Condenser Approach Drift on a Water-Cooled Chiller
Two 800 kW water-cooled centrifugal chillers serve a 600 kW IT load. Design condenser-water supply/return is 29/35°C with a chiller condenser approach of 1.5 K. At 75% load on one chiller, the BMS shows condenser water 32/38°C, approach 5 K, chilled-water 8/14°C, and chiller power 0.68 kW/ton against a 0.58 kW/ton design. Condenser-water flow is 100 m³/h, essentially design. Cooling-tower fans are at 100%; the tower fill is three years old; water treatment logs show cycles of concentration at 5.5 against a target of 4.0, and the last tube inspection was 18 months ago. The site can clean tubes or rebuild the tower fill, but not both this quarter. Wet-bulb is 26°C. The operator wants to increase condenser-water flow to 120 m³/h. The second chiller is off. No vibration or surge alarms.
1. Separate tower performance from chiller fouling using the available numbers.
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At 75% load, chiller cooling = 0.75×800 = 600 kW. Input at 0.68 kW/ton: 600 kW = 170.6 ton; 0.68×170.6 ≈ 116 kW. Heat rejection ≈ 716 kW. With 100 m³/h (27.8 kg/s), range = 716/(4.186×27.8) ≈ 6.2 K, matching 32/38°C. Tower supply of 32°C at 26°C wet-bulb is a 6 K approach; design 29°C at the same wet-bulb implies about 3 K, so the tower is underperforming. The chiller approach of 5 K versus 1.5 K, with design flow, points to tube fouling or reduced heat transfer. Both tower and chiller are degraded; flow is not the main explanation.
2. Estimate the energy penalty and recommend which repair to prioritize this quarter, with justification.
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A 3.5 K approach penalty on a centrifugal chiller is roughly 7–10% chiller power, or about 8–12 kW at this load, before the tower penalty. High cycles of concentration (5.5 vs 4.0) and an overdue tube inspection support scaling or fouling. Clean the tubes and restore water treatment first: fouling is active, can damage tubes, and directly raises condensing temperature. Then rebuild or clean tower fill and verify fan and water distribution. Increasing condenser flow to 120 m³/h would raise pump energy and only slightly reduce range; it does not correct the 5 K approach or the 6 K tower approach.
3. Explain why raising condenser-water flow is not the right primary fix, and list follow-up operating checks.
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Do not increase flow as the primary fix. First verify flow and temperature sensors, then clean condenser tubes, restore cycles to target, and inspect tower fill and nozzles. After tube cleaning, re-test approach at design flow. If tower supply remains above design, rebuild fill or improve tower air and water distribution. Operationally, stage chillers to avoid low-load operation, reset condenser-water temperature with wet-bulb, and trend approach weekly. The second chiller being off is appropriate at 75% load if one chiller can carry it, but confirm N+1 and electrical limits.