Cooling Tower Calculator
Calculate cooling tower range, approach, and effectiveness from water and air conditions. Enter values for instant results with step-by-step formulas.
Formula
Effectiveness = Range / (Range + Approach) x 100
Where Range equals hot water temperature minus cold water temperature (actual cooling), and Approach equals cold water temperature minus wet bulb temperature (proximity to theoretical limit). Heat rejection in BTU/hr equals flow rate (GPM) times 500 times Range.
Worked Examples
Example 1: HVAC Chiller Plant Cooling Tower
Problem: An HVAC system has 1,000 GPM flow, hot water at 95F, cold water at 85F, and 78F wet bulb temperature. Calculate range, approach, effectiveness, and water consumption.
Solution: Range = 95 - 85 = 10 F\nApproach = 85 - 78 = 7 F\nEffectiveness = 10 / (10 + 7) x 100 = 58.8%\n\nHeat rejection = 1,000 x 500 x 10 = 5,000,000 BTU/hr = 416.7 tons\nEvaporation = 1,000 x 10 x 0.001 = 10 GPM\nBlowdown (5 cycles) = 10 / 4 = 2.5 GPM\nMakeup = 10 + 0.2 + 2.5 = 12.7 GPM
Result: Range: 10F | Approach: 7F | Effectiveness: 58.8% | Makeup: 12.7 GPM
Example 2: Industrial Process Cooling Tower
Problem: A refinery cooling tower handles 5,000 GPM with hot water at 120F cooled to 90F. Wet bulb temperature is 80F. Calculate heat rejection and water requirements.
Solution: Range = 120 - 90 = 30 F\nApproach = 90 - 80 = 10 F\nEffectiveness = 30 / (30 + 10) x 100 = 75%\n\nHeat rejection = 5,000 x 500 x 30 = 75,000,000 BTU/hr = 6,250 tons\nEvaporation = 5,000 x 30 x 0.001 = 150 GPM\nBlowdown (5 cycles) = 150 / 4 = 37.5 GPM\nMakeup = 150 + 1.0 + 37.5 = 188.5 GPM
Result: Range: 30F | Heat: 6,250 tons | Effectiveness: 75% | Makeup: 188.5 GPM
Frequently Asked Questions
What are cooling tower range and approach, and why are they important?
Range is the temperature difference between the hot water entering the cooling tower and the cold water leaving, measured in degrees Fahrenheit or Celsius. It represents the actual cooling achieved by the tower. Approach is the temperature difference between the cold water leaving the tower and the ambient wet bulb temperature, representing how close the tower can cool the water toward the theoretical minimum (wet bulb temperature). A smaller approach indicates a more effective tower but requires exponentially more tower capacity to achieve. Typical approach values range from 5 to 10 degrees Fahrenheit for well-designed towers. Together, range and approach define the cooling tower performance and are the primary parameters used for tower selection and sizing.
How does wet bulb temperature affect cooling tower performance?
Wet bulb temperature is the single most important ambient condition affecting cooling tower performance because it represents the lowest temperature to which water can theoretically be cooled by evaporation. As wet bulb temperature increases, the driving force for evaporative cooling decreases, and the tower produces warmer cold water. Conversely, lower wet bulb temperatures allow better cooling performance. The wet bulb temperature depends on both the dry bulb temperature and the relative humidity of the ambient air. In arid climates with low humidity, the wet bulb temperature is significantly below the dry bulb temperature, making cooling towers very effective. In humid climates, the wet bulb and dry bulb temperatures are close together, reducing tower effectiveness substantially.
What is cooling tower effectiveness and how is it calculated?
Cooling tower effectiveness is a performance metric that expresses the actual cooling achieved as a percentage of the maximum theoretically possible cooling. It is calculated as the range divided by the sum of the range and approach, multiplied by 100. Alternatively, it equals the range divided by the difference between the hot water temperature and the wet bulb temperature. For example, if hot water enters at 105 degrees Fahrenheit, cold water leaves at 85 degrees, and the wet bulb is 78 degrees, the effectiveness is 20 divided by (105 minus 78) times 100, which equals 74.1 percent. Typical cooling tower effectiveness ranges from 60 to 80 percent. Higher effectiveness requires larger and more expensive towers with greater fan power consumption.
How much water does a cooling tower consume through evaporation?
Cooling towers consume water primarily through evaporation, which is the mechanism that actually removes heat from the water. The evaporation rate is approximately 1 percent of the circulation rate for every 10 degrees Fahrenheit of cooling range. For example, a 1,000 GPM system with a 20-degree range evaporates approximately 20 GPM or 2 percent of the circulation rate. In addition to evaporation, water is lost through drift (small droplets carried out by the air stream, typically 0.001 to 0.01 percent with modern drift eliminators) and blowdown (water intentionally discharged to control dissolved solids concentration). Total makeup water requirements are typically 2 to 5 percent of the circulation rate depending on the range and cycles of concentration.
What is the difference between crossflow and counterflow cooling towers?
Crossflow and counterflow are the two primary configurations describing the relative direction of air and water flow through the cooling tower fill. In a crossflow tower, air flows horizontally across the fill while water flows vertically downward by gravity. This design uses gravity-fed distribution basins that are simpler and require less pump head, but the tower has a larger footprint. In a counterflow tower, air flows vertically upward through the fill while water flows downward, creating true counter-current contact that is thermodynamically more efficient. Counterflow towers are typically taller and narrower, require spray nozzle distribution systems with higher pump pressure, but achieve better heat transfer per unit of fill volume. Counterflow designs are generally preferred for large industrial applications.
How do you calculate the required makeup water for a cooling tower?
Makeup water is the total water that must be added to the cooling tower system to replace all water losses. It is the sum of three components: evaporation loss, drift loss, and blowdown. Evaporation loss in GPM equals the circulation rate times the range in degrees Fahrenheit times 0.001 (or approximately 1 percent per 10 degrees of range). Drift loss with modern eliminators is typically 0.001 to 0.005 percent of the circulation rate. Blowdown equals the evaporation rate divided by the cycles of concentration minus 1. The total makeup water is therefore approximately 1.5 to 3 percent of the circulation rate for typical systems. Accurate makeup water estimation is important for water supply planning, water treatment chemical dosing, and operating cost calculations.