Heat exchangers are devices that transfer heat to achieve desired heating or cooling. An important design aspect of heat exchanger technology is the selection of appropriate materials to conduct and transfer heat fast and efficiently.
Copper has many desirable properties for thermally efficient and durable heat exchangers. First and foremost, copper is an excellent conductor of heat. This means that copper's high thermal conductivity allows heat to pass through it quickly. Other desirable properties of copper in heat exchangers include its corrosion resistance, biofouling resistance, maximum allowable stress and internal pressure, creep rupture strength, fatigue strength, hardness, thermal expansion, specific heat, antimicrobial properties, tensile strength, yield strength, high melting point, alloy, ease of fabrication, and ease of joining.
The combination of these properties enable copper to be specified for heat exchangers in industrial facilities, HVAC systems, vehicular coolers and radiators, and as heat sinks to cool computers, disk drives, televisions, computer monitors, and other electronic equipment. Copper is also incorporated into the bottoms of high-quality cookware because the metal conducts heat quickly and distributes it evenly.
Non-copper heat exchangers are also available. Some alternative materials include aluminum, carbon steel, stainless steel, nickel alloys, and titanium.
This article focuses on beneficial properties and common applications of copper in heat exchangers. New copper heat exchanger technologies for specific applications are also introduced.
History Heat exchangers using copper and its alloys have evolved along with heat transfer technologies over the past several hundred years. Copper condenser tubes were first used in 1769 for steam engines. Initially, the tubes were made of unalloyed copper. By 1870, Muntz metal, a 60% Cu-40% Zn brass alloy, was used for condensers in seawater cooling. Admiralty metal, a 70% Cu-30% Zn yellow brass alloy with 1% tin added to improve corrosion resistance, was introduced in 1890 for seawater service.Gaffoglio, Carl J., Copper alloy surface condenser tube application and service considerations; CDA Heat Exchange Seminars; Copper Development Association By the 1920s, a 70% Cu-30% Ni alloy was developed for naval condensers. Soon afterwards, a 2% manganese and 2% iron copper alloy was introduced for better erosion resistance. A 90% Cu-10% Ni alloy first became available in the 1950s, initially for seawater piping. This alloy is now the most widely used copper-nickel alloy in marine heat exchangers.
Today, steam, evaporator, and condenser coils are made from copper and copper alloys. These heat exchangers are used in air conditioning and refrigeration systems, industrial and central heating and cooling systems, radiators, hot water tanks, and under-floor heating systems.
Copper-based heat exchangers can be manufactured with copper tube/aluminum fin, cupro-nickel, or all-copper constructions. Various coatings can be applied to enhance corrosion resistance of the tubes and fins.10 tips on getting the most from your coil; Super Radiator Coils; http://www.srcoils.com/wp-content/blogs.dir/1/files/2010/05/T003-10-Tips.pdf
Beneficial properties of copper heat exchangers
Thermal conductivity Thermal conductivity (k, also denoted as λ or κ) is a measure of a material's ability to conduct heat. Heat transfer across materials of high thermal conductivity occurs at a higher rate than across materials of low thermal conductivity. In the International System of Units (SI), thermal conductivity is measured in watts per meter Kelvin (W/(m•K)). In the Imperial System of Measurement (British Imperial, or Imperial units), thermal conductivity is measured in Btu/(hr•ft⋅F).
Copper has a thermal conductivity of 231 Btu/(hr-ft-F). This is higher than all other metals except silver, a precious metal. Copper has a 60% better thermal conductivity rating than aluminum and has almost 30 times more thermal conductivity than stainless steel.
+ Thermal conductivity of some common metals Metal Thermal conductivity (Btu/(hr-ft-F)) (W/(m•K)) Silver 247.87 429 Copper 231 399 Gold 183 316 Aluminium 136 235 Yellow brass 69.33 120 Cast iron 46.33 80.1 Stainless steel 8.1 14.0
Further information about the thermal conductivity of selected metals is available.
Corrosion resistance Corrosion resistance is essential in heat transfer applications where fluids are involved, such as in hot water tanks, radiators, etc. The only affordable material that has similar corrosion resistance to copper is stainless steel. However, the thermal conductivity of stainless steel is 1/30th times than that of copper. Aluminum tubes are not suitable for potable or untreated water applications because it corrodes at pH<7.0 and so it releases hydrogen gas.Finned coil heat exchangers (water to air heat exchangers); Brazetek; http://www.brazetek.com/water-to-air-heat-exchangersT.E. Larson, Corrosion by domestic waters; ISWS-75 Bulletin 59; State of Illinois Department of Registration and Education; Page 29.D.N. Fultonberg; Corrosion of aluminum in water; Westinghouse Electric Corp. Contract NAS 3-5215, prepared for the Lewis Research Center, U.S. National Aeronautics and Space Administration; Page 3.
Protective films can be applied to the inner surface of copper alloy tubes to increase corrosion resistance. For certain applications, the film is composed of iron. In power plant condensers, duplex tubes consisting of an inner titanium layer with outer copper-nickel alloys are employed. This enables the use of copper’s beneficial mechanical and chemical properties (e.g., stress corrosion cracking, ammonia attack) along with titanium’s excellent corrosion resistance. A duplex tube with inner aluminium brass or copper-nickel and outer stainless or mild steel can be used for cooling in the oil refining and petrochemical industries.Kobelco: Copper alloy tubes for heat-exchanger; Shinko Metal Products, Japan; http://www.shinkometal.co.jp/catalog/copperalloy-en-sc.pdf
Biofouling resistance Copper and copper-nickel alloys have a high natural resistance to biofouling relative to alternative materials. Other metals used in heat exchangers, such as steel, titanium and aluminum, foul readily. Protection against biofouling, particularly in marine structures, can be accomplished over long periods of time with copper metals.
Copper-nickel alloys have been proven over many years in sea water pipework and other marine applications. These alloys resist biofouling in open seas where they do not allow microbial slime to build up and support macrofouling.Powell, C.A.; Preventing biofouling with copper-nickel; Copper Development Association, October 2002;
Researchers attribute copper's resistance to biofouling, even in temperate waters, to two possible mechanisms: 1) a retarding sequence of colonization through slow release of copper ions during the corrosion process, thereby inhibiting the attachment of microbial layers to marine surfaces;Sutherland, I.W., 1983, Microbial exopolysaccarides: Their role in microbial adhesion in aqueous systems, Critical Reviews in Microbiology, Vol. 10, pp.173–201 and/or, 2) separating layers that contain corrosive products and the larvae of macro-encrusting organisms.Edding, Mario E., Flores, Hector, and Miranda, Claudio, (1995), Experimental Usage of Copper-Nickel Alloy Mesh in Mariculture. Part 1: Feasibility of usage in a temperate zone; Part 2: Demonstration of usage in a cold zone; Final report to the International Copper Association Ltd. The latter mechanism deters the settlement of pelagic larval stages on the metal surface, rather than killing the organisms.
Antimicrobial properties Due to copper’s strong antimicrobial properties, copper fins can inhibit bacterial, fungal and viral growths that commonly build up in air conditioning systems. Hence, the surfaces of copper-based heat exchangers are cleaner for longer periods of time than heat exchangers made from other metals. This benefit offers a greatly expanded heat exchanger service life and contributes to improved air quality. Heat exchangers fabricated separately from antimicrobial copper and aluminum in a full-scale HVAC system have been evaluated for their ability to limit microbial growth under conditions of normal flow rates using single-pass outside air. Commonly used aluminum components developed stable biofilms of bacteria and fungi within four weeks of operation. During the same time period, antimicrobial copper was able to limit bacterial loads associated with the copper heat exchanger fins by 99.99% and fungal loads by 99.74%.Michels, H. (2011). Copper air quality program. Annual Report #4, prepared for U.S. Army Medical Research and Materiel Command, Financial Times. Detrick, Maryland.Copper Helps Shanghai Bus Users Breathe Easy: http://www.microgroove.net/press/copper-helps-shanghai-bus-users-breathe-easy
Copper fin air conditioners have been deployed on buses in Shanghai to rapidly and completely kill bacteria, viruses and fungi that were previously thriving on non-copper fins and permitted to circulate around the systems. The decision to replace aluminum with copper followed antimicrobial tests by the Shanghai Municipal Center for Disease Control and Prevention (SCDC) from 2010 to 2012. The study found that microbial levels on copper fin surfaces were significantly lower than on aluminum, thereby helping to protect the health of bus passengers.Jiangping, C. (2011). Year 2011 Research Report for the Comparative Analysis of Antimicrobial Capability for Copper and Aluminum Fin Radiators in Air Conditioners of Public Buses, Shanghai Municipal Center for Disease Control and Prevention, Environmental Health Section, International Copper Association.
Further information about the benefits of antimicrobial copper in HVAC systems is available.Michel, J. 2012. Your new weapon in the fight against hospital-acquired infections; Antimicrobial copper webinar presented by Modern Healthcare, 12 September 2012; Webinar ID: 883-480-666.Feigley, C. 2011. Copper heat exchangers for improving indoor ait quality: Cooling season at Ft. Jackson. Paper #919, Proceedings of Indoor Air 2011. 12th International Conference on Indoor Air Quality and Climate; Austin, Texas, USA, June 2011
Ease of inner grooving
Internally grooved copper tube of smaller diameters is more thermally efficient, materially efficient, and easier to bend and flare and otherwise work with. It is generally easier to make inner grooved tubes out of copper, a very soft metal.
Common applications for copper heat exchangers
Industrial facilities and power plants Copper alloys are extensively used as heat exchanger tubing in fossil and nuclear steam generating electric power plants, chemical and petrochemical plants, marine services, and desalination plants.
The largest use of copper alloy heat exchanger tubing on a per unit basis is in utility power plants. These plants contain surface condensers, heaters, and coolers, all of which contain copper tubing. The main surface condenser that accepts turbine-steam discharges uses the most copper.
Copper nickel is the group of alloys that are commonly specified in heat exchanger or condenser tubes in evaporators of desalination plants, process industry plants, air cooling zones of thermal power plants, high-pressure feed water heaters, and sea water piping in ships. The composition of the alloys can vary from 90% Cu–10% Ni to 70% Cu–30% Ni.