Datacenter
Data center free cooling and economizer field guide
Using the outdoor air and water to cool the hall without the compressor: airside and waterside economizers, partial and full free cooling, the changeover, and the commissioning that proves the hours.
Direct answer
Free cooling uses cool outdoor conditions to cool a data center with little or no chiller compressor work, cutting the largest non-IT energy load and the PUE. An economizer is the equipment that does it, airside or waterside, and the cooler the climate the more hours it runs. ASHRAE 90.4 and the project documents control the design.
Key takeaways
- Free cooling uses cool outdoor air or water so the chiller compressor runs less or not at all, cutting the largest non-IT energy load and the PUE.
- Airside economizers bring in filtered outdoor air; waterside economizers make cold water with the cooling tower and a plate-and-frame heat exchanger, chiller off.
- A waterside economizer triggers on outdoor wet-bulb, often available once the tower makes condenser water a few degrees below the chilled-water setpoint, near mid-40s F.
- Pipe the heat exchanger in series with the chiller, not parallel, to capture partial free-cooling hours where most annual savings accumulate.
- Force the changeover under load at commissioning with a wide enough deadband; a plant hunting across the boundary burns compressors instead of saving.
Free cooling, and what it actually is
Free cooling is using cool outdoor air or water to remove the heat a data center makes, so the chiller's compressor runs less or not at all. The compressor is the part that costs the money. Take it out of the loop for the hours the weather allows, and the cooling energy drops hard, which is the whole point. Nothing about it is actually free. You still run fans, pumps, and tower water, and you still bought the economizer hardware.
What you stop paying for is the lift, the work the compressor does to pump heat from a cold space out to a warmer outside. When the outside is already cold, that work is wasted, and free cooling is the design that refuses to pay for it.
The device that makes it happen is an economizer, and there are two families. An airside economizer brings cool outdoor air into the hall or the air handler. A waterside economizer makes cold water with the cooling tower and hands the cooling to the chilled-water loop through a heat exchanger, with the chiller off. The cooling pillar guide covers how the heat leaves the building in general; this guide is about the hours you get to do it without the compressor.
The input that decides how much you save is climate. A hall in a cool, dry climate can run free cooling most of the year. The same design in a hot, humid one gets a fraction of the hours. That is why siting and the free-cooling hours go into the business case before the first chiller is sized.
Why does free cooling matter for PUE?
Cooling is the biggest non-IT load in a data center, so cutting cooling energy is the fastest way to move PUE. PUE is total facility energy divided by the energy that reaches the IT gear, and in an older hall the cooling plant, fans, pumps, and heat rejection can eat nearly as much power as the servers. Drive that down and the ratio follows.
Free cooling goes after the largest single piece of that overhead, the compressor lift. A chiller spends most of its energy pumping heat uphill against the temperature difference between the chilled water and the outside. Run the tower or the outdoor air instead and that lift falls toward zero for those hours. A plant that free-cools a large share of the year holds an annualized PUE well below what the same plant on full mechanical cooling could ever reach.
The number that matters is annualized, not the reading on a cold day. A design can post a great instantaneous PUE in February and a poor one in August, and the operating cost is the weighted average across the year. The cooling pillar guide treats PUE as the operational scorecard. Here the point is narrower. Free-cooling hours are the input that drives the annual number, and they are set by the climate and the supply temperature you can hold.
The airside economizer
An airside economizer brings filtered outdoor air into the data center, either straight into the hall or through the air handler, and exhausts the hot return instead of recirculating and re-cooling it. When the outdoor air is cool enough, the dampers open, the outdoor air does the cooling, and the mechanical refrigeration backs off or shuts down. That is economizer mode.
The hardware is dampers and controls more than anything exotic: an outdoor-air damper, a return-air damper, a relief or exhaust path, the filter bank, and the actuators and sensors that decide when to open them and how far. A direct airside economizer dumps the outdoor air into the space. An indirect one keeps the outdoor air separate and moves the heat across a heat exchanger, which gets its own section below.
The trade-offs are filtration, humidity, and contamination, and they are real, not footnotes. Outdoor air carries particulate and, depending on the site, corrosive gases, salt near the coast, and smoke during fire season, so the filter bank steps up from what a recirculating room needs. Outdoor air also brings its own moisture, so the control has to watch dew point, not just temperature, or it will swing the hall's humidity around. Get the filtration or the humidity logic wrong and the energy you saved comes back as failed hardware or a humidity excursion. The contamination and humidity sections cover what the design has to handle.
How does a waterside economizer work?
A waterside economizer makes cold water with the cooling tower in cold weather and transfers that cooling to the chilled-water loop through a plate-and-frame heat exchanger, so the chiller's compressor sits off. The tower rejects heat to the outdoors the way it always does, but when the outdoor wet-bulb is low enough, the condenser water comes off the tower cold enough to cool the chilled-water side directly. The plate-and-frame heat exchanger is the bridge: condenser water on one side, chilled water on the other, heat crossing the plates without the two streams mixing.
The trigger is the wet-bulb, not the dry-bulb, because the tower is an evaporative device. A common rule of thumb is that free cooling becomes available once the tower can make condenser water a few degrees below the required chilled-water supply temperature, which on many designs lines up with outdoor wet-bulb in the mid-40s F or lower. Warmer chilled-water setpoints push that threshold higher and buy more hours.
The chiller does not get removed; it gets bypassed. Valves route the chilled water through the heat exchanger instead of, or in series with, the evaporator, the compressor idles, and the pumps and tower fans carry the load. That is where most of the savings live, because on a chilled-water plant the compressor is the largest energy user by a wide margin. The chilled-water hydro guide covers proving that loop holds before any of this runs; the economizer rides on the same piping.
Partial, full, and integrated free cooling
Free cooling is not a single switch. As the outdoor conditions cool, a well-designed plant moves through three states. In full mechanical cooling the chiller carries everything and the economizer is off. In partial free cooling the economizer pre-cools the return water and the chiller finishes the job at part load, unloading its compressor as the outdoor conditions improve. In full free cooling the economizer carries the whole load and the compressor is off entirely.
The partial range is where most of the annual savings actually accumulate, because the hours when it is cold enough for full free cooling are fewer than the hours when it is cool enough to help. A plant that only free-cools when it can carry the whole load leaves a lot of money on the table. The design move that captures the partial hours is piping the heat exchanger in series with the chiller on the chilled-water side, so the economizer takes the first bite out of the return water and the chiller trims the rest. Series piping gives far more economizer hours than parallel, where the economizer only helps once it can carry everything.
This is what integrated economizer means: the economizer and the chiller working at the same time, not one or the other. A non-integrated economizer is a wasted opportunity for a large part of the year. The control has to hand off smoothly, which is the changeover, and a bad changeover that lets the economizer and the chiller fight each other burns more than it saves. The controls section covers that handoff.
Adiabatic and evaporative free cooling
Evaporative cooling extends the free-cooling hours by using water to drop the air temperature, and it earns its keep in dry climates where the air has room to take on moisture. Spray or wet-media evaporation cools the air as the water evaporates, so a hall that would otherwise need the chiller on a warm, dry afternoon can stay on free cooling because the evaporative stage knocks the supply air back down. Adiabatic cooling is the same idea used as a pre-cool: wet the incoming air or the heat-exchanger surface, drop its temperature toward the wet-bulb, and stretch the economizer hours past where dry free cooling alone would quit.
Direct evaporative adds the moisture straight to the supply air going to the hall, which cools it but raises its humidity, so it suits dry climates and has to watch the dew point it delivers. Indirect evaporative wets the outdoor side of a heat exchanger instead, cooling the hall air without adding moisture to it, which is the next section.
The cost is water, and it shows up in WUE, water usage effectiveness, the water used per kWh of IT energy. Evaporative free cooling trades electricity for water, and in a water-stressed region that trade is not automatically a win. Adiabatic pre-cooling, run only when it is needed rather than continuously, can hold most of the energy benefit while cutting the water draw sharply against full-time evaporation. The design has to weigh PUE against WUE for the actual site, because the cheapest answer on energy can be the worst on water.
Indirect evaporative and the air-to-air heat exchanger
Indirect evaporative cooling gets the benefit of outdoor air without letting that air into the white space, which is the contamination answer. Two airstreams pass on opposite sides of a heat exchanger, an air-to-air core or a heat wheel, so the hall's own air is cooled by the outdoor air across a wall it never crosses. The data center air recirculates in a closed loop; the outdoor air, with its dust, salt, and corrosive gases, stays on the other side and gets exhausted.
Add water to the outdoor side and the system goes from dry indirect to indirect evaporative: a thin film of water sprayed on the outdoor side of the core evaporates, pulls the surface temperature down toward the wet-bulb, and lets the core reject more heat from the hall air. The hall air still never touches the water or the outdoor air. That is the appeal for a dirty or coastal site, where a direct economizer would pull contamination into the room. You get a large share of the free-cooling benefit with the white space sealed.
The price is a heat exchanger between the two streams, which costs some efficiency against direct outdoor air, plus the water for the evaporative stage and its treatment. For a clean inland site a direct economizer can be simpler and cheaper. For a site with bad outdoor air, indirect is often the only economizer worth running, because the alternative is filtering and scrubbing the entire outdoor airflow.
How does the ASHRAE envelope enable more free cooling?
The wider the temperature and humidity the IT gear is allowed to see, the more hours the outdoor air or water can carry the load, so the ASHRAE envelope is the lever that sets the free-cooling ceiling. ASHRAE Technical Committee 9.9 publishes the Thermal Guidelines for Data Processing Environments, with a recommended server-inlet band, commonly given near 18 to 27 C, and wider allowable envelopes tied to equipment classes A1 through A4. Higher classes let the hall run warmer, and every degree warmer is more hours the economizer stays on.
The reason is direct. Free cooling works whenever the outdoor condition is cooler than what you need to deliver. Raise the chilled-water setpoint or the allowable server-inlet temperature and you have lowered the bar the outdoor air or wet-bulb has to clear, so more of the year qualifies. The expansion of the allowable envelopes to A3 and A4 in earlier editions was aimed squarely at this, opening near full-time economizer operation across much of the world's climate. The fifth edition is the current reference as of this review.
The trap is treating the wider envelope as free margin. Running warmer cuts your thermal ride-through when a chiller drops, because the hall starts closer to the limit and heats up faster. The cooling pillar guide covers the envelope and the ride-through gap in depth. The economizer point is narrower: confirm the actual equipment class and the edition the design referenced, then set the warmest supply the gear and the warranty allow, because that setting is what buys the hours.
Climate, bin data, and free-cooling hours
How many hours a year a design free-cools is a climate calculation, and it is done with bin data, the hour-by-hour record of outdoor dry-bulb and wet-bulb for the site. You take the supply condition the hall can hold, count the hours the local outdoor condition beats it, and that count, split into full and partial free cooling, is the number the energy model and the business case run on. A cool, dry climate can show thousands of full free-cooling hours. A hot, humid one shows far fewer, and most of its savings come from the partial range.
Wet-bulb drives the count for any evaporative or tower path, dry-bulb for a direct dry economizer, so the right bin data depends on the cooling type. A waterside or evaporative design lives on the wet-bulb bins, which is why a dry desert with high dry-bulb but low wet-bulb can still free-cool well, and a humid coast with moderate dry-bulb but high wet-bulb does worse than the thermometer suggests.
This is why siting decides so much before any equipment is picked. The same plant earns a different PUE in two cities, and the difference is the free-cooling hours the climate hands it. Run the bin analysis for the actual site and the actual supply temperature, not a generic map, because the hours are what the whole economizer investment is justified against.
Controls and the economizer changeover
The economizer is only as good as the control that decides when to switch it on, and the decision rides on outdoor conditions. The changeover is the point where the plant moves between mechanical cooling, partial free cooling, and full free cooling. Set it badly and you either leave free-cooling hours unused or you let the economizer and the chiller fight each other across the boundary.
There are three common control strategies for an airside economizer, and they differ in how honestly they read the outdoor air. Dry-bulb control switches on outdoor temperature alone, which is simple and works in a dry climate where humidity is rarely the problem. Enthalpy control reads the total heat content of the air, temperature and humidity together, so it does not open the dampers to air that is cool but so humid it would cost more to dehumidify than it saves. Differential or comparative control compares the outdoor air to the return air directly and economizes only when the outdoor air is genuinely better. Enthalpy and differential beat fixed dry-bulb in humid climates; dry-bulb is fine where the air stays dry. On a waterside plant the equivalent trigger is the outdoor wet-bulb against the chilled-water setpoint.
The failure that wastes the most money is the economizer and the chiller working against each other. A clean control unloads the compressor as the economizer picks up load and holds a stable handoff with enough deadband that the plant is not hunting across the changeover every few minutes. No fighting the chiller. That handoff is a commissioning item, not just a design intent, and it is the first thing to put under load and watch.
Contamination, corrosive gas, and filtration
A direct airside economizer brings the outdoor air's contamination indoors with the cooling, so the filtration steps up from what a sealed, recirculating hall needs. The two threats are particulate and gas. Fine particulate fouls boards and connectors and bridges contacts. Corrosive gases attack the copper and silver in the electronics and drive creep corrosion that kills hardware over months, not minutes.
ASHRAE addresses this in its gaseous and particulate contamination guidance for data centers. A common filtration approach is to filter continuously recirculated room air to around MERV 8 and to filter any outdoor air brought in for economizing to a higher class, commonly MERV 11 to 13, with carbon or chemical filtration added where the gaseous load is high. Confirm the current recommendation and class against the published guideline, because the numbers and the document revise.
The site decides how hard this is. Near a coast you fight salt. In an industrial corridor or a region with high sulfur air you fight hydrogen sulfide and other corrosive gases. During wildfire season you fight smoke, which can force a direct economizer offline and back onto mechanical cooling for days. High-contamination sites monitor corrosion with copper and silver coupons to track how aggressive the air actually is, and a result past the guideline severity is the signal to add filtration or move to an indirect economizer that keeps the outdoor air out. The cooling pillar guide covers contamination as a general envelope concern; the economizer-specific point is that direct outdoor air is the thing that makes it a daily question.
Humidity, dew point, and free cooling
Outdoor air carries its own moisture, so an airside economizer is also a humidity control problem, and the number that governs is dew point, not relative humidity. Pull in cool, damp outdoor air and you can drive the hall's humidity up. Pull in cold, dry winter air and you can drive it down toward the static-discharge risk at the low end. Either way the economizer control has to hold the hall inside the moisture band, which on a wide modern design is a dew-point range with a relative-humidity ceiling.
The expensive mistake is letting the economizer chase temperature while ignoring what it does to moisture. Cold winter outdoor air is bone dry, and free-cooling on it without any humidification can pull the hall below its low dew-point limit, which raises electrostatic-discharge risk on the floor. Damp shoulder-season air can carry the hall up toward condensation if the control is not watching dew point. Condensation tracks dew point, not relative humidity, so a hall held by relative humidity alone can still sweat a cold coil or a cold surface.
The modern answer is to control to a wide dew-point band and let relative humidity float, which is also what saves the energy the old narrow-humidity habit wasted. The cooling pillar guide covers the envelope's humidity limits. For the economizer, the point is that bringing outdoor air in is what makes humidity a live variable instead of a sealed-room setpoint.
Commissioning the economizer
Commissioning the economizer means proving three things under real conditions: that it enters economizer mode at the right outdoor condition, that the changeover between mechanical, partial, and full free cooling is clean, and that the free-cooling capacity is actually there when the plant calls for it. Reading the sequence of operations is not proof. You have to make the plant do it.
The problem is that you often cannot wait for the weather. Economizer changeover happens at outdoor conditions you may not have on the day of the test, so the functional test simulates the trigger. You override the outdoor sensor or the setpoint to force the plant across the changeover and watch the compressor unload, the valves stroke, and the tower or dampers pick up the load without the hall leaving its envelope. Then you confirm the capacity: in full free cooling, can the economizer alone hold the load it is rated for at the design outdoor condition, or as close as the season allows, corrected back to design.
The keystone is the integrated test, where the cooling plant runs on load banks through utility-loss and failure scenarios. The economizer belongs in that test, because the changeover and the chiller restart interact. A plant in free cooling that loses power has to get the compressors back if the weather turns, and the ride-through has to cover the gap. The cooling pillar and the chilled-water hydro guide cover the broader commissioning sequence; the economizer's piece is the changeover and the free-cooling capacity, proven rather than assumed, with the as-left changeover setpoints recorded for operations.
High-density AI loads and warm-water free cooling
High-density AI racks make free cooling easier, not harder, because liquid cooling lets the plant run warm water, and warm water free-cools for far more hours. A direct-to-chip cold plate can take heat away with water in the 30s to 40s C, well above the low-40s F a legacy chilled-water coil wanted, and the warmer the water you have to make, the lower the bar the outdoor wet-bulb has to clear. A loop that delivers warm water can free-cool with the tower across most of the year in many climates.
This flips the usual worry about AI density. The racks pull 40, 80, past 100 kW, which is a heat problem air cannot solve, but once the heat is in a warm-water loop the economizer story gets better, not worse. The compressor lift shrinks because the temperature difference the chiller would have to pump across is smaller, and on warm-water designs the chiller can be a trim device or, in the right climate, nearly idle.
The liquid-cooling work, the cold plates, the coolant distribution units, and the loop chemistry, is its own commissioning cluster covered elsewhere. The free-cooling point is that warm-water cooling and free cooling pull in the same direction. The warmer the supply the load tolerates, the more the outdoor conditions can carry it, and AI's appetite for liquid is what makes the warm loop normal.
The energy code and the economizer requirement
On most data center projects an economizer is effectively driven by the energy code, with the exact obligation set by the adopted standard and the climate zone. ASHRAE Standard 90.4, the energy standard for data centers, is the data-center-specific path, and it bounds the mechanical and electrical overhead by climate zone using a maximum mechanical load component rather than dictating one piece of equipment. ASHRAE Standard 90.1, the general building energy standard, references 90.4 as an alternative compliance path for large computer rooms and carries economizer provisions that apply across the climate zones, with narrow exceptions for the warmest, most humid zones.
What that means on the ground is that in most climate zones the design has to either provide an economizer or hit the energy targets another way, and free cooling is usually the cheapest way to hit them. The warmest tropical zones are the common exception, where outdoor conditions rarely beat the supply and an economizer earns little. Confirm the adopted edition of 90.4 and 90.1, the climate zone from ASHRAE Standard 169, and any local amendment, because the compliance paths and the economizer language move between cycles and the jurisdiction controls which edition applies.
Treat the code as the floor, not the target. Meeting the minimum economizer provision is not the same as designing for the free-cooling hours the site actually offers, and the operating cost rewards going past the code minimum where the climate pays for it.
What the owner has to maintain
An economizer adds moving parts and wetted surfaces that the operations team has to maintain, and a neglected economizer quietly stops saving money. The owner inherits three maintenance items the recirculating-only plant did not have.
Dampers and actuators are first. An airside economizer's outdoor, return, and relief dampers cycle constantly, and a seized actuator or a damper that no longer seals leaves the plant stuck in one mode, either missing free-cooling hours or leaking unconditioned outdoor air into the hall. They get exercised and inspected, not assumed. Filters belong here too: the heavier outdoor-air filtration loads up faster than a sealed room's and has to be changed on a real schedule, or the pressure drop eats the fan energy the economizer was supposed to save.
The heat exchanger and the water side are the second and third. A plate-and-frame economizer heat exchanger fouls on the condenser-water side and loses transfer if the water treatment slips, so the same tower water chemistry, biocide, and cleaning the chilled-water plant needs now also protects the free-cooling capacity. An evaporative or adiabatic stage adds wet media, spray nozzles, and the treatment that keeps scale and biological growth, including Legionella risk, under control. Skip the water treatment and you lose capacity to fouling and inherit a health problem. The free-cooling savings are real, but they are earned every year through the maintenance, not banked once at commissioning.
Field example: a series waterside economizer in a cool climate
A 2 MW chilled-water hall in a cool, dry climate was built with a plate-and-frame economizer piped in series with the chillers and a chilled-water supply setpoint raised from a legacy 44 F to 55 F to widen the free-cooling window. The question at commissioning was simple: how many hours would the compressors actually be off, and would the changeover hold under load.
The bin analysis for the site, run against a 55 F supply and the tower's approach to wet-bulb, projected full free cooling for roughly 4,000 hours a year and partial free cooling for several thousand more, with full mechanical cooling needed only in the warmest weeks. The functional test could not wait for winter, so the outdoor wet-bulb signal was overridden to walk the plant across both changeovers. In partial mode the economizer pre-cooled the return water and the lead chiller dropped to part load as expected. In simulated full free cooling the compressors went off and the tower and pumps held the load on the heat exchanger alone.
The catch the test found was the changeover deadband. The first settings let the plant hunt across the partial-to-full boundary, starting and stopping a compressor every few minutes when the wet-bulb sat right at the threshold. Widening the deadband and slowing the handoff stopped the hunting. As left, the plant moved between modes cleanly, and the as-found and as-left changeover setpoints went into the turnover so operations would not undo the fix. The hours and the capacity were the headline. The deadband was the thing that would have burned compressors if nobody had watched the handoff.
| Item | Value |
|---|---|
| Hall IT load | about 2 MW |
| Climate | cool, dry |
| Economizer type | waterside, plate-and-frame, series with chillers |
| Chilled-water supply setpoint | raised 44 F to 55 F |
| Projected full free-cooling hours | about 4,000 hr/yr (bin analysis) |
| Changeover trigger | outdoor wet-bulb vs chilled-water setpoint |
| Issue found | hunting across partial-to-full changeover |
| Fix | widened deadband, slowed handoff |
What to document
A free-cooling design that was modeled but never documented leaves operations unable to tell whether the plant is hitting its hours or quietly running compressors it should not need. The record is what tells the next engineer how the economizer is supposed to behave and what it was proven to do.
Capture the economizer type and configuration, each operating mode and the outdoor condition that triggers it, the changeover setpoints and deadbands as left, the projected and the verified free-cooling hours, the free-cooling capacity proven at the test condition, and the water use if the design is evaporative or adiabatic. Tie the changeover setpoints to the sequence of operations so a future control change does not silently undo the commissioning. If the plant uses evaporative free cooling, record the WUE basis alongside the PUE, because the two trade against each other and the owner needs both to run the site.
| What to record | Why it matters |
|---|---|
| Economizer type and configuration | Airside, waterside, indirect, or evaporative sets everything downstream |
| Operating mode and changeover trigger | Defines when the plant should be free-cooling |
| Changeover setpoints and deadband, as left | Stops a future control change from undoing the commissioning |
| Projected vs verified free-cooling hours | The basis the energy and cost case were built on |
| Free-cooling capacity at the test condition | Proves the economizer can carry the load it was sized for |
| Water use / WUE for evaporative or adiabatic | Tracks the water traded for the energy saved |
Common mistakes
- Running no economizer where the climate would have paid for one, wasting free-cooling hours all year.
- Pulling unfiltered or under-filtered outdoor air into the hall and bringing in particulate and corrosive gas.
- Letting the economizer and the chiller fight across a changeover with no deadband, so the plant hunts and burns compressors.
- Controlling an airside economizer on dry-bulb in a humid climate, opening the dampers to cool but damp air.
- Ignoring dew point, so winter free-cooling air pulls the hall too dry or shoulder-season air drives it toward condensation.
- Piping the heat exchanger in parallel instead of series, so the plant free-cools only at full load and loses the partial hours.
- Undersizing the plate-and-frame heat exchanger, so the economizer cannot carry the load it was credited with.
- Running evaporative free cooling without weighing WUE, trading an energy win for a water problem in a stressed region.
- Skipping the water treatment on the tower, heat exchanger, or evaporative media, losing capacity to fouling and inheriting a Legionella risk.
- Treating the economizer sequence as proven from the drawings instead of forcing the changeover under load at commissioning.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The central thermal reference is ASHRAE Technical Committee 9.9 and its Thermal Guidelines for Data Processing Environments, which set the recommended and allowable temperature and humidity envelopes that decide how warm the hall can run and therefore how many free-cooling hours are available. The energy side is ASHRAE Standard 90.4, the energy standard for data centers, which bounds the mechanical and electrical overhead by climate zone, with ASHRAE Standard 90.1 referencing it as a compliance path and carrying the economizer provisions that apply across most climate zones. The climate zones themselves come from ASHRAE Standard 169.
Contamination is governed by ASHRAE's gaseous and particulate contamination guidance for data centers, which informs the outdoor-air filtration class and the corrosion monitoring an economizing hall needs. The test-and-balance and functional testing draw on the AABC and NEBB procedures and the ASHRAE commissioning guidance, and where a facility chases a tier, the Uptime Institute Tier standards drive the witnessed redundancy and integrated-test demonstrations. The chilled-water and condenser-water piping a waterside economizer rides on is pressure-tested to the applicable ASME B31 section, which the chilled-water hydro guide covers.
Edition numbers, envelope values, filtration classes, and the economizer language all move between cycles, so confirm the current edition and the actual numbers against the published standards, the equipment manufacturer, and the project specification before citing them on a submittal. ASHRAE gives the framework; the equipment ratings and the contract documents control the limits you design and commission to.
Units, terms, and acronyms
Free cooling carries vocabulary from HVAC, from the central plant, and from data center efficiency, and the same idea reads differently across an energy model, a chiller submittal, and a controls sequence. The terms below travel across the whole economizer scope.
- Free cooling
- Cooling a data center with cool outdoor air or water so the chiller compressor runs less or not at all
- Economizer
- The dampers, valves, and heat exchanger that let outdoor conditions carry the cooling load
- Airside economizer
- An economizer that brings filtered outdoor air into the hall or the air handler
- Waterside economizer
- An economizer that makes cold water with the cooling tower and a plate-and-frame heat exchanger, chiller off
- Partial / full free cooling
- Partial unloads the chiller while the economizer helps; full carries the whole load with the compressor off
- Integrated economizer
- Economizer and chiller running at the same time, capturing the partial-cooling hours
- Dry-bulb / wet-bulb / enthalpy
- The outdoor measurements that trigger changeover; enthalpy includes humidity, wet-bulb drives evaporative systems
- PUE
- Power usage effectiveness, total facility energy divided by IT energy; free cooling drives it down
- WUE
- Water usage effectiveness, water used per kWh of IT energy; evaporative free cooling raises it
- Adiabatic / evaporative cooling
- Using water evaporation to drop air or surface temperature and extend the free-cooling hours
- Approach
- How close the tower can bring the water to the outdoor wet-bulb; sets the waterside free-cooling threshold
FAQ
What is free cooling in a data center?
Free cooling uses cool outdoor air or water to remove the heat a data center makes, so the chiller's compressor runs less or not at all. It is not literally free, since fans, pumps, and tower water still run. What you stop paying for is the compressor lift, which is the largest cooling energy cost.
What is a data center economizer?
A data center economizer is the equipment that lets cool outdoor conditions carry the cooling load instead of the chiller compressor. An airside economizer brings filtered outdoor air into the hall; a waterside economizer makes cold water with the cooling tower and a heat exchanger. Both cut energy and PUE when the climate is cool enough.
Airside vs waterside economizer: what is the difference?
An airside economizer brings filtered outdoor air into the hall or air handler and exhausts the hot air, so filtration and humidity matter. A waterside economizer keeps the air sealed and makes cold water with the cooling tower and a plate-and-frame heat exchanger. Airside suits clean climates; waterside avoids pulling outdoor contamination indoors.
How does a waterside economizer work?
A waterside economizer runs the cooling tower in cold weather to make condenser water cold enough to cool the chilled-water loop directly through a plate-and-frame heat exchanger, with the chiller compressor off. The two water streams cross the plates without mixing. It starts working when the outdoor wet-bulb drops a few degrees below the chilled-water setpoint.
What is the difference between partial and full free cooling?
In partial free cooling the economizer pre-cools the water and the chiller runs at part load to finish the job, unloading as it gets colder out. In full free cooling the economizer carries the whole load and the compressor is off. Most annual savings come from the partial range, which series piping captures.
How many free-cooling hours can a data center get?
It depends on climate and supply temperature, and you size it from bin data for the actual site. A cool, dry climate can free-cool most of the year, while a hot, humid one gets far fewer hours and leans on partial mode. Raising the chilled-water setpoint widens the window in any climate.
Does free cooling require humidity and contamination control?
A direct airside economizer does, because outdoor air brings moisture and contamination indoors. Filtration steps up, commonly toward MERV 11 to 13 on outdoor air with gas-phase filters where needed, and the control holds dew point. A waterside or indirect economizer sidesteps this by keeping the outdoor air out of the white space.
Is an economizer required by code for data centers?
In most climate zones an economizer is effectively required to meet the energy code. ASHRAE 90.4 sets data center overhead limits by climate zone and 90.1 carries economizer provisions across the zones, with narrow exceptions for the warmest, most humid ones. Confirm the adopted edition, the climate zone, and local amendments.
What is indirect evaporative cooling for data centers?
Indirect evaporative cooling passes the hall's air and the outdoor air on opposite sides of an air-to-air heat exchanger, so the outdoor air cools the hall air without ever mixing with it. Spraying water on the outdoor side boosts the cooling. It keeps dust, salt, and corrosive gas out of the white space.
Does free cooling save water or use more?
It depends on the type. A dry airside or dry waterside economizer saves both energy and water, but evaporative and adiabatic free cooling trade electricity for water, which shows up as a higher WUE. In a water-stressed region, weigh PUE against WUE; running adiabatic pre-cooling only when needed cuts the water draw.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.