BRASS AND COPPER TUBE SELECTION GUIDE
Condenser and Heat Exchanger Tubes are required to transfer heat in a wide variety of operating conditions and to resist corrosion for the longest period of time possible. Operating conditions may cover temperature ranges from sub zero to 1000F, fluid velocities from 1-15 feet per second and pressures from vacuum to 1000 psi. Depending upon the exchanger design, purpose and location, the media being heated or cooled may be corrosive. The media is normally (1) cooling water, fresh, sea, brackish or chemically treated (2) steam and condensate, or (3) chemicals and petrochemicals exposed to the inside or outside surface of the tube. Copper alloy tubes have been used for heat exchangers for over 100 years being specified in the British Admiralty in 1870. Under some operating conditions units have been known to give good service for 25 yrs. The continued use of copper alloys has been the result of their proven reliability and recognition. No single tube alloy will give equally satisfactory performance under all operating conditions, however, the copper alloys are outstanding for their anti fouling characteristics. They do not support marine growth.
2.2 SOME FACTORS AFFECTING TUBE MATERIAL SELECTION
The copper alloys have good mechanical, physical and joining characteristics in addition to their excellent corrosive resistance.
Mechanically, the copper alloys have excellent ductility to permit easy tube installation by expansion, belling, or rolling to insure a tight seal of tubes into tube sheets. They have high strength and rigidity which allow easy handling and feeding into tube sheet holes free of bending, kinking and denting. The availability of copper alloy integral finned and U bend tubes are examples of their ductility capabilities. The excellent physical properties of copper alloys give them higher thermal conductivities than most alloys and a predictable coefficient of expansion. Also these alloys can be readily joined by brazing or welding in assembling many types of heat exchanger equipment.
The selection of the best tube alloy for a particular application is normally based upon alloy corrosion resistant characteristics in addition to resistance to operating temperatures and pressures. Each of these factors is described in the following paragraphs, and a quick reference tube environment summary is presented in Table 2.1.
In making an alloy selection based upon the corrosive environment, additional factors that must be considered include:
- Past performance or life of materials under similar service conditions.
- Chemical composition of the media contacting the surfaces of the tubes.
- Operating and tube wall temperatures.
- Fluid velocities.
- Unusual contamination of water supplies — silt, sand, flyash, marine life.
- Location in unit of most prevalent failures.
- Study of the condition of baffles, plates and tube sheet holes.
- Study of the type of corrosion experienced in similar units.
The copper alloys offer groups of materials that fall into alloy families that have selective resistance to certain corrosive environments. Basically the industry offers coppers, tin brasses or admiralties, aluminum brasses and copper nickels.