Copper Heat Exchange Coils in Evaporative Cooling
The heat exchange coil is the most critical component of a closed-circuit fluid cooler. This application demands a coil made from the highest-quality material to provide years of continuous service at maximum thermal efficiency. It also demands that the materials used in the coil provide the best possible corrosion resistance to maximize service life.
Carbon-steel tubing, galvanized externally after fabrication, is the standard material for heat exchange coils used by many manufacturers. While galvanized zinc is an effective coating for many condenser components, it is, in fact, only that-a coating. The carbon-steel tubes used to manufacture coils can rust as the galvanized coating fails. In addition, because the inside of the tubes are not coated, a steel coil is susceptible to undetectable corrosion on the internal surfaces.
Because of the severe environment in many condenser applications, certain standard components are often upgraded to higher-quality materials to extend condenser life. For example, stainless, rather than galvanized steel is often 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 there is a higher-quality alternative to a galvanized coil. While these alternative materials may be more costly, the initial cost penalty is quickly justified if a premature coil replacement is required. Not only is a replacement coil expensive, but owners must also deal with field labor and hoisting requirements in addition to the cost of being temporarily out of service. Sometimes these costs are so high that replacing the entire condenser ahead of schedule may be the most economic decision.
Copper had long been the material of choice for tubing and piping in the HVAC, refrigeration and construction industries. For example, in HVAC applications, other components of hydraulic systems such as heat pumps typically have copper heat exchangers. Copper possesses many desirable features, including high strength, good workability, excellent heat-transfer characteristics, lower weight and resistance to corrosion. And, unlike steel coils, copper will not fail just because a galvanized coating fails. Let's look at an in-depth comparison of the physical properties of copper and steel.
Thermal conductivity of a metal is a very significant factor in the equation determining the capacity of a heat rejection device. The heat rejected to the atmosphere is merely the product of the coil surface area, the temperature difference between the fluid in the closed loop and the water exposed to the atmosphere, and the heat transfer coefficient or thermal conductivity:
Q = U x A x LMTD
Q is the quantity of heat transferred in time;
U is the heat transfer coefficient;
A is the area of the coil surface; and
LMTD is the log mean temperature difference between the open and closed loop.
Since the thermal conductivity of copper is several times that of steel, it follows that the surface area of a copper coil is smaller than that of a steel coil for the same heat transfer capacity. Similarly, if the areas of copper and steel coils are the same, the heat transfer capacity of the copper coil will be greater. This leads to a host of other significant advantages for using copper in heat exchange applications, including smaller size and lower weight for the same rated heat rejection.
Resistance to corrosion is the principal consideration that determines the service life of a heat exchange coil in evaporative cooling applications. The inner surface of the coil and the outer surface that contacts the air/water mixture must be considered.
The corrosion rates for common metals contacting a variety of cooling fluids are given in the table below. Note that copper offers a distinctively better corrosion resistance to water than steel.
Comparative Corrosion Effects of Fluids on
Source: Dow Chemical Company
Combined exposure to air and water does not significantly increase the corrosion rates for copper coils. The corrosion of untreated steel coils, however, increases significantly in a mixed air/water environment. This is the reason that the entire steel coil is hot dipped in zinc, providing a much more corrosion-resistant coating over the entire external surface. Zinc itself exhibits a rate of corrosion about four times that of copper, but this is susceptible to degradation of pH varies widely or carbonate, sodium or other salts are present in the water. Since the zinc is just a thin coating on the surface of the steel tubing, any voids, scratches or deformations on the coil can create an area that will exhibit accelerated corrosion.
In operation, it is inevitable that air will become entrained in the coolant within the closed-loop, regardless of precautions taken. With the air/coolant mixture contacting the inner walls of the steel coil, corrosion will again accelerate significantly over copper coils. Since it is difficult and expensive to hot-dip galvanize the internal walls of the steel tubing, hidden internal corrosion is a major failure mechanism that does not exist in copper coil coolers.
Because of the properties described above, we have found that copper coils are smaller in size and lighter in weight than steel coils of equivalent heat transfer capacity. This benefit provides Recold units their traditional low profile and can save the owner money in both supporting steel and lifting costs. Another difference between steel and copper coil units is that, due to their size, steel coil units often ship in two sections and require extra field rigging before they are completely factory-assembled and tested to ensure a clean and simple installation.
Copper also allows Recold the flexibility to split the coil into a number of independent circuits where separate fluid cooling is required in the same unit. The use of copper also allows tube wall thickness to be easily increased, depending on the application and customer needs.
Quality and value
Today, more than ever, customers are demanding more value and life from their HVAC equipment. The use of copper heat transfer coils offers the opportunity to choose an evaporative cooling unit that is of higher quality, lighter weight and longer life than can be fabricated with steel coils.
For additional discussion of the benefits of copper in HVAC systems, see Below the Surface.