Copper heat exchanger designation

Copper-base alloys will corrode in aerated conditions. It is, therefore, sometimes appropriate to consider cathodic protection. It becomes particularly relevant when the flow rates are high or when the design of an item causes the copper to be an anode in a galvanic cell (e.g. a copper alloy tube plate in a titanium-tubed heat exchanger). Corrosion can be controlled by polarisation to approximately — 0-6V (vs. CU/CUSO4) and may be achieved using soft iron sacrificial anodes. 

Many boilers are fitted with a heat exchanger-type water sampling coil that permits the collection of a representative and cooled BW sample. The design generally provides for a coil of copper or stainless steel (SS) fitted inside a small SS shell. The unit is fitted with gate valves to control the flow of cooling water and BW. 

Heat exchangers may be termed, for example, a condenser (where condensing a process vapor) or an evaporator (where vaporizing a liquid), and they are, of course, designed with a variety of metals and alloys having high heat-transfer coefficient U values, such as copper-based alloys. 

Design such that the components that are most liable to corrosion are easy to replace. Special parts may be installed for attracting the corrosion. An example is shown in Figure 10.6 a spool of carbon steel is inserted between a heat exchanger and the adjacent copper pipeline for the purpose that copper shall be deposited before the water enters the heat exchanger (see Section 7.3.2). 

The mass and heat transfer in the bulk of the flow is rapid, via molecular diffusion and not by convection in the boundary layer. This results in a delay in the heat and mass transfer. Therefore, the concept and knowledge of the boundary layer are important for designing heat exchangers and any discussions of chemical processes controlled by mass transfer. In the electrochemical field, there are many processes controlled by mass transfer, mostly the diffusion of a reacting species in the vicinity of the working electrode. An example is copper deposition on the cathode from a copper sulfate solution. 

Carbon dioxide is often ignored in steam systems. However, when absorbed in water, it forms carbonic acid, which can be corrosive to all parts of the steam and condensate system. Its potential presence is frequently overlooked in the design of heat exchangers, steam traps, condensate systems, deaerators, and water-treating systems. Most steam systems require continual addition of makeup water to replace losses. Makeup water must be adequately treated, by demineralization or distillation, to remove carbonates and bicarbonates. If these are not removed, they can be thermally decomposed to carbon dioxide gas and carbonate and hydroxide ions. The ions will normally remain in the boiler water, but the caron dioxide will pass off with the steam as a gas. When the steam is condensed, the carbon dioxide will accumulate since is is noncondensable) be passed as a gas by the steam trap or if the condensate and carbon dioxide are not freely passed by the steam trap, become dissolved in the condensate and form carbonic acid. If carbonic acid is formed it can have a pH approaching 4 and be very corrosive to copper and steel. Even if both the gas and condensate are passed freely by the steam trap, the gas will become soluble in the condensate when subcooling occurs. If oxygen is present, the corrosion rate Is accelerated. 

The basic design data consisted of the Fanning friction factor and Colburn modulus versus Reynolds number characteristics (/versus Re and i versus Re) for gas flow both normal to the tube bundle and inside the turbulated tubes. The data for flow normal to the tubes were taken from a geometrically similar surface described by Kays and London [1] in their Fig. 45 and may be expressed by the equations / = 0.285 Re and j = 0.320 Re" , where 300 < Re < 15,000. Data for the turbulated tube were obtained partly from steam-air tests on a small shell and turbulated tube heat exchanger, and partly from transient tests with nitrogen on a hollow copper cylinder which contained a turbulator. The turbulated tube is described in Fig. 4, and the basic data are shown in Fig. 6. The data may also be expressed by the equations / 0.05 and j =0.05 Re" , where Re > 6000. 

Substitution of carbon or low-aUoy steel tubes for those of more expensive copper alloys in heat exchanger service results in marked savings in the initial costs. Because Admiralty tubes are roughly 60% more expensive than carbon steel, the designer must be assured of reasonably long and trouble-free service if the additional cost of the copper alloy tubing is to be justified. The tubes must resist the build-up of corrosion products that will interfere with heat transfer and flow, as well as accelerate the development of leaks. 

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