Why Copper Plates Perform Well in Heat Exchangers
Copper plates are used in heat exchangers as tube sheets, baffles, plate coils, stamped exchanger plates and fin stock because copper does a combination of jobs at once: it conducts heat faster than almost any other common engineering metal, it resists corrosion in water and many process fluids, it stays ductile enough to be formed and expanded, and it tolerates the thermal cycling of start-up and shut-down without cracking.
The sections below compare copper with the other materials normally considered for exchanger plate and sheet, and set out the service limits that designers should respect.
Thermal Conductivity Compared with Other Materials
| Material | Typical thermal conductivity, W/(m·K) | Relative performance |
|---|---|---|
| Copper (C11000, annealed) | Approximately 386 | Reference material; fastest heat transfer of the common metals |
| Aluminium | Approximately 237 | About 60% of copper; the usual alternative where weight matters |
| Carbon steel | Approximately 50 | Roughly one seventh of copper |
| Austenitic stainless steel | Approximately 16 | Roughly one twenty-fourth of copper |
Under the same conditions and with the same wall thickness, a copper plate therefore transfers heat from one side to the other much faster than aluminium, carbon steel or stainless steel, which lowers the thermal resistance of the wall and reduces the surface area needed for a given duty. Alternatively, for a fixed area the copper exchanger runs with a smaller temperature difference between the hot and cold streams, which is valuable in refrigeration, air conditioning and low-grade heat recovery applications where the driving temperature difference is small.
Corrosion Behaviour and Service Limits
In corrosive environments such as those found in chemical and pharmaceutical plants, copper forms a dense, adherent oxide film that slows further attack by the process medium and extends the service life of the exchanger. Copper plate also performs well in fresh water, in most neutral aqueous solutions and in the condensate and refrigerant circuits of cooling systems.
Copper is not universal, and the limits are worth stating plainly. Copper is attacked by ammonia and ammonium compounds, which can cause rapid stress-corrosion cracking of stressed plate, so copper exchangers should not be used where ammonia, ammonium hydroxide or ammoniacal cleaning agents are present. Copper is also unsuitable for strongly oxidising acids, for sulfide-bearing streams and for mercury contact. Where water velocity is high or where air is entrained, copper plate can suffer impingement attack, particularly at inlet areas and around baffle edges, so inlet velocities and flow distribution should be controlled. Above roughly 200 °C the allowable design stress of annealed copper falls sharply, and high-temperature duties are usually better served by steel or stainless steel.
Strength, Fabrication and Design Considerations
Copper plate has moderate strength and high ductility. The relatively high strength and hardness of work-hardened copper let it withstand the pressures and mechanical loads of exchanger service without deforming easily, which matters because exchangers must absorb pressure fluctuations and the thermal expansion and contraction of every operating cycle while keeping the tube-to-tubesheet joints tight and the structural stability of the unit intact.
Copper also processes well. It can be rolled to tight thickness tolerances, blanked and stamped into complex plate geometries, pressed into fin and plate patterns, and formed into coils and headers. Welding, brazing and soldering are all used; copper-to-copper joints are normally brazed with copper-phosphorus filler, while copper to steel joints need a filler and flux combination suited to the dissimilar metals. Because copper work hardens, severe forming operations should be planned with intermediate annealing, and plate for tube sheets should be ordered in a temper that keeps the ligament material ductile enough for roller expansion.
Standard supply for exchanger plate and sheet includes C11000 electrolytic tough pitch copper, C10200 oxygen-free copper and C12200 deoxidised high phosphorus copper, in thicknesses from thin sheet to heavy plate and in a range of tempers. Flat rolled copper and copper alloy products for general purposes are covered by ASTM B152/B152M and by GB/T 2040 and EN 1652 for plate in the corresponding grades and tempers.
Applications
Shell-and-tube exchangers: copper tube sheets, support plates and baffles, usually paired with copper or copper-nickel tube.
Plate heat exchangers and brazed plate units: stamped plates, and copper foil and sheet used as the brazing medium in brazed assemblies.
Refrigeration and air conditioning: plate coils, accumulator and receiver internals, and fin stock for coils.
Industrial and construction: heat transfer plates for pools and district heating, tank coils, and electrical busbar where copper plate doubles as a conductor and a heat spreader.
Marine and offshore: plate for seawater systems where the chloride stress-corrosion cracking risk of austenitic stainless steel is unacceptable and the duty calls for a copper alloy.
Common Pitfalls and Inspection Points
Most failures of copper plate in exchangers come from design and workmanship rather than from the material itself: plate ordered in the wrong temper for forming or expansion, sharp internal corners that concentrate stress, welding or brazing procedures that overheat the parent metal, dissimilar metal joints left unprotected against galvanic corrosion, and cleaning regimes that use ammoniacal or strongly oxidising chemicals. Selecting the plate grade to match the process fluid, specifying the temper with the fabrication route in mind, and keeping the water chemistry and velocity inside the design envelope will prevent the majority of these problems.
Inspection of incoming plate typically covers grade and heat-number verification, chemical analysis, tensile testing for the ordered temper, thickness and flatness measurement, ultrasonic or visual checks for laminations and edge cracks, and a grain-size check where the plate will be subsequently formed. For tubesheets, add hardness testing to confirm expandability and a dimensional check of tube-hole pitch, diameter and ligament width.
FAQ
Q: How much better is copper than aluminium in a heat exchanger?
Copper's thermal conductivity is approximately 386 W/(m·K) against about 237 W/(m·K) for aluminium, so copper transfers heat appreciably faster and needs less surface area for the same duty. Aluminium remains attractive where weight or first cost dominates and the fluid is compatible.
Q: Can copper plates be used in chemical plants?
Yes, in many chemical and pharmaceutical duties where copper forms a protective oxide film and resists the process medium. They should not be used with ammonia, ammonium compounds, strongly oxidising acids or sulfide-bearing streams, which attack copper rapidly.
Q: What grade of copper plate is used for tubesheets?
C11000 electrolytic tough pitch copper, C10200 oxygen-free copper and C12200 deoxidised high phosphorus copper are the usual choices. The grade and temper are selected from the process fluid, the expansion or welding method used for the tube joints and the design pressure.
Q: Is copper plate suitable for brazed plate heat exchangers?
Yes. Brazed plate exchangers are built up from stamped stainless or other plates joined with copper or copper alloy brazing material, and copper sheet and foil are also used for the heat transfer plates themselves in specially designed units. The limiting factor is usually the compatibility of copper with the process fluid.
Q: What temperature limit applies to copper plate exchangers?
Copper retains good ductility over a wide temperature range, but the allowable design stress of annealed copper drops steeply above roughly 200 °C, and the material softens. High-temperature duties are normally handled by steel or stainless steel instead.
Q: How should copper plate be cleaned in service?
Use cleaning chemicals that are compatible with copper and avoid ammonia and ammonium-based formulations. Mechanical cleaning should not score the plate surface, and the cleaning procedure should be checked against the exchanger manufacturer's guidance and the plate grade specification.
