Consider a stainless steel tube, aluminum fin coil rather than a galvanized steel coil for your next ammonia evaporator.

Table 1. Density, thermal conductivity, specific heat capacity and tensile strength are compared for stainless steel, aluminum, carbon steel and zinc.
Air-cooling evaporators (air coolers) used in ammonia systems traditionally have been made using galvanized (zinc-coated) carbon steel. There is an alternative technology, though, that utilizes stainless steel tubes and aluminum fins (figure 1). Advantages of this technology include cost, weight, heat transfer performance, energy-efficient defrosting, corrosion reistance, cleanability and reliability.

Table 1 shows the properties of several metals, including stainless steel, aluminum, carbon steel and zinc. Because galvanized steel is obtained by dipping carbon steel in a bath of molten zinc, the two base metals are shown in the table. The density of the metal directly affects the weight of the heat exchanger, and when multiplied by the specific heat capacity, the product indicates the amount of energy required to heat and cool the heat exchanger during a defrost cycle.

The thermal conductivity of the metal affects the thermal performance of the heat exchanger as well as the speed and effectiveness of defrost.

Figure 1. This ammonia evaporator uses an alternative technology that uses stainless steel tubes and aluminum fins.
Working Pressure at Low Temperatures. The tensile strength of the metal will determine the burst pressures of the heat exchanger tubes and headers for a given wall thickness. Different metals behave differently at low temperatures. Carbon steel becomes brittle at temperatures below -20 oF (-29oC). Special allowances must be made when designing with carbon steel below -20oF such as using special impact-tested material, increasing the wall thickness of the pipe and post-weld heat treating to avoid failures caused by embrittlement of the metal.

Stainless steel and aluminum perform well in low temperature freezer applications compared to galvanized steel (table 2).

Cost Benefits. On a per pound basis, carbon steel is lower in cost than both stainless steel and aluminum. This cost differential is offset for aluminum by the metal’s low density, so that the cost of aluminum fins is approximately the same as the cost of carbon steel fins. Because stainless steel has high tensile strength (table 1), the wall thickness of the stainless steel tubing can be reduced safely, which reduces the tubing cost per foot accordingly. Hot dip galvanizing is not required for stainless tube/aluminum fin construction, which further offsets the higher cost per pound of these metals compared to carbon steel.

Lightweight Construction. The very low density of aluminum makes it an ideal metal to use for heat exchanger fins when weight is a concern. In a refrigeration evaporator, the fins represent approximately one-half the total weight of the coil block. Most of the remaining weight of the coil block is contributed by the tubes and headers.

Using appropriately selected stainless steel tubing with aluminum fins produces a coil block that is significantly lighter in weight than the same size galvanized steel coil block. Air coolers often are mounted on the ceiling or roof of the refrigerated building. The weight of the air coolers has a significant impact on the structural design of the building and is particularly important in high seismic areas.

Table 2. Stainless steel and aluminum offer good performance in low temperature freezer applications.
Performance. The thermal conductivity of aluminum is 4.5 times higher than steel, and two times higher than zinc. Thermal conductivity of the fin material has a direct effect on heat transfer efficiency; the higher, the better. Aluminum is superior to galvanized steel for efficient heat transfer. Calculated performance of an ammonia evaporator having stainless tubes and aluminum fins is approximately 5 percent to 8 percent higher than a galvanized steel evaporator having the same dimensions.

Energy-Efficient Defrost. The high thermal conductivity of aluminum fins also produces faster defrosts compared to galvanized steel. Stainless tube/aluminum fin evaporators also perform better than galvanized steel during defrost on an energy basis. A substantial amount of energy is expended during defrost to heat the mass of metal in a refrigeration evaporator up to the defrost temperature, then to cool the metal back down to operating temperature after defrost. When the density of the metal is multiplied by the thermal conductivity the product indicates the amount of energy required to heat or cool a heat exchanger of a given volume by one degree.

Figure 2 shows the amount of energy required to heat the coil block from suction temperature to 50oF and cool it back down again. This difference in energy consumption can be converted to cost savings by making assumptions for number of defrosts per day, days of operation per year, and the electric utility rate.

Corrosion Resistance. Corrosion of heat exchangers by contact with or proximity to foodstuffs is a concern in food processing facilities. All foodstuffs are mildly acidic. Aluminum and stainless steel are more corrosion resistant than galvanized steel when exposed to acetic and citric acids (dairy or citrus products), fatty acids (anti-caking agents, lubricants) and lactic acids (bread, confections, beverages, fermentation and blood). Aluminum is more corrosion resistant than galvanized steel in the presence of sodium chloride (preservation of meats and vegetables) and sulfur dioxide (grape storage). Neither galvanized steel nor aluminum is recommended for exposure to nitrites (cured and smoked meats). Stainless steel is the suggested material to use in the presence of nitrites.

Cleaning. The ability to clean equipment in food processing facilities, including evaporators, has become an increasingly important issue. The smooth, hard surfaces of stainless tube/aluminum fin evaporators facilitate effective cleaning in food processing equipment and facilities.

Reliability. The quality and integrity of the evaporator tubing and headers will have a direct impact on reliability of the heat exchanger. Only ASME-grade tubing should be used. In addition, 100 percent eddy current testing of the tubing is recommended to ensure zero tube defects prior to assembly. All tube joints should be carefully joined by a TIG welding, then pressure tested at 500 psig (35 bar) to ensure absolutely zero leaks.