Understanding the Cooling Tower Key Formula: A Complete Calculation Guide
As an engineer or industry professional, you know that cooling tower efficiency is not just about physical size, but also about precise calculations. Understanding the basic cooling tower formula is essential for designing, operating, and optimising your cooling system.
At PT Jaya Sakti Internusa and BHL Cooling Tower, we believe that knowledge is power.
Therefore, we present a comprehensive guide to key cooling tower formulas that will help you understand every important aspect of unit operation.
1. Cooling Tower Capacity: Tonnes of Refrigeration (TR)
Cooling tower capacity is often expressed in tonnes of refrigeration (TR), although this is often associated with chiller load. For cooling towers alone, we often calculate the heat load being dissipated.
1 Ton Refrigeration (TR) = 12,000 BTU/hour = 3024 kCal/hour = 3,517 kW
The total heat load discharged by the cooling tower (Q) can be calculated with:
Q=m×Cp×ΔT
Where:
Q = Heat Load (kCal/hour or BTU/hour)
m = Water mass (kg/hour or lb/hour)
Cp = Specific heat capacity of water (approximately 1 kCal/kg.∘C or 1 BTU/lb.∘F)
ΔT = Range (water temperature drop, Tin − Tout)
Then, to convert to TR cooling tower: TR Cooling Tower = Heat Load (BTU/hour) / 15,000 BTU/hour (Note: 15,000 BTU/hour is often used for cooling towers because it takes into account the latent heat load from compression)
2. Range: How Much Heat is Removed
Range is the main indicator of a cooling tower's ability to remove heat.
Range= Thot−Tcold
Where:
Thot = Hot water temperature entering the cooling tower (∘C or ∘F)
Tcold = Cold water temperature exiting the cooling tower (∘C or ∘F)
3. Approach: Efficiency Approaching the Thermodynamic Limit
Approach indicates how close the cooling tower can cool the water to the wet-bulb temperature of the air.
Approach = Tcold − Twet-bulb
Where:
Tcold = Temperature of cold water leaving the cooling tower (∘C or ∘F)
Twet-bulb = Wet-bulb temperature of the air (∘C or ∘F)
A lower approach (< 5∘C) indicates excellent cooling tower performance.
4. Evaporation Loss (E): Water Loss Due to Cooling
This is the unavoidable loss of water due to the evaporative cooling process.
E=(Flow Rate×Range×Evaporation Factor)
The Evaporation Factor is approximately 0.00085 GPM/TR or 0.00085% of the flow rate per degree Fahrenheit Range.
Simply put, for every 10∘F Range, the evaporation loss is approximately 1% of the circulation flow rate.
If in m³/hour units: E(m³/hour)≈0.0017×Flow Rate (m³/hour)×Range (∘C)
5. Drift Loss (D): Water Loss Due to Droplets
Drift loss is water in the form of droplets carried away by air flow. This can be minimised with an efficient drift eliminator.
D=Flow Rate×Drift Factor
Drift Factor: Usually very small, around 0.0001 (0.01%) to 0.0005 (0.05%) for modern cooling towers with good drift eliminators.
6. Blowdown (B): Water Disposal for Concentration Control
Blowdown is water that is deliberately discharged to prevent the accumulation of minerals and dissolved solids.
B = E/(COC−1)
Where:
E = Evaporation Loss
COC = Cycles of Concentration
7. Makeup Water (M): Additional Water Requirements
Makeup water is the total amount of water that must be added to the system to replace all losses.
M=E+D+B
Where:
E = Evaporation Loss
D = Drift Loss
B = Blowdown
8. Cycles of Concentration (COC): Water Management Indicator
COC is the ratio between the mineral concentration in cooling tower water and the concentration in makeup water. A higher COC means more efficient water usage (less blowdown).
COC=TDS Concentration in Makeup WaterTDS Concentration in Cooling Tower Water
Or:
COC=(D+B)E
Conclusion
Understanding these formulas is a fundamental step in optimising the performance of your cooling tower. With accurate calculations, you can make better decisions in water treatment, maintenance, and operations to achieve maximum efficiency and cost savings.
PT Jaya Sakti Internusa and BHL Cooling Tower not only provide high-quality products, but also technical expertise to support your operations. Please feel free to contact our team of engineers for further consultation or assistance in analysing your cooling tower needs.