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
- 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
- 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