Air Cooler Exchanger Construction

Air Cooled Exchanger Construction Module 6

EXC 6-1

Heat Exchangers - Course Content Module 6 - Air Cooled Exchanger Construction

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Typical Arrangement Header Types Standards Materials of Construction Finned Tubing Types Fan Arrangements Fans and Drives Temperature Control EXC 6-2

Typical Arrangement Nozzles Header Tube Bundle

Plenum

Fan Deck

Fan

Fan Ring

Drive Assembly Supporting Columns EXC 6-3

Header Types Tubesheet

Removable cover Gasket

Removable Cover Plate

Advantages – Easy cleaning – Easy access to tubes and tubesheet(easy inspection and repair) „ Disadvantages – Gasket requirements – Mechanical design of long flanges is difficult for high pressure and temperature (>300 psig and >350°F) „

EXC 6-4

Header Types (continued) Gaskets Tubesheet

Removable bonnet

Removable Bonnet

Advantages – Good cleaning of header and tubesheet – Header only or tubesheet only can be replaced „ Disadvantages – Removal of piping necessary for tubesheet inspection – Gasket requirements – Mechanical design of bonnet is difficult for high pressure and high temperature (>300 psig and >350°F) „

EXC 6-5

Header Types (continued)

Plug headers

Plug

Tubesheet

Advantages – Easy cleaning of tubes (brush, hydroblasting) – No gaskets – No long-flange design – Fewer bolts (many plugs) – Good for high pressure design (up to 3000 psi) – Good for hydrogen service „ Disadvantages – headers have to be chemically cleaned (water, steam, light hydrocarbon) „

EXC 6-6

Header Types (continued)

For large temperature differences > 200°F One must deal with differential expansion of top and bottom tubes „ Example „

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First pass at 325°F, second pass at 120°F, Carbon Steel, 30 ft cold length (360 inches at 70°F) α is about 6.3x10-6 °F-1 for carbon steel ( is the thermal expansion coefficient) IN

360.6 inches 325°F 120°F

OUT

360.1 inches

differential expansion is 0.5"

EXC 6-7

Header Types (continued)

Split headers

All headers require vents and drains - high and low points EXC 6-8

Standards

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API 661 / ISO 13706 Petroleum and Natural Gas Industries -- Air-Cooled Heat Exchangers Mechanical Design – Winterization – Noise Measurement –

EXC 6-9

Materials of Construction „

Tubes and Headers – Carbon steel – Killed carbon steel (H2, wet H2S service) – Chrome Moly steels – Stainless steel

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Fins – Aluminum

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Plenums, Fan Rings and Supports – Structural carbon steel (often galvanized)

EXC 6-10

Finned Tubing Types „

Process temperature constrictions are dependent – Fin material – Fin construction - thermal contact of fin to tube

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Problems – Separation of fin and tube – Thermal expansion – Poor heat transfer

EXC 6-11

Finned Tubing Types (continued)

thickness

Embedded Fins (< 750°F process temp)

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Description – Helical groove is cut into the outside of the tube wall. – Metal is displaced on each side of the groove. – Fins are wound into groove. – Displaced metal forced against each side of the fin.

height

CL EXC 6-12

Finned Tubing Types (continued)

Integral/Extruded (< 550°F process temp)

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Description – Fins are formed by extrusion from an aluminum outlet tube – Mechanically bonded to the tube

CL

EXC 6-13

Finned Tubing Types (continued)

Footed (L-foot) (< 350°F process temp)

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Description: – Footed fins are tension wrapped around tube – Each fin butts up against the adjacent fin to cover the complete tube – Shields tube against atmospheric corrosion

CL EXC 6-14

Finned Tubing Types (continued)

Footed (L-foot) (< 350°F process temp)

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Variations: – Overlapped footed -Next fin on top of previous L leg. – Knurled footed - Tube and fins are ribbed for better holding power.

CL EXC 6-15

Finned Tubing Types (continued)

Externally Bonded (<250°F process temp) „

Description – Fin is bonded to outside of tube by welding, brazing or hot-dip galvanizing. CL

EXC 6-16

Finned Tubing Types (continued)

Tension wrapped (<250°F process temp) uop n o t a llo w e d S p e c 4 -1 3

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Description – Fin is tension wounded around tube.

CL

EXC 6-17

Finned Tubing Types (continued)

Cost scale from least expensive to most expensive – Tension wrapped – Footed – Embedded – Extruded „ Typical Tube Sizing – Tubes — 1" OD tubes — 12 BWG (0.109" thick) — 2-1/2" triangular pitch – Fins — Height 0.6" — Thickness 0.12", usually aluminum — About 9-11 fins per inch — Finned area: bare tube area 18-20 : 1 „

EXC 6-18

Finned Tubing Types (continued)

2.5" 0.6"

60°

2.2" EXC 6-19

Fan Arrangements and Types „

Forced Draft - air is forced or pushed across tubes. Fan is placed below the bundle.

bundle

Advantages Lower horsepower (cool air) Better accessibility of motor and fan Easily adaptable to recirculation for winterization

Disadvantages Air distribution not as good as induced. Hot air velocity away from bundle is small. Possibility of recirculation of hot air. Low natural draft capability, unless skirt is specified EXC 6-20

Fan Arrangements and Types (continued)

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Induced Draft - air is pulled across tubes. Fan is placed above the bundle.

bundle

Advantages good air distribution across bundle hot air is blown far away from bundle bundle is protected from above by plenum and fan has 'stack' to improve natural draft (fan failure)

Disadvantages higher fan-power required (air is hot) air outlet temperatures must be limited (prevent fan damage) fan maintenance - damage to bundle fan maintenance difficult (hot air, accessibility)

EXC 6-21

Fan Arrangements Induced Draft

EXC 6-22

Fan Types „

Fixed-pitch fans (2 types) – Blade pitch is permanent – Blade pitch can be adjusted manually when fan is

stopped „

Variable pitch blades – Automatic or operator controlled during operation – Greater operational flexibility – Used for power conservation during turndown or low

ambient air temperature – Control of process outlet temperature

EXC 6-23

Driver Types „

Standard Motors – Single Speed ™ Limited turndown/winterization capability ™ Poor temperature control ™ Low price – Two Speed Motors ™ Two forward operating speeds ™ Better turndown/winterization capability

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Variable-Speed Motors – Vary electrical frequency to control motor speed ™ Used to vary air flow rate for temperature

control or energy conservation ™ Used instead of AV fans EXC 6-24

Driver Types (continued)

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Steam Turbines – Vary steam flow rate to vary speed of turbine — Used to vary air flow rate — Same advantages as variable speed motors — Very high cost for equipment — Poor steam turbine efficiencies — Very infrequently used

EXC 6-25

Fan Blade Angle Inspection

EXC 6-26

Drive Belts „

Use Direct Drive Cog Type Belt for Motors Greater Than 20 HP

EXC 6-27

Temperature Control „

Control temperature is accomplished by – Varying the air flow (vary ∆Tcorr)

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Louvers – Controlled manually – Controlled automatically — Disadvantage: — Snow, ice or corrosion affects operation of

louvers

EXC 6-28

Temperature Control Control temperature is accomplished by „

Two Speed Motors – Motor can be run at fast and slow speed as required

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Auto-Variable Pitch Fans – Blade pitch adjusted to vary air flow rate as ambient

air temperature increases „

Variable Speed Motors – Fan speed various when process or ambient conditions

changes „

Louvers – Not recommended for temperature control EXC 6-29