The document discusses air cooled heat exchangers. It describes how air cooled heat exchangers work by using air as the cooling medium, making them useful when water supply is limited. The document outlines the main components of air cooled heat exchangers, including axial fans, tube bundles, headers, fins and nozzles. It also discusses types of fans, headers, fins, factors that affect performance like fouling, and considerations for inspection and design of air cooled heat exchangers.

1. NATIONAL REFINERY LIMITED
AIR COOLED HEAT
EXCHANGER
Process & Maintenance
AHSAN SHAKIL (SENIOR MANAGEMENT TRAINEE)
9/22/2015

2. Air Cooled Heat Exchanger
It belongs to that type of heat exchanger in which air is used as a
cooling medium for liquids and vapors, commonly used where
there is shortage of water or in other case when entire water
supply is being utilized for heat exchange and still further coolant
is required. Thus employing air cooled heat exchanger enables to
get rid of extensive water treatment techniques and cooling
water circuit. On the other hand, they are more expensive than
shell and tube heat exchanger and comprised of large foot print.
Thus they are usually installed on pipe racks. The minimum
temperature to which the process stream can be cooled is 5C
above the ambient air temperature.
Types
 Forced Draught (push)
 Induced Draught (pull)
Induced Draught is favored over forced draught when:
 Hot air recirculation is to be avoided
 Uniform air distribution is required
 Less affected by climatic conditions (rain, snow)
because of top covered
Disadvantages associated with induced draught include:
 Higher power consumption
 Fan maintenance more difficult
Main Components of Air Cooled Heat Exchanger
 Axial fan
 Tube bundle
 Header
 Tube supports
 Fins
 Nozzles

3.  Partition plates for passes
 Drive assembly
 Plenum
The header can be either plate type or plug type i.e.
one of their face is attached to the tube sheet
whereas the other one is either bolted down or
welded. If it is welded, plugs are provided for
cleaning of tubes. In addition to this, removable
cover and removable bonnet type header assembly
also exist. Removable cover enables removing the
cover without disturbing header pipeline whereas
removable bonnet permits bonnet removal with
minimum dismantling of header pipe line.
The different type of fins used in air cooled heat exchanger includes:
Fin Type Temperature Range Illustration
Wrapped 120C
Extruded 260C
Embedded 400C
Footed 150C

4. Factors
 Since air is used as a coolant having low thermal conductivity, heat
transfer is promoted using extended surfaces called fins, which
increases the surface area and ultimately the air side heat transfer
coefficient. Fin efficiency depends on thermal conductivity and
geometry of fins.
 Since air moves in perpendicular direction to tube side,
considering first row, the air temperature varies in
longitudinal direction because heat transferred varies from
point to point. Similarly for the second row, temperature
differences also vary and temperature changes experienced
by the air are different from preceding row.
Fowling
Fowling refers to uniform accumulation of suspended particles on a
surface. It does not involve any sort of reaction and settlement is
temporary I.e. it can removed by high velocity stream. External
surface of the tubes start to fowl as a result of environmental dust,
thus from maintenance point of view, compressed air having pressure
up to 600psi is used.
Inspection
Inspection of following parts/condition is carried out for smooth operation of air cooled heat exchanger:
 Header/Bolts
 Gasket
 Corrosion/Erosion
 Damaged fin
 Leakage

5. Design
For sizing/designing of air cooled heat exchanger, ACHEX proves to be fruitful.
References
 API 661
 Heat transfer in Process Engineering (Eduardo Cao)