1.0 Introduction to Air Cooled Heat Exchanger
This kind of heat exchanger are used in petrochemical and refineries to utilize the atmospheric air to cool the hydrocarbon, process and utility fluids by means of indirect heat transfer from fluid (within the tube) to be cooled by air being circulated by means of forces / induced draft fan. In order to increase the heat transfer area, fins are also attached to the periphery of tubes. These heat exchangers are generally designed, inspected and tested as per API661 standard.
These exchangers are similar to the radiator of a motor car. The air cooled heat exchangers are mainly used where the heat load is very high and conventional heat exchanger becomes extremely big with large water requirement. By using air cooled heat exchanger, water requirement reduces drastically and cooling tower size comes down. The air cooler are of very large size as the heat transfer co-efficient with air is very low. Due to the large size, they are mounted on top of the pipe rack to save space.
2.0 Types of Air Cooled Heat Exchanger
There are three types of air cooled heat exchanger:
a) Forces Draft
b) Induced Draft
c) Natural Draft (used for applications like transformer oil cooling)
The outline sketch and components are as identified in figure 1A & B.
3.0 Definitions used for Air Cooled Heat Exchanger
The general terms used for air cooled heat exchangers are defined below:
a) Bank
A bank is one or more bays including one or more items arranged on a continuous structure.
b) Bare tube surface
Total area of the outside surface of the tubes in sq. meter.
c) Bay
A bay is one or more tube bundle serviced by two or more fans and that include structure, plenum and other attendant equipment.
d) Finned Surface
The finned surface of a tube is the total area of the outside surface of fins exposed to air.
e) Forced Draft exchanger
This is the one designed with the tube bundles located in the discharge side of the fan.
f) Induced Draft exchanger
This is the one designed with the tube bundles located on the suction side of the fan.
g) Tube bundle
This is an assembly of headers, tubes and frames.
Refer figure 1A, 1B, 2, 3, 4, 5 and 6 to understand the above definition.
4.0 Content of Enquiry Documents (Special Requirements)
The enquiry documents shall specify any special requirements concerning location of airflow, any obstruction to airflow and other heat sources.
5.0 Vendors Responsibilties
The vendor’s proposal shall include the following:
a) A proposal drawing showing major dimensions in plan, elevation, nozzle size and orientation
b) The proposal shall state whether the vertically mounted electric motors are to be shaft up or shaft down.
c) It should show the supporting arrangement of the tube bundle / plenum, fan and fan drive assembly.
d) It should show the relation of the tube bundle part of the bay with respect to the plenum to depict the movement of the nozzle installed on inlet and outlet headers.
e) It should show the platform and ladder for accessibility, operatibility and maintenance of fan, drive assembly and piping hooked up to the nozzles located on the inlet and outlet header.
f) It should show the weight of components, part of fan and drive assembly to derive the lifting beams, monorail and hoist arrangement to be permanently installed for maintenance of such components.
g) The supporting column shall be located such that it matches the supporting structure to house the air cooled heat exchanger e.g. pipe rack or building structure.
h) Allowable nozzle loading standard or data must be furnished to meet the same during detailed engineering.
i) The fixed point of the tube bundle shall be defined such that the inlet nozzles (hot fluid side) experience minimum movements in comparison to (cold fluid side) outlet nozzle to maintain the nozzle loading within allowable limits.
j) In order to achieve the smooth movement of the tube bundle headers and nozzles over plenum, a friction between the tube bundle and plenum must be means of providing smooth surface such as Teflon pads, stainless steel plate, graphite pads or similar material which can withstand the system temperature and compressive load of component at support.
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6.0 Approvers Responsibilities
The approver shall approve following information received from vendor :
- Maximum and minimum design temperature.
- Overall dimensions.
- Dimension and location of supports.
- Nozzle size, rating, facing, location, projection, allowable movements and loading on nozzle (forces and moments).
- Weight of components for erection and maintenance.
- Drive mounting details.
- Screen platform and ladders.
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