Air cooled heat exchanger perform their cooling function by flowing large quantity of cooled air around a bank of finned tubes by the help of large fans. The combination of FINned tube and the air circulating FAN has made FIN-FAN cooler a common term used for air coolers.
an overview of how Air cooled heat exchanger work can be seen in below animation
CODE USED FOR AIR COOLED HEAT EXCHANGER:
ANSI/API Standard 661 – air cooled heat exchanger for general refinery services.
ISO 13706-1:2005(identical) – petroleum, petrochemical and natural gas industries-Air Cooled Heat Exchanger.
Parts of Air cooled Heat Exchanger (ACHE):
A typical air cooled heat exchanger consists of following components.
1. Tube bundle
2. An air pumping device such as an axial flow fan or blower.
3. A support structure high enough to allow air to enter beneath the ACHE at a reasonable flow rate.
4. Optional feature like (a) louvers for process outlet temperature control (b) recirculation ducts and chambers for protection against freezing or solidification of high pour point fluids in cold weather (c) variable frequency drive for temperature control
Headers are the boxes at the end of the tubes which distribute the fluid from piping to the tubes. Welded Rectangular box type is used most primly in all of them. Some of the common header types are shown in pic below.
Orientation of tube bundle:
ACHE can be installed either in horizontally, vertically or in sloped angle. The most common orientation is the horizontal one. A considerable reduction in ground area can be made if bundles are vertically mounted but performance of the unit is greatly dependent on prevailing wind speed. As ACHE Fans push air in one direction and if prevailing wind in opposing direction it greatly reduces the performance, that’s why use of vertically mounted bundles is confined to small, packaged units.
A compromise, which requires around half the ground area of horizontal unit, is the A-frame or V-frame unit. In this type, two bundles sloped at 45°-60° from the horizontal are joined by their headers at top or bottom to form the sloping side of the A or V type, respectively. The A-frame type with forced draft fans is the more common and is used in steam condensing application.
Summarizing above air cooled heat exchangers can be classification into below types.
1. Horizontal forced draft Air cooled heat exchanger
2. Horizontal induced draft Air cooled heat exchanger H
3. Vertical Air cooled heat exchanger.
4. A frame air cooled heat exchanger.
Forced draft VS Induced draft:
The moving of air across the tube bundle may be either forced draft or induced draft. Both arrangements are shown in above figure.
Forced draft- in this arrangement fan is located below the tube bundle and air is forced through the fin tubes. The majority of air cooled heat exchangers are forced draft type.
(A) As forced draft fans are at air inlet of ACHE, these have the advantage of handling cold air entering the exchanger.
(B) As air at outlet is hotter it has increased volume at outlet. Being at inlet forced draft requires moving smaller volumes of air and requires less horsepower than induced draft.
(C) It generally offer batter maintenance access as the tube bundle is mounted on top and can easily be removed.
(D) A common problem with forced draft coolers is accidental warm air recirculation. For more on this see https://www.calgavin.com/heat-exchanger-problems/7304/heat-exchanger-problems-part-3
Induced Draft: In this arrangement fan is located above the tube bundle and air is induced or pulled through the fin tubes.
Compared with forced draft design, induced draft design has the below advantages.
(A) Easier to shop assemble, ship and install.
(B) The hoods offer protection from weather and hailstone protection.
(C) Induced draft with fan above them can be mounted directly on the pipe rack, eliminating the structural steel support which results in lower material cost as well as simplified field installation.
(C) With the fan above cell, hot air exits the top of the unit at 2.5 times the velocity possible in the forced draft unit so it is less likely to be affected by hot air recirculation.
(D) Batter air distribution over the tube bundle.
And some of the disadvantages are
(A) More difficult to remove bundle for maintenance.
(B) High temperature service is limited due to effect of hot air on the fans.
(C) More difficult to work on the fan assembly due to heat from the bundle and due to their location.
Natural Draft: Natural ventilation does not need a fan to operate. Air circulates because of the temperature difference between the inside and outside of ACHE and also due to difference in height. A chimney is provided above the tube bundle to create the draft necessary to suck and push air through the tube bundle. That’s why it is called “chimney effect”. One of the advantages of natural draft is that of a silent and economical unit.
Recirculation: Air cooled heat exchanger with internal recirculation systems are used in extremely cold climates (e.g. Canada, Siberia etc). This system is used to control the cooling air temperature regardless of ambient air temperature. This prevents clogging of the fluids to be cooled. Internal recirculation system requires the use of positive and negative step auto variable fans.
Air cooled exchanger with air humidification by flow or spray: For certain extreme cases in hot countries with a fluid outlet temperature very close to the ambient air temperature, it is necessary to use water humidification system by flow (humidifier) or high pressure spraying (peak cooling/moisturizing). Air cooled heat exchanger with air humidification by flow currently used marginally and they are intended to be installed in tropical countries where it is necessary to use the latent heat of evaporation of water to cool the ambient air. Much ancillary equipment, such as the circulation pump, the recovery sump and the humidifiers, is added to the air cooled exchanger to allow the humidification of intake air.
When the existing dry air cooled heat exchangers is no longer powerful enough due to change in climatic condition or process changes the humidification system made up of humidification sprayers fed by high pressure pumps. The fineness of the droplets allows thermal exchange with the ambient air. This system generally operates without a recovery tank.
Some advantages and disadvantages of Air cooled heat exchanger vs water cooled exchangers are provided below
Air cooling vs water cooling
1. Air is available free in abundant quantity with no preparation costs.
2. ACHE design is well established, and can perform well with a reasonable degree of reliability.
3. Water is corrosive and requires treatment to control both scaling and deposition of dirt, whereas air is mostly noncorrosive. Therefore, material selection is governed by process fluids routed through the tube side.
4. Mechanical design problems are eased with ACHEs since the process fluid is always on the tube side.
5. Danger of process fluid contamination is much greater with water-cooled system.
6. Air-side fouling can be periodically cleaned by air blowing, and chemical cleaning can be carried out either during half-yearly or yearly attention. Water-cooled systems may require frequent cleaning.
7. Maintenance costs for ACHEs are about 20%–30% of those for water-cooled system. Operating costs for water coolers are higher, because of higher cooling water circulation pump horsepower and water treatment costs.
8. Air cooling eliminates the environmental problems like heating up of lakes, rivers, etc., blow down, and washout.
Air cooling has the following disadvantages:
1. ACHEs require large surfaces because of their low heat transfer coefficient on the airside and the low specific heat of air. Water coolers require much less heat transfer surface.
2. ACHEs cannot be located next to large obstructions if air recirculation is to be avoided.
3. Because of air’s low specific heat, and dependence on the dry-bulb temperature, air cannot usually cool a process fluid to low temperatures. Water can usually cool a process fluid from 10°F to 5°F lower than air, and recycled water can be cooled to near the wet-bulb temperature of the site in a cooling tower
Kuppan Thulukkanam, 2nd edition,2013, Heat Exchanger Design Handbook, crc press, new york
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