The heat exchange coil is one of the most critical components of an evaporative condenser or closed-circuit fluid cooler. This metal coil is where all of the elements of evaporative systems - heat, pressure, water and air - combine to provide process cooling performance and any energy-saving benefits. Process cooling applications demand that the materials used in the heat exchange coil provide the best possible heat transfer performance to help optimize the transfer of heat out of the system, as well as the best possible corrosion resistance to promote a long service life and reduce maintenance and replacement costs.
Bare steel tubing, galvanized after fabrication, is the standard material used for heat exchange coils in most evaporative condensers. While galvanized zinc is an effective coating for many condenser components, it is only that - a coating. The carbon steel tubes used to manufacture coils can rust quickly as the galvanized coating fails. In addition, because the inside of the tubes cannot be coated, a steel coil is susceptible to undetectable rust and corrosion from the inside.
Because of the severe environment in many condenser applications, certain standard components often are upgraded to higher-quality materials to extend condenser life. For example, stainless rather than galvanized steel often is specified for the sump pan or condenser casing. While many people recognize the benefits of using superior materials in critical condenser areas, they often do not realize that there is a higher-quality alternative to a galvanized coil. This oversight is unfortunate because a coil failure, and its subsequent replacement, can be extremely costly. Replacement coils can be expensive, and often owners must deal with field labor and hoisting requirements in addition to the unplanned costs of being temporarily out of service. At times these costs are so high that a company chooses to replace the entire condenser ahead of schedule.
Copper has long been the material of choice for tubing and piping in the industrial refrigeration industry and other process cooling applications. Copper has many desirable features that make it a superior material compared to steel, including high strength, good workability, excellent heat transfer characteristics, lower weight and better corrosion resistance. Additionally, unlike steel coils, copper will not fail just because a galvanized coating fails.
An in-depth comparison of the key physical properties of copper and steel illustrates the benefits of copper.
Thermal Transfer CharacteristicsThe heat transfer efficiency in a heat exchange coil is directly proportional to the thermal conductivity of the metal used in the coil’s fabrication. Any college physics textbook will give the thermal conductivity of copper as 380 to 400 W/m•K and the same value for steel as 45 to 65 W/m•K. This difference in thermal conductivity means that for heat transfer coils with the same area, pipe wall thickness and temperature gradient, copper tubing will conduct heat more than seven times as efficiently as steel tubing. In addition, it means that smaller copper coils are required to remove the same amount of heat over time when compared with steel heat exchanger coils. The smaller size requirement leads to other advantages when using copper in heat exchange applications, including a physically smaller system with a lower weight for the same rated heat conduction capacity.
Corrosion ResistanceResistance to corrosion is the principal factor that determines the service life of a heat exchange coil in evaporative cooling applications. Fluids contacting the coil begin to erode the tubing surface the minute the heat exchanger is turned on. Both the inner coil surface that is in direct contact with the water or other fluids and the outer surface that is in contact with water-based mists in the air must be considered.
Internal coil corrosion depends on the fluid used to transfer heat within the closed system. The corrosion rates for common metals contacting several types of cooling fluids are given in table 1. It shows that copper offers better corrosion resistance to water than steel, by a factor of more than 100. These data mean that copper tubing with a wall thickness of 100 mils in contact with water would have a projected lifetime of more than 1,000 years, while a similar ungalvanized steel pipe in contact with water could be expected to fail in just over 10 years. Because it is inevitable that air will eventually be mixed with water, even in a closed-loop fluid circulating system, these corrosion rates would be somewhat lower than those encountered in real-life evaporative cooling applications. In such an environment, copper lifetimes would remain in the hundreds of years, while plain steel would drop to less than five years in most cases.
Corrosion resistance is equally important on the external surface of the coil, which is subjected to an airborne water aerosol to promote evaporation and coil cooling. The combined exposure to air and water does not significantly increase the corrosion rates for copper coils, even if they are installed near the ocean and experience an increase in salt exposure from the surrounding environment. However, the corrosion of untreated steel coils increases significantly in a mixed air/water environment, just as internal corrosion increases when air is entrained in the water or other fluid within the closed-loop coolant system. For this reason, the entire steel heat transfer coil is always hot dipped in zinc, which provides a much more corrosion-resistant coating over the entire external surface. Zinc exhibits a corrosion rate about four times that of copper, but its corrosion resistance is susceptible to degradation if the pH of the cooling fluid varies widely, or if carbonate, sodium or other salts are present in the water/air mixture. Additionally, because the zinc is just a thin coating on the surface of the steel tubing, any voids, scratches or deformations of the coil can create an area that will exhibit accelerated corrosion.
Other BenefitsUsing copper allows the coil to be split into a large number of independent circuits. This capability is often a distinct advantage for refrigeration applications in which different condensing circuits are desired for each compressor or a separate fluid-cooling circuit is required. The copper headers also can be placed externally to the sidewall casing to give the company the flexibility to modify circuiting in the future if manufacturing requirements change.
The use of copper also allows the tube wall thickness to be increased easily, which can be beneficial in applications that experience higher-than-standard inlet fluid temperatures (up to 150°F [66°C]) such as furnace and dynamometer cooling.
More than ever, today’s manufacturers are demanding more value and life from their evaporative cooling equipment. The use of copper heat transfer coils offers the opportunity to choose an evaporative cooling unit with higher quality, lower weight and a longer service life than can be fabricated with steel coils.