How a Copper Plate Heat Exchanger Works
A plate heat exchanger consists of a stack of thin, corrugated metal plates clamped in a frame or sealed into a brazed assembly. The two fluids flow through the alternating channels between the plates, and heat passes directly through the plate wall from the hot stream to the cold stream. Because the flow is broken up by the corrugation pattern, the boundary layer is continuously disturbed and the heat transfer coefficient is high even at modest velocities.
Copper is the plate material of choice whenever the process fluids are compatible with it. The plates are made from wrought high-conductivity copper such as C11000 or, where higher corrosion resistance is needed, from a copper alloy or a copper-nickel grade with the same working characteristics.
Why Copper Is Selected for the Plates
| Property | Copper, wrought high-conductivity | Consequence for the exchanger |
|---|---|---|
| Thermal conductivity | About 400 W/m.K at 20 C | Very little temperature drop is lost through the plate wall |
| Formability | Excellent in the annealed and half-hard tempers | Deep corrugations can be pressed without cracking, giving a large secondary surface |
| Corrosion behaviour | Protective surface film in water and in the atmosphere | Long life in fresh water, steam condensate and many process streams |
| Fouling behaviour | Smooth surface, naturally protective film | Biological and scale deposits form more slowly than on many alternatives |
| Brazing | Readily brazed with copper or silver filler metals | Compact brazed plate units can be produced without a frame and gaskets |
The combination of a very high thermal conductivity and a thin wall is what makes the plate concept work: heat has to cross only the plate thickness, so the overall heat transfer coefficient is governed mainly by the two fluid films rather than by the metal. Copper's pressed corrugations then multiply the effective area and induce turbulence, so a copper plate pack can deliver a large duty in a fraction of the floor space of an equivalent shell and tube unit.
Design Variants and Construction
Gasketed plate units. Plates are clamped between two end covers with elastomer gaskets, so the pack can be opened for cleaning and plates can be added or removed to change the duty. Copper plates are used in gasketed units where the process fluid and the gasket compound are compatible.
Brazed plate units. The plates are vacuum brazed with a copper-based filler to form a sealed, very compact unit with no gaskets. Brazed copper plate units are common for refrigeration, air conditioning and small district heating sub-stations.
Welded and semi-welded units. Used when gasket materials cannot tolerate the medium, with welded pairs of channels for the aggressive side.
Plate and frame with copper-alloy plates. Where the fluid is mildly aggressive, an aluminium brass, admiralty brass or copper-nickel plate is substituted while keeping the same frame and channel geometry.
Performance Characteristics
The compact stacked-channel layout produces a close temperature approach between the two streams, which is the reason plate exchangers appear so often in heat recovery duties. A close approach means that more of the available temperature difference is turned into useful duty, so the thermal load can be met with less heating or cooling energy and with a lower flow of the utility medium.
The narrow channels also mean a low hold-up volume, which reduces the process fluid inventory and the residence time. Response to load changes is quick because there is little mass to heat or cool. Counter-current flow is easily arranged, and where several duties must be served, a single frame can be built up in sections so that one unit performs several heat transfer steps.
Typical Applications
Chemical processing. Heating and cooling of reaction streams, condensing of solvents and heat recovery between process streams.
Power generation. Boiler feedwater heating, turbine exhaust condensing, generator and auxiliary cooling circuits.
Refrigeration and air conditioning. Evaporators and condensers in chiller packages and in air conditioning equipment.
HVAC and district heating. Domestic hot water production and sub-station heat exchange where potable water must be separated from the primary circuit.
Marine and offshore. Central cooling circuits, using copper-nickel or aluminium brass plates in seawater service.
Food and beverage. Pasteurisation and process cooling duties where copper-compatible fluids are handled.
Selection, Service Limits and Maintenance
Copper performs best in fresh water, steam condensate, demineralised water and neutral process streams. Two limits should be checked before the plates are ordered. First, chloride content and flow velocity in the water circuit determine whether copper is acceptable or whether a copper-nickel grade is required for seawater and brackish water. Second, the fluids must not contain ammonia or ammonium compounds, which attack copper alloys, and the pH and dissolved oxygen levels should be within the range accepted for copper.
Maintenance is based on keeping the channels clean. Fouling increases the pressure drop and reduces the heat transfer coefficient at the same time, and heavy scale can also cause under-deposit corrosion. Fouling is monitored by tracking the pressure drop across the unit against its clean baseline, and cleaning is carried out by circulating a suitable cleaning solution in place or by opening a gasketed pack and washing the plates individually. Gaskets are a wear item and are replaced at the intervals recommended by the manufacturer. Plate edges and port areas should be inspected for erosion whenever the unit is opened, and the clamping dimension should be restored exactly as specified when the pack is closed, because both over-tightening and under-tightening shorten gasket life.
FAQ
Q: Why is copper used as the plate material in a plate heat exchanger?
Copper combines very high thermal conductivity with excellent formability, so thin plates can be pressed into deep corrugations that create a large surface area and turbulent flow without a large temperature drop through the wall.
Q: What is the advantage of a plate exchanger over a shell and tube unit?
A plate pack achieves the same duty in much less space and with a closer temperature approach, because the corrugated channels produce high heat transfer coefficients and the flow is counter-current.
Q: Can copper plates be used with seawater?
Plain copper is not the best choice for seawater. A copper-nickel or aluminium brass plate grade should be selected for seawater and brackish water duties, keeping the same frame and channel geometry.
Q: Which fluids should be kept away from copper plates?
Ammonia and ammonium compounds, and strongly acidic or strongly alkaline streams, attack copper. The pH, chloride level and dissolved oxygen content should be checked against the acceptable range for the grade before ordering.
Q: How is a plate heat exchanger cleaned?
Either by circulating a compatible cleaning solution in place or by opening a gasketed unit and cleaning the plates individually. Cleaning frequency is set by the rise in pressure drop compared with the clean baseline.
Q: What are the most important operating limits?
Fluid compatibility with the plate and gasket materials, the working pressure and temperature of the gasket compound, chloride content in water circuits, and the flow velocity at the plate inlet ports, where excessive velocity causes erosion.
