In Houston and other U.S. cities, a new health-care-delivery model is emerging: Large centrally located urban hospitals are giving way to smaller community-based facilities intended to attract patients by being more accessible. ASHRAE has supported this movement through publication of “Advanced Energy Design Guide for Small Hospitals and Healthcare Facilities: 30% Energy Savings.”1 One of the guide’s recommendations to reduce operational costs and greenhouse-gas emissions is to utilize air-cooled chillers with efficiencies of at least 10.0 EER/11.5 IPLV.

The Discovery Process

Recent technology innovations, combined with advances in manufacturing practices, have resulted in considerable improvements in air-cooled-chiller performance, particularly in terms of efficiency, sound, and footprint. What has changed?

Mono-rotor screw compressor. Enhanced component tolerances, less oil in circulation, the elimination of metal-to-metal sealing surfaces, and balanced component radial forces contribute to a design maximizing compression efficiency and reducing mechanical vibration and noise, leading to improved performance and reliability.

Factory-mounted variable-frequency drives (VFDs). VFDs long have been credited with helping to reduce the cost of operating both variable-torque (dynamic) and constant-torque (positive displacement) prime movers. With a chiller, VFDs help to match required refrigeration capacity to compressor output, allowing reductions in motor speed that take advantage of accompanying reductions in motor horsepower. One advantage of the screw compressor (positive displacement) is the ability to deliver high torque (lift) at very low speeds. This results in an operational window wider than that of centrifugal chillers, the avoidance of surge, and the minimization of motor horsepower at low turndowns.

Factory-installed refrigerant economizer. Used on centrifugal chillers for many years, refrigerant economizers recently became available on some screw chillers. Through the addition of a brazed-plate heat exchanger and thermostatic expansion valve on each refrigerant circuit, refrigerant is both subcooled for additional capacity and diverted to the interstage of the compressor for increased efficiency. The increase in refrigeration capacity has led to the development of large-tonnage screw chillers with remarkably small footprints (Figure 1).