of repairing aluminum have appeared on the market, some work well. A compression type of fitting is being used where a joint of steel to aluminum is made. The device works fairly well. Another type of repair is coupling two pieces of aluminum with the use of a sleeve that is sealed in the line once in position by an epoxy. This adhesive is activated by placing heat on the fitting.