BRASS AND COPPER TUBE SELECTION GUIDE
2.1 INTRODUCTION
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.
TABLE 2.1 TUBE ENVIRONMENT
|
Alloy
Number |
Material
|
Fresh
Water (Lakes Rivers) |
Salt
or Brackish Water |
Steam
or Concentrate |
Hydrogen
Sulphide & Active Sulpher Compounds |
Weak
Mineral Acids – Acid Mine Water |
Weak
Alkalis Except Ammonia |
Velocity
|
|
|
Under
7.0 ft per sec |
Over
7.0 ft. per sec |
||||||||
|
122
|
Deoxidized Copper
|
X
|
|
X
|
|
X
|
X
|
X
|
|
|
142
|
Arsenical Copper
|
X
|
|
X
|
|
X
|
X
|
X
|
|
|
443
|
Arsenical Admiralty
|
X
|
X
|
X
|
X
|
X
|
X
|
X
|
|
|
445
|
Phosphorized Admiralty
|
X
|
X
|
X
|
X
|
X
|
X
|
X
|
|
|
687
|
Aluminum Brass
|
|
X
|
X
|
X
|
|
|
X
|
X
|
|
706
|
Copper Nickel, 10%
|
X
|
X
|
X
|
|
X
|
X
|
X
|
X
|
|
715
|
Copper Nickel, 30%
|
X
|
X
|
X
|
X
|
X
|
X
|
X
|
X
|
* An “X” under a particular heading indicates that the material has served satisfactorily under the outlined condition
2.2.1 COPPER AND BRASS ALLOYS AVAILABLE FOR SELECTION AS
HEAT EXCHANGER TUBE MATERIALS
Table 2.1.1 lists the nominal composition and some properties of standard alloys for heat exchanger tube. A brief description of these alloys and general applications are listed.
2.2.1.1 Deoxidized Copper — Alloy No. 122 Nom. Composition Copper 99.90% min., Phosphorus 0.02%.
Deoxidized copper is commercially pure copper deoxidized with phosphorus. Tubes of this material have found wide usage in sugar refining, chemical processing, feed water heaters and condensers, air-conditioners and refrigerators, cryogenic exchangers and evaporators and distribution lines for hot and cold waters in domestic, industrial and commercial applications. Deoxidized copper has the highest thermal conductivity of any of the standard heat exchanger alloys.
Deoxidized copper has excellent resistance to many chemicals, such as organic acids, alkalis and neutral salts as well as fresh water of low velocities. Where the acid or alkali solutions are aerated, the velocities are high or considerable turbulence is experienced, the corrosion of deoxidized copper may be more severe. Deoxidized copper is generally not recommended for handling liquids, vapors, or gases having a relatively high concentration of hydrogen sulfide or similar sulfur compounds or in those installations where appreciable concentrations of ammonia or ammonium compounds are in contact with the tubes.
2.2.1.2 Arsenical Copper — Alloy No. 142 Nominal composition Copper 99.7% min. Arsenic 0.3%
Arsenical copper has corrosion-resistant properties comparable to those of Deoxidized Copper and is used in similar applications. This alloy is particularly popular in units using clean, fresh river or lake water, such as feed water heaters operating at low-to-medium temperatures, velocities and pressures. Alloy 142 serves well in circulating systems using spring, ponds and cooling towers. The alloy is not recommended for use in sea or brackish water or waters containing ammonia, carbon dioxide or sulphur compounds.
Advantages over Deoxidized Copper are
- somewhat higher strength at temperatures up to 570F as well as ambient temperatures
- slightly better resistance to erosion and corrosion
- better resistance to corrosion by localized pitting.
- The thermal and electrical conductivities are lower than the corresponding values for Deoxidized Copper.
2.2.1 COPPER AND BRASS ALLOYS AVAILABLE FOR SELECTION AS
HEAT EXCHANGER TUBE MATERIALS
2.2.1.3 Phosphorized Admiralty Type D Alloy No. 445 Nom. composition Copper 71%, Tin 1%, Phosphorus 0.03% Zinc remainder.
The Admiralty gave their name to an alloy of 70 percent copper and 29 percent zinc and 1 percent tin after research verified improved corrosion resistance. In the 1930’s it was found that small additions of an inhibitor resulted in marked increase in resistance of this alloy to dezincification.
Inhibitors which are currently used are phosphorus, arsenic or antimony and the term Inhibited Admiralty would mean one of the inhibitors is present. Phosphorized Admiralty developed and patented by Scovill contains 0.03 phosphorus. Our tests and service experience of several years has verified the outstanding resistance of this material to dezincification in diverse service applications. The alloy was specifically developed for installations where severe dezincification had occurred but where the desirable characteristics of Admiralty were generally accepted for a wide variety of service conditions. Phosphorized Admiralty tubes are often used in equipment operating at temperatures of 400F or higher. The alloy has a tensile strength of approximately 55,000 psi in the finish annealed condition together with excellent ductility and retains its strength up to a temperature of approximately 570F. Generally, this alloy performs best with fluid velocities under 6-7 ft/sec.
2.2.1.4 Arsenical Admiralty Type B Alloy No. 443 Nominal Composition: Copper 71%, Tin 1%, Arsenic 0.03%, Zinc Remainder.
The uses, properties and applications for Arsenical Admiralty are the same as those for Phosphorized Admiralty. The alloys can be used interchangeably for most applications. The only difference between alloy 443 and 445 is that Arsenic is added as the inhibitor instead of Phosphorus to control resistance to dezincification. The choice of whether Phosphorized or Arsenical Admiralty tube is used in an application is usually dictated by the purchaser of the tube, however, we believe that Phosphorized Admiralty is superior. Under some conditions of service, particularly in contact with slightly acid, low velocity media at temperatures over 100F, Arsenical Admiralty may undergo a severe type of integranular dezincification which may cause accelerated pitting or corrosion-fatigue cracking. Under these conditions we recommend Phosphorized Admiralty.
2.2.1.5 — Aluminum Brass (Type B) — Alloy No. 687 Nom. Composition Copper 77%, Aluminum 2%, Arsenic 0.03%, Zinc remainder.
The addition of 2 percent aluminum to a copper-zinc alloy to make Aluminum Brass (Type B) significantly improves the resistance to corrosion by salt or brackish waters and to combined erosion and corrosion in salt water service. The excellent corrosion resistance of this alloy can be attributed to the nature of the film which develops over the surface of the tube. In service, this film is thin, adherent, continuous and compact and gives excellent protection to the metal surface even under very abrasive type water conditions.
In the case of a localized rupture of this film as might occur through abrasion or erosion of the tube surface, this film is found to be self-healing.
This characteristic gives added protection to Aluminum Brass in marine and tidewater steam condensers where circulating water velocities of 8-9 feet per second, together with turbulence and air impingement, have a particularly severe local wearing action. Aluminum Brass tubes have also given good service in polluted sewage and industrial wastes containing sulphides and oil refining applications where sea water is the coolant. This alloy has been found to give satisfactory service in some desalination installations.
2.2.1 COPPER AND BRASS ALLOYS AVAILABLE FOR SELECTION AS
HEAT EXCHANGER TUBE MATERIALS
2.2.1.6 Copper Nickel, 30% — Alloy No. 715 Nom. composition Copper 69.5%, Nickel 30%, Iron 0.6%
Copper Nickel, 30% give the best service life under the most adverse conditions. in some condensers the tubes at the top of the bundles or tubes in the non-condensible gas sections are made from Copper Nickel, 30% whereas the balance of a unit can be tubed with an Admiralty alloy.
In the text books listing conductivities Copper Nickel, 30% has about one-third the thermal conductivity of Phosphorized Admiralty but experience has indicated that under operating conditions they have nearly equal heat transfer. A design engineer may provide about 10 percent additional surface in a unit when using copper nickel tubes as compared to Phosphorized Admiralty or Aluminum Brass.
In Naval vessels operating at high speeds, Copper Nickel, 30% has excellent resistance to high velocity and sea water turbulence.
Copper Nickel, 30% tubes are used in heat exchangers where severe erosion and corrosion have been experienced, especially at elevated temperatures, high velocities (12-15 feet per second) and extreme turbulence of the circulating media. In feed water heaters and in some oil refining units operated at relatively high temperatures in contact with corrosive media, especially chlorides good service results have been found. This alloy is one of the preferred alloys for use in desalination installations.
Copper Nickel, 30% has the highest resistance to stress-corrosion cracking
2.2.1.7 Copper Nickel, 10% — Alloy No. 706 Nom. Composition Copper 88.7%, Iron 1.3, Nickel 10%
Copper Nickel, 10% has shown excellent resistance to corrosion by sea and brackish as well as high resistance to erosion-corrosion and air-impingement attack. The deliberate addition of 1.3 percent iron to this alloy has resulted in Copper Nickel, 10% performing as well as Copper Nickel, 30% in salt water service heat exchangers. It is especially suited for high velocities (10-12 feet per second) only slightly lower than Copper Nickel, 30%. It is only slightly less resistant to stress-corrosion cracking than Copper Nickel, 30%. The alloy has only fair resistance to corrosion by hydrogen sulfide and similar sulfur compounds.
Copper Nickel, 10% has good strength and ductility at ordinary temperatures and relatively high strength and other mechanical properties at elevated temperatures. The U.S. Navy has made widespread use of the alloy in condenser tubes and salt water piping. This alloy is commonly used in high temperature applications as high pressure, high temperature feed water heaters where the optimum properties of Copper Nickel, 30% are not required. It is expected that this alloy will find widespread use in desalination installations
TABLE 2.1.1 NOMINAL COMPOSITION AND PROPERTIES OF HEAT EXCHANGER TUBE ALLOYS
(See Section 2.6 for detailed mechanical properties)
|
Alloy
Number |
Material
|
Nominal
Chemical Composition1 percent |
Annealed Temper (0.015mm)
|
Fatigue
Strength at 100,000,000 cycles psi |
Percent
Expansion3 minimum |
Linear Expansion
|
Thermal
Conductivity at 68 F Btu per ft per sq ft per hr per degree F |
||||
|
Tensile
Strength |
Yield
Strength (0.5% Ext. under Load) psi |
Elonga-
ation in 2 inches percent |
Density
at 68F lb per cu in. |
Coefficient
per degree F from 68 to 572 F x 10 |
in
10 ft length per 10 deg F inches |
||||||
|
122
|
Deoxidized Copper
|
Copper 99.90 min
Phosphorus 0.02 |
40,0002
|
32,0002
|
252
|
13,0002
|
202
|
0.323
|
9.8
|
0.0118
|
196
|
|
142
|
Arsenical Copper
|
Copper 99.7 min.
Arsenic 0.3 Phosphorus 0.2 |
41,0002
|
32,0002
|
242
|
13,0002
|
20
|
0.323
|
9.8
|
0.0118
|
112
|
|
443
|
Arsenical Admiralty
|
Copper 71
Zinc 28 Tin 1 Arsenic 0.04 |
56,000
|
23,000
|
60
|
16,000
|
20
|
0.308
|
11.2
|
0.0134
|
64
|
|
445
|
Phosphorizeed Admiralty
|
Copper 71
Zinc 28 Tin 1 Phosphorus 0.04 |
56,000
|
23,000
|
60
|
16,000
|
20
|
0.308
|
11.2
|
0.0134
|
64
|
|
687
|
Aluminum Brass
|
Copper 77.5
Zinc 20.5 Aluminum 2 Arsenic 0.05 |
62,000
|
28,000
|
54
|
17,000
|
20
|
0.301
|
10.3
|
0.0124
|
58
|
|
706
|
Copper Nickel, 10%
|
Copper 88.7
Nickel 10 Iron 1.3 |
47,000
|
18,000
|
43
|
19,000
|
30
|
0.323
|
9.5
|
0.016
|
26
|
|
715
|
Copper Nickel, 30%
|
Copper 69.4
Nickel 30 Iron 0.6 |
62,000
|
26,000
|
46
|
20,000
|
30
|
0.323
|
9.0
|
0.0108
|
17
|
1 For chemical limits see ASTM Specification Bill, Table 2
2 Values of Tensile Strength, Yield Strength, Elongation, Fatigue Strength and Expansion for Deoxidized Copper — No. 122 and Arsenical Copper — No. 142 only are for Light Drawn Temper
3 Expansion of the tube inside diameter versus original inside diameter; pin shall have a taper of 1-1/2 inches/foot (approximately 7 degree included angle) from ASTM