Air-Cooled Heat Exchanger

What an Air-Cooled Heat Exchanger Is and What Is Being Decided

The purchase is bigger than a tube bundle. An air-cooled heat exchanger arrives as a structural bay carrying one or two tube bundles, plenums, a fan deck with axial fans, electric motors, and right angle gear drives or belt drives, plus the header boxes that connect the process to the tubes. API 661, with ISO 13706 as its international mirror, governs the minimum requirements for petroleum and natural gas service: thermal and mechanical design, materials, fan and drive construction, noise, testing, and the data sheets on which vendors must quote. Buying without the standard means buying without a common language for any of those items, and the chapters below assume the API 661 data sheet discipline throughout.

What is actually being decided in this purchase is whether air is the right coolant for the duty at all, and if it is, four construction commitments follow: draft configuration, fin type, header type, and capacity control. Those four, plus the design dry bulb temperature and a noise limit, define the machine far more than any catalogue model number. A bay chosen only for its summer duty freezes its water service in January. A fin type chosen only on first cost loses its bond at hot oil temperature. A control scheme chosen casually either cooks the process in summer or splits tubes in winter. Each of those failure paths is preventable by a specification line, and each line is developed in the chapters that follow.

The boundary against the water cooled neighbors is worth drawing early, with the full comparison reserved for the selection chapter. Where water, plot space, and a drainage path exist, a shell and tube heat exchanger backed by a cooling tower rejects heat at lower first cost per kilowatt, and the circulating loop it needs is an ordinary centrifugal pump duty, with a multistage pump on the return leg where the site elevation demands it. A plate heat exchanger is a compact liquid to liquid machine for close approach duty inside such a water circuit, not a heat rejection device. A chiller is the instrument for below ambient cooling, and the air-cooled heat exchanger is not: it cannot cool any process below the ambient dry bulb, and a realistic outlet sits 10 to 15 deg C above it. Those two sentences settle half of the feasibility questions before any vendor is contacted.

Four documents organize the datasheet. API 661 fixes the air-cooled heat exchanger itself, design, construction, testing, and the vendor data sheets, and a serious purchase is normally accompanied by those data sheets filled line by line. Header boxes are pressure parts, designed to ASME VIII Division 1 where the mechanical code stamp is specified, and process piping to and from the bank follows ASME B31.3. Fan performance and sound data trace to AMCA rating practice. Where the service is not petroleum, the same standard is still the strongest available reference, and an RFQ that cites it explicitly, with the ASME VIII scope for headers, transfers the burden of proof to the vendor.

The exchanger never works alone, and the RFQ should name its station. Process fluid arrives through piping that carries its own thermal growth, and the header nozzles are usually the weakest link in that negotiation. Commissioning practice puts a temporary Y strainer in the inlet line because weld bead and construction scale are exactly what lodges in a finned tube bank during first circulation. Operating data comes from instruments on the inlet and outlet, and the winter package may add steam coils and tracing whose condensate handling returns in chapters 3 and 6. Naming the station in the RFQ lets one supplier check nozzle loads, isolation hardware, and instrument mounts in a single pass instead of four arguments at delivery.

How Air-Side Physics Sets the Design

The air side dominates. Inside a tube carrying liquid or condensing vapor, the heat transfer coefficient runs from several hundred to several thousand watts per square metre kelvin. On the bare outside of that same tube in moving air, the coefficient is an order of magnitude smaller. The fin exists to spend cheap aluminium surface against that stubborn air film: a typical bundle carries 15 to 25 times more finned surface than bare tube surface, and the fins work at a fin efficiency of 60 to 90 percent because fin metal has finite conductivity and the bond to the tube has finite resistance. The honest result is an overall heat transfer coefficient around 30 to 60 W/m2K referred to the total finned surface, and every thermal quotation should state which surface area its coefficient refers to, because referred to bare tube area the same hardware appears to perform fifteen times better than it does.

Mean temperature difference is where thermal honesty is won or lost. Air enters at the design dry bulb and leaves typically 10 to 30 deg C hotter, so the effective temperature difference driving the duty is far below the process temperature minus ambient, and the correction depends on the number of tube passes and the flow arrangement. The design dry bulb itself should be selected on a 1 to 2 percent exceedance basis for the exact site: the air temperature that the local climate exceeds only that fraction of hours per year. Quote the record summer maximum and the exchanger grows, with fan power it will almost never use; quote the annual mean and the process runs hot every summer afternoon. The RFQ should also state the site mean minimum temperature, because the winterization package is designed against that number, not against the summer data.

Air side numbers close the physics. Face velocity across the bundle face runs 2.5 to 3.5 m/s in normal design, low enough to keep air side pressure drop and fan power sane, high enough to carry the duty. Axial fans of 8 to 18 ft diameter, roughly 2.4 to 5.5 m, cover most process bays, with larger fans to 30 ft on big utility banks. Fan tip speed is capped near 61 m/s on acoustic grounds, which is why a high flow bank takes more fans or wider blades rather than faster ones. Bundles run 2 to 4 rows deep in most services, and each added row buys surface at the price of air side pressure drop and fan energy for the life of the machine.

Duty follows air, and air follows control. Because the air side film dominates, capacity swings with ambient temperature and airflow: the same bank that barely holds process temperature in August overcools the same fluid by 40 deg C in January. Capacity control therefore lives on the air side first, fan speed through a two speed motor or a variable frequency drive, or blade pitch through auto variable pitch hubs, and on the process side second, where a winter bypass around the bundle with a control valve blends hot fluid back into the outlet line. On loops that must hold a tight outlet temperature year round, the trim element is often a small globe control valve on that bypass, deliberately sized for the January case rather than the summer design case, because that is the condition where it does all of its work.

Recirculation is the failure physics of the plot plan. Wind carrying hot exit air back into the fan intake raises the effective inlet temperature well above the meteorological dry bulb, and the bank loses capacity exactly when the wind is worst. Induced draft places the fans above the exit stream and resists recirculation; forced draft bays sit lower and need separation, wind walls, or careful plot planning. The exit air of an induced draft bay reaches 60 to 80 deg C at the fan, which is why its drivetrain ratings, bearing selection, and maintenance access all differ from the forced draft bay standing beside it.

Instrumentation closes the loop on this physics. The controlling variable is process outlet temperature, and the instrument that feeds the fan controls is a temperature transmitter in the outlet line. The second most useful reading is the process side differential pressure across the bundle, which trends internal fouling between turnarounds and separates a dirty bundle from a failing fan when summer capacity falls short. Both belong in the RFQ instrument list with tag numbers, not in a startup afterthought, and chapter 6 gives them their installation rules.

Draft Types, Construction, and Duty Boundaries

Draft configuration is the first construction commitment. In a forced draft bay the fans sit below the tube bundle and push air up through it; the bundle rides in an outlet plenum on top, and the fans, motors, gear drives, and bearings all live at grade in cool air, which makes every maintenance visit cheap. The trade is aerodynamic: air arrives at the bundle face less evenly, and the low discharge point invites hot air recirculation under wind. In an induced draft bay the fans sit on top and pull air through the bundle; distribution across the face is more uniform, the exit air leaves with enough stack velocity to defeat recirculation, and the plot footprint shrinks, but the drivetrain works in exit air up to about 80 deg C and the structure grows taller to put it there. The table below compresses the choice.

Finned tube construction is the second commitment, and wall temperature decides it. Extruded bimetallic fins, an aluminium muff rolled onto the base tube with an L or LL shaped foot, are the volume standard to about 180 deg C, with the LL wrap covering the fin root against atmospheric corrosion at the tube face. Embedded fins, an aluminium strip rolled into a machined groove in the base tube, hold a mechanically protected bond to about 350 deg C. High frequency welded steel fins carry hot oil and similar 350 to 400 deg C service with an all steel bond at the price of less area per metre. Fin pitch runs 2.3 to 3.2 mm, which is roughly 310 to 435 fins per metre, and the base tube is 1 inch OD carbon steel in most refinery catalogues, with stainless and duplex tube materials where the process chemistry demands them.

Header construction is the third commitment. The plug header, the refinery standard, is a welded box whose plug sheet carries a threaded plug opposite every tube: a leaking tube is located, re-rolled, or capped through its plug hole without breaking the piping or the structure. The removable cover plate header replaces one wall of the box with a bolted, gasketed cover, giving full tube sheet access for chemically dirty services that need periodic cleaning; its gasket becomes a maintenance item that the plug header does not have. Bonnet and pipe headers with U-bend tube bundles serve high pressure gas coolers, and D-type headers wrap themselves into the corners of A-frame condensers. Nozzles are flanged to ASME B16.5 drilling, and every flange with its gasket set is part of the pipe stress conversation, not an afterthought bolted on at installation.

Bundle orientation follows the service. Horizontal bundles are the default for liquid coolers and gas coolers. A-frame bundles mounted at 45 to 60 degrees from horizontal are the refinery overhead condenser standard, because condensate drains positively through them and they present a large face to the airflow in a small plot. True vertical bundles appear in compact and retrofit units. Multiple bundles make a bay, multiple bays make a bank, and the structure, platforms, and fan deck are engineered as part of the machine rather than as site steelwork. Process piping arrives with its own thermal growth, and where that growth cannot be absorbed by pipe flexibility alone, an expansion joint in the inlet line protects the header nozzles, whose allowable loads are among the smallest on the plot.

Fans and drives finish the hardware. Axial fans run 8 to 18 ft in most bays with aluminium or fiberglass blades, mounted on fixed pitch hubs for economy or auto variable pitch hubs where continuous airflow control is wanted. The driver is usually a TEFC electric motor through a right angle gear drive, with belt drives on smaller economy units and variable frequency drives where the duty swings across seasons. Every drivetrain is a rotating machine with its own vibration limits, lubrication, and guarding, and API 661 treats the fan and drive assembly with the same seriousness as the thermal design.

Winterization is a construction package, not a startup trick. Air side protection stacks three devices: recirculation ducts that mix hot exit air back into the inlet when ambient approaches freezing, steam or electric coils that warm the inlet face on cold start-up, and fan control that derates airflow before the bundle sees damaging cold. Water and condensing services add process side protection: continuous drainage, siphon arrangements that empty idle passes, and heat tracing cable on drain, vent, and instrument lines so no liquid pocket can freeze and split a tube or a gauge line. Steam coils and tracing discharge their condensate through a steam trap at each low point, and the trap is itself a winter failure item: a trap stuck open wastes the coil it feeds, and a trap stuck closed floods and then freezes it.

Key Specification Parameters

The table below decodes the datasheet lines that decide whether a quoted exchanger matches the duty. Each row is purchase defining; the paragraphs after it explain what to demand and what a weak answer looks like.

The design dry bulb row deserves its own warning because it is the row most often fudged. Every thermal number in a quotation is conditional on the stated air temperature, so a vendor quoting at a 35 deg C design temperature against a competitor quoting at 40 deg C is not quoting the same machine. Fix the design dry bulb in the RFQ on a 1 to 2 percent exceedance basis, and require each bidder to state thermal performance at that temperature and at the expected summer maximum. The companion line is the approach, the process outlet temperature minus the design inlet air temperature: approaches below 10 deg C cost exponential surface, and 10 to 15 deg C is the band where most honest designs land.

Mechanical rows are code rows. Design pressure and temperature fix the header boxes, which are pressure vessels: specify the ASME VIII Division 1 scope with hydrotest where the plant standard requires it, and state corrosion allowance explicitly. Process side nozzles receive the piping loads calculated under ASME B31.3, and the vendor drawing should tabulate allowable nozzle loads rather than leaving them to a startup discovery. Isolation hardware at the nozzles is usually a butterfly valve on the large gas and vapor lines and a ball valve on vents, drains, and instrument connections. Liquid full bundles that can be locked in between closed valves need a thermal relief path, a small safety relief valve discharging to a safe location, because sunshine on a blocked liquid full bundle builds pressure quickly.

Instrument rows turn the machine into a controlled unit. The outlet temperature transmitter closes the fan control loop; a pressure transmitter on each side of the process pass, read as a pair, trends fouling between turnarounds; and the winter package adds low ambient switches and coil proof signals. These are line items with tag numbers in the RFQ, because retrofitting instruments onto an operating bank costs a multiple of buying them built onto the machine. The construction table below then pairs the common services with the fin, header, and orientation decisions they imply.

Selection Decision Factors and RFQ Language

The decision sequence below runs as a checklist in a specification meeting, and each step eliminates a class of quotations before any price comparison starts.

1. Fix the duty honestly: heat duty, process inlet and outlet temperatures, and the design dry bulb on a 1 to 2 percent exceedance basis, plus the site mean minimum for winter design.
2. Check the approach: if the outlet must sit less than 10 to 15 deg C above the design dry bulb, air cooling is the wrong physics and the water or refrigeration alternative must be priced instead.
3. Choose draft against plot and climate: forced draft for grade level maintenance and economy, induced draft for condensers and recirculation prone plots.
4. Choose fin type by wall temperature and header type by fouling behavior, using the construction table in chapter 4.
5. Set the control scheme for both seasons: two speed or VFD fans, auto variable pitch hubs, a winter bypass with a control valve, and a globe control valve for trim where the outlet temperature must hold year round.
6. Write the noise limit and its measurement point into the RFQ, commonly 85 dBA at the plot boundary, lower near residential edges.
7. Specify the winterization package against the mean minimum temperature: recirculation ducts, coils, tracing, drainage, and fan control settings.
8. Demand API 661 data sheets filled and returned, the ASME VIII scope for headers where specified, and performance statements at the stated design conditions.

Against the water cooled alternatives, the comparison a specifier actually runs is total cost of heat rejection, and it belongs here in the selection chapter rather than in the page title. A shell and tube heat exchanger plus a cooling tower rejects heat at lower cost per kilowatt where water, plot, and drainage exist, but it buys water treatment chemicals, blowdown handling, drift, year round freeze protection, and a circulating loop whose centrifugal pump runs whether the process needs cooling or not, with a multistage pump on tall return legs where the site elevation demands it. The air-cooled bank trades higher summer process temperatures for a waterless heat balance, and in arid or freezing climates that trade is usually the end of the argument. A chiller holds outlet temperature below ambient and belongs in the specification only when the process genuinely needs it, because its energy cost per kilowatt rejected is an order above moving air. A plate heat exchanger serves close approach liquid to liquid duty inside a water circuit, not heat rejection to atmosphere. The honest rule is simple: if the process outlet temperature can float with ambient and water is scarce, the fin fan cooler wins the total cost argument, and if the outlet cannot float, no fan arrangement fixes that.

The application map below turns that comparison into a table the project file can carry.

The RFQ block should be written once and reused. A complete block reads: service description and fluid with fouling behavior; heat duty at design conditions; process inlet and outlet temperatures; design dry bulb on a stated exceedance basis and the site mean minimum; design pressure and temperature with corrosion allowance; tube material and size with fin type and pitch and rows deep; draft configuration and bundle orientation; bay and fan count with fan diameter and control method, fixed pitch, auto variable pitch, two speed, or variable frequency drive; the noise limit at a stated measurement point; nozzle sizes, ratings, and allowable loads; isolation and relief hardware, the butterfly valve or ball valve positions and any thermal safety relief valve; the instrument list with temperature transmitter and pressure transmitter tags; the winterization package contents; and the API 661 data sheets returned with the quotation. A quotation returned without the design dry bulb standing behind its thermal numbers is incomplete at any price.

Installation, Standards, Failure Modes, and Maintenance

Installation quality decides whether the bank reaches its design life. Set the bays per the plot plan with the separations the recirculation study assumed, and never let later construction crowd the intake side. Piping must arrive without forcing the header nozzles: spring supports or an expansion joint near the inlet absorb the thermal growth, flange bolts come up in stages to the torque table, and the gasket at every flange is specified for the process temperature rather than whatever the fitter had on the truck. Give each gear drive its correct oil, and fit a Y strainer on any external oil or water make-up line so the gearbox never eats construction debris.

Commissioning verifies four things. Air side: fan rotation, blade pitch setting, tip clearance, and a vibration baseline on every drivetrain. Process side: a temporary Y strainer during first circulation, removed once the line proves clean, and a walk-down of every drain, vent, and siphon. Control: the outlet temperature transmitter loop exercised through its full fan control range, with winter settings proven as far as the season allows. Paper: gearbox oil sampled and filed as the baseline, hub bolt torques recorded, and the vendor performance package checked against the purchase conditions. Four hours of commissioning discipline buys years of clean baseline data.

The standards frame is short and citable. API 661 governs air cooled heat exchangers for petroleum and natural gas service, and ISO 13706 carries the same requirements internationally; either citation obliges the vendor to the full data sheet, noise, and testing discipline. Header boxes are pressure parts under ASME VIII Division 1 where specified, connected piping follows ASME B31.3, and fan and drive performance ratings trace to AMCA practice. Site engineering standards usually layer their own sound limits and winter design temperatures on top, and those two lines, noise and minimum ambient, are where owner specifications most often diverge from the catalogue defaults.

Failure modes in service follow a short and predictable list. Fin damage leads the statistics: hail, ladders leaned against bundles, and careless pressure washing flatten fins, and combed fins recover most of the loss while ignored damage costs capacity permanently. Tube to header joint leaks appear at the plugs on plug header construction, re-rolled through the plug hole, or at the cover gasket on cover plate construction. Fan vibration follows blade erosion, bearing wear, or hub bolt relaxation, and the gear drive contributes oil leaks and, rarely, tooth failure between overhauls. Winter writes its own list: frozen and split tubes on water services with dead legs, a failed steam trap flooding a coil, tracing that has lost continuity, and hydrostatic lock on a blocked liquid full bundle that a small safety relief valve would have saved. Internal fouling shows up as rising process side differential pressure between turnarounds, and cleaning rods work through plug access where the construction allows.

Maintenance intervals scale with duty severity. Yearly: gearbox oil sample, hub and foundation bolt torque check, bearing lubrication, and a fin combing pass over any hail or wash damage. Each turnaround: pull plugs opposite sampled tube rows, inspect and re-roll joints, and rod out fouled tubes where access permits. Before every freeze season: prove the steam coil and each steam trap, verify heat tracing cable continuity on drains and instrument lines, confirm that drainage and siphon arrangements hold water out of idle passes, and exercise the recirculation dampers through their stroke. The pressure transmitter pair across the process pass belongs in the monthly readings, because the fouling trend it writes is the honest argument for or against chemical cleaning at the next shutdown, and the record it leaves is what turns the next RFQ from a guess into a specification.