AIR COOLED HEAT EXCHANGERS: WHAT TO KNOW

Air cooled heat exchangers (ACHEs) are vital devices used for cooling process fluids and condensing vapors by utilizing air as the cooling medium. They are commonly used where water availability is limited. Let's look at what ACHEs are, their working, types, applications, pros & cons.

What is an Air Cooled Heat Exchanger?

An air cooled heat exchanger consists of fins and tubes with air flowing over the finned tubes and a process fluid flowing inside the tubes. It works like a car radiator, using air to remove heat from the process fluid. The large surface area of the fins efficiently dissipates heat to the ambient air via convection.

How Does an ACHE Work?

The hot process fluid flows inside the tubes while atmospheric air flows over the finned tubes. The fins conduct heat from the tubes and increase the effective heat transfer area exposed to air. The air absorbs heat from the fins and tubes and gets warmed up while the process fluid gets cooled. Fans are used to force air over the fins to improve heat transfer when needed.

Types of Air Cooled Heat Exchangers

Air cooled heat exchangers (ACHEs) use air to remove heat from process fluids and are commonly used where water cooling is not available. The main types of ACHEs are:

1. Natural Draft Dry Cooling Towers

These very large ACHEs utilize tall radiator-like heat exchanger bundles and the natural draft created by the density difference between the warm air inside and cooler ambient air. Fans are not required. Used for large power plants.

2. Forced Draft ACHEs

Utilize axial or centrifugal fans to force ambient air across finned tube bundles to enhance heat transfer. Can be used for capacities up to 150 MW.

3. Induced Draft ACHEs

Have fans mounted at the exit to pull air through the core of the bundle. This avoids fouling of fan blades. Better for dusty environments.

4. Horizontal ACHEs

Have horizontal finned tubes bundled in a box shape with air blown horizontally across the tubes. Compact and suitable for smaller duties.

5. A-frame or V-frame ACHEs

Have finned tubes arranged in an A-shape or V-shape bundle. Allows easy access to tubes for cleaning. The angled tubes promote air flow and heat transfer.

6. Coil Type ACHEs

Use finned coil loops arranged in a cylindrical shape with air blown through the center or over the coils. Used for condensers, evaporators, HVAC.

7. Plate and Shell ACHEs

Finned tubes or plate bundles enclosed in a duct or shell with air forced through the core. The shell helps optimize air flow.

The choice of ACHE depends on factors like capacity, allowable pressure drop, fouling risk, maintenance access, cost and space constraints.

Applications of ACHEs

ACHES are used for:

• Cooling turbine exhaust in power plants
• Cooling & condensing refrigerants
• Cooling oils lubricants and process fluids
• HVAC systems for facilities
• Pros of Air Cooled Heat Exchangers
• Eliminate need for cooling water
• Can be used where water supply is scarce
• Avoid wastewater treatment of cooling water
• Lower risk of corrosion and scaling
• Compact footprint, easy to install
• Cons of Air Cooled Heat Exchangers
• Performance limited by wet bulb temperature
• Larger and more expensive than water cooled exchangers
• Re require more maintenance - fan motors, fins, filters
• More prone to fouling by airborne particulates

Conclusion

Heat exchanger, including air-cooled heat exchangers, play a crucial role in numerous industrial operations. These devices enable the efficient transfer of heat from one fluid to another, all while preventing the mixing of the fluids. While there are different designs of heat exchangers, they all operate on the fundamental principle of facilitating heat exchange between hot and cold fluids.