Chiller Plant Sequencing After a Compressor Failure
A 12-story office building has two 400-ton centrifugal chillers, Ch-1 and Ch-2, with a design load of 620 tons on a 95 F design day. Ch-2 tripped on a high-discharge-pressure fault at 2:10 PM and cannot be reset; the building is at 78 F and rising, and the outdoor air is 96 F. The lead-lag controller is set to run Ch-1 as lead with a 45 F supply-water setpoint and a 10-minute stage-up delay. The only available portable unit is a 150-ton air-cooled rental chiller with a 5 F approach and a maximum condenser ambient of 105 F. The building manager wants to keep all tenant floors occupied until 6 PM. You have 30 minutes of on-site technician labor before overtime rates begin.
1. Evaluate whether the rental chiller can meaningfully offset the loss of Ch-2, and state what you would do with the remaining on-site labor before overtime begins.
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Capacity gap. Design load is 620 tons; with Ch-2 down the plant can deliver about 400 tons from Ch-1, leaving roughly 220 tons of unmet load at design conditions. The rental unit is rated 150 tons at its standard rating condition, so even at full output it covers only about two-thirds of the gap, and derating matters.
Derate the rental. At 96 F ambient the air-cooled condenser is near the top of its envelope. A 5 F approach means the rental's supply water will be about 5 F warmer than the water it returns, and its net capacity falls as ambient rises. Assume a realistic 10-20% derate, so plan on 120-135 tons of usable capacity, not 150.
Conclusion. The rental cannot carry the building alone, but it can slow the temperature rise and protect the most heat-sensitive spaces (server rooms, tenant comfort complaints). Pair it with aggressive demand-side measures: raise the chilled-water setpoint on Ch-1 from 45 F to 48-50 F (each degree of reset adds roughly 2-3% chiller capacity), stage off non-critical loads, and pre-cool where possible.
Use the 30 minutes well. Dispatch the technician to verify Ch-1 is at full load and its condenser water is clean and at design flow, confirm the rental's electrical and piping tie-in point, and document the Ch-2 fault for the morning service call. Do not start overtime on a repair you cannot finish before 6 PM; instead, authorize overtime only if the rental tie-in can be completed and commissioned within the window.
2. The building manager insists on keeping all floors occupied until 6 PM. What operational and communication actions would you take, and what criteria would trigger partial evacuation or floor shutdown?
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Set explicit thresholds. Define a trigger temperature (for example, 80-82 F in occupied space) and a rate-of-rise trigger (more than 1 F per hour sustained). At 78 F and rising with 96 F outdoor air, the building is already close to the first threshold.
Operational actions.
- Reset Ch-1 supply water to 48-50 F to gain capacity.
- Reduce ventilation to minimum code-required outside air to cut latent load; monitor CO2 to stay within acceptable limits.
- Turn off non-essential lighting and plug loads on the hottest floors.
- Use the rental chiller to serve the highest-priority zones (server rooms, south/west perimeter floors).
- Close blinds and use any available spot cooling.
Communication. Notify tenants early with a clear timeline and a single point of contact. Tell them the building is on a partial-cooling contingency and give them the trigger conditions for floor shutdown so there are no surprises.
Shutdown criteria. If any occupied floor exceeds the trigger temperature and cannot be held, or if Ch-1 shows signs of distress (high condenser pressure, low evaporator pressure, surging), shut down the affected floors rather than risk losing the entire plant. Losing Ch-1 would be far worse than losing two floors.