TL;DR

Reduced capacity is usually scaled fill blocking air-water contact, low water flow, or a fan running below design speed. Inspect fill, measure flow, and verify fan RPM.

What you might see

leaving water temperature above design point
condenser supply water warmer than normal
reduced approach temperature to wet bulb
process cooling inadequate under normal load

Likely causes

Tap to mark as ruled out

Scaled or fouled fill media blocking air-water contact surface
Low circulating water flow from a worn pump, throttled valve, or clogged strainer
Fan running below design speed from belt slip, worn sheaves, or motor underspeed
Clogged spray nozzles causing uneven water distribution over fill

Required tools

Calibrated thermometer or thermocouple
Wet-bulb thermometer or sling psychrometer
Tachometer (contact or non-contact)
LOTO kit
Nozzle pick or small wire brush
Flow meter or pump curve data

Safety first

Lock out the fan motor before entering the tower plenum. Fan blades are not guarded inside the tower.
Cooling tower basins may contain biological hazards including Legionella. Wear gloves and eye protection when working near basin water.

Procedure

9 steps · ~60 min

1
Record the entering and leaving water temperatures and the ambient wet-bulb temperature. Compare them to the original tower design conditions.
2
Lock out the fan motor and circulating pump before any physical inspection.
Warning: Rotating fan blades cause severe injury. Lock out before entering the tower plenum.
3
Inspect the fill media from above and below. Look for scale deposits, biological growth, or collapsed sections that restrict air flow.[1]
4
Measure the circulating water flow rate at the pump discharge. Compare to the design GPM on the Marley NC nameplate.
5
Check the inlet strainer on the pump suction for debris. Clean it if partially blocked.
6
Inspect each spray nozzle or distribution basin orifice for scale or debris. Clear blocked nozzles with a brush or nozzle pick.
7
Re-energize the fan and measure the fan RPM with a tachometer. Compare to the design RPM on the fan curve.
8
If fan speed is low, check belt tension and sheave wear. Adjust or replace belt as needed.[1]
9
If fill is heavily scaled, schedule a chemical descaling or fill replacement during the next planned shutdown.

Sources

Marley NC Cooling Tower User Manual, Marley Cooling (SPX Cooling Technologies)
Marley NC Cooling Tower Installation, Operation and Maintenance manual, performance and fill inspection procedures (general)
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