9.5 Cooling Towers, Condenser Water Loops & Legionella Water Treatment
Key Takeaways
- Cooling towers reject heat through evaporation (~1,000 BTU/lb of water evaporated), rejecting ~15,000 BTU/hr per ton of chiller capacity at a standard flow rate of 3.0 GPM/ton.
- Cooling tower Range is the temperature difference between entering hot water and leaving cold water (typically 10°F: 95°F -> 85°F); Approach is the difference between leaving cold water and entering ambient wet-bulb temperature (typically 7°F–10°F).
- Cycles of Concentration (CoC) measure mineral accumulation in the sump; Arizona's hard water requires automated conductivity blowdown to maintain CoC between 3.0 and 5.0 to prevent scale fouling.
- ASHRAE Standard 188 mandates water management plans to control Legionella pneumophila bacteria, requiring dual oxidizing and non-oxidizing biocide treatments, high-efficiency drift eliminators (<0.005%), and regular sump cleanings.
Cooling Towers, Condenser Loops & Water Treatment
Cooling towers are specialized direct-contact evaporative heat exchangers that reject heat from commercial water-cooled chiller condensers into the outdoor atmosphere. In the arid desert climate of Arizona, cooling towers take advantage of low ambient wet-bulb temperatures to produce cold condenser water far below outdoor dry-bulb temperatures.
Thermodynamic Principles of Evaporative Heat Rejection
A cooling tower cools water primarily through latent heat of vaporization: as warm condenser water cascades across the tower fill pack, a small portion (approximately 1% to 1.5%) evaporates into the moving air stream. Each pound of water evaporated absorbs approximately 1,000 BTU of latent heat from the remaining water stream.
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| CONDENSER HEAT REJECTION THERMODYNAMICS |
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| PARAMETER | VALUE / CALCULATION |
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| Chiller Evaporator Heat Load | 12,000 BTU/hr per Ton of refrigeration |
| Compressor Heat of Compression | ~3,000 BTU/hr per Ton (Work of compression energy) |
| Total Condenser Heat Rejection | ~15,000 BTU/hr per Ton (12,000 + 3,000 = 15,000 BTU/hr) |
| Design Condenser Flow Rate | 3.0 GPM / Ton (@ 10°F ΔT: 15,000 / [500 × 10] = 3.0 GPM) |
| Standard Temperature Profile | 95°F Hot Condenser Water In -> 85°F Cold Condenser Water Out|
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Critical Cooling Tower Metrics: Range and Approach
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| COOLING TOWER THERMAL METRICS |
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| METRIC | DEFINITION & FORMULA | TYPICAL DESIGN VALUES |
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| RANGE | Temperature drop across the tower: | 10.0°F |
| | Range = T_hot_in - T_cold_out | (95°F In - 85°F Out = 10°F) |
| | | |
| APPROACH | Difference between leaving cold water and ambient | 7.0°F to 10.0°F |
| | wet-bulb: Approach = T_cold_out - T_ambient_WB | (85°F Out - 76°F WB = 9°F) |
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The Arizona Desert Wet-Bulb Advantage: In Phoenix, when the summer dry-bulb temperature is 115°F, the design wet-bulb temperature is typically only 72°F to 76°F WB due to low atmospheric humidity. With a 9°F approach, the cooling tower easily produces 85°F cold condenser water, allowing the water-cooled chiller to operate at peak efficiency despite the extreme exterior heatwave.
Mechanical Configurations: Draft & Airflow Topologies
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| COOLING TOWER CONFIGURATION COMPARISON |
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| DESIGN FEATURE | INDUCED DRAFT | FORCED DRAFT |
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| Fan Location | Top discharge cowl (Pulls air) | Bottom / side air intake (Pushes air) |
| Air Velocity Profile | High discharge velocity (No recir| Lower velocity; susceptible to recirc |
| Fan Power Efficiency | Higher aerodynamic efficiency | Lower efficiency (Higher static loss) |
| Motor Environment | In moist discharge air stream | In dry ambient entering air stream |
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| FLOW PATTERN | CROSSFLOW | COUNTERFLOW |
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| Air/Water Direction | Air moves horizontally across | Air moves vertically upward directly |
| | vertically falling water | against vertically falling water |
| Water Distribution | Gravity-feed open top basins | Pressurized pipe headers & spray nozzle|
| Physical Footprint | Larger plan footprint; lower ht | Compact footprint; taller height |
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Condenser Water Chemistry & Cycles of Concentration (CoC)
As pure water (H2O) evaporates from the tower, dissolved mineral salts—including calcium carbonate (CaCO3), magnesium, silica, and chlorides—are left behind in the sump basin, concentrating over time.
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| TOWER WATER BALANCE & CONCENTRATION FORMULAS |
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| PARAMETER / METRIC | MATHEMATICAL FORMULA |
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| Cycles of Concentration | CoC = Tower Water Conductivity / Makeup Water Conductivity |
| (CoC) | CoC = [Chlorides in Tower Water] / [Chlorides in Makeup Water] |
| Evaporation Rate (E) | E = GPM_circulating × 0.0008 × Range (°F) |
| Blowdown / Bleed Rate (B) | B = E / (CoC - 1) |
| Total Makeup Water (M) | M = E + B + Drift (D) |
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Scale Formation in Arizona Municipal Water
Arizona municipal water sources (e.g., Central Arizona Project / Salt River Project water in Phoenix) are extremely hard, with total dissolved solids (TDS) exceeding 500 to 800 ppm and calcium hardness exceeding 250 to 350 ppm.
- Scaling Consequences: When Cycles of Concentration exceed critical saturation indices (Langelier Saturation Index LSI > +1.5), calcium carbonate rapidly precipitates onto the hot copper tubes of the chiller condenser. A scale layer just 1/32 in. thick increases compressor energy consumption by 15% to 20% and leads to high head-pressure trips.
- Automated Conductivity Blowdown: An automated conductivity controller continuously samples sump water. When electrical conductivity exceeds the setpoint (e.g., 1,800 to 2,200 μS/cm, corresponding to 3.0 to 4.0 CoC), the controller opens a motorized bleed valve to dump mineral-laden water to the sewer while fresh makeup water is added.
Biological Control & Legionella Prevention (ASHRAE Standard 188)
Cooling towers provide an ideal incubator for microbiological pathogens, most notably Legionella pneumophila, the causative bacterium of Legionnaires' disease (a severe, potentially fatal bacterial pneumonia) and Pontiac fever.
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| LEGIONELLA BACTERIOLOGICAL GROWTH PARAMETERS |
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| TEMPERATURE RANGE | BACTERIAL BEHAVIOR & CELLULAR STATUS |
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| Below 68°F (20°C) | Bacteria dormant (survives but does not multiply) |
| 68°F to 122°F (20°C to 50°C) | ACTIVE GROWTH & MULTIPLICATION ZONE |
| 95°F to 115°F (35°C to 46°C) | OPTIMUM PROLIFERATION ZONE (Matches Cooling Tower Basins!) |
| Above 140°F (60°C) | Bacteria dies within 2 minutes (Disinfection threshold) |
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Transmission Vector
Legionella bacteria do not spread person-to-person or through drinking water; transmission occurs exclusively via inhalation of aerosolized water mists or respirable droplets (less than 5 μm in diameter) generated by cooling tower fans, drift, or evaporative condensers.
ASHRAE Standard 188 Mandates & Water Management Program (WMP)
Under ASHRAE Standard 188 (Legionellosis: Risk Management for Building Water Systems) and CDC guidelines, commercial HVAC contractors and building operators must implement a formal Water Management Program:
- Dual Biocide Treatment Program:
- Oxidizing Biocides: Continuous or automated slug dosing of chlorine (sodium hypochlorite), stabilized bromine, or chlorine dioxide. Oxidizers rapidly penetrate and disrupt microbial cell walls and biofilm. Bromine is preferred in cooling towers because it remains an active, potent biocide at alkaline pH levels (8.0 to 9.0) typical of concentrated tower water, whereas chlorine effectiveness drops drastically above pH 7.5.
- Non-Oxidizing Biocides: Alternated weekly or bi-weekly with non-oxidizing compounds (e.g., isothiazolone, glutaraldehyde, DBNPA, or polymeric quaternary ammonium). Alternating biocides prevents bacteria from developing cellular immunity.
- High-Efficiency Drift Eliminators:
- Mechanical PVC multi-pass chevron or cellular drift eliminators mounted across the fan discharge capture water droplets from the exiting air stream.
- Performance Standard: Must restrict drift loss to less than 0.005% (and down to 0.001% on modern installations) of total circulating condenser water flow rate.
- Routine Cleaning & Disinfection Protocol:
- Physical semi-annual draining, mechanical pressure-washing, and sediment removal from tower basins and fill packs.
- Emergency Hyperhalogenation (Shock Chlorination): In the event of elevated Legionella counts (>1,000 CFU/mL) or system startup after seasonal stagnation, the tower must be dosed to maintain 10 to 50 ppm free residual chlorine with a dispersant for 6 to 24 hours prior to draining and refilling.
In a cooling tower operating with a water temperature entering at 95°F, water leaving at 85°F, and an ambient wet-bulb temperature of 76°F, what are the Range and Approach?
According to ASHRAE Standard 188, what is the primary transmission mode of Legionella pneumophila from commercial cooling towers to humans?
Why is bromine generally preferred over chlorine as the primary oxidizing biocide in commercial cooling tower water treatment programs?