HVAC
Cooling tower types and how they work field guide
What a cooling tower does, why the wet-bulb is the floor it can never beat, and how open, closed-circuit, crossflow, counterflow, induced, and forced-draft towers actually differ on the job.
Direct answer
A cooling tower rejects building or process heat to the air by evaporating a small fraction of its water, cooling the condenser water a chiller dumps heat into. It can cool that water close to the ambient wet-bulb temperature but never below it. The design wet-bulb, the project specification, and the load control the selection.
Key takeaways
- A cooling tower rejects heat by evaporating roughly 1 percent of its flow per 10 degrees F of range, leaving the rest cooled.
- Wet-bulb temperature is the floor: a tower approaches it but can never cool water below it, so towers are sized to a design wet-bulb.
- Range equals hot water in minus cold water out (set by the load); approach equals cold water out minus entering wet-bulb, commonly 5 to 7 degrees F for HVAC.
- Cycles of concentration equals makeup divided by blowdown; condenser-water programs typically run 3 to 6 cycles, set by makeup chemistry.
- Cooling towers are a recognized Legionella source via drift; ASHRAE Standard 188 requires a written water management program to control it.
What a cooling tower does
A cooling tower rejects heat to the atmosphere by evaporating a small part of the water that runs through it. Warm water comes off a load, usually the condenser of a chiller, and the tower spreads it out so air can move across it. A fraction of that water flashes to vapor, the vapor carries the heat away, and the cooled water collects in the basin and goes back to pick up the next load. That loop is the heat sink the whole plant leans on.
Most of the time the load is a water-cooled chiller. The chiller pulls heat out of the building's chilled water, adds the work its own compressor does, and dumps the total into the condenser-water loop. The tower is the only place that heat goes. Lose the tower and the chiller has nowhere to reject to, head pressure climbs, and the machine trips on high condenser pressure within minutes. The chiller-plant guide covers that side of the loop in depth.
The thing to keep straight from the start is that the tower does not make cold. It moves heat out of the water by turning some of the water into steam. Evaporation is the engine, the air just carries the vapor off. Get that one idea right and every type, every term, and every water problem in this guide follows from it.
How does a cooling tower work?
A cooling tower works by evaporative cooling. When water evaporates it pulls its latent heat of vaporization out of the water left behind, and that is what cools the bulk of the stream. It takes roughly 1,000 Btu to boil off a pound of water, so evaporating even a small fraction of the flow carries off a large amount of heat. The breeze through the tower is not doing the cooling. It is hauling away the humid air so more water can evaporate.
The limit on how cold the water can get is the wet-bulb temperature of the entering air, not the dry-bulb you read on a thermometer. Wet-bulb is the temperature air reaches when it has taken on all the moisture it can hold, and it is the lowest temperature evaporation can drive water toward. A tower can get the cold water close to the wet-bulb but it can never go below it. Pick a hot, humid afternoon and the wet-bulb is high, so the coldest water you can make is warm, and the chiller behind it works harder. Reading wet-bulb off a psychrometric chart is its own skill, and it is the language this whole cold-water limit is written in.
This is why towers are sized to a design wet-bulb for the site, not to an air temperature. A 95 degree F dry day at a low wet-bulb is an easy day for the tower. A 78 degree F wet-bulb day, common in a humid summer, is the design case. The wet-bulb is the floor, and every performance number in the next sections is measured against it.
Open (direct) cooling towers
In an open, or direct, cooling tower the process water itself is sprayed over the fill and falls through the moving air in open contact with it. The water you are cooling is the same water that touches the atmosphere. This is the common condenser-water tower on most chiller plants, and it is the most efficient way to reject heat because nothing sits between the water and the air.
The cost of that efficiency is exposure. Open water scrubs the air as it falls, so it picks up dust, pollen, and whatever else is blowing past, and it concentrates dissolved minerals as it evaporates. That fouls the fill, feeds biological growth, and is exactly the environment Legionella likes. An open tower has to run on a real water treatment program or it scales, corrodes, and turns into a health risk. The commissioning and water-treatment guide is the place that program gets built.
Open towers also expose the condenser side of the chiller to that dirty water, so the condenser tubes foul and have to be cleaned on a cycle. You accept all of that because the open tower makes the closest approach to the wet-bulb for the least fan and pump energy. On a large plant that efficiency usually wins, which is why open towers dominate.
What is the difference between an open and closed cooling tower?
The difference between an open and a closed cooling tower is whether the process fluid touches the air. In an open tower the process water is sprayed into the air directly. In a closed-circuit tower, also called a fluid cooler or an indirect tower, the process fluid stays sealed inside a coil, and a separate spray-water loop wets the outside of that coil while air pulls through. The spray water and air cool the coil, the coil cools the clean fluid inside it, and the two never mix.
A closed-circuit tower keeps the process loop clean. The water or glycol going out to the chiller, the heat pumps, or the process never sees the atmosphere, so it does not foul the condenser tubes and it can carry antifreeze for winter operation. That clean loop is the reason to choose one: sensitive equipment, freeze-prone outdoor piping, or a process that cannot tolerate fouling.
You pay for it two ways. A closed tower costs more up front because you are buying a coil and a second pump, and it runs a few degrees less efficient because the heat now crosses a coil wall instead of contacting the air directly, which usually means a larger approach for the same conditions. The spray-water loop outside the coil still evaporates, still concentrates, and still needs treatment and freeze protection. People buy a closed tower thinking they have escaped water treatment. They have only moved it to the spray side.
| Feature | Open (direct) tower | Closed-circuit (indirect) tower |
|---|---|---|
| Process fluid | Sprayed into the air directly | Sealed in a coil, never touches air |
| Efficiency | Highest, closest approach | Slightly lower, larger approach typical |
| First cost | Lower | Higher (coil plus second pump) |
| Loop cleanliness | Fouls condenser, needs tube cleaning | Process loop stays clean |
| Water treatment | Required on the process water | Still required on the spray water |
| Freeze handling | Drain or basin heat | Can run glycol in the closed loop |
Crossflow vs counterflow
Crossflow and counterflow describe how the air moves relative to the falling water. In a crossflow tower the air travels horizontally across the water as it falls. In a counterflow tower the air travels straight up, against the falling water. Both work. They trade off in ways that show up in footprint, fan energy, and how miserable the maintenance is.
Counterflow is the more compact and usually the more thermally efficient layout, because the coldest water meets the driest incoming air at the bottom, which makes a tight approach. The distribution is a pressurized spray header firing down into the fill, and the air path is more restricted, so it tends to want more fan power and a taller profile. The spray nozzles are inside the unit, which makes them harder to get at when one clogs.
Crossflow tends to run lower fan power because the air path is shorter and more open, and it uses a gravity distribution basin on top where the water just falls through metering holes, so you can open the louvers and see the fill and the distribution while the tower runs. That access is a real maintenance advantage. The catch is footprint, since crossflow towers are usually longer and wider for the same capacity, and the open inlet louvers make them a bit more prone to freezing and to sunlight feeding algae in the hot deck. Pick counterflow when the roof space is tight, crossflow when you want easy service access and lower fan energy and you have the room.
Induced draft vs forced draft
Induced and forced draft describe where the fan sits and which way it works the air. An induced-draft tower has the fan up top, pulling air through the fill and discharging it straight up at high velocity. A forced-draft tower has the fan at the base or the side, pushing air into the unit. Most field-erected and large packaged towers are induced draft. Forced draft shows up more on smaller packaged units and where the fan needs to be down low for sound or service reasons.
The big practical difference is recirculation. An induced-draft tower throws the hot, saturated discharge well clear of the unit at high velocity, so very little of it gets pulled back into the inlet. A forced-draft tower discharges at low velocity right where the warm plume can roll back around to the intake, and when the tower breathes its own exhaust the entering wet-bulb at the fan climbs above the ambient wet-bulb. That raises the approach and quietly robs capacity, and it is one of the harder problems to diagnose because the tower is working fine, it is just being fed bad air.
Forced draft has its own arguments. The fan and motor sit in the dry incoming air instead of the warm wet discharge, which can mean a longer motor life and easier access to the drive. The fan handles denser entering air. But the low-velocity discharge and the recirculation risk are why induced draft is the default on most large condenser-water towers. Whichever you have, the discharge has to clear the intake. The siting section covers how that goes wrong.
The fill
The fill is the media inside the tower that the water falls through, and its only job is to create as much water surface and contact time with the air as possible, because evaporation happens at the air-water interface. More surface and more contact time means more evaporation in the same box, which means a tighter approach. Fill is where most of the thermal work of the tower actually gets done.
There are two families. Film fill is closely spaced sheets of PVC that spread the water into thin films across a large surface, and it is the more efficient type because it packs enormous surface into a small volume. Splash fill is bars or slats that break the water into droplets as it cascades down, less efficient per cubic foot but far more forgiving of dirty water because there are no narrow passages to plug. The tradeoff is fouling. Film fill's tight passages are exactly where suspended solids, scale, and biological slime bridge across and choke the air path, and fouled film fill is a known harborage for Legionella biofilm.
On dirty or hard water, or on a tower that runs without tight treatment, splash fill or a wide-spaced low-fouling film is the honest choice even though it costs efficiency. Put high-efficiency film fill on a tower with poor water and you will be cutting collapsed, slimed fill out of it in a couple of seasons. The fill you can keep clean beats the fill that looks best on the selection sheet.
What is approach and range?
Range and approach are the two numbers that tell you how a cooling tower is performing. Range is the hot water temperature coming in minus the cold water temperature going out, so it is how many degrees the tower actually pulled out of the water. Approach is the cold water temperature going out minus the entering air wet-bulb, so it is how close the tower got to the theoretical floor. Range measures the work done. Approach measures the tower's quality.
Range is set by the load and the flow, not by the tower. For a given gallons per minute, more heat coming in means a wider range. A tower does not control its own range, the load does. Approach is the tower's report card, because it tells you how near the wet-bulb the unit can drive the water, and a tower can approach the wet-bulb but it can never reach or beat it. A smaller approach is a better, and a more expensive, tower.
Design approach for HVAC condenser-water towers is commonly in the range of about 5 to 7 degrees F, but treat that as a design selection, not a law. The exact approach lives in the project specification and the manufacturer's selection at the design wet-bulb and flow. A tighter approach buys colder condenser water and a more efficient chiller, but it costs more tower. When you go to verify a tower is making its performance, you prove it makes its design approach at the design wet-bulb, which is the acceptance test the commissioning guide walks through.
| Term | How it is figured | What it tells you |
|---|---|---|
| Range | Hot water in minus cold water out | How much heat the tower pulled out, set by the load |
| Approach | Cold water out minus entering wet-bulb | How close to the floor the tower got, the tower's quality |
| Wet-bulb | Lowest temp evaporation can reach | The floor the tower can approach but never beat |
Where the water goes: evaporation, drift, and blowdown
A running tower loses water three ways, and you have to make all three up. Evaporation is the useful loss, the water that turned to vapor and carried the heat off. As a rule of thumb, evaporation runs about 1 percent of the circulating flow for every 10 degrees F of range, so a tower with a 10 degree range evaporates roughly 1 percent of its flow continuously. That water leaves clean. Everything dissolved in it stays behind.
Drift, sometimes called windage, is liquid water blown out of the tower as fine droplets in the discharge air. Unlike evaporation, drift carries dissolved solids and whatever is living in the water out with it, which is why drift is the path that can spread Legionella downwind of the tower. Drift eliminators, the baffled plastic media in the discharge, catch those droplets and drop them back into the basin. Modern eliminators cut drift to a small fraction of flow, commonly on the order of 0.001 to 0.005 percent depending on the unit and the program, where older towers leaked far more. Damaged or missing drift eliminators are both a water-loss problem and a public-health problem.
Blowdown, also called bleed, is water you deliberately dump and replace to keep the dissolved solids from concentrating without limit, since evaporation leaves the minerals behind. Makeup water is the sum of all three: evaporation plus drift plus blowdown. Get the makeup meter and the blowdown right and you can account for every gallon. When the makeup runs far above evaporation plus drift, you are blowing down too hard or you have a leak, and both cost water and chemical.
| Loss | What it is | Carries minerals out? |
|---|---|---|
| Evaporation | Water flashed to vapor, ~1% of flow per 10 F of range | No, leaves clean |
| Drift | Liquid droplets blown out, minimized by drift eliminators | Yes, and can carry Legionella |
| Blowdown (bleed) | Water purged on purpose to control concentration | Yes, by design |
| Makeup | Evaporation plus drift plus blowdown | Replaces all three |
What is cycles of concentration?
Cycles of concentration is how many times the dissolved minerals in the circulating water have concentrated above the makeup water. As the tower evaporates pure water, everything dissolved stays behind and the water gets harder. Cycles is the ratio of the concentration in the tower to the concentration in the makeup, and in practice it equals the makeup flow divided by the blowdown flow. Run two cycles and the circulating water is twice as concentrated as the city water you feed it.
Cycles is the dial between wasting water and scaling the tower. Run too few cycles and you blow down hard, dumping treated water and chemical down the drain for nothing. Run too many and the minerals concentrate past their solubility, drop out as scale on the fill and the condenser tubes, and the whole plant loses efficiency under a chalk coating. The blowdown that sets your cycles follows from the evaporation: blowdown equals evaporation divided by the quantity (cycles minus one), so higher cycles means proportionally less blowdown.
Most condenser-water programs run somewhere in the range of about 3 to 6 cycles, but the right number is set by the makeup water chemistry and the treatment program, not by a number off a guide. Soft makeup water tolerates more cycles before it scales. Hard makeup forces you lower. Pushing cycles up saves a real amount of water, but only as far as the chemistry allows, and that line is the treatment specialist's call. The commissioning and water-treatment guide is where the cycles target and the chemistry get set together.
Why a tower needs water treatment
An open tower is a chemistry problem that happens to reject heat. It evaporates clean water and concentrates everything else, it scrubs dirt out of the air, it runs warm, and it sits in the sun. That is the recipe for three failures at once, and treatment exists to hold off all three.
Scale is mineral, mostly calcium carbonate, dropping out of concentrated water onto hot surfaces. Scale on the condenser tubes acts like insulation, so the chiller has to run higher head to push its heat through it, and efficiency falls measurably for a thin layer. Corrosion is the opposite failure, the water eating the metal, and it eats basins, piping, and tube sheets if the chemistry runs aggressive. The treatment program balances the water so it neither scales nor corrodes, often tracked with an index like the Langelier Saturation Index, and it carries inhibitors for the metals in the system.
The one that gets people hurt is biological. Warm nutrient-rich water is ideal for bacteria, and the one that matters is Legionella, which grows in the biofilm on fouled fill and basins and rides out of the tower on drift to be breathed in downwind. A cooling tower is a recognized Legionella source, and the response is a written water management program: keep the system clean, hold a biocide regime, monitor, and act on the numbers. ASHRAE Standard 188 is the framework for that program, and the commissioning and water-treatment guide is where it gets built and run. This is not optional housekeeping. It is the difference between a maintenance line item and an outbreak with your tower's name on it.
The basin, the sump, and freezing weather
The cold water basin is the pan at the bottom that catches the cooled water and holds the pump's suction. The sump is the low point where the makeup float valve sits, the strainer protects the pump, and the blowdown and overflow connect. A clean basin matters more than it looks, because sediment, sludge, and biofilm collect there and the basin is one of the prime spots a Legionella program targets for cleaning.
Cold weather is where towers get hurt or killed. The water in the basin can freeze, the fill can ice over from drift in the cold air, and ice loading can collapse fill and break louvers. The first line of defense for the basin is a basin heater, usually an electric immersion heater on a thermostat that turns on around 40 degrees F to keep the standing water above freezing. Size that heater for the coldest design temperature the site sees, not for an average winter night. Understand its limit, though: a basin heater protects the water in the basin only. It does nothing for the fill or the exposed piping.
Cold-weather operation is a strategy, not just a heater. A remote indoor sump keeps the water inside where it cannot freeze. Heat trace and insulation protect exposed pipe. Running the tower with the fans cycled or reversed, capacity-controlled to keep water moving and warm, and managing the load to avoid icing all belong to it. On a closed-circuit tower the process loop can carry glycol so the sealed side cannot freeze, but the spray side still has to be drained or heated. Plan freeze protection in design. Discovering you needed it during the first hard freeze means you discover it as a split basin and a flooded mechanical room.
Capacity control: matching the tower to the load
A tower is sized for the worst design day, and it spends almost all its life at part load on cooler, drier days, so it needs a way to throttle back. The control target is usually a condenser-water supply temperature, and the tower modulates to hold it. The order of cheap to expensive control is fan air, then water flow, then bypass.
Fan control is the main lever and the order of efficiency is clear. A single-speed fan only cycles on and off, which swings the water temperature and wears the motor and gearbox with every start. A two-speed fan gives a low step that covers a lot of mild-weather hours at much lower power. A variable frequency drive is the best of the three because fan power falls roughly with the cube of speed, so running the fan at half speed pulls close to an eighth of the power, and the VFD holds a steady water temperature instead of hunting. On a multi-cell tower, the smart move is to run more cells at low fan speed rather than fewer cells at full speed, because spreading the air over more fill at low speed beats a few cells working hard.
Below what the fans can do, you bypass. A bypass valve routes some condenser water around the tower and back to the basin so the supply temperature does not crash on a cold day, which protects the chiller from condenser water that is too cold and protects the tower from freezing. The thing not to do is starve the tower of water to control it, because uneven or low flow over the fill leaves dry spots that scale and ice. Control the air and bypass the water. Do not throttle the water across the fill to make temperature.
The tower, the condenser loop, and the chiller
On a water-cooled plant the tower exists to serve the chiller's condenser. The condenser-water loop carries the chiller's rejected heat out to the tower, the tower cools it, and the cooled water returns to the condenser. The temperature the tower delivers is not just a tower number. It sets how hard the chiller has to work.
Colder condenser water is the single biggest free efficiency lever on a chiller. Every degree of condenser water you can shave drops the compressor lift, the pressure difference the machine has to pump against, and the chiller's kilowatts per ton fall with it. Warm condenser water does the reverse: head pressure climbs, efficiency tanks, and if it climbs far enough the chiller trips on high condenser pressure to protect itself. A tower that has scaled, fouled its fill, or lost capacity to recirculation shows up as a chiller that costs too much to run and trips on hot days. People blame the chiller. The bill was written at the tower.
There is a floor, set by the chiller, not the tower. Most chillers need a minimum condenser water temperature to keep enough refrigerant pressure difference for oil return and proper operation, often somewhere around 65 to 70 degrees F depending on the machine, so on a cold day you stop chasing colder water and hold it at the minimum with bypass. This is the same loop that data centers run for heat rejection, where the condenser-water tower carries a continuous, year-round computing load and colder tower water directly cuts the power bill on equipment that never shuts off. The chiller-plant guide covers the machine side of this handoff.
Waterside economizer and free cooling
When the weather gets cold enough, the tower can make chilled water without running the chiller at all, and that is a waterside economizer, also called free cooling. On a cold, dry day the wet-bulb drops far enough that the tower can produce condenser water cold enough to cool the building's loop directly, so you shut the compressor off and let the tower and pumps do the cooling for the price of fan and pump power alone.
There are two common ways to plumb it. A heat exchanger between the condenser loop and the chilled-water loop lets the cold tower water cool the building water without mixing the dirty open tower water into the clean loop, which is the usual choice with an open tower. A closed-circuit tower can sometimes feed the loop more directly because its process side is already clean. Either way the win is large in a cold climate, where hundreds of hours a year of compressor-off cooling is real money on the energy bill.
Free cooling is also where a closed-circuit tower and a tight approach earn back some of their cost, because more economizer hours come from making colder water at a higher wet-bulb. The control gets handed off carefully, blending from full mechanical cooling, to partial, to full free cooling as the wet-bulb falls, and freeze protection becomes a live concern because you are running the tower hard in cold weather. The economizer and free-cooling control sequence is a deep subject on its own, and the waterside-economizer side is where a tower does the most work in cold weather.
Siting, recirculation, and the visible plume
Where you put a tower decides whether it makes its rating. The failure mode is recirculation: the warm, saturated discharge gets pulled back into the air intake, so the tower breathes its own exhaust, the entering wet-bulb at the inlet rises above the ambient wet-bulb, and the approach grows. The tower is working fine. It is being fed hot, wet air. You see it as a tower that cannot make its cold water temperature on a still, humid day with no obvious cause.
The cures are all about clearance and air paths. Keep the discharge clear of the intakes, hold the manufacturer's clearance off walls and adjacent towers so each unit can draw fresh air, and watch for a wall, a parapet, or another rooftop unit downwind that can bounce the plume back. Two towers set too close starve and recirculate into each other. A tower tucked into a screened well with no room to breathe will never make its number no matter how good the fill is. Get the airflow study or the manufacturer's layout right before the steel goes up, because moving a tower after the fact is the most expensive way to learn this.
The visible plume is the white cloud of condensed water vapor off the discharge on a cold day, and it is not smoke or a malfunction. It is the saturated warm air hitting cold ambient air and the moisture condensing, the same as your breath in winter. It is usually only a nuisance, fogging a road, icing a walk, or drawing complaints near a public space. Plume-abated towers mix a stream of warmer, drier air with the saturated discharge so the leaving air is below saturation and the plume does not form. You pay for that with added cost and some performance, so you only specify abatement where the plume is a real problem, not by default.
Maintenance overview
Tower maintenance is short to list and unforgiving to skip. Keep the basin clean of sediment and biofilm, keep the fill clear and the spray distribution even, keep the drift eliminators intact, and keep the treatment program running and monitored. Those four are the whole job, and three of them are also Legionella controls.
On a cycle, inspect the fill for scale, slime, collapse, and UV damage, clean the basin and the strainer, check the float valve and the makeup and blowdown for correct operation, verify the drift eliminators are seated and undamaged, and service the fans, drives, gearbox or belts, and bearings. A drifting condenser water temperature, a climbing chiller head pressure, or a makeup rate that no longer matches the math are all early signals that something in here has slipped. The cleaning intervals and the full program belong to the water management plan, and the commissioning and water-treatment guide carries the detail. The point for this guide is narrow: a tower is not fit-and-forget equipment, and the cost of treating it that way is paid in chiller efficiency, water, and risk.
What to document
A tower runs on a handful of numbers, and the value of recording them is that drift away from the baseline is the first sign of a problem you can still cheaply fix. The record is what tells you the approach has crept, the makeup no longer matches evaporation, or the cycles have wandered off target.
| Term | Definition | Why it matters |
|---|---|---|
| Range | Hot water in minus cold water out | Shows the heat load the tower is actually rejecting |
| Approach | Cold water out minus entering wet-bulb | The tower's performance against the floor; creep means fouling or recirculation |
| Wet-bulb | Entering air wet-bulb temperature | The floor the tower is measured against; needed to judge approach |
| Evaporation | Water lost to vapor, ~1% of flow per 10 F range | The useful loss and the basis for the water balance |
| Drift | Droplets blown out past the eliminators | Water and minerals lost; a Legionella exposure path |
| Blowdown | Water purged to control concentration | Sets cycles; too much wastes water, too little scales |
| Makeup | Evaporation plus drift plus blowdown | Mismatch against the math flags leaks or over-bleed |
| Cycles of concentration | Makeup divided by blowdown | The dial between wasting water and scaling the plant |
| Condenser water supply temp | Cold water leaving the tower to the chiller | Drives chiller kW per ton; the main control target |
Common mistakes
- Running too few cycles of concentration, blowing down hard and wasting treated water for nothing.
- Running too many cycles past what the makeup chemistry allows, scaling the fill and the condenser tubes.
- Treating drift as a water-loss line item and ignoring that damaged drift eliminators are a Legionella exposure path.
- Siting the tower where its own discharge recirculates into the intake, raising the entering wet-bulb and the approach.
- Leaving an open tower with no basin heater or freeze plan, then finding the split basin after the first hard freeze.
- Letting condenser water run warm from a fouled or recirculating tower and blaming the chiller for high head pressure and bad kW per ton.
- Buying a closed-circuit tower and assuming water treatment is no longer needed, when the spray side still concentrates and grows Legionella.
- Throttling water flow across the fill to control temperature instead of controlling the fans and bypassing, leaving dry spots that scale and ice.
- Specifying high-efficiency film fill on hard or dirty water that fouls and collapses it within a couple of seasons.
- Neglecting the written water management program until a Legionella case forces 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.
Standards and references
The Cooling Technology Institute, the CTI, is the body that sets the test and certification standards for thermal performance, so a tower's rating and any field thermal test trace back to CTI methods. When a spec calls for a CTI-certified rating or a CTI-witnessed performance test, that is the standard governing whether the tower makes its approach at the design conditions.
ASHRAE is where the design conditions and the systems guidance live: the design wet-bulb for the site, the energy provisions in ASHRAE Standard 90.1 that push efficient towers and economizers, and the equipment coverage in the ASHRAE Handbook. The standard that carries real legal weight is ASHRAE Standard 188, which requires a written water management program for building water systems including cooling towers to manage Legionella risk. Some jurisdictions and codes adopt or reference it, so confirm what the authority having jurisdiction actually requires.
The manufacturer's selection and instructions govern the specifics: the design approach and range, the required water flow and distribution, the fill type, clearances for airflow, and the freeze-protection and capacity-control requirements for that exact model. The approach figures and the cycles-of-concentration targets in this guide are typical design and treatment-program values, not fixed rules. The project specification, the manufacturer's selection at the site design wet-bulb, and the water treatment program control the real numbers. Verify them against the documents, not against a rule of thumb.
Units and terms
Cooling tower work mixes temperature, flow, and water-chemistry units, and the same idea reads differently across a selection sheet, a treatment report, and a controls screen.
Temperatures are in degrees F on most US selections and degrees C on metric ones, and wet-bulb and dry-bulb are both temperatures, so always note which one a number is. Water flow is gallons per minute, gpm, or cubic meters per hour on metric jobs. Heat rejection shows up in Btu per hour or in tons, where a ton is 12,000 Btu per hour of building cooling, though the tower actually rejects more than that because it also carries the compressor's work. Water chemistry runs in parts per million of dissolved solids or hardness, and conductivity in microsiemens is often the live signal a controller uses to trigger blowdown.
- Range
- Hot water temperature entering the tower minus cold water temperature leaving it
- Approach
- Cold water temperature leaving the tower minus the entering air wet-bulb
- Wet-bulb
- The lowest temperature evaporation can drive water toward, the floor a tower can approach but never beat
- Drift
- Liquid water droplets blown out of the tower, minimized by drift eliminators, a carrier of minerals and Legionella
- Blowdown / bleed
- Water purged on purpose to keep dissolved solids from concentrating without limit
- Makeup
- Water added to replace evaporation plus drift plus blowdown
- Cycles of concentration
- Ratio of dissolved solids in the circulating water to the makeup, equal to makeup divided by blowdown
- Fill
- The splash or film media that spreads the water for maximum contact with the air
FAQ
How does a cooling tower work?
A cooling tower works by evaporative cooling. It spreads warm water over fill while air moves through, and a small fraction of the water evaporates. Evaporation carries off latent heat, cooling the water left behind. The cooled water collects in the basin and returns to the load. Wet-bulb sets the lowest temperature it can reach.
What is the difference between an open and closed cooling tower?
In an open tower the process water is sprayed directly into the air, so it is efficient but exposed to fouling and treatment. In a closed-circuit tower, or fluid cooler, the process fluid stays sealed in a coil while spray water and air cool the coil outside. That keeps the loop clean but costs more and runs slightly less efficient.
What is approach and range on a cooling tower?
Range is the hot water entering minus the cold water leaving, so it measures the heat the tower removed, set by the load. Approach is the cold water leaving minus the entering wet-bulb, so it measures how close the tower got to the floor. A tower can approach the wet-bulb but never reach or beat it.
What is cycles of concentration in a cooling tower?
Cycles of concentration is how many times the dissolved minerals in the circulating water have concentrated above the makeup water, and it equals makeup flow divided by blowdown flow. Too few cycles wastes water on excess blowdown. Too many scales the fill and condenser tubes. Most programs run about 3 to 6 cycles, set by the makeup chemistry.
Why can't a cooling tower cool water below the wet-bulb temperature?
Because evaporation is the cooling mechanism, and the wet-bulb is the lowest temperature evaporation can reach. Wet-bulb is the temperature air hits when it is fully saturated with moisture. Once the air around the water is saturated, no more water evaporates, so the water cannot get colder. A tower approaches the wet-bulb but never beats it.
What is the difference between induced draft and forced draft cooling towers?
An induced-draft tower has the fan on top pulling air through and discharging it upward at high velocity, which throws the plume clear and limits recirculation. A forced-draft tower has the fan at the base pushing air in, with the motor in dry air but a low-velocity discharge that is more prone to recirculating into the intake.
Crossflow vs counterflow: which cooling tower is better?
Neither wins outright. Counterflow is more compact and often more thermally efficient but wants more fan power and buries the spray nozzles. Crossflow uses lower fan power and gives open access to the fill for easier service, but it has a larger footprint and is a bit more prone to freezing. Footprint and service access usually decide it.
How much water does a cooling tower lose?
A tower loses water to evaporation, drift, and blowdown. Evaporation runs about 1 percent of circulating flow per 10 degrees F of range and leaves clean. Drift is a small fraction caught by drift eliminators. Blowdown is purged on purpose to control concentration. Makeup replaces all three, so makeup equals evaporation plus drift plus blowdown.
Are cooling towers a Legionella risk?
Yes. A cooling tower holds warm nutrient-rich water and produces drift droplets that can carry Legionella downwind to be breathed in, so it is a recognized source. The control is a written water management program: keep the fill and basin clean, hold a biocide regime, monitor, and act on results. ASHRAE Standard 188 is the framework for that program.
Why does warm condenser water hurt chiller efficiency?
Warmer condenser water from the tower raises the chiller's condenser pressure, so the compressor works against a larger lift and kilowatts per ton climb. Push it far enough and the chiller trips on high condenser pressure. Colder tower water is the biggest free efficiency lever, down to the chiller's minimum condenser water temperature, where you hold it with bypass.
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Codes cited in this guide
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