Dry Cooler

What a Dry Cooler Is and What Is Being Decided

Begin with the loop. A closed carrier circuit, water or water and glycol, leaves the process or the heat recovery point at temperature, runs to the unit, and passes through coils whose outside surface is extended by fins into the airstream; one or more axial fans, direct or belt driven, pull or push ambient air across that surface; the air leaves a few kelvin warmer and the fluid leaves a few kelvin cooler, and a centrifugal pump on the loop makes the circuit continuous. Because the carrier never sees the atmosphere, it stays clean: no airborne fouling of the wet side, no open tank, no biocide program, and the loop water that reaches a downstream plate heat exchanger or a coolant distribution unit is the same water that left it. That closed loop cleanliness is half the reason the machine exists; the other half is that it needs no water at all to operate.

The boundary against the wet machine must be drawn early because the two are routinely confused at budget stage. An evaporative cooling tower rejects heat by evaporating a fraction of its circulating water, and evaporation lets the leaving water temperature approach the wet bulb temperature, typically within a few kelvin, which is nearly always colder than the dry bulb at the same hour and buys a smaller machine for the same load. The price is everything the dry cooler avoids: makeup water consumption on the order of kilograms per kilowatt hour rejected, blowdown and chemical dosing, drift and a visible plume in cold weather, and a registered legionella risk program in most jurisdictions. A dry cooler rejects the same kilowatts to the dry bulb only, runs 3 to 10 K above it at design, consumes no water in operation, and owns none of those obligations. Choosing between them is a water, climate, and permitting calculation, and the selection chapter treats it as one.

The boundary against the process machine is just as sharp. An air-cooled heat exchanger in the refinery sense is an API 661 engineered product: bare or finned tubes carrying process gas or condensing vapor, bank structures, header nozzles, and winterization logic designed per process code, sized by the process licensor. The dry cooler of this page is a liquid loop product for HVAC, refrigeration, and light process duty, built from standard coil families and fan modules and sold on a capacity at stated conditions. Neither replaces the other, and a quotation that answers a process duty with an HVAC product has not understood the duty. For completeness inside the same family tree: a shell and tube heat exchanger is the water side workhorse when cooling water exists to be warmed, and a desuperheater conditions steam by injecting water into it, which is a metering job rather than a heat rejection job; neither is a closed loop air cooled anything.

What is actually being decided in this purchase is five commitments at once. First, the honest design point: heat load, entering and leaving fluid temperature, and the ambient dry bulb the unit must reject that load at, chosen as a 0.4 to 1 percent exceedance value rather than an annual average. Second, the fan technology: fixed speed AC motors with a VFD, or integrated EC fan arrays with their native speed control and the 30 to 50 percent class energy advantage at part load. Third, the freeze strategy: glycol fraction, drain capability, or heat tracing, because a split coil in the first hard winter is the most expensive failure this product family owns. Fourth, the sound envelope at the property line and at night, which constrains fan speed and therefore capacity margin. Fifth, the siting: clearance and enclosure geometry that keep discharge air from recirculating into the intake, because recirculation converts a rated unit into a smaller one without touching the nameplate.

One naming caution belongs here because it misleads searches and specifications alike. Some catalogs use the term fluid cooler for closed circuit evaporative coolers, machines that spray water over a closed coil to get wet bulb approach while keeping the process fluid sealed. Those are wet machines, a subtype of the evaporative tower family, and this page does not cover or claim them; the dry cooler rejects heat to air with no water on the coil, ever. The aliases that genuinely belong here are dry air cooler and free cooling unit, and the free cooling name records the economics: every degree of ambient below the fluid requirement is compression work the plant does not have to do, which is why the machine earns its floor space in cold climates and data centers.

How Closed Loop Sensible Heat Rejection Works

Both sides of the coil obey the same balance: heat rate equals mass flow times specific heat times temperature change. On the fluid side, a 500 kW duty on plain water with a 6 K temperature drop needs about 20 kg per second, roughly 72 m3/h of circulating flow, and that flow sets the centrifugal pump size and the pipe. On the air side, taking the same 500 kW out of air with an 8 K temperature rise needs about 62 kg per second of air, near 53 m3/s at sea level density, which is why the product looks like a wall of fans: the air side carries roughly a thousand times less heat per kilogram than the water side, and the machine spends its surface area and its motor kilowatts compensating for that ratio. Every selection argument in this chapter is a consequence of those two mass flows.

Approach is the single number that bounds the machine. Approach means the leaving fluid temperature minus the entering air dry bulb, and at design the families deliver 3 to 10 K: tight approach means more coil surface, more fans, more cost, and more fan energy forever; loose approach means the fluid leaves warmer, which is harmless if the process tolerates it and expensive if it forces a chiller to run. A capacity number is meaningless without its conditions, so every datasheet line must be read as a quadruple: kilowatts at entering fluid temperature, leaving fluid temperature, ambient dry bulb, and fluid flow. Two vendors quoting the same 500 kW at different approach values are quoting different machines, and the RFQ comparison in chapter 5 normalizes them before price is discussed.

The fan array is where the operating cost lives. Fan power scales with the cube of speed, so a unit running its fans at 60 percent speed to hold a set fluid temperature draws roughly a fifth of full speed power, and the shape of the load curve, cold most of the year, hot briefly at design, means the average operating point sits far below design. AC induction motors with a VFD per fan row or per unit deliver this turndown with conventional hardware. Electronically commutated EC fan arrays integrate the variable speed drive into each fan motor: control resolution down to the 10 percent speed class, family energy savings of 30 to 50 percent against fixed speed AC operation on typical duty cycles, native 0 to 10 V or Modbus control, and redundancy, because one fan of a dozen failing costs capacity margin rather than the whole machine. The premium for EC hardware is repaid in two to five years on continuous duty, faster where electricity is expensive, and slower where the unit runs only in summer peaks; the payback arithmetic belongs in the RFQ file with the local tariff attached.

Free cooling is the same physics read as an economy. A chiller holding a 15 deg C fluid needs compression whenever the ambient is too warm for the dry cooler to deliver 15 deg C, and needs nothing but fans once ambient falls far enough below it, because the unit can hold the fluid at ambient plus approach. Between those states the two machines share the load, either in series through a plate heat exchanger that lets the dry loop pre-cool the chilled circuit, or on a common loop where the chiller staging controller sees the dry cooler as the first stage. The design exercise is counting hours: with local hourly dry bulb data, the free cooling hours per year decide whether the dry cooler is the heat rejection plant or a supplement to it, and the answer differs completely between a Shanghai summer profile and a northern European one. Data centers make the arithmetic easiest, because the coolant distribution unit loops run fluid temperatures high relative to ambient for most of the year and the compression displacement is nearly total in cold climates.

The control chain is deliberately short. A temperature controller, or the building management system acting as one, compares the leaving fluid temperature against setpoint and modulates fan speed; a temperature transmitter on each fluid inlet and outlet gives it the data, a pressure transmitter or differential sensor across the pump proves circulation, and on legacy AC units the VFD takes the speed signal while EC arrays take it directly. Where the loop feeds critical equipment, a flow meter adds the third verification signal and the trend history that commissioning and disputes both use. Control resolution matters at night: on EC arrays the speed floor is low enough to hold setpoint quietly, while AC units cycling at a minimum VFD speed can hunt, and the sound complaint usually arrives before the temperature one.

Glycol changes the numbers and belongs in the mechanism chapter, not a footnote. Adding propylene or ethylene glycol lowers specific heat, raises viscosity, and lowers thermal conductivity, so at 30 percent fraction the same unit rejects roughly 5 to 10 percent less heat at the same conditions, and at 50 percent the derate grows further while pump power rises through viscosity; makers publish capacity correction tables by fraction and temperature, and a quotation compared without applying the same correction to both units is not a comparison. The compensation is real capacity margin or larger surface, and the freeze protection it buys is quantified in chapter 4 alongside the drain and tracing alternatives.

Types, Construction, and Duty Boundaries

Two coil geometries cover the catalog. The V arrangement, two coil banks meeting at the top with the fan bays drawing horizontally through the V faces, packages a lot of surface into a low box and dominates the smaller commercial sizes, rooftop units, and packaged free cooling sets where height is the constraint and the airflow pattern is horizontal discharge. The horizontal coil, flat or slightly A-framed with vertical airflow blown upward, dominates the larger modules and modular arrays: discharge rises away from the unit, which helps against recirculation in tight yards, and the coil drains toward its headers, which helps winterization. Both geometries carry the same fin materials and the same fan technology; the choice between them is usually made by the roof or yard the project has, not by thermodynamics.

Single box units ship complete: one casing, one or two fan rows, coil, and controls, in capacities from tens of kilowatts to a few hundred. Modular arrays bolt box modules into banks, the Baltimore Aircoil TrilliumSeries TDFS class is one reference execution of this architecture, and deliver from a hundred kilowatts to multi megawatt plants with three practical advantages: rigging in pieces that a roof structure or a crane schedule can accept, capacity staged as the load grows in phased construction, and fan redundancy distributed across the array. The Gunter GFD family illustrates the single box end of the same market, and its installation manual is worth reading before writing any RFQ, because it states the clearances, minimum flow rates, and freeze practices that the datasheet only implies.

Fan and drive technology splits the families again. Direct drive AC axial fans with a shared VFD per row are the economical baseline and the retrofit default. EC fan arrays, one intelligent motor per fan with integrated electronics, are the current standard on premium units and data center packages for the energy, turndown, and redundancy reasons in chapter 2. Belt driven fans survive in heavy industrial executions and at some large diameters, trading a maintenance item, belts and sheaves, against motor availability; a belt check belongs in the maintenance plan of any unit that ships with them. Motor classes run from fractional kilowatt on small fans to the 5 kW class on large ones, with external rotor motors typical on EC and IE3 or IE4 induction on AC.

Adiabatic assist exists as an option family and must be scoped as one, not as the product. In hot spells a pad or a fine spray pre-cools the entering air toward its wet bulb, lifting capacity 20 to 40 percent class exactly when the design day arrives, and for the rest of the year the unit runs dry with the pads dry or the nozzles off. The option reintroduces a small seasonal water and hygiene obligation, which is why it belongs in the specification as a clause, with its water quality limit, its drain, and its hygiene regime, rather than as a surprise in the vendor proposal. A dry cooler with adiabatic assist is still a dry cooler most of the year; it must not quietly become an evaporative machine on the drawings.

Materials and coatings set the service life. The baseline is copper tube with aluminum fin, mechanically bonded, in a hot dip galvanized G3 class casing, with stainless casing and hardware as the coastal and hygiene upgrade. All-aluminum microchannel coils trade higher air side efficiency and lower refrigerant or fluid charge for a repair philosophy of replacement rather than soldering, and they suit clean air sites best. Where the air carries salt, hydrogen sulfide, or industrial pollution, factory coatings on the coil, the ePIXAL and epoxy and Blygold class families, buy years of fin life, and the coating clause belongs in the RFQ whenever the site is within sight of the sea or a stack. Duty boundaries close the chapter: this product is not a refrigerant condenser, not an API 661 air-cooled heat exchanger, and not a wet closed circuit cooler, and a specification that needs any of those three should name them instead.

The table below compresses the configurations into the form a specification meeting argues from; values are the common spans of the established families and must be confirmed against the model datasheet before order.

Key Specification Parameters

The datasheet rows below decide whether a quoted unit matches the duty; values are stated as the common spans of the established families and must be confirmed against the model datasheet, at the project's own conditions, before order.

Rating practice deserves a paragraph because it decides whether two capacity numbers can be compared at all. European families rate under the EN 328 framework for forced convection air cooled liquid coolers, with declared conditions and tolerances, and Eurovent certification audits the declared performance against test, so a certified catalogue carries an independent floor under its numbers; the ASHRAE family of methods plays the same role in North American practice. Sound is declared to EN 12102 with measurement roots in the ISO 3744 sound power method, and the honest comparison is sound power at full speed plus the speed the unit actually runs at night, because a 65 dB(A) at 10 m unit at 40 percent speed is a quiet neighbor. A quotation without stated rating conditions on the capacity line and a stated sound basis is a number without a definition.

The correction table below gathers the derates and planning factors that turn a catalogue number into a site number; each row is a place where real projects lose capacity silently.

The fluid side rows close the specification. Working pressure in the 6 to 16 bar family covers typical hydronic and refrigeration condensing loop pressures, with the coil, headers, and frame rated and PED classified accordingly; higher pressures exist as engineered versions and cost as such. The fluid volume matters for freeze arithmetic, because the glycol charge or the drain time scales with it, and for the expansion vessel sizing that keeps the closed loop whole across its temperature range. Electrical and control rows bind the unit into the plant: a VFD room or integral EC controllers, the bus or analog the BMS speaks, and the staging logic that hands load between this machine and the chiller it partners with.

Selection Decision Factors and RFQ Language

The selection sequence runs as a checklist, and each line eliminates a class of quotations before price comparison begins.

1. Fix the heat load and the fluid temperatures the process actually needs, not the coldest water anyone might want; every kelvin of leaving fluid temperature below need is surface area and fan power spent for nothing.
2. Fix the design dry bulb honestly: the 0.4 to 1 percent exceedance value from the site climate data, with the altitude correction applied, and the wet bulb as well if adiabatic assist is contemplated.
3. Compute the free cooling hours against the fluid requirement with hourly data, and let that number, not a brochure climate map, decide the machine's role in the plant.
4. Choose the freeze strategy: glycol fraction matched to the coldest credible ambient, drain provisions for idle winters, or heat tracing on exposed runs and small bore piping; state it in the RFQ so every vendor prices the same winter.
5. Fix the sound limit at the property line and at night, then check the fan speed reserve the limit leaves; a sound capped unit is a smaller unit.
6. Lay out the siting: wall and enclosure clearances per the maker, discharge direction against prevailing wind, and the recirculation margin; a yard that cannot breathe needs a different machine or a different yard.
7. Fix the fluid side hardware in the same purchase: the circulating centrifugal pump, a Y strainer ahead of it, isolation valves for every serviceable item, and a balancing valve with flow meter verification on the loop.
8. Fix the evidence: rating basis and certification, sound data with its measurement standard, the glycol correction table, and the rigging and commissioning documentation.

Against the neighboring equipment the dry cooler holds specific ground, and the comparison belongs here in the selection chapter rather than in a page title. Against the cooling tower the trade is water and approach: the tower wins every equal-load sizing because wet bulb beats dry bulb, and the dry cooler wins every water constrained, plume sensitive, or hygiene registered site, and in between the arithmetic on water cost, chemical program, and winter free cooling decides. Against a chiller the question is not either or but partnership: the chiller makes cold when ambient refuses, the dry cooler makes it free when ambient allows, and the series arrangement through a plate heat exchanger or a common loop decides how much compression the year contains. On the water side, a shell and tube heat exchanger remains the right tool when a river or plant circuit can absorb the heat, and it does the job with a fraction of the fan kilowatts, at the price of the water it warms. A desuperheater, stated once more because procurement documents mix them up, conditions steam and is not heat rejection hardware. The accessories decide commissioning honesty: an isolation butterfly valve or gate valve at the unit, a ball valve at each instrument, a check valve where standby pumps share the loop, and the strainer and balancing hardware from the checklist, so that the flow meter reading that closes commissioning is taken through hardware that can be maintained.

The application map below compresses the practice into the form a project file carries.

The RFQ block should be written once and reused: heat load in kW; entering and leaving fluid temperature with the design flow; ambient design dry bulb and its exceedance basis, wet bulb if adiabatic assist is offered; glycol type and fraction or the alternative freeze strategy; coil construction and casing materials with the coating clause where the air requires it; fan type, fan count, and the sound power and pressure data with its measurement basis; electrical supply and control interface to the BMS or temperature controller; weight, footprint, and rigging state; winter provisions, drain ports, tracing, low ambient fan control; the accessories, pump head and strainer and isolation valves; and the documentation set, rating basis with certification, correction tables, and the installation and startup manuals. A quotation that answers the capacity line without stating its conditions, or the sound line without its standard, is incomplete at any price.

Installation, Standards, Failure Modes, and Maintenance

Installation starts with siting, because recirculation is the capacity thief that no datasheet row catches. The unit needs the clearance the maker states, on the order of one fan diameter class from walls and adjacent banks, discharge directed with the prevailing wind rather than into a dead corner, and screens and louvers sized for the airflow rather than for the elevation drawing; an enclosed yard that pulls its own discharge back through the intake can raise entering air several kelvin and quietly remove a quarter of the capacity. Rigging follows the manufacturer's instructions, the TrilliumSeries rigging and assembly documents are a public reference for how modular units are lifted, spread, and set, and wind and seismic bracing belongs on the structural drawings rather than in the installer's judgment. Leave service access on the fan face and the coil face; a unit boxed against a parapet gets its maintenance deferred, and deferred maintenance on this family compounds. On the fluid side, commission the loop before the controls: fill, vent, and clean through the strainer, set the circulating flow with a balancing valve, and record the flow meter reading as the baseline every later season is judged against.

Freeze protection is a design deliverable, not a season. The glycol fraction must match the coldest credible ambient with margin: family values for ethylene glycol run near minus 15 deg C protection at 30 percent and minus 33 deg C at 50 percent, with propylene similar but slightly weaker per percent, and the maker of the fluid, not the cooler, owns the exact table. Where glycol is rejected, drain ports must actually drain the coil and its low corners when the unit idles, which means slopes and drain routing checked at commissioning, and any dead leg, a small bore sample line, an instrument connection, the fill and vent points, needs heat tracing cable or it will split in the first hard frost while the main coil survives. The circulating pump needs the same winter discipline: idle pumps freeze first, a check valve on each standby leg stops convection circulating cold water through the idle branch, and the winterization round in late autumn, tracing energized, drains verified, glycol refractometer reading logged against the specification, is the cheapest insurance the plant owns.

The electrical installation carries one failure mode worth naming because it is new to many sites: bearing currents on variable frequency drives. A VFD or an EC controller's switching waveform induces voltage on motor shafts, and the resulting current passes through bearings and flutes the races within a season or two; the remedies are shaft grounding rings, insulated bearings on larger motors, and correct cable screening per the drive maker, and the RFQ should state which one each vendor fits. Harmonics from drive rectifiers belong on the same electrical sheet, with filters or low harmonic EC arrays where the site standard demands them, and motor protection must be coordinated with the fan array's control so a tripped fan reads as an alarm and a staging event rather than a silent capacity loss.

The standards frame is short and citation earns its place in the RFQ. Performance rating for forced convection air cooled liquid coolers follows the EN 328 framework in the European market, with ASHRAE methods playing the parallel role in North America, and Eurovent certification provides third party audit of declared capacity, sound, and power consumption. Sound declaration follows EN 12102 with measurement roots in the ISO 3744 sound power method. The pressure retaining coil and headers fall under the PED in the European market at their classification, and structural and electrical work falls under the ordinary building and low voltage codes of the site. None of these is exotic; citing them moves the burden of proof onto the quotation, which is where a buyer wants it.

Failure modes follow a short and predictable list. Fin fouling: airborne dust, seed fluff, and kitchen or diesel exhaust pack the fin face, air side resistance rises, and leaving fluid temperature climbs at constant load; washing and fin combing restore it, and the trend line on the temperature transmitter catches it months before the alarm. Fan motor bearing failure: the dominant moving part failure, heard first as a whine and seen in current trend; on EC arrays it is a fan swap, on belt drives it is a bearing or belt change done from the safe side of the guard. Freeze damage: the split tube or burst corner from the strategy that was specified but never commissioned, or the tracing breaker that someone opened in spring and no one closed in autumn. Coil leaks: from vibration fatigue at unsupported headers, from galvanic attack where dissimilar metals meet at tube to header joints in a wet or salty environment, and from careless pressure testing; the symptom is make up water or falling loop pressure on the pressure transmitter, and a pressure gauge on the loop makes the slow leak visible between services. Recirculation: capacity that fell on installation day and never came back, cured with ducts, screens, or relocation rather than parts. And service isolation neglect: a butterfly valve seized open or a Y strainer packed solid means the simplest service becomes a plant shutdown, which is why the isolation and strainer hardware in chapter 5 is specified as part of the machine. Plus sensor drift: a drifted fluid sensor lets the controller hold the wrong speed for a season, which is why the instrumentation calibration round belongs to this machine's maintenance file as much as to the plant's.

Maintenance scales with the air more than the water, which is the family's quiet advantage: there is no water treatment, no biocide, no tower register. The routine is fin face inspection and washing on the air side season, fan and motor checks, belt and bearing service on belt drive units, an autumn winterization round covering glycol, drains, and tracing, and a spring round covering the reverse. Instrumentation, the temperature transmitter pair, the pressure transmitter, and the loop flow meter where fitted, is calibrated on the plant's ordinary cycle, and the control setpoints are audited against the free cooling logic once a year so the machine keeps earning the hours it was bought for. A unit that gets this much attention runs its coils for twenty years; a unit that gets none gives its first warning as a winter freeze claim and its second as a summer capacity shortfall, both of which cost more than the maintenance ever would.