Cooling Tower Delivering 87°F Cold Water at a 74°F Wet Bulb

Actual performance:

• Range = 95 − 87 = 8°F
• Approach = 87 − 74 = 13°F
• Effectiveness = range/(range + approach) = 8/(8 + 13) = 0.38
• Heat rejected = 3,000 × 500 × 8 = 12.0 MMBtu/h, matching the chiller's stated rejection.

Design performance:

• Range = 10°F, approach = 7°F, effectiveness = 10/17 = 0.59
• Design heat rejection = 3,000 × 500 × 10 = 15.0 MMBtu/h

Attribution test: at the current 74°F wet bulb, a tower still delivering design effectiveness with the design 10°F range would produce approach = 10 × (1 − 0.59)/0.59 = 7.0°F, so cold water would be about 81°F. Even at the actual reduced 8°F range, design effectiveness would give approach = 8 × 0.41/0.59 = 5.6°F and cold water near 79.6°F.

The tower is producing 87°F instead of roughly 80–81°F. A colder wet bulb should have improved performance, and the heat load is 20% below design. Neither the weather nor the load explains the result: the tower has lost effectiveness, from 0.59 to 0.38. Adding cells would add capacity at the same degraded effectiveness per cell; the first task is to find why this tower is not performing.

Interpretation:

• No visible spray from the laterals points to plugged or partially plugged distribution nozzles. Water is then channeled through wetted paths rather than spread uniformly over the fill, so a large fraction of the fill area is dry and contributes no mass transfer. Effectiveness falls sharply even though fan and pump run.
• Basin sediment is consistent with scale and solids carried from the fill and drifting into the basin. Sediment also indicates inadequate blowdown and water treatment, which usually means the fill surfaces are scaled as well; scale on fill is the most common cause of lost tower effectiveness.
• Fan current 5% below design is a smaller clue. Only part of that is explained by the cooler, slightly denser air (denser air raises power, so it does not explain a drop). It more likely indicates slightly reduced airflow from debris on the air inlet screens, partially closed dampers, or fill that is blocked and redistributing air.
• Rising condenser approach is the downstream symptom: colder-than-design condenser water would reduce approach, so the worsening approach is consistent with the tower's failure and possible tube-side fouling.

Diagnostics in order: (1) shut down and inspect the hot-water distribution basin, laterals, and nozzles; (2) inspect fill for scale, mud, and collapsed or channeled sections and check the fill's cleanliness from the air inlet side; (3) measure fan pitch, belt condition, and static pressure drop across the fill/eliminators; (4) verify pump flow and head against the pump curve (a worn pump impeller can pass a meter reading while delivering less than expected at the tower); (5) check water treatment logs, cycles of concentration, and blowdown rate; (6) check the condenser tubes on the chiller side separately.

Do not adjust fan speed or add cells until the distribution and fill are inspected; if fill scaling is the cause, the additional air will not fix the dry-area problem.

Evaluate against the physics. Tower cold-water temperature is bounded by the wet bulb; approach is set by the ratio of air to water flow (L/G), the contact area, and the fill's thermal performance. Effectiveness near 0.38 at design airflow means the tower is not converting the available air into evaporation. Adding cells increases both air and wetted area, so it works - but it is the expensive way to buy something this tower already owns.

Expected result of restoring the tower: with the distribution and fill repaired and design effectiveness of about 0.59, at a 74°F wet bulb and the current 12 MMBtu/h load (8°F range), the tower should return cold water near 74 + 8 × 0.41/0.59 ≈ 79.6°F. Condenser water 5–7°F colder typically improves chiller efficiency by roughly 1.5–2% per °F on the condenser side, so the chiller kW reduction is large relative to the cost of cleaning and re-nozzling.

Recommendation:

1. Repair distribution nozzles and clean or replace scaled fill; restore water treatment and blowdown (target cycles set by the makeup water chemistry, not by convenience).
2. Verify airflow and pump performance.
3. Re-test effectiveness against wet bulb using a short performance test - a single measurement of range, approach, and wet bulb is enough with a flow meter and a calibrated wet-bulb reading.
4. Only after the tower is back near 0.55–0.59 effectiveness should added capacity be considered. If added capacity is still needed at the true design wet bulb of 78°F, then a third cell or a larger fill is justified.

Do not simply raise fan speed. More airflow lowers approach modestly (and raises drift and evaporation) but cannot compensate for dry, scaled fill; the fan current data suggest airflow is already near design. Also confirm the condenser tube side separately, since the 4°F rise in condenser approach may be a chiller issue rather than a tower issue.