SIZING A COOLING TOWER

Core Concept

Sizing a cooling tower involves determining its required thermal capacity (heat rejection rate) and physical dimensions to meet specific process cooling needs under design ambient conditions, while balancing capital and operating costs.

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1. KEY INPUTS FOR SIZING (Design Parameters)

A. Thermal Load (Heat to be Rejected)

• Primary Input. Determines the tower’s capacity.
• Sources:
  • From Chiller Condenser: Q (kW or tons) = Chiller Capacity × (1/COP + 1) (approx.)
  • From Process: Direct measurement or calculation:
    Q = m × Cp × ΔT
    where:
    m = Water flow rate (kg/s or gpm)
    Cp = Specific heat of water (~4.18 kJ/kg·°C or 1 BTU/lb·°F)
    ΔT = Temperature rise in process (Range)
• Units:
  • Refrigeration Tons (RT): 1 RT = 12,000 BTU/hr = 3.516 kW
  • kW or MW for large industrial.

B. Water Flow Rate

• Often predetermined by process or system design.
• Typical Range: 3 gpm/ton for standard HVAC (~0.054 l/s per kW) to higher flows for industrial processes.

C. Design Wet-Bulb Temperature (Design WBT)

• The most critical environmental factor.
• Based on historical weather data for the location (e.g., 0.4%, 1%, or 2.5% annual exceedance values).
  • 0.4% WBT: Temperature exceeded only 0.4% of hours in a year (35 hrs). More conservative, larger tower.
  • 2.5% WBT: Exceeded 2.5% of hours (219 hrs). More common, smaller tower.
• Selection impacts cost significantly. A 1°C lower design WBT can increase tower size 10-15%.

D. Water Temperatures

1. Hot Water Temperature (HWT): Temperature from process/condenser to tower inlet.
   Typical HVAC: 35°C (95°F)
   Industrial: Can be 45-50°C+.
2. Cold Water Temperature (CWT): Required temperature returning to process.
   Typical HVAC: 29.4°C (85°F)
   Industrial: Varies by process need.

E. Key Calculated Parameters

From above inputs:

• Range = HWT – CWT
  Typical HVAC: 5.5°C (10°F)
  Industrial: Can be 8-15°C.
• Approach = CWT – Design WBT
  Typical: 2.8-4.5°C (5-8°F). Closer approach requires larger tower.

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2. SIZING METHODS & CALCULATIONS

Method 1: Using Manufacturer’s Ratings & Selection Software

• Most common method. Use published rating tables or software from vendors (BAC, SPX, Evapco, etc.).
• Process:

1. Input: GPM, HWT, CWT, Design WBT.
   1. Output: Tower Model, Fan Motor HP, Physical Dimensions.
   1. Select the smallest tower that meets or exceeds required capacity at design conditions.

Method 2: Cooling Tower “Ton” Rating

• Definition: 1 Cooling Tower Ton = Rejection of 15,000 BTU/hr when cooling water from 35°C to 29.4°C (95°F to 85°F) at a Design WBT of 26.7°C (80°F).
• Sizing Formula (Approximate):
  Required Tower Tons = (Actual Heat Load in BTU/hr) / 15,000
  • Then apply correction factors for different approach and WBT using manufacturer’s curves.

Method 3: Psychrometric & Merkel Theory (Engineering Design)

• Uses the Merkel Equation to calculate required KaV/L value (Tower Characteristic).
• KaV/L = f(Range, Approach, WBT)
• This value is used to select fill volume and tower geometry.
• Software/Tools: CTI (Cooling Tower Institute) standards, proprietary sizing programs.

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. PRACTICAL SIZING STEPS

Step 1 – Define Operating Conditions:

• Determine design WBT, HWT, CWT, water flow rate (GPM), heat load.

Step 2 – Calculate Range & Approach:

• Range = HWT – CWT
• Approach = CWT – WBT

Step 3 – Preliminary Selection:

• Use manufacturer’s catalog. Find a model where at the design WBT and given GPM, the catalogued CWT is ≤ your required CWT.
• Example Selection Table Snippet (Induced Draft, Counter flow):