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Chiller plant sizing fundamentals
Chiller plant sizing starts with the building peak cooling load but doesn't end there. Distribution losses from piping, pumps, and AHU fans add 5-10% to what the chiller must actually produce. Diversity reduces the required plant capacity below the sum of zone peaks. Future growth adds capacity for expansion. The result is the net plant tonnage required.
Single vs. multiple chillers
A single large chiller is simpler and less expensive than multiple smaller ones. Multiple chillers provide redundancy and better part-load efficiency. At 50% load, 2 chillers running at 50% each is more efficient than 1 chiller at 50% load (centrifugal chillers are most efficient at 70-80% load). For critical facilities, N+1 redundancy means installing one extra chiller so the plant can handle full load if any one chiller fails.
kW/ton efficiency and ASHRAE 90.1
ASHRAE 90.1 sets minimum chiller efficiency in kW/ton (full load and IPLV). A centrifugal chiller over 300 tons must achieve at most 0.634 kW/ton full load and 0.368 kW/ton IPLV. Modern high-efficiency centrifugal chillers reach 0.40-0.50 kW/ton full load and 0.25-0.35 kW/ton IPLV. The difference in annual electricity cost between a minimum-code and high-efficiency 200-ton chiller running 2,000 hours per year at $0.13/kWh is roughly $7,000-12,000/year.
Frequently Asked Questions
Chiller plant capacity is calculated from the total building cooling load plus system losses: Q_chiller = (sensible cooling load + latent cooling load + pump heat + chiller auxiliaries) / COP. The sensible and latent loads come from a full building cooling load calculation per ASHRAE. Pump heat and auxiliary loads typically add 5-10% to the cooling load. The chiller is then selected to handle the peak load with 10-15% safety margin per ASHRAE 90.1 guidelines.
Modern centrifugal chillers achieve COP of 5.0-7.0 at full load and IPLV ratings of 6.0-9.0. Screw chillers typically achieve COP of 4.0-5.5. Reciprocating and scroll chillers range from 3.0-4.5 COP. At part load, variable-speed centrifugal chillers can achieve much higher effective COP because they operate at reduced lift. ASHRAE 90.1 minimum efficiency requirements for water-cooled centrifugal chillers above 300 tons is COP 6.10 at full load.