Aluminum Versus Steel Coils For Ammonia

Section 6.9 has already listed the most popular choices of materials of construction for air coils, including copper tubes and aluminum fins for halocarbon coils and steel or aluminum for ammonia coils. Both all-steel galvanized coils and allaluminum are widely used in industrial refrigeration and each has advantages and disadvantages. The early aluminum coils suffered bad publicity because of tubes and tube-bends rupturing, but aluminum coils manufactured today are more rugged than those of the past. The density of aluminum is one-third that of steel, which reflects directly on the weight of the coil and the strength of the required supporting members. The thermal conductivity of aluminum is 4–1/2 to 5 times that of steel, so it is expected that an aluminum coil of given construction will transfer a greater rate of heat than a steel coil. The advantages and disadvantages of aluminum for air coils can be summarized as follows:

Advantages of aluminum:
– lighter weight
– better heat transfer. While aluminum may possess five times the conductivity of steel, the influence of this factor applies only to heat transfer through the fins and through the tubes. The air-side and refrigerant-side heat-transfer coefficients dominate, so the favorable conductivity of aluminum results in a 10–12% improvement in heat transfer rate for a coil of given construction.
– less corrosive in acidic and dry SO2 atmospheres (but see also disadvantages of aluminum)
– achieves more rapid defrost

Disadvantages of aluminum:
– inability to handle stress and physical blows
– higher cost than galvanized steel
– more difficult to repair in the field
– more corrosive when subjected to chlorine in cleaning solutions and when in contact with calcium chloride brine

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