Quick answer: A cooling tower is sized on heat rejection, not just chiller tonnage. The three numbers that define it are range (hot water temp − cold water temp), approach (cold water temp − ambient wet-bulb), and flow rate (GPM). The industry standard is the nominal cooling tower ton = 15,000 BTU/h, based on 3 GPM of water cooled from 95°F to 85°F at a 78°F wet-bulb. Because a tower rejects both the cooling load and the compressor heat, a tower is sized larger than the chiller’s cooling tons. Size yours with the cooling tower sizing calculator.
What a cooling tower does
A cooling tower rejects heat from a building’s water-cooled system to the atmosphere by evaporating a small fraction of the circulating water. Warm “condenser water” from the chiller is sprayed over fill media while air is drawn through; a little water evaporates, and evaporation carries away heat, cooling the rest of the water so it can return to the chiller’s condenser. It is the same physics as sweat cooling your skin — which is why the wet-bulb temperature (a measure of how much evaporative cooling the air can accept), not the dry-bulb, sets the limit on how cold the tower can make the water.
The three defining numbers
| Term | Definition | Typical value |
|---|---|---|
| Range | Hot water temp − cold water temp (how much the tower cools the water) | 10°F (95→85°F) |
| Approach | Cold water temp − ambient wet-bulb (how close the tower gets to the theoretical limit) | 7°F (85°F − 78°F wet-bulb) |
| Flow rate | Condenser water circulated | 3 GPM per nominal ton |
Range is set by the load and flow — it’s how many degrees the water must drop. Approach is the true measure of tower performance and cost: the smaller the approach, the bigger (and more expensive) the tower, because getting the cold water temperature closer to the wet-bulb is progressively harder. You cannot cool below the wet-bulb, and getting within a few degrees of it takes a very large tower. A 7°F approach is a common economical design point.
The nominal cooling tower ton
Here is the definition that anchors all tower sizing. A nominal cooling tower ton is:
3 GPM of water cooled from 95°F to 85°F at a 78°F entering wet-bulb — which rejects 15,000 BTU/h.
Note that a cooling tower ton (15,000 BTU/h) is larger than a refrigeration ton (12,000 BTU/h). That’s deliberate: the tower must reject not only the building’s cooling load but also the heat of compression added by the chiller’s compressor. The extra 3,000 BTU/h — about 25% — represents that compressor work.
Why the tower is bigger than the chiller
This is the concept that trips people up. A 100-ton chiller does not pair with a 100-refrigeration-ton tower. The condenser must dump the evaporator load plus the compressor’s energy input. As a rule of thumb:
Heat rejected ≈ Cooling load × 1.25
So a 100-ton (1,200,000 BTU/h cooling) chiller rejects roughly 1,500,000 BTU/h — which at 15,000 BTU/h per nominal tower ton is 100 nominal cooling-tower tons. Conveniently, because the nominal tower ton already bakes in the ~25% factor, a 100-ton chiller pairs with a 100-nominal-ton tower — but only at the standard conditions (95/85/78). Change the wet-bulb, range, or approach and the tower’s real capacity changes, which is where careful sizing comes in.
The heat-rejection formula
The fundamental relationship between flow, range, and heat rejected is:
Heat rejected (BTU/h) = 500 × GPM × Range (°F)
The 500 comes from water’s properties (8.33 lb/gal × 60 min/h × 1 BTU/lb·°F). This is the same equation used for any water-side heat transfer, and it lets you move between load, flow, and range:
- Know the load and the range → solve for required GPM.
- Know the flow and the load → solve for the range.
Worked example: sizing a tower for a chiller
A 300-ton water-cooled chiller needs a cooling tower. Design wet-bulb for the location is 78°F; the chiller wants 85°F condenser water supply, and the condenser flow is 3 GPM/ton.
Step 1 — heat rejection: 300 tons × 15,000 BTU/h = 4,500,000 BTU/h (using the tower ton, which already includes compressor heat).
Step 2 — condenser flow: 300 tons × 3 GPM/ton = 900 GPM.
Step 3 — check range: Range = Heat ÷ (500 × GPM) = 4,500,000 ÷ (500 × 900) = 10°F — the standard 95→85°F range.
Step 4 — approach: 85°F cold water − 78°F wet-bulb = 7°F approach.
So we specify a tower for 900 GPM, 10°F range, 7°F approach at 78°F wet-bulb — a complete, manufacturer-ready selection. Give a tower vendor those four numbers and they can pick a model. The cooling tower sizing calculator produces them from the load and design conditions, and you can cross-check the chiller side with the chiller tonnage calculator.
Wet-bulb is everything
Because a tower cannot cool below the ambient wet-bulb, the design wet-bulb temperature for your location is the most important environmental input. Use the ASHRAE 1% or 0.4% design wet-bulb for the site, not the dry-bulb and not a national average. A tower sized for a 75°F wet-bulb will fail to hit 85°F water on a muggy 80°F wet-bulb afternoon — the approach would have to go negative, which is impossible. Humid climates (Gulf Coast, Southeast) have high design wet-bulbs and need larger towers or warmer condenser water than dry climates at the same tonnage.
Don’t forget water losses
A cooling tower continuously loses water three ways, and the make-up must be sized for all of them:
- Evaporation — the useful loss that does the cooling, roughly 1% of flow per 10°F of range (about 1.8 GPM per 100 GPM at a 10°F range).
- Drift — tiny droplets carried out in the air stream; minimized by drift eliminators, typically well under 0.1% of flow.
- Blowdown (bleed) — water deliberately drained to keep dissolved solids from concentrating as pure water evaporates away; set by the “cycles of concentration” of the water treatment.
Total make-up is typically 2–4% of the circulating flow. Water treatment to control scale, corrosion, and biological growth (including Legionella) is essential and code-regulated in many jurisdictions.
Tower types: which one you’re sizing
The sizing numbers are the same, but the tower type affects selection and footprint:
| Type | How it works | Best for |
|---|---|---|
| Open (direct) | Condenser water contacts the air directly over the fill | Lowest first cost and best thermal performance; water needs treatment because it’s exposed |
| Closed-circuit (indirect) | Process water stays in a coil; sprayed water evaporates over the coil | Keeps the condenser loop clean and closed; costlier and slightly less efficient |
| Counterflow | Air moves up, water falls down — opposite directions | Compact footprint, good performance, less prone to sunlight/algae in the basin |
| Crossflow | Air moves horizontally across falling water | Lower fan power and easy maintenance access; larger footprint |
Most large HVAC chiller plants use open counterflow or crossflow induced-draft towers; data centers and industrial processes that must protect a clean loop often choose closed-circuit towers. The choice affects fan energy, footprint, and water treatment, but the four sizing numbers — flow, range, approach, and wet-bulb — still define the duty either way.
Free cooling and part-load
A well-sized tower does more than reject heat at design conditions — it enables waterside economizer (free cooling) in mild weather. When the outdoor wet-bulb drops low enough, the tower can produce condenser water cold enough to cool the building directly through a heat exchanger, letting the chiller compressor shut off entirely. This is a major energy saver in climates with cool seasons, and it’s another reason approach and wet-bulb matter: a tower with a tight approach reaches free-cooling temperatures for more hours of the year. Variable-speed tower fans further cut energy at part load, since fan power drops sharply as the fans slow to match the reduced heat rejection on cooler days.
Common cooling tower sizing mistakes
- Sizing to refrigeration tons, not heat rejection. Forgetting the ~25% compressor heat undersizes the tower.
- Using dry-bulb instead of wet-bulb. The wet-bulb sets the achievable cold-water temperature.
- Specifying too small an approach without paying for it. A tight approach means a much larger tower.
- Ignoring local design wet-bulb. Humid sites need bigger towers for the same tonnage.
- Undersizing make-up water and treatment. Evaporation, drift, and blowdown all draw make-up; treatment prevents scale and Legionella.
- Forgetting freeze protection. Cold-climate towers need basin heaters and controls.
Standards and references
| Reference | What it covers |
|---|---|
| CTI (Cooling Technology Institute) | Tower performance rating & testing standards |
| ASHRAE Handbook — HVAC Systems & Equipment | Cooling tower theory, sizing, water treatment |
| ASHRAE 188 / CTI guidance | Legionella risk management for cooling towers |
| Key constants | Tower ton = 15,000 BTU/h; 3 GPM/ton; Heat = 500 × GPM × Range |
The bottom line
Cooling tower sizing comes down to four numbers: flow (GPM), range, approach, and the design wet-bulb — all built on the nominal tower ton of 15,000 BTU/h, which already accounts for the compressor heat that makes a tower larger than its chiller’s cooling tonnage. Use your local design wet-bulb, pick an economical approach, and size make-up water and treatment for evaporation, drift, and blowdown. Generate a manufacturer-ready selection with the cooling tower sizing calculator, then confirm the model and water treatment with the tower vendor and a licensed mechanical engineer.
Frequently asked questions
How do you size a cooling tower?
Size it on heat rejection, not just chiller tonnage, using four numbers: the flow rate (GPM), the range (hot minus cold water temperature), the approach (cold water minus ambient wet-bulb), and the design wet-bulb. Heat rejected equals 500 times GPM times range. Convert to nominal cooling-tower tons at 15,000 BTU/h per ton, based on 3 GPM cooled from 95 to 85 F at a 78 F wet-bulb. Give a vendor the flow, range, approach, and wet-bulb to select a model.
What is range and approach in a cooling tower?
Range is the temperature drop the tower produces — the hot water temperature entering minus the cold water temperature leaving, typically 10 F. Approach is how close the cold water gets to the ambient wet-bulb — the cold water temperature minus the wet-bulb, typically about 7 F. Range is set by the load and flow; approach measures tower performance and cost, because a smaller approach requires a much larger, more expensive tower.
Why is a cooling tower ton bigger than a refrigeration ton?
A nominal cooling-tower ton is 15,000 BTU/h, while a refrigeration ton is 12,000 BTU/h. The tower must reject not only the building's cooling load but also the heat of compression added by the chiller's compressor — roughly an extra 25%. That extra 3,000 BTU/h represents the compressor's work, which is why a tower is sized for more heat rejection than the chiller's cooling tonnage.
Why does the tower depend on wet-bulb temperature?
Because a cooling tower cools by evaporation, and the wet-bulb temperature measures how much evaporative cooling the air can accept. The tower can never cool the water below the ambient wet-bulb, so the design wet-bulb sets the achievable cold-water temperature. You must use the local ASHRAE design wet-bulb, not the dry-bulb; humid climates have high wet-bulbs and need larger towers or warmer condenser water for the same tonnage.
How much condenser water flow does a cooling tower need?
The standard design flow is 3 GPM per nominal ton, which corresponds to a 10 F range (since heat rejected equals 500 times GPM times range, and 500 x 3 x 10 = 15,000 BTU/h per ton). So a 300-ton tower circulates about 900 GPM. Some low-flow designs use a larger range with less water, but 3 GPM per ton at a 10 F range is the classic baseline for selection.
How much water does a cooling tower lose?
A tower loses water three ways: evaporation (the useful loss that does the cooling, roughly 1% of flow per 10 F of range), drift (tiny droplets carried out in the air, minimized by drift eliminators to well under 0.1%), and blowdown or bleed (water drained to keep dissolved solids from concentrating). Total make-up is typically 2 to 4% of circulating flow, and water treatment is essential to control scale, corrosion, and Legionella.
What is a nominal cooling tower ton?
A nominal cooling-tower ton is a standardized rating: 3 GPM of water cooled from 95 F to 85 F at a 78 F entering wet-bulb, which rejects 15,000 BTU/h. It provides a common basis for comparing towers. Because it already includes the roughly 25% compressor-heat allowance, a chiller and tower of the same nominal tonnage pair correctly at those standard conditions — but real capacity changes if the wet-bulb, range, or approach differ.