Air cooled heat exchangers design good practices

Air cooled heat exchangers design good practices for temperature selection, Air re-circulation, Leaks, Approach temperature, Mean-temperature-difference (MTD) correction factor, Maintenance Cost, and Operating Costs.

• The dry-bulb temperature that is selected should be more than 2.5 percent of the warmest ambient temperatures with an addition of 5.4 ०F (3 ०C) as a contingency.
• Air recirculation: Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation.
• During winter or extreme weather conditions, shelter needs to be considered for heavy rain, strong winds, freezing of moisture upon the fins, etc.
• More the number of fans, more the noise pollution. To reduce this noise pollution exchanger position, the reflective surfaces near the fan, the hardness of these surfaces, and noise from adjacent equipment needs to be looked into.
• Space requirement consideration of air cooled heat exchanger is not similar to water cooled heat exchanger since these can be installed above other equipment as well.

Typical Air Cooled Exchanger or Aerial Cooler - Alfa Laval

• Leaks in air-cooled units end up directly to the atmosphere which can cause fire or fumes hazards. But due to the large amount of air-flow, concentration of toxic fluids reduces drastically.
• Avoid locating the air-cooled heat exchangers close to corrosive vapors and fumes from vent stacks.
• There is negligible fouling on the air-side. This is because the outside fouling factor, based on tube surface, must be divided by the surface ratio (bare to finned) which results in a very small figure and thus negligible in most cases.
• Tube side cleaning is done easily be either mechanical or chemical means.
• The thickness of the tube is normally small enough with sufficiently high conductivity.
• The high-pressure process fluid is always in the tubes. Tube-side headers are relatively small as compared with water-cooled units when the high pressure is generally on the shell side.
• Bond resistance needs to be take care of during design.
• Approach temperature: The approach temperature, which is the difference between the process-fluid outlet temperature and the design dry-bulb air temperature, has a practical minimum of 8 to 14°C (15 to 25°F). When a lower process-fluid outlet temperature is required, an air-humidification chamber can be provided to reduce the inlet air temperature toward the wet-bulb temperature. A 5.6°C (10°F) approach is feasible. Since typical summer wet-bulb design temperatures in the United States are 8.3°C (15°F) lower than dry- bulb temperatures, the outlet process-fluid temperature can be 3°C (5°F) below the dry-bulb temperature.
• Mean-temperature-difference (MTD) correction factor: When the outlet temperatures of both fluids are identical, the MTD correction factor for a 1:2 shell-and-tube exchanger (one pass shell side, two or more passes tube side) is approximately 0.8. For a single- pass air-cooled heat exchanger the factor is 0.91. A two-pass exchanger has a factor of 0.96, while a three-pass exchanger has a factor of 0.99 when passes are arranged for counterflow.
• Maintenance Cost: Maintenance for air-cooled equipment as compared with shell-and-tube coolers (complete with cooling-tower costs) indicates that air-cooling maintenance costs are approximately 0.3 to 0.5 those for water-cooled equipment.
• Operating Costs: Power requirements for air-cooled heat exchangers can be lower than at the summer design condition provided that an adequate means of air-flow control is used. The annual power requirement for an exchanger is a function of the means of air- flow control, the exchanger service, the air-temperature rise, and the approach temperature. When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Ref 1].  Alternatively, when fans have two-speed motors, these deliver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition.

References:

Frank L. Rubin, “Power Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans,” Oil Gas J., 165–167 (Oct. 11, 1982).