Heat Transfer Process Equipment

Air Coolers

Figure 1 - Air cooler

An air cooled heat exchanger, or ACHE, is simply a pressure vessel which cools a circulating fluid within finned tubes by forcing ambient air over the exterior of the tubes. A common example of an air cooler is a car’s radiator.

Air coolers are used in industrial applications when reliable source of water is not available. Even if water is available, in some cases, air cooled exchangers are favored for economic & operational reasons – because dedicated cooling water circuits, pumps, water cooling systems & water conditioning system add complexity & capital cost, as well as operating & maintenance cost.

In moderate climate, air cooler will be a best choice for minimum process temperature above 650C & water cooling for minimum process temperature below 500C.

Advantages of using air coolers

  • Air is always available.
  • Maintenance cost normally less than for water cooled systems.
  • In event of power failure they can transfer some heat because of natural convection.
  • Mechanical design is normally simpler due to pressure on air side always being closer to atmospheric.
  • Fouling on air side can normally be ignored.
  • No problem arising from thermal & chemical pollution of cooling fluids.

Disadvantages of air coolers

  • Noise – Low noise fans are reducing this problem but at cost of fan efficiency & hence higher energy costs.
  • May need special features for cold weather protection (high pour point liquids freezes in cold weather).
  • Can not cool to same low temperature as cooling tower.

Types of air coolers

Forced draft type air cooler

Figure 2 - Forced draft type air cooler

  • Low power requirement for air Temperature rise greater than 100C.
  • Adaptable for winterization, pour point Recirculation schemes.
  • More readily accessible for maintenance.
  • Exchangers are easy to remove for repairs.
  • No mechanical equipment exposed to hot air. Whereas induced draft is subjected to much higher temperature.
  • Less structural support required.
  • Isolated support for mechanical equipments.

Induced draft type air cooler

Figure 3 - Induced type air cooler
  • Requires less power for an air temperature rise less than 10 oC.
  • Less hot air circulation as exhaust air velocity Is about times that of forced draft.
  • Offers bundle protection from adverse weather.
  • Better suited for cases with close approach temperatures between inlet air & outlet fluid.
  • Air distribution over exchanger is better.
  • Will transfer more heat by natural convection because of stack effect.
  • Connecting piping usually less

    Figure 4 - Types of tube bundle or manifold arrangements for air coolers


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