Centrifugal Chiller With Both Approaches Elevated

Approaches

• Evaporator approach = leaving chilled water − evaporating saturation = 44 − 36 = 8°F (design 2°F)
• Condenser approach = condensing saturation − leaving condenser water = 101 − 95 = 6°F (design 2°F)
• Excess lift ≈ (8 − 2) + (6 − 2) = 12°F

Efficiency

kW/ton = 260/400 = 0.65 kW/ton, versus 0.55 rated - an 18% penalty.

Consistency check

The industry rule of thumb is roughly 1.5-2% compressor kW per °F of excess saturated lift. Twelve degrees of excess lift predicts 18-24% penalty; the measured 18% sits at the low end of that band. The kW reading and the two approach readings tell the same story, so this is not a single drifting temperature sensor. A bad sensor would move one approach without moving the kW in the direction and magnitude predicted.

Evaporator approach elevated can come from: fouled evaporator tubes, low refrigerant charge, or low evaporator water flow. Check chilled-water flow (pressure drop across the bundle versus the manufacturer's curve, plus the pump curve and the balance valve position) and the refrigerant liquid level and sight glass. Low charge typically lowers the condenser approach while raising the evaporator approach, which is not what is being seen here.

Condenser approach elevated can come from: fouled condenser tubes, non-condensables, overcharge, or low condenser water flow. Check condenser water flow and tower performance, then:

• Non-condensables: purge run time and history. A purge that runs continuously with a high discharge pressure and normal-to-high subcooling points to air in the machine.
• Overcharge: liquid subcooling well above the manufacturer's expected value (for example above 15-20°F at these conditions) with a high condenser approach.
• Fouling: normal subcooling, no purge activity, normal flows, and a documented history of no mechanical cleaning on an open tower for five years.

Because both approaches are elevated, the leading hypothesis is fouled tubes on both bundles. Overcharge alone would not raise the evaporator approach, and non-condensables alone would not either. Confirm with an eddy-current or brush inspection before spending anything on refrigerant-side work.

Penalty

• Energy: 0.10 kW/ton × 400 tons = 40 kW continuous at this load. At 2,000 equivalent full-load hours: 80,000 kWh/yr × $0.11 ≈ $8,800/yr.
• Demand: 40 kW × roughly $12/kW-month × 12 ≈ $5,800/yr, depending on the tariff.
• Total avoided cost is about $14,000-15,000/yr with the approaches restored.

Decision sequence (cheapest and most diagnostic first)

1. Verify water flows on both bundles and confirm the tower is delivering design condenser water temperature. Low flow mimics fouling exactly and costs nothing to fix.
2. Check refrigerant subcooling and purge run time to rule in or out overcharge and non-condensables.
3. Pull the heads and brush or eddy-current both bundles. A cleaning on a 500-ton machine typically runs $20,000-40,000, so the payback against $14,000/yr is under three years - and it restores capacity the owner is currently buying from a spare machine.
4. Fix the root cause: open towers without effective mechanical cleaning foul regardless of chemistry. Add annual approach trending to the O&M contract and alarm on a 1°F rise in either approach; that is the cheapest reliability measure available.

On replacement: an 18% efficiency penalty does not justify a new chiller. Replacement becomes defensible only if the tube inspection finds erosion, wall loss, or a failed tube sheet, or if the machine cannot be brought back within about 1°F of design approach after cleaning.