Abstract—Air conditioning system is an energy-consuming unit in most buildings. For hospitals, air conditioners are always turned on to provide reliable and continuous service for patients, leading to high amount of energy usage and great expense. In this research, chiller operations in HRH Princess Maha Chakri Sirindhorn Medical Center, Nakhon Nayok, Thailand, were investigated to determine cooling load of chillers and calculate cooling performance. Recommendations were provided to help improve chillers’ efficiency and reduce energy consumption in the future. The system comprised 2 chillers, using centrifugal constant speed compressor, with the cooling capacity of 600 tons of refrigeration (TR). These chillers run simultaneously during the day while only one chiller was operated at night. Data was acquired by using internal monitoring software and external instruments which are ultrasonic flow meter, thermocouple and power meter. Results showed that the operation can be divided into 2 periods for different level of cooling loads, which are peak load hours (8 a.m.-6 p.m., 10 hours) and off-peak hours (6 p.m.-8 a.m., 14 hours). Highest cooling loads of CH1 and CH2 chillers were 478 and 438 TR respectively. The average cooling performance during peak load hours from both chillers were 0.735 and 0.967 kW/TR respectively. This indicates that the performance of the CH1 chiller was better compared with that of the CH2 chiller. In most cases, 600-ton chillers would work on part load condition, where cooling load is lower than cooling capacity, especially during the off-peak period; this makes inefficient energy consumption. Plotting results on pressure-enthalpy diagram suggested that the efficiency of chiller could be improved by lowering cooling water temperature, cleaning evaporator and condenser. Also, more energy could be saved when considering the use of variable speed drive (VSD) chiller over constant speed chiller.