HVAC
Cooling tower commissioning and water treatment field guide
Prove the tower makes its approach to the wet-bulb, hold the cycles of concentration, and stand up the water management plan that keeps it from scaling and from growing Legionella.
Direct answer
A cooling tower rejects building heat by evaporating water, cooling the condenser water the chiller dumps heat into. Commissioning proves it makes its thermal performance, measured as approach to the wet-bulb, while the water management plan keeps it from scaling up and from growing Legionella. The project specification and ASHRAE 188 control the program.
Key takeaways
- Approach equals cold water out minus ambient wet-bulb and is the real performance number; a tower cannot cool below the wet-bulb.
- CTI ATC-105 is the field acceptance test; a result at or above 100 percent of predicted capability passes, and test conditions must sit near design wet-bulb.
- Cycles of concentration equals circulating conductivity divided by makeup conductivity; many systems run 2 to 6 cycles, set conductivity setpoint to makeup times target cycles.
- Most cooling programs hold a slightly positive LSI, typically near 0 to plus 0.5; negative LSI is corrosive, positive is scaling.
- ANSI/ASHRAE 188 requires the owner to run a written water management plan; CDC guidance points to locating the tower at least 25 feet from building air intakes.
The cooling tower, and the two risks you commission against
A cooling tower rejects heat by evaporating a small part of the water that runs through it. Warm condenser water comes back from the chiller, gets spread over the fill, and air pulls through it. A fraction of that water flashes to vapor, and evaporation is what carries the heat away, not the breeze. The cooled water collects in the basin and goes back to the chiller to pick up the next load. That is the whole machine.
Commissioning a tower is two jobs that people treat as one and then short one of them. The first is thermal: prove the tower actually cools the water to the temperature the design promised, at the design flow and the design wet-bulb. The second is water management: prove the chemistry and the controls will keep the tower from scaling up, corroding out, and growing Legionella once it runs every day with nobody watching.
The thermal side fails loud. The plant cannot hold the chillers and somebody calls before the building is full. The water side fails quiet. Scale builds on the fill and the condenser tubes for a year before the approach creeps and the energy bill climbs, or the bacteria count walks up until a culture comes back hot and now it is a public-health problem, not a maintenance ticket. The tower is the classic source of a Legionnaires' outbreak, so the water management plan is not paperwork you skip to make a date.
Open or closed-circuit: which tower is on the job?
An open tower puts the process water in direct contact with the air. The condenser water itself is what gets sprayed over the fill and evaporated, so it picks up whatever the air carries: dust, spores, organics, and the bacteria that ride in on all of it. Open towers are cheaper, smaller for the same duty, and cool closer to the wet-bulb, which is why most condenser-water plants use them.
A closed-circuit tower, also called a fluid cooler, keeps the process fluid sealed inside a coil. A separate spray-water loop wets the outside of the coil and evaporates off it, so the heat moves from the clean fluid, through the coil wall, into the spray water and out as vapor. The fluid in the coil never touches the air, so it stays clean. That matters for a glycol loop, a process loop you do not want fouling, or a chiller you do not want scaling on the condenser side.
The trade is real and it has a lean. Closed-circuit costs more up front, is larger and heavier for the same tonnage, and runs a few degrees warmer on approach because you have added a coil wall between the fluid and the air. You take that penalty when keeping the process side clean is worth more than the first cost and the efficiency, which on a critical or a glycol system it often is. Note that a closed-circuit unit still has an open spray-water loop, so it still grows Legionella and still needs the full water management plan. Closing the process loop does not close the Legionella loop.
What are range and approach?
Range is the hot water in minus the cold water out. It is how many degrees the tower pulled off the water on one pass, and it is set by the heat load and the flow, not by the tower. A bigger load or a slower flow gives you a wider range. Range tells you how hard the building is working the loop.
Approach is the cold water out minus the ambient wet-bulb. It is the gap between the coldest water the tower made and the coldest water it could theoretically make, and that gap is the performance number. A tower running a 5 degree F approach is cooling much closer to the limit than one running 12, at the same wet-bulb. When somebody says a tower is undersized or fouled, what they mean, whether they say it this way or not, is the approach is wider than design.
Here is the physics that anchors all of it: a cooling tower cannot cool the water below the ambient wet-bulb temperature. Evaporative cooling is driven by the difference between the water and the wet-bulb, and as the water approaches the wet-bulb that driving force goes to zero. So approach can get small but never reaches zero, and a design with a tight approach, say 5 to 7 degrees F, buys a much larger tower than a loose one. This is also why a tower is rated at a wet-bulb, not a dry-bulb or an air temperature. The wet-bulb is the floor, and the whole machine is judged by how close it gets to it.
Range = Thot water in − Tcold water outApproach = Tcold water out − Tambient wet-bulbHow is the tower's thermal performance proven?
You prove it by comparing the tower against its design curve at the conditions it actually sees. The Cooling Technology Institute, CTI, publishes the methods the industry trusts. CTI ATC-105, the Acceptance Test Code for Water Cooling Towers, is the field acceptance test: you measure the hot and cold water temperatures, the circulating flow, the ambient wet-bulb, and the fan power, then work out the tower's thermal capability as a percent of the predicted capability from the manufacturer's curve. A result at or above 100 percent of predicted capability passes.
ATC-105 gives two methods, the characteristic-curve method and the performance-curve method, and the test conditions have to land inside a tolerance band around design or the result is not valid. You cannot run an acceptance test on a 55 degree F wet-bulb day for a tower rated at 78 and call the math good. A licensed test agency usually runs the formal CTI test because the instrumentation and the flow measurement have to be tight.
Separate from the field test is CTI STD-201, the thermal performance certification program. A certified tower model has had its rating verified by CTI, so the curve you are testing against is one a third party already stood behind. Confirm whether the spec calls for a CTI-certified model, a witnessed ATC-105 field test, or just a functional check at the conditions available. Those are three different levels of proof, and the contract decides which one you owe. When you cannot get design wet-bulb during the commissioning window, record the actual conditions, correct to the curve, and flag that a full acceptance test waits for weather.
The components: fill, drift eliminators, distribution, and basin
The fill is where the heat transfer happens. It breaks the water into thin films or small droplets so the air has the most surface to work on. Film fill, the tight PVC honeycomb, gives the most surface in the least space and the best approach, but it plugs with debris, scale, and biofilm on dirty water. Splash fill, the staggered bars the water cascades over, is far harder to foul and is what you use on water that will not stay clean. The fill choice is a bet on how good the water treatment will be, and on a tower that gets neglected, film fill is the first thing to clog and steal the approach.
The drift eliminators sit above the fill and the distribution. Drift is the water that leaves as fine droplets carried out in the air stream, separate from the vapor of evaporation, and those droplets carry whatever is in the basin, including Legionella. Modern eliminators force the air through several sharp direction changes so the droplets impact and drain back, cutting drift to under 0.001 percent of the circulating flow. Treat them as the last line of defense for the bacteria the water treatment did not catch. A panel that is cracked, sagging, or left out after a fill cleaning bypasses that defense and throws contaminated aerosol no matter how good the chemistry is.
The distribution spreads water evenly over the fill, either pressurized spray nozzles in a counterflow tower or open gravity troughs in a crossflow tower. Uneven distribution starves part of the fill and kills the approach. The cold-water basin collects the cooled water and holds the makeup float valve, the suction, the bleed line, the strainer, and on most outdoor units the basin heater. The fan and drive pull the air, on a belt or gear drive or increasingly a direct-drive motor, often on a VFD so the controls can modulate airflow to hold a leaving-water setpoint.
What are cycles of concentration?
Cycles of concentration is the ratio of dissolved solids in the circulating water to the dissolved solids in the makeup water. Evaporation leaves the minerals behind. The water vapor that carries the heat away is nearly pure, so every gallon that evaporates concentrates the salts in the gallons that stay. Left alone, the tower would keep concentrating until it scaled solid. You hold it in check by bleeding off a fraction of the concentrated water, the blowdown, and replacing it with fresh makeup.
Cycles is the lever between two failures. Run too few cycles and you blow down too much, wasting water and the chemicals dissolved in it. Run too many and the water gets so concentrated it scales the fill and the condenser tubes, or turns corrosive. Many systems run between 2 and 6 cycles, and the right number depends on the makeup water chemistry, not on a rule of thumb. Going from 3 cycles to 6 can cut makeup water roughly 20 percent and blowdown roughly 50 percent, which is why the water-conscious move is to run the highest cycles the chemistry safely allows.
In the field you do not measure dissolved solids directly, you measure conductivity, in microsiemens per centimeter, because conductivity tracks the dissolved salts closely and a probe reads it continuously. A conductivity controller watches the circulating water and opens the blowdown valve when conductivity climbs past the setpoint, then closes it when fresh makeup brings it back down. Set the setpoint as the makeup conductivity times your target cycles. Commissioning the conductivity controller, and proving the blowdown valve actually strokes, is one of the steps that quietly decides whether the tower scales in its first year.
Cycles = Conductivitycirculating / Conductivitymakeup- Cycles of concentration
- Circulating dissolved solids divided by makeup dissolved solids
- Blowdown / bleed
- Concentrated water drained off to hold the cycles down
- Makeup
- Fresh water added to replace evaporation, drift, and blowdown
Scale, corrosion, and the LSI balance
Scale and corrosion are the two ways the chemistry goes wrong, and they pull in opposite directions. Scale is mineral dropping out of solution and plating onto hot surfaces, mostly calcium carbonate, and it insulates the condenser tubes and chokes the fill. Corrosion is the metal going into solution, eating the basin, the piping, and the tube walls. Water that is too scaling on one tower is too corrosive on the next, and the treatment program walks the line between them.
The tool for that line is the Langelier saturation index, the LSI, which measures how saturated the water is with calcium carbonate using pH, calcium hardness, alkalinity, temperature, and dissolved solids. A negative LSI means the water is hungry and corrosive. A positive LSI means it is scaling. Most cooling programs hold a slightly positive LSI, typically near 0 to plus 0.5, on the scaling side of neutral; inhibitor-stabilized programs may push higher, up to around plus 2.0 to plus 2.5, with scale inhibitors keeping the deposit from actually forming. The LSI only speaks to calcium carbonate, though. It says nothing about calcium phosphate, silica, or sulfate, so it is a guide, not the whole picture.
A real treatment program runs three chemistries together. A scale inhibitor, usually a phosphonate or polymer, holds the hardness in solution past where it would normally drop out. A corrosion inhibitor builds a protective film on the metal so the water does not eat it. A dispersant keeps suspended solids and the start of biofilm from settling and packing into the fill. These come as a blended product matched to the makeup water, the cycles, and the metallurgy. The water treatment vendor sets the program and the targets, and commissioning is where you confirm the feed equipment actually doses it and the test results land in the band the vendor specified.
Why do cooling towers grow Legionella?
A cooling tower is a near-perfect incubator for Legionella, and that is not bad luck, it is the design conditions. The water sits in the warm zone the bacteria love, roughly the range of a comfortable bath, with sunlight, dissolved nutrients, and the surface area of all that fill for biofilm to colonize. Legionella lives inside that biofilm and inside the amoebae that graze on it, protected from biocide. Then the tower does the one thing that turns a contaminated basin into a public-health event: it makes aerosol and throws it into the air, where someone downwind breathes it. The tower is the textbook source of Legionnaires' disease outbreaks for exactly this chain.
Biological control runs on biocide, and the program uses two kinds. An oxidizing biocide, chlorine or bromine compounds or sometimes ozone, kills fast and leaves a measurable residual you can read in the field to prove the water is protected right now. A non-oxidizing biocide, such as isothiazolone, glutaraldehyde, or DBNPA, kills by a different mechanism and gets into the biofilm the oxidizer struggles to penetrate. The usual program runs an oxidizer for continuous residual and rotates a non-oxidizer on a schedule so the population never adapts to a single chemistry.
Cleaning backs up the chemistry, because biocide does not dissolve the deposit the bacteria hide under. OSHA guidance points to cleaning and disinfecting the system at least twice a year, and CDC guidance to an offline clean and disinfection at least annually, with more often where conditions or testing demand it. The point that gets missed: chemistry controls the planktonic bacteria in the water, but the biofilm on the surfaces is the reservoir, and only physical cleaning plus a penetrating biocide gets at it.
What is an ASHRAE 188 water management plan?
ANSI/ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, is the standard that frames how a building controls Legionella, and the cooling tower is one of the systems it covers. It does not hand you a chemical recipe. It requires the owner to build and run a written water management plan, a WMP, specific to the building and its equipment, and to keep the records that prove the plan is being followed. In a growing number of jurisdictions, cooling-tower registration and a Legionella management plan are also a legal requirement, so confirm the local rule on top of the standard.
The plan is built around a set of elements. A program team that owns the decisions. A written description and a flow diagram of the water systems, so everyone is working from the same picture. An analysis of where Legionella can grow and spread, which identifies the control points. Control measures with defined limits at each point, the things you monitor and the numbers that mean in or out of control. Monitoring procedures that say what gets checked, how, and how often. Corrective actions for when a limit is exceeded. And verification and validation, plus the documentation that the program is doing what it is supposed to.
For a cooling tower, the control points are the ones you commission: the biocide residual, the conductivity and cycles, the pH and inhibitor levels, the bacterial and Legionella testing, and the condition of the drift eliminators and the basin. The plan sets the limit on each and the action when it is breached. The commissioning agent's job is to hand the owner a tower that is set up to run inside the WMP and the documentation that proves where it started. The plan itself is the owner's to run for the life of the tower, and the standard is explicit that it is an ongoing program, not a one-time startup check.
Startup: cleaning, disinfection, and passivation
A new tower is dirty inside. Construction leaves oil, grease, pipe dope, mill scale, and dust in the basin and the loop, and that debris feeds the first biofilm and shields it from biocide. So the startup sequence is clean, disinfect, then treat. Flush the condenser-water piping to clear the construction debris before it lands in the tower, the same discipline the chilled-water side uses on its flush, then pre-clean the tower with a phosphate or surfactant cleaner, and disinfect with a biocide before the system carries normal load. CDC and the water-treatment industry both call for a pre-commissioning disinfection, because the first weeks are when an untreated system seeds the biofilm it will fight for years.
A galvanized tower needs one more step that crews skip and pay for later: passivation. New galvanized steel will form white rust, a powdery zinc oxidation, if it runs in alkaline water before the zinc surface has stabilized. Passivation is letting a dull gray protective zinc-carbonate layer build instead. The way you get it is to hold the circulating water mildly, roughly pH 6.5 to 8.0 and below about 8.3, for the first several weeks to a few months of operation, often with controlled acid feed, until the gray passivated film is visibly established. Run a new galvanized tower hard and alkaline out of the gate and you grow white rust instead, and you have damaged the corrosion protection you paid for.
The initial chemical feed has to be live before the tower runs at load. Inhibitors and biocide dosed, the conductivity controller bleeding to target cycles, and the first water sample pulled and logged as the baseline. That baseline is the number every later test gets measured against, so it belongs in the commissioning record, not in someone's memory.
Commissioning the controls
The controls are where the thermal side and the water side meet, and each loop gets proven, not assumed. The fan control modulates airflow to hold the leaving condenser-water temperature, usually a VFD ramping the fan or staging cells on a multi-cell tower. Confirm it actually holds the setpoint and that the staging and the VFD do not fight each other or short-cycle. On many plants the setpoint resets against the wet-bulb or the approach, so the tower makes the coldest water it can without overspending fan energy. Prove the reset moves the setpoint the right direction.
The water-side controls each get a functional test. The makeup float or valve holds the basin level and does not overflow or run the basin dry. The conductivity controller reads correctly and strokes the blowdown valve at the setpoint, which you verify by watching conductivity drop when it opens, not by trusting the display. The chemical feed pumps for inhibitor and biocide dose on their timers or their water-meter signal, and the biocide rotation is programmed. The basin heater energizes on its thermostat and is interlocked so it cannot fire on a dry basin.
Then prove the alarms, because an alarm nobody tested is an alarm that will not save the tower. Low basin level, low or high conductivity, chemical drum low or empty, vibration on the fan, and basin-heater fault should each annunciate where someone will see it. The alarm that matters most over the tower's life is the one that says the water treatment has stopped working, the empty chemical drum or the failed feed pump, because a tower running with no biocide is the failure that grows into a Legionella problem quietly.
Freeze protection and winter operation
A tower that runs through winter, or just sits idle in a cold climate, will freeze its basin and crack things if nobody protected it. The cold-water basin is the first casualty, so outdoor towers carry an electric basin heater sized for the local design low, with a thermostat that brings it on below about 40 degrees F and a low-water cutoff so it cannot energize on a dry basin and burn out. The heater protects the standing water during shutdown. It is not there to fight ice while the tower is running.
While the tower runs in the cold, the risk is ice on the fill and the inlet louvers as cold air meets the water. The controls fight it by managing airflow, cycling or reversing fans to de-ice, and by using a basin bypass that dumps warm return water straight into the basin instead of over the fill when the load is light, keeping the basin and the suction warm. Run a tower at light winter load with the fan blind to the cold and you build ice that loads the structure and blocks the air.
The cleaner answer where the design allows it is to move the water indoors. A remote or indoor sump keeps the working volume in a heated space, so the basin outdoors drains down when the tower is off and there is nothing outside to freeze. Whichever approach the design used, commission it: prove the basin heater and its cutoff, the bypass, and the de-ice logic actually function before the first hard freeze, not during it.
Drift, plume, and placement away from air intakes
Two things leave the top of a tower, and only one of them is a hazard. The plume is visible water vapor condensing in cold air, the same as your breath on a winter morning, and it is harmless. Drift is the fine liquid droplets carried out in the air stream, and drift carries whatever is in the basin, Legionella included. Good drift eliminators hold drift under 0.001 percent of circulating flow, which is the engineered control. Placement is the other control, and it is set at design, not at commissioning, so if it is wrong you flag it rather than fix it.
The placement rule that matters is keeping the tower discharge away from anything that breathes air into the building. CDC guidance points to locating a tower at least 25 feet from building air intakes so the drift plume is not pulled into the ventilation system, and to account for prevailing wind, which can push the effective distance well past that. A tower set close to a fresh-air intake, or upwind of an operable window or a public space, turns every gap in the water treatment into an exposure path straight into the building.
On commissioning, you cannot move a tower, but you can document the as-built relationship to intakes and operable openings and raise it if it is short. You also confirm the drift eliminators are fully installed and undamaged after any fill or basin work, because that is the controllable half of the drift risk and it is the half that gets left out after maintenance.
The condenser water loop and the chiller
The tower does not work alone. It is the heat-rejection end of the condenser-water loop, and the chiller is the other end. The chiller pulls heat out of the building's chilled water and dumps it, plus the compressor's own work, into the condenser water. The condenser pump pushes that warm water out to the tower, the tower rejects it to the air, and the cooled water comes back to the condenser. Commissioning the tower means commissioning it as part of that loop, not as a box on a roof.
The numbers have to line up across the loop. The condenser-water flow has to match what the chiller and the tower were both selected for, commonly near 3 gallons per minute per ton on a standard design, though the project selection controls the actual figure. The tower's range should match the loop's design delta-T at design load, because the range is just the loop delta-T seen from the tower. If the tower shows a narrow range when the chiller is loaded, the flow is high or the load is low, and if the approach is wide at design wet-bulb, the tower is the problem. Balancing the condenser flow and confirming the delta-T is what ties the tower's performance to the chiller's, and it is where a hydronic balance on the condenser side earns its place. The chilled-water side of the same plant gets proven its own way, with a witnessed pressure test before it is ever insulated.
Watch the condenser approach on the chiller too, the gap between the leaving condenser water and the refrigerant condensing temperature. When that approach widens over time at steady conditions, the condenser tubes are scaling or fouling, which is the water treatment talking through the chiller. The tower and the chiller share one water problem.
Cooling towers in the data center cooling plant
In a data center, the tower or the condenser-water plant is often the single largest water user on the site, and that has put it under a different kind of scrutiny. The metric is water usage effectiveness, WUE, the liters of water used per kilowatt-hour of IT energy. A conventional evaporative plant rejects heat cheaply on energy but spends water doing it, through evaporation and the blowdown that holds the cycles. In a water-stressed region, that water number can decide the cooling architecture before the energy number does.
That pressure has pushed two answers onto data-center jobs. Hybrid towers run dry most of the year, rejecting heat with air alone and keeping WUE near zero, then switch to evaporative mode only on the hottest design days when dry cooling cannot hold the loop. Pure dry coolers use no process water at all, trading water for a higher energy bill and a warmer approach, since a dry cooler is limited by the dry-bulb, not the wet-bulb, and the dry-bulb is always higher. The choice between an evaporative tower, a hybrid, and a dry cooler is a water-versus-energy trade that the site and the climate decide.
Whichever the plant uses, the commissioning rigor is the same. A hybrid still has a wet mode with a spray-water loop, so it still grows Legionella and still needs the full water management plan in its wet operation. The cooling side of the data-center plant, how the rejected heat actually leaves the building, is its own subject, and the tower is one stage of it.
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.
What to document
The commissioning record is what the owner runs the water management plan against, and what a public-health investigator asks for if a culture ever comes back hot. Capture the tower as-built and the conditions it was proven at, the water-side baseline, and the control setpoints, so the day-one numbers are on paper and not in a memory that left with the startup tech.
Record the tower make, model, type, and whether it is CTI certified. Record the test conditions and results: range, approach, flow, ambient wet-bulb, fan power, and the percent of predicted capability. Record the water baseline: cycles, conductivity setpoint, pH, inhibitor and biocide levels, and the first Legionella and bacterial counts. Record that the water management plan exists, who owns it, and that the commissioned limits match it. The tower the building gets is only as good as the record that proves where it started.
| Field to record | Why it matters |
|---|---|
| Tower type and model, CTI certified | Sets which proof and which curve apply |
| Range at test | Confirms the load and flow seen by the tower |
| Approach vs design at actual wet-bulb | The real performance number, corrected to conditions |
| Ambient wet-bulb at test | The floor the tower is judged against; validates the test |
| Circulating flow and delta-T | Ties the tower to the chiller and condenser loop |
| Cycles of concentration and conductivity setpoint | The scale-versus-water-waste balance, baselined |
| Biocide residual and Legionella baseline | The day-one biological condition for the WMP |
| Water management plan owner and control limits | Proves the ASHRAE 188 program is in place |
Common mistakes
- Handing over a tower with no written water management plan, so the Legionella controls have no owner and no limits.
- Leaving drift eliminators cracked, sagging, or out after a fill cleaning, which bypasses the last barrier against contaminated aerosol.
- Running cycles of concentration too low, wasting water and chemical, or too high, scaling the fill and condenser tubes and turning the water corrosive.
- Skipping the pre-startup cleaning and disinfection, so the first biofilm seeds before treatment ever starts.
- Running a new galvanized tower hard and alkaline before passivation, growing white rust and wrecking the corrosion protection.
- Accepting a thermal result at an off-design wet-bulb without correcting to the curve, so the tower looks fine and is not.
- Commissioning a tower set too close to a fresh-air intake or upwind of occupied space and not flagging the placement.
- Leaving the basin heater, low-water cutoff, or bypass uncommissioned and discovering it during the first hard freeze.
Standards and references
ANSI/ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, is the central standard for the water side. It requires the owner to develop and maintain a written water management plan for building water systems including cooling towers, built around a program team, system description and flow diagram, hazard analysis and control points, control limits, monitoring, corrective action, and documentation. Confirm the current edition, since the standard is revised and the program requirements are updated.
On the thermal side, the Cooling Technology Institute, CTI, owns the performance documents. CTI ATC-105 is the field acceptance test code for water cooling towers, and CTI STD-201 is the thermal performance certification program for the equipment. ASHRAE Standard 90.1 sets minimum efficiency for heat-rejection equipment, which the tower selection has to meet. Use the standard that governs the point, and let the project specification override a rule of thumb when it is stricter.
Public-health and regulatory guidance backs the standard. CDC publishes cooling-tower-specific Legionella control guidance, including pre-commissioning disinfection, cleaning frequency, drift-eliminator use, and the 25-foot separation from air intakes. OSHA addresses Legionella as a workplace hazard, including cleaning and disinfection frequency. A growing number of state and local jurisdictions now require cooling-tower registration and a Legionella management plan, so confirm the local rule on top of the standard. The water-treatment vendor sets the chemical program and the test limits for the specific makeup water and metallurgy, and the project specification and equipment submittals control the numbers.
Units, terms, and conversions
Cooling-tower work mixes thermal terms with water-chemistry terms, and the same quantities show up in different units across a manufacturer sheet, a balancing report, and a water-treatment log.
Temperatures and the wet-bulb run in degrees F on most US drawings and degrees C on metric and many manufacturer documents. Flow is gallons per minute, often tied to tons of cooling at the condenser. Conductivity, the field stand-in for dissolved solids, is microsiemens per centimeter, and dissolved solids themselves are parts per million. The LSI is a dimensionless number where negative is corrosive and positive is scaling. Drift is a percentage of circulating flow. Cycles of concentration is a ratio, dimensionless. Keep each in the unit the document that controls it uses, and convert deliberately rather than eyeballing it.
- Range
- Hot water in minus cold water out, in degrees, set by load and flow
- Approach
- Cold water out minus ambient wet-bulb; the tower's performance measure
- Wet-bulb
- Lowest temperature evaporative cooling can reach; the tower is rated against it
- Cycles of concentration
- Ratio of circulating to makeup dissolved solids, held with blowdown
- Blowdown / bleed
- Concentrated water drained off to control cycles and scaling
- LSI
- Langelier saturation index; negative is corrosive, positive is scaling
- Drift
- Liquid droplets carried out in the air stream, the Legionella aerosol path
- ASHRAE 188
- The standard requiring a written water management plan for Legionella risk
FAQ
What are range and approach on a cooling tower?
Range is the hot water entering minus the cold water leaving, set by the load and the flow. Approach is the cold water leaving minus the ambient wet-bulb, and it is the performance measure. A smaller approach means the tower cools closer to the physical limit, since a tower cannot cool below the wet-bulb.
What is an ASHRAE 188 water management plan?
ASHRAE Standard 188 requires building owners to keep a written water management plan controlling Legionella risk in systems like cooling towers. It sets a program team, a flow diagram, hazard analysis and control points, monitoring with limits, corrective actions, and documentation. Confirm the current edition and any local cooling-tower registration rule.
What are cycles of concentration in a cooling tower?
Cycles of concentration is the ratio of dissolved solids in the circulating water to the makeup water. Evaporation concentrates the salts, so you bleed off concentrated water as blowdown and add fresh makeup to hold the cycles. Many systems run 2 to 6 cycles; too few wastes water, too many scales and corrodes.
Why do cooling towers grow Legionella?
A tower holds warm water with nutrients and huge wetted surface area for biofilm, which is ideal for Legionella, and it then makes aerosol and throws it into the air. That makes the tower the classic Legionnaires' disease source. Control needs biocide, cleaning, drift eliminators, and a water management plan, not chemistry alone.
What is the difference between an open and a closed-circuit cooling tower?
An open tower evaporates the process water directly, so it contacts the air and picks up contamination, but it is cheaper and cools closer to the wet-bulb. A closed-circuit tower keeps the process fluid in a sealed coil and evaporates a separate spray loop over it, staying clean at higher cost and a warmer approach.
How is a cooling tower's thermal performance tested?
You compare the tower to its design curve using CTI ATC-105, measuring hot and cold water temperatures, flow, ambient wet-bulb, and fan power, then computing capability as a percent of predicted. CTI STD-201 certifies the model's rating. Test conditions must sit near design wet-bulb, or the result is corrected to the curve.
What is the LSI in cooling tower water treatment?
The Langelier saturation index measures how saturated the water is with calcium carbonate using pH, hardness, alkalinity, temperature, and dissolved solids. Negative LSI is corrosive, positive is scaling. Cooling programs run slightly scaling, typically an LSI near 0 to plus 0.5, with inhibitor-stabilized programs going as high as about plus 2.0 to plus 2.5 while inhibitors hold the scale off. LSI covers only calcium carbonate.
How far should a cooling tower be from air intakes?
CDC guidance points to at least 25 feet between a cooling tower discharge and building air intakes, so the drift plume is not pulled into the ventilation system, and to account for prevailing wind that can push the needed distance further. Placement is set at design, so flag a short distance rather than accept it.
What do I do before starting up a new galvanized cooling tower?
Flush the piping, pre-clean and disinfect the tower, then passivate the galvanized surface. Hold the water mildly acidic to neutral, roughly pH 6.5 to 8.0 and below about 8.3, for the first several weeks until a gray zinc-carbonate film forms. Run it alkaline early and you grow white rust instead.
How does a conductivity controller manage cooling tower blowdown?
A conductivity probe reads the circulating water continuously, in microsiemens per centimeter, as a stand-in for dissolved solids. The controller opens the blowdown valve when conductivity passes the setpoint and closes it as fresh makeup dilutes it. Set the setpoint to makeup conductivity times your target cycles, and verify the valve actually strokes.
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.