A Discussion About Application of Double Wall Brazed Steel Tubing in Air Conditioning

1. Future of Air Conditioning

After decades of opening policy and striving of Chinese AC (air condition) manufacturers, China has been recognized as the center of AC industry. The total yield of air conditioners of China in 2011 reached 139 million sets according to reports, about 80% of the global production. Meanwhile, the demand increase is being continuously driven by the urbanization policy, global warming etc.

There are two types of tubing used in the air conditioners in general:

a. Tubing for heat exchange, such as tubing for condensers, evaporators;

b. Tubing for applications other than heat exchange, such as the outdoor set tubing and connecting tubes between the indoor and outdoor sets.

The latter does need to exchange heat, thus offers possibility of replace the copper tubing by double wall copper brazed steel tubing (heat conductivity of steel is 1/8.5 of that of copper, thus steel is less likely to lose heat than copper and more energy saving).

Roughly estimating, each set of air conditioner needs 1.5kg copper tubes. The outdoor set needs 0.5kg each and the connecting tubes needs 1kg. Based on the yeild of 2011, the market is about 200,000 metric tons. If they are all replaced by double wall steel tubing, it is a great opportunity for the double wall steel tubing manufacturers and saves huge amount of copper resources.

2. Charactoristics of Bundy Tubes For AC

A. Aging Effect

It is usually a long period for AC tubing, from purchase to installation. It is even longer for indoor set tubing and the connecting tubes. Thus the aging affect of the double wall copper brazed steel tubing must be minimized to ensure flexibility for easy bending.

B. Hardening Effect

Higher hardening index n means high elongation of the tubes, and good for cold forming. On the other hand, fast hardening would get it more difficult to bend and flare. So, Bundy tubes for AC requires moderately lower hardening index for acceptable elongation.

C. Mechanical Performance

a. Tensile is not a problem. It’s 290Mpa min for the steel tubing while 270MPa min for the copper tubes.

b. Yield strength of the steel tubing is 180MPa min, while the AC industry prefers lower yield strength for easy forming of the tubes, ideally below 140Mpa.

c. Elongation: 25% is required for steel tubing, while the AC wants 38% min, and 40% preferably.

D. Chemical Compostion

According to the mechanical performance requirements, ultra low carbon steel (carbon less than 0.004%) would acheive the goals. The original carbon compsotion of the double wall steel tubing is 0.02%-0.08%.

E. Bending Test

The original standards for the steel tubing defines 3 times of the diameter as the diameter of the bending mold, for tubing no larger than 8mm. It’s 6 times for tubing diameter larger than 8mm. However, the bending of the tubes in AC is mostly less than 3 times of the tube diameter.

F. Deforming Ratio

The deforming of the ratio of the copper tubes used for AC is normally required to be 1.15-1.23. (The deforming ratio is defined as the ratio between the major axis and the minor axis of the ellipse tube section resulted from the bending).

G. Corrossion Protection

a. NSS Test

Many of the tubes are exposed in the outside atmosphere and all weathers, and it’s very common that water condensening occures on the outside surface of the tubes. Mostly 500 hr min is requested for tubes used for AC. Some manufacturers require even 1000 hour.

b. Welding Effect

The tubes are sometimes jointed by welding, The welding temperature is high above 800℃, and the temperature of the flames are even higher. The outside finish of the tubes are not to be melted or compromised.

c. Elongation

The tubes are to be coated with protective coatings against corrosion, and the coatings should not fail after the tubes are bended, flared or shrinked.

d. Weldability

The outside surface must be able to be welded with the copper pipes. This requirements limits the outside finish types and also raised the difficulty of welding.

H. Inside Cleanness

The residual of the brazed steel tubing is 160mg/m2, while AC requires 25~38mg/m2.

3. Conclusion

The AC copper pipes are manually welded. The welding quality depends on operators experience and skills, and there is a high possibility of cold solder, due to since manual welding can not be controlled exactly in temperature and welding time. The effect of welding to the surface finishes also might become a major problem for the replacement.

We have done some study and experiments . Samples were tested on AC and a report of over 40 pages was concluded. Recognition  has come from major AC makers after real product tests and we are looking forward to next stage of small quantity application.

Based on the above discussion, our conclusion is that there is high feasibility of replacing the copper tubes with double wall brazed steel tubes. However, there are tech obstacles to be solved and they challenge the whole industry.

Meanwhile, it takes a longer time for the AC industry to verify and accept new techs. To replace the copper tubes w ith double wall copper brazed steel tubes, there is a long way ahead.


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