Finned-Tube Corrosion

The location of equipment on site may also have an influence on the cleanliness of a heat exchanger from the point of view of fouling. For instance an air cooled heat exchanger placed where it is susceptible to particulate matter and dust, could lose efficiency due to fouling on the finned tubes. Corrosive fumes from nearby processing may also be responsible for air cooled heat exchanger fouling. 

This is the exchanger where heat flows from the room return or mixed air to cold refrigerant or to chilled water. It is an arrangement of finned tubes normally of aluminum fins on copper tubes, but copper fins can be specified for corrosive atmosphere. Performance characteristics are controlled by fin and tube spacing. If the room rh is high, dehumidification may be brought into use by operating the coil or one of a number of parallel coils at a low temperature. If the room s sensible heat load is low reheat must be allowed to operate at the same time. 

The L-foot fin covers the tube more or less completely to protect the base tube against corrosive attack, but still leaves a potential corrosive site at the base of the fin adjacent to the preceding fin. The double L-foot is intended to provide complete coverage of the tube, where corrosion would otherwise be a problem. Where corrosion is troublesome, soldered or galvanized tubes may offer a solution. The dimensions of finned tubes are results of past experience in the design of air cooled heat exchangers. Tube diameters range from about 1.905 cm (0.75 in.) to 5.08 cm (2.0 in.). 

In this equation, K and hi represent the outside and inside film coefficients of heat transfer of the two fluids Ly, and K the thickness and conductivity of the partition wall and Ri the resistances due to corrosion, dirt, or roughness of the surfaces and Ao, A, and Ay, represent the areas of the wall at the outside, at the inside, and at about the mean of the two (A ). Two fouling resistances are used throughout this book, but most of the literature fails to indicate whether one or both factors are being reported. Accordingly, a single fouling resistance Rd, which is the sum of Ro and iZ will be used in this derivation and no correction for outside and inside surfaces will be made except in such necessary cases as fin-tube resistances (see Example 17-8). 

Many low- to moderate-strength aluminum alloys (primarily in the 1000-, 3000-, 5000- and 60(X)-series) have useful corrosion resistance. They are used in mildly corrosive atmospheres (such as those offshore or at seacoasts and for cable trays and fins on air cooler tubes). Several 5000-series alloys are moderately resistant to chloride pitting and find applications in clean seawater and other aqueous chloride services. They are very resistant to corrosion by wet CO2 and are compatible with organics such as acetic acid. 

Water is circulated through a heat exchanger above the burner. The 15 heat exchanger is made of tubes of cast iron or copper, which resist corrosion. Both types use fins to increase the surface area in contact with water, which improves the transfer of heat. A thermostat located in the boiler causes the gas control valve to shut off when the water temperature reaches the pre-set level. 

For better heat transfer rates for gas/liquid cooling by air or for preheating air by steam or hot gasses inside the tubes. The area required for heat transfer can be less if gasses are clean. Dirty, corrosive, dust-laden gasses should not be put on the fin-side as they can get deposited between the fins and advantage of additional area is lost. 

The airside of the radiator, including the fins and the outside of the tubes, is exposed to the road climate. The atmospheric corrosion of radiators is influenced hy the following parameters [28] the time of wetness as corrosion of practical importance generally will take place only when the metal surface is. covered with a moisture film the t3fp>es and contents of air pollutants from urban and industrial sources splashing from the road. 

In adjusting the operation several of a wide range of equipment constraints may be encountered. These include condenser duty which may limit because of high coolant inlet temperature (on air-fins in hot weather) or because there is a maximum permitted coolant exit temperature (e.g. corrosion by salt water). The condenser limit might be approached because the column pressure is too low such that the dew point at the top of the column approaches the coolant temperature. High feed enthalpy can similarly overload the condenser, as can fouling on either the tube or shell side. 

The 230°F water inlet may cause the flue gas temperature to drop below 450°E This will precipitate a weak H SO mist, which will aggressively corrode the fins or studs of the economizer tube bank. If low sulfur (less than 100 ppm HjS is being fired), H SO corrosion should not be a problem. 

---