How L-foot, knurled, embedded, extruded and welded fins differ, and which tube materials they suit. Welded finned tube for new coolers and re-tubing comes from Spiro-Gills Thermal Products in Stafford, supplied in the GCC by Petromarsh.
Tap a part to see what it does. The red line is the hot fluid inside the tube, the blue lines are air passing between the fins.
Base tube. Carries the process fluid and the pressure. Its material is chosen for the fluid inside, its wall for pressure and corrosion allowance.
Fins. Thin discs wound or welded in a helix on the tube. They add surface on the air side, where heat transfer is weakest.
Fin root and bond. Where the fin meets the tube. The way it is held, by a foot, a groove, an extruded sleeve or a weld, decides the temperature limit and how long the bond lasts.
Fin pitch, fin height and tube OD. Closer pitch and taller fins add surface but raise air pressure drop and trap dust. A common air cooler tube is 25.4 mm OD with fins about 16 mm high.
End view. Looking along the tube. The fin is a disc around the tube, and air flows across it, between the fins.
A plain tube in air transfers little heat, because air is a poor heat carrier. Wrapping the tube in fins multiplies the outside surface. The fin is only useful while it stays in tight contact with the tube, so the bond is what you are really choosing.
Fins raise the outside area many times over. That lets the air side keep up with the much better heat transfer inside the tube.
Heat must cross from tube to fin. Tension-wound fins rely on grip, embedded and extruded fins are locked in, and welded fins are fused to the tube.
As the tube heats, it expands more than aluminium can grip, and the bond loosens. Moisture and salt then attack the gap. Each fin type has a typical temperature limit.
Temperatures are typical industry limits for the tube metal, not guarantees. The exact value depends on the tube, fin material and maker. Tap a type to see when to use it.
An aluminium strip is wound under tension with an L-shaped foot that sits on the tube. The cheapest fin, but the grip relies on tension alone.
Low-temperature coolers in dry inland air.
Like the L-foot, but each foot overlaps the next, so the tube is fully covered by aluminium.
A step up from L-foot where some protection of the tube is needed.
An overlapped L-foot fin pressed into a knurled tube surface, so the fin locks into the tube. Better bond and higher temperature than plain L-foot.
Medium-temperature process coolers.
A helical groove is cut in the tube wall and the fin is rolled into it, then the groove is closed. The strongest mechanical bond. The tube between fins is exposed.
Hot inlets and cyclic duty, inland or with corrosion-resistant tubes.
An aluminium sleeve over the tube is rolled out into fins, so the liner is sealed over the whole tube. Very good bond and the best protection of the base tube.
Coastal and offshore sites, and anywhere the bundle is washed often.
A carbon or stainless steel strip is welded in a helix to the tube, solid or cut into serrations. Very high temperature and very robust, but heavier with lower fin efficiency than aluminium.
Hot gas, flue gas, waste heat recovery and air pre-heaters.
Any fin type can sit on several tube materials. The tube is chosen for the fluid inside, the fin for temperature and the air outside.
| Tube | Aluminium fins | Steel fins (welded) | Copper fins |
|---|---|---|---|
| Carbon steel tube, SA-179 · SA-214 · SA-334 | L, LL, KL, G or extrudedthe usual air cooler tube | Carbon steel, galvanisedhot gas and heavy duty | Possible, rarely usedcheck galvanic effects |
| Stainless tube, SA-213 · SA-249 | L, LL, KL, G or extrudedcorrosive process fluid | Stainless steel finshot or corrosive air | Not usual |
| Duplex tube, SA-789 | G or extrudedchlorides, sour duty | Stainless finson request | Not usual |
| Copper alloy tube, SB-111 | L or extrudedwater and oil coolers | Not usual | Copper finshigh conductivity, compact coolers |
Tube material standards set the base tube. API 661 sets fin and tube requirements for process air coolers.
Fin types, fin material, minimum fin thickness, bond and tube requirements for air-cooled heat exchangers.
Seamless cold-drawn and welded carbon steel heat exchanger tubes.
Carbon and alloy steel tubes for low-temperature service, impact tested.
Seamless and welded austenitic stainless steel heat exchanger tubes.
Seamless and welded ferritic-austenitic stainless steel tubes.
Copper and copper alloy seamless condenser tubes, including admiralty and copper-nickel.
Hot-dip galvanised coating on steel welded fins.
Non-destructive testing methods, and certified mill certificates for every pressure part.
Rates the finned tube in the bundle: fin efficiency, air pressure drop and the effect of fouling.
Your engineers red-line our drawings, you approve the design twice before production starts, and you can witness the tests. Tap a step to see what happens and which documents you receive.
Spiro-Gills Thermal Products has designed and built bespoke air-cooled heat exchangers and finned tube in Stafford since 1946, and has supplied them to more than 100 countries. Thermal design is done in HTRI, welding is to ASTM code, and the company holds ISO 9001:2015. Petromarsh is their exclusive representative in the GCC.
For replacement tubes, a sample or the old bundle drawing is ideal. For new duty, the items on the right are enough.
