Air Cooled Heat Exchanger

Air cooled heat exchanger is a finned tube heat exchanger that uses the air for cooling medium. It is required a wide surface area for heat transfer. It is applied to finned tube for increasing the heat transfer surface and compact design of construction area

An Air cooled heat exchanger installation includes the finned tube heat exchanger header, steel support structure, fan drive equipment, including axial fans, V-belt drivers, bearing shafts and electric motors.

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BASARANLAR produces industrial air cooled heat exchanger that are designed to meet specific customer requirements.

BASARANLAR offers air cooled heat exchanger that vary in size, ranging from small vertical air coolers to large horizontal air coolers. Unit configurations include forced draft, induced draft and v-type designs. Air cooled heat exchanger are commonly used for cooling compressed gas, oil and other working fluids.

Air cooled heat exchanger are used for many Industrial Applications, such as Power, Chemical, ORC Plant, Oil & Gas, Steel Industry and many other Applications.

Although air cooled heat exchangers have higher initial investment costs compared to water-cooled systems, their operating costs are cheaper. If we count the operating costs, these are make-up water, water purification systems and pump costs.

The operating cost of dry type coolers consists only of the power consumed by the fans that provide air to the heat exchangers. When all these initial investment and operating costs are added together, after a certain period of time, dry type coolers can be more advantageous than water-cooled systems.

Small air cooled heat exchangers are used in computers and other electronic devices, vehicles (radiators, oil coolers, intercoolers), air conditioning and refrigeration plants (condenser), etc. It finds application in many areas such as. Larger air-cooled heat exchangers are used in chemical plants, various process industries and power plants. The movement of cooling air is provided by mechanical equipments and fans.

Air cooled heat exchangers can be forced draft or induced draft types. In forced draft types, fans are assemblied at the bottom of the finned tube heat exchanger. As a result, the power consumption for a given mass airflow is less than in a forced airflow configuration. The fans and motors that provide cooling airflow at the bottom of the unit are also easier to maintain, and the fans are not exposed to high temperatures, making the choice of construction material less critical.

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What parts does an air cooled heat exchanger consist of?

• A finned-tube heat exchanger that provides one or more heat transfers. ( Extruded, L, KL, G-Fin types )
• An axial fan that provides air flow.
• An electric motor to mechanically rotate the fan.
• A galvanized steel construction, high enough to allow air to enter under the air cooled heat exchanger at a reasonable rate.
• Collector and fan-motor maintenance platform and ladder.
• Optional louvers for process outlet temperature control.
• Fan system where optional blade angles can be changed for temperature control and power saving.

The heat exchanger consists of the combination of finned tubes, collector, steel frames and tube supports as shown in the figure.

How to choose fins type?

The selection of fins types is critical in air cooled heat exchangers. This choice is affected by cost, operating temperatures and atmospheric conditions. Each type has different heat transfer and pressure loss characteristics.

Extruded finned tube provides the best protection of the main tube against atmospheric corrosion as well as consistent heat transfer from initial installation and throughout the life of the cooler.

This is the preferred finned tube for operating temperatures up to 315°C.Type G fin provides predictable heat transfer and should be used for all cooling operating above 315°C and below 400°C.

L Fin type  can be used below 120°C; However, the bond between the fin and the tube loosens over time and heat transfer cannot be predicted with certainty over the life of the cooler.

How to choose collector types?

Plug type header

It is a very frequently used collector type. The tubes are fixed to the tubesheets by welding or by expantion methods. When the tubes want to be cleaned, the bolts called plugs are removed and the inside of the tubes are cleaned, then they are tightened again with bolts called plugs to prevent any air leakage.

It is especially preferred in high pressure applications (up to 211 bar) and applications involving clean water. The most important disadvantage is that in processes involving dirty water, great time is lost during the removal and installation of bolts to clean the inside of the tubes

Removable bolted header

It is preferred because it is easier to remove than plug types, especially in applications where the inner surfaces of the tubes need to be cleaned frequently due to dirty process water. Not suitable for applications above 40 bar.

Forced Draft Cooler Design:

Fans are easier to maintain. Fans blow air at ambient temperature with higher efficiency.

Air Cooled Heat Exchanger Advantages:

• Fans are very easy to access for maintenance.
• Since the plenium are located under the bundle, it is very easy to remove and install the bundles.
• It is the perfect solution when the process fluid is very hot, because if the process fluid is very hot, the fans will be directly exposed to the outside air since they are located below; whereas, if the fans were at the top, the heated air would take the temperature of the process fluid and cause some thermal deformations in the fans.
• It is suitable for operation at high process fluid inlet temperatures.

Air Cooled Heat Exchanger Disdvantages:

• It is weaker for the air to pass equally across all surfaces of the bundle than for those with top fans.
• Since the exit speed of hot air from the bundle surface is low, there is a possibility of recirculation.
• In case of failure of the fans, the natural ability to produce traction is poor.
• Since the surface of the serpentines is open, the finned tubes are exposed to all kinds of sun, rain and hail, making it difficult for the process to continue in a controlled and stable manner.

Induced Draft Cooler Design:

The fan stack is mounted on top of the bundle, thus protecting the finned tubes surface against wind, rain, snow and partly hail. The location of the fans also ensures optimum air distribution, limiting air recirculation and allowing maintenance lower at walkway level. Fans deliver hot air.

FAN STACKS

• Maximizes fan efficiency.
• Reduces sound levels near the fan system.
• The stack prevents the fan from running backwards due to outside air interference.
• The inlet bell provides smooth airflow to the fan.
• Air-cooled heat exchangers are provided with an axial flow, propeller-type fan that ensures the flow of air through the heat exchanger. A heat exchanger or set of heat exchangers is usually included with two fans to provide redundancy in the event of failure of a mechanical unit and to provide basic control that can be achieved by operating one or two fans.

In air cooled heat exchangers, fans can be very large in size, from 1.8 m to 18 m in diameter, and consist of 2 to 20 blades. Blades can be made of aluminum or fiberglass-reinforced plastic and can be solid or hollow.

In air-cooled heat exchangers, fans can be driven by electric motors. The most popular speed reducer is the high torque positive type belt drive, which uses gears that match the timing belt sprockets.

They are used in motors up to 50 or 60 horsepower and fans up to a diameter of approximately 5.4 m. Belt V-belts are still used on small to medium sized fans, and gear drives are used on very large motors and fan diameters.

Fan speed is adjusted by using a suitable combination of sprocket or pulley sizes with timing belts or V-belts and selecting a suitable reduction ratio with the sprockets. In air-cooled heat exchangers the fan tip speed should not exceed 58 m/s for mechanical reasons and to achieve lower noise levels can be reduced for .

WHAT ARE THE AIR COOLED HEAT EXCHANGER DESIGN CRITERIA?

• In air cooled heat exchangers, single-fan product design should be avoided as much as possible. In applications with two fans, the system can continue to operate with a single fan even if a single fan fails.
• In air cooled heat exchanger, fans must be axial type and cover at least 40% of the heat exchanger surface.
• The width of each group (bay) should be half the tube
• In air-cooled heat exchangers, the angle between the fan blade tip and the heat exchanger edge center point should not exceed 45º for proper air distribution.
• The distance between the fan blade tip and the fan circle should not exceed 0.5% of the fan diameter or the smallest value of 19 mm should be accepted.
• Fan blade tip speed should not exceed 58 m/s.
• The minimum distance between the fan and the air-cooled heat exchanger should be half the fan diameter in forced draft units and one third in induced draft units.
• Fan ring height should be taken as one-sixth of the fan diameter in both forced draft and induced draft units.
• Gear reducers should be used for direct drive motors for fans of 5 kW and below, and for motor powers of less than 3.7 kW, for V-belt motors with motor powers of 22 kW and below, and for fans of 37 kW and above.
• Tube diameter must be at least 1′ (25.4mm).
• Fluid speed in the tubes is min 1m/s and max. It should be 2.5-3.0 m/s.
• Fin heights should be 9.5mm, 12.7mm, 15.875 mm.
• The number of fins should be between 276-433 pieces/m.
• Although the number of pipe rows in air-cooled heat exchangers is 4-6, there are also dry coolers with 8 or even 10 rows. However, it should be taken into consideration that when increasing the number of rows, the motor power will increase accordingly.

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