Designers and installers of industrial ammonia evaporators for low-temperature applications must be concerned with the weight, performance, and reliability of the equipment being specified. Additionally, there may be requirements for corrosion resistance, cleanability, and defrosting characteristics that need to be considered.
Ammonia and aluminum are very compatible and have been used in ammonia heat exchangers for several decades.
COMPARISON OF PROPERTIESTable 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. Hence, these two base metals are shown in Table 1.
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 up and cool down the heat exchanger during a defrost cycle. The thermal conductivity of the metal affects the thermal performance of the heat exchanger and the speed and effectiveness of defrost.
The very low density of aluminum results in a light heat exchanger. The higher thermal conductivity aids thermal performance.
The lighter weight of aluminum evaporators reduces structural requirements for buildings where units are ceiling hung, an especially important feature in high seismic areas. Rigging and handling are also made easier with lighter- weight aluminum evaporators.
PERFORMANCEThe thermal conductivity of aluminum has a direct effect on heat transfer efficiency. Aluminum can provide efficient heat transfer.
The cooling capacity of aluminum evaporators allows the designer the choice between selecting an evaporator having fewer rows and/or wider fin spacing for lower first cost, or using the same size unit (same rows and fin spacing) and operating at higher suction pressures, which results in reduced operating costs, compared to galvanized steel.
DEFROSTThe high thermal conductivity of aluminum results in fast, effective defrosts. A substantial amount of energy is expended during defrost to heat the mass of metal in a refrigeration evaporator up to the melting point of ice (32°F), than to cool the metal back down 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.
Generally, an aluminum evaporator may require less energy than other technologies to heat up and cool down during every defrost cycle. This component of defrost energy becomes very significant at lower temperatures (as in freezers). Using aluminum evaporators can produce significant 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 surface of the metal of impurities and promotes the formation of the normal protective oxide layer. Thousands of industrial aluminum ammonia evaporators have been installed and operated successfully since the 1970s.
Corrosion of heat exchangers by contact with, or proximity to, foodstuffs is a concern in food processing facilities. All foodstuffs are mildly acidic. Aluminum is corrosion resistant when exposed to acetic and citric acids (dairy products, citrus products), fatty acids (anti-caking agents, lubricants), and lactic acids (bread, confections, beverages, fermentation, blood).
Aluminum is corrosion resistant 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.
Generally speaking, aluminum is recommended where there is concern about corrosion due to contact with most foodstuffs.
CLEANABILITYCleanability of equipment, including evaporators, in food processing facilities 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 smooth, hard surfaces of aluminum evaporators pay dividends for effective cleaning in food processing equipment and facilities.
There are four basic types of cleaners used in the food processing industry: acidic, strongly alkaline, mildly alkaline, and chlorine based.
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 (such as Base-511, manufactured by Great Western Chemical) is recommended for both aluminum and galvanized steel surfaces.
If caustic soda (strong alkaline) must be used for clean up, then galvanized steel is preferred over aluminum, since zinc has higher resistance than aluminum to mildly alkaline solutions.