STRATEGIES TO RAISE THE OFF-DESIGN EFFICIENCY OF CHILLER MACHINES

Most HVAC systems in large buildings are equipped with centrifugal chi llers which are typically designed for maximum efficiency at 70% to 80 % of their full-load. But, below about 30% full-load, their part-load efficiency starts to deteriorate rapidly (increasing kilowatts per ton ). For older centrifugal chillers, this rapid drop may start at even h igher part-load, rendering their operation at low part-load undesirabl e. In many cases chillers are over-sized, forcing operation below 50% of the full-load most of the time. Furthermore, the field performance of most chiller machines is generally not documented, and there is no tradition of recording chiller performance history. There is growing i ncentive-driven interest in chiller monitoring. But, field visits, dis cussions with HVAC engineers and opinions from manufacturers have prov en that virtually no chiller systems are currently monitored for kilow atts per ton. Only recently has field data gathering been initiated, a nd part-load performance of chillers in the field started drawing seri ous attention. The problem, however, still remains poorly disclosed to the end-user. As a result, chiller machines actually operate at much lower efficiency than the design values. There are opportunities for p rofoundly improving their field performance. Part-load operation is pa rticularly poor for sites with single centrifugal chillers. This resea rch is an appraisal of strategies for improving the part-load performa nce of chiller systems. A review of simulation results and field data indicate that chiller machines operate at higher kilowatts per ton tha n allowed for by current technology. Energy consumption of various chi ller types and configurations for a generic building indicate that the re are possibilities for lowering chiller energy consumption, and thes e opportunities can be harvested with simple measures.