Though single-compressor chiller de-signs are common, two-compressor systems with brazed plate heat exchangers can prove reliable and demonstrate improved temperature control and reduced energy use. Two-circuit, two-compressor heat exchangers are gaining acceptance in many applications, including use as liquid chillers, condensers and heat pumps.
Two-compressor chiller designs demonstrate improved temperature control due to compressor cycling that lessens capacity. Also, two-compressor systems provide 50% load backup (figure 1). This allows operational standby and eliminates potential downtime. Lower use of hot gas bypass, used for capacity and temperature control, provides an additional 20% to 60% in savings on annual kW-hr chiller consumption.
Compressors above 20 tons are more expensive than smaller units and often can be difficult to obtain. Because of the two-compressor heat exchanger, chiller manufacturers can utilize 3 to 20 ton hermetic compressors off the shelf to build 3 to 40 ton systems. This means that manufacturers can lower compressor costs while utilizing the latest scroll compressor technology up to 20 tons. Some chiller manufacturers are now building four-compressor chillers with 80-ton capabilities that have exhibited further improvements in reliability and backup capacity.
How It WorksStandard brazed plate heat exchangers typically use a single 316L stainless steel plate that is embossed with a heat transfer surface and copper braze material. The plates are stacked up to 200 plates high and brazed in a vacuum furnace for a solid, rugged leak-tight assembly.
The two-circuit, two-compressor design is different in that two refrigerant circuits and one water circuit are integral to the plate design (figure 2). As the name implies, there are two compressor connections on the front of the unit and water connections out the back. Each compressor has its own independent circuit. In addition, the refrigerant channels are interlaced with water channels in such a way that when just one compressor is operating, 100% of the water is being chilled. Interlaced refrigerant circuits allow improved temperature control with one or two compressor operation, and the package chiller is less prone to freezing due to higher suction temperatures needed at part-load. The heat exchanger's integral thermal well connections also make it possible for precise fluid temperature measurement.
Reliable PerformanceBrazed plate heat exchangers have a history of reliability. Fluid turbulence from the heat transfer surface has a self-cleaning effect on heat exchangers. Self-cleaning action prevents residual buildup. This reduces maintenance and cleaning requirements while improving efficiency. The high heat transfer turbulence of glycols, oils and certain heat transfer fluids allows performance over a wide range of operating conditions.
Typical cooling applications can be as low as -60°F (-51.1°C). Brazed plate systems can be up to 60% smaller than a comparable shell-and-tube heat exchanger with similar capacity levels. For reliability assurance, each brazed plate heat exchanger is helium leak tested.
An approach temperature (entering fluid A temperature minus fluid B temperature) of 10°F (-12.2°C) is considered cost effective. Approach temperatures for brazed plate systems can be as low as 3°F (-16.1°C) for fluid-to-fluid applications. Most refrigeration systems design for 9 to 10°F (-12.8 to 12.2°C) approaches, with 6°F (-14.4°C) approaches possible.
Brazed plate heat exchangers are suited for industrial applications in process cooling, hydraulic or engine oil coolers and transmission coolers. Other brazed plate applications include hydronic models for use as shell/tube replacement.