Variable Flow and Volume Refrigerant System

efrigerant systems that vary flow and volume (VRF/V) are basically large-capacity versions of ductless multisplit air-conditioning or heat pump systems. In contrast to conventional HVAC systems, they circulate refrigerant directly to multiple evaporator units, rather than using water (in chillers) or air (in ducted DX systems) to achieve heat transfer to the conditioned space. VRF/V systems are extremely flexible and can have a single condensing unit connected to multiple indoor units of varying capacity and configuration. The number of indoor units varies, but a typical manufacturer connects up to 16 indoor units to one condensing unit, or up to 30 indoor units on a single refrigerant circuit supplied by three condensing units. Each condensing unit uses two or three compressors, including an inverter-driven variable-speed compressor. Larger systems combine multiple condensing units to achieve system capacities of up to several hundred tons. VRF/V systems first appeared in Japan in 1982 and are now used throughout the world, with the notable exception of the U.S. Energy Savings Potential VRF/V systems can reduce HVAC energy consumption in at least three ways. They have high part-load efficiencies because they consist of multiple compressors, including variable speed compressors. Consequently, a typical dual compressor system can operate at 21 capacity steps. HVAC systems usually operate at between 30% and 70% of their maximum capacity. VRF/V systems have high efficiencies in this range and, thus, achieve excellent seasonal energy efficiencies. VRF/V systems also provide effective zone control because they have many evaporator units. The system can turn off individual indoor units in locations that do not need cooling or heating while continuing to operate efficiently. Relative to all-air systems, VRF/V systems use less energy to distribute heating and cooling to conditioned spaces. Finally, VRF/V systems can recover heat in buildings that require simultaneous heating and cooling. A three-pipe heat recovery system circulates refrigerant discharged from the evaporators in the cooled space to the zones needing heat and vice versa. The energy savings potential of VRF/V systems varies significantly from one application to another, making it difficult to develop a definitive estimate of energy savings. A VRF/V consumed 38% less energy than a rooftop VAV in one installation; however, the test details needed to determine whether the test was a true “apples-to-apples” comparison were not available. 1 A yearlong hourly simulation compared the performance of a 538-ton (1892 kW) VRF/V to recent screw and centrifugal chillers (2 at 240 tons each [844 kW each]) in a moderate climate. The VRF/V system reduced energy consumption from about 30% in summer to more than 60% in winter. 2 These savings seem unusually high and likely reflect the high part-load efficiency of the VRF/V in a moderate climate. Initial estimates of U.S. energy savings relative to efficient conventional systems range from 5% to 15%, with higher savings in hot, humid climates and lower savings in cold climates due to the primary energy efficiency advantage of gas heating. Rigorous field tests are needed to better establish the energy-savings potential of VRF/V systems.