Mechanical Draft Tower Design
Design wet bulb: use 1% or 2.5% ASHRAE summer design value
Typical: 3-7. Higher = less blowdown but more scaling risk.
Understand Merkel equation, tower types, water treatment, and thermal design principles
Approach temperature is the difference between the cold water temperature leaving the tower and the ambient wet bulb temperature. A typical approach is 7-10°F. Smaller approach (5-7°F) requires a larger, more expensive tower. Below 5°F approach is generally impractical and uneconomical.
Makeup water equals evaporation loss plus blowdown plus drift loss. Evaporation is approximately 1% of circulation rate per 10°F of cooling range. Blowdown equals evaporation divided by (cycles of concentration minus 1). Drift is typically 0.005% of circulation rate for modern towers with drift eliminators.
L/G ratio is the liquid-to-gas mass flow ratio (lb water / lb dry air). It typically ranges from 0.75 to 1.5 for mechanical draft towers. Lower L/G means more air per unit of water, requiring larger fans but a shorter tower. Higher L/G means less air, requiring a taller tower with more fill volume.
KaV/L is the Merkel number or tower characteristic that quantifies cooling tower thermal performance. It represents the number of transfer units (NTU) and is calculated by integrating dT/(h_s - h_a) over the cooling range using the Chebyshev 4-point numerical method. Higher KaV/L means more difficult cooling duty.