When to Choose Air cooled heat exchanger for Specific Application?
Deciding when to choose an Air-Cooled Heat Exchanger (ACHE) over a water-cooled system (like a shell and tube with a cooling tower) is a fundamental engineering and economic decision.
The choice isn’t just about technical feasibility; it involves a careful analysis of resources, costs, environment, and safety.
Here is a structured guide on when to choose an air-cooled heat exchanger for a specific application.
The Core Decision Philosophy
The primary rule of thumb is:
Choose air cooling when it is technically feasible and offers a lower lifetime cost, or when water cooling is impractical, too expensive, or poses a safety risk.
Air is free, but the equipment to use it is large and expensive. Water is an excellent coolant, but the water itself, its treatment, and the associated systems have significant costs and environmental implications.
Key Decision Factors: When to Choose Air-Cooling
1. Water Scarcity and Cost
This is often the most decisive factor.
- Choose ACHE when: The plant is located in an arid, desert, or semi-arid region where fresh water is scarce or very expensive.
- Why: ACHEs consume negligible amounts of water (only minor losses for cleaning). A water-cooled system, which relies on a cooling tower, loses significant water through evaporation, drift, and blowdown.
2. Environmental & Regulatory Constraints
- Choose ACHE when: Environmental regulations are strict regarding water withdrawal, thermal discharge (heating up natural water bodies), or chemical treatment discharge (blowdown).
- Why: ACHEs eliminate the need for a large water withdrawal permit and the problem of disposing of chemically treated blowdown from a cooling tower. They also avoid “thermal pollution” of waterways.
3. Process Temperature Requirements
- Choose ACHE when: You need to cool a process fluid to a temperature that is ~10-20°C (20-30°F) above the maximum ambient dry-bulb temperature.
- Why: The process outlet temperature is limited by the ambient air temperature. For example, if you need to cool a stream to 50°C and your site’s max ambient is 40°C, air cooling is feasible. If you need to cool to 30°C, it is not.
- Also Choose ACHE when: The process has a high approach temperature (difference between process outlet and ambient air temperature). This makes ACHEs more efficient and cost-effective.
4. Maintenance and Operational Simplicity
- Choose ACHE when: You want to minimize maintenance related to fouling and corrosion, or when operational simplicity is valued.
- Why: Water-side fouling (scaling, biological growth) in a cooling tower and shell-and-tube exchanger is a significant maintenance issue. ACHEs have air-side fouling (dust), which is generally easier to manage with on-line cleaning (air lances). The system is also simpler, with no pumps, water treatment systems, or cooling towers to maintain.
5. Safety and Reliability
- Choose ACHE when: The process fluid is at a high pressure or temperature, or is flammable.
- Why: A leak in a water-cooled exchanger could lead to water entering the process stream, potentially causing a violent reaction, corrosion, or process upset. A leak in an ACHE simply releases to the atmosphere, which, while not ideal, is often safer for hydrocarbon and chemical services. They are also less vulnerable to freezing in cold climates compared to water systems.
6. Plot Space and Layout
- Choose ACHE when: You have ample available plot space at grade level, but space on pipe racks or near water sources is limited.
- Why: ACHEs are large and must be located in open areas with good air circulation. They cannot be placed in congested areas.
When to Avoid Air-Cooled Heat Exchangers
It is equally important to know when not to choose an ACHE.
- Low Process Temperature Required: If the process fluid must be cooled to within a few degrees of the ambient temperature, ACHEs are not suitable. The wet-bulb temperature in a cooling tower system is always lower than the dry-bulb temperature for air, allowing water cooling to achieve a colder outlet temperature.
- High-Precision Temperature Control: The large thermal inertia of air and its diurnal (day-night) temperature swings make precise temperature control more challenging with an ACHE compared to a water system.
- Limited Plot Space: If the plant site is very compact and expensive (e.g., on an offshore platform or in a dense urban area), the large footprint of an ACHE may be prohibitive.
- Extremely High Heat Duty with Space Constraints: For a very large heat duty, a single water-cooled exchanger might be more compact than a vast battery of ACHEs.
- Noisy or Sensitive Environments: The large fans and motors of an ACHE can generate significant noise, which may be an issue in residential areas or noise-sensitive facilities.
- Dirty or Corrosive Air Environment: If the ambient air is full of abrasive dust, fibrous material, or highly corrosive chemicals, the maintenance on the finned tubes and fans can become excessive.
Practical Examples in Industry
- Oil & Gas (Refineries, Gas Plants): The default choice for cooling hydrocarbons (e.g., overhead condensers, product coolers). Water scarcity and safety (fire hazard) are key drivers.
- Power Plants: Used as the main condenser in “dry cooling” systems for turbines in water-scarce regions.
- Chemical & Petrochemical Plants: Used for similar reasons as oil & gas, especially for intermediate and final product cooling.
- Compressor Aftercoolers: Very common, as the approach temperature is usually high and the duty is manageable.