Ideally, heat should not be released into the atmosphere and it should be recovered for process use. Many times process is not capable of utilizing heat and heat needs to be rejected to the atmosphere to keep the process operational.
As in the case of cooling tower, cooling water temperature cannot be reduced to below wet-bulb temperature, in air-cooled heat exchanger process temperature cannot be reduced to below dry bulb temperature. Typically in the air-cooled condenser process cooling can be achieved as low as 4 deg C above dry bulb temperature.
In conventional heat exchangers, heat is extracted to cooling water, where heat is ultimately rejected at the cooling tower. The major challenge with this type of cooling system is the use of water. As we know there is a scarcity of water in almost all countries. We need to think of other options for process cooling. Air-cooled condensers can be the best option for reducing water consumption. In air-cooled exchanger’s heat is directly rejected to the atmosphere through the air. It works on the principle of convection and conduction to dissipate heat from process fluid to air. The process fluid passes through the tubes and air stream is passed over the tubes to carry away the heat; air streams are created by the fans mounted on the unit
The tubes which carry the process fluid may be manufactured easily with carbon steel or stainless steel with aluminium high fins to maximize the heat transfer. Airstream can be generated with the use of induced or forced draft fans. Depending on the application fan can be direct or gearbox coupled.
Air cooled heat exchanges can be used for both cooling and condensing application :-
- Air Cooled Cooler: Air-cooled heat exchangers can be utilized for process cooling either stand-alone or in the combination of air-cooled condenser and air & water-cooled heat exchanger where the process needs to be cooled to lower temperature. In the air-cooled type heat exchanger process fluid is passed through tubes. Tube with fins can be installed in different passes and circulation movements based on the property of process fluid. Air is circulated to accelerate heat transfer. For air circulation forced and induced type of fan can be installed for the process cooler.
- Air Cooled Condensers: Air-cooled condensers can be used where the exchange of latent heat is at a higher temperature than dry bulb temperature. Air-cooled condensers for steam applications are very common and used in various process industries including power plants. It must be noted that power generation is reduced with the use of air-cooled condensers as a condensing point in air-cooled condenser shifted above dry bulb temperature. In air-cooled condensers, vapors are fed through the duct and cooling tubes with fins are installed in an inclined / vertical direction to allow condensed fluid to flow down with the use of gravity. An air circulation fan is used accelerate heat transfer.
Benefit of air cooled heat exchangers :
- No water consumption for cooling applications.
- No chemical and wastewater treatment required as cooling water is eliminated
- No periodic cleaning required.
- The system is corrosion and fouling free
- Maintenance is negligible because of the non-involvement of water
Limitation of air cooled heat exchanger:
- Cooling can be done above 4 Deg C of dry-bulb temperature.
- Not suitable for low-temperature cooling application
- Increases compressor power consumption in refrigeration application reduced power in power plant etc.
- Higher noise in comparison to the cooling water system.
- The installation area needs to be properly ventilated.
- Higher Size of heat exchanger.