Air-cooling evaporators used in ammonia refrigeration systems traditionally have been made using galvanized (zinc-coated) carbon steel. However, other metals, including aluminum, are compatible with ammonia. Aluminum has been used in ammonia heat exchangers for several decades; its properties make it a suitable metal to use in ammonia refrigeration applications.
Designers and installers of industrial ammonia evaporators for low-temperature applications must consider the weight and performance of the equipment being specified. Corrosion resistance, cleanability and defrosting characteristics also should be evaluated. Table 1 compares several properties of aluminum to those of carbon steel and zinc. (Galvanized steel is obtained by dipping carbon steel in a bath of molten zinc.)
The density of the metal directly affects the weight of the heat exchanger. When the density is 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.
In these three areas -- weight, performance and defrost -- as well as corrosion resistance and cleanability, aluminum has distinct advantages over galvanized steel.
WeightThe low density of aluminum compared to steel and zinc results in a lighter heat exchanger. A galvanized steel ammonia evaporator will weigh three times more than an aluminum evaporator that has the same dimensions. The higher thermal conductivity of aluminum also provides better thermal performance compared to galvanized steel. When the effect of this increased performance is taken into account, the galvanized steel version will weigh three and a half times more than an aluminum evaporator that has the same cooling capacity.
The lighter weight of aluminum evaporators reduces structural requirements for buildings where units are ceiling-hung -- an important feature in high seismic areas. Rigging and handling also are made easier with lighter weight aluminum evaporators. It is easy to visualize the advantages of mounting a 2,000 lb aluminum evaporator compared to a 6,000 lb steel evaporator in a room with a 30' ceiling.
PerformanceThe thermal conductivity of aluminum is four and a half times higher than steel, and two times higher than zinc. Thermal conductivity has a direct effect on heat transfer efficiency -- the higher, the better. Tests have proven that an ammonia evaporator made with aluminum tubes and fins will have a cooling capacity that is 12 to 15 percent higher than a galvanized steel evaporator with the same dimensions.
The higher cooling capacity of aluminum evaporators allows the designer the choice between selecting an evaporator with fewer rows or wider fin spacing for a lower first cost; or using the same size unit (same rows and fin spacing) and operating at higher suction pressures for reduced operating costs compared to galvanized steel.
DefrostThe much higher thermal conductivity of aluminum also results in faster, more effective defrosts compared to galvanized steel coils.
During defrost, a substantial amount of energy is expended to heat the mass of metal in a refrigeration evaporator to the melting point of ice (32oF [0oC]) and then to cool the metal to operating temperature. When the density of the metal is multiplied by the thermal conductivity, the resulting product indicates the amount of energy required to heat or cool a heat exchanger of a given volume by one degree.
Based on this analysis, a galvanized steel evaporator will require 70 percent more energy than the same size aluminum evaporator to heat up and cool down during every defrost cycle. This component of defrost energy becomes significant at lower temperatures (i.e., in freezers). Using aluminum evaporators can produce savings in operating costs over the course of a year, especially at freezer temperatures.
Corrosion ResistancePure ammonia naturally passivates aluminum surfaces. The passivation process cleans the metal surface of impurities and promotes the formation of the normal protective oxide layer.
Corrosion of heat exchangers by contact with or proximity to foodstuffs is a concern in food processing facilities because all foodstuffs are mildly acidic. Aluminum is more corrosion resistant than galvanized steel when exposed to acetic and citric acids (e.g., dairy and citrus products), fatty acids (e.g., anti-caking agents and lubricants) and lactic acids (e.g., bread, confections, beverages, fermentation and blood).
Aluminum is also more corrosion resistant than galvanized steel in the presence of sodium chloride (used in the preservation of meats and vegetables) and sulfur dioxide (used in grape storage). Neither galvanized steel nor aluminum is recommended for exposure to nitrites (used in cured and smoked meats). In these cases, stainless steel is recommended.
Generally speaking, aluminum is a better metal to use than galvanized steel where there is concern about corrosion due to contact with most foodstuffs.
CleanabilityCleanability of equipment in food processing facilities, including evaporators, has become an increasingly important issue. Bacterial contaminants must be removed with regular cleaning, and locations where they can accumulate need to be minimized.
The relatively rough and porous surfaces of galvanized steel evaporators can present difficult cleaning problems for operators in facilities where bacteria-free operation is critical. Conversely, the relatively smooth and hard surfaces of aluminum evaporators accommodate effective cleaning in food processing equipment and facilities.
Another important issue is the effect of various cleaners on aluminum and galvanized steel. Both aluminum and galvanized steel are attacked by acidic, strongly alkaline and chlorine-based cleaners. These types of cleaners are not recommended for use on any ammonia evaporators. A foaming type mildly alkaline cleaner is recommended for both aluminum and galvanized steel surfaces.
If caustic soda (strong alkaline) must be used for cleanup, then galvanized steel is preferred over aluminum.