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issue: January 2008 APPLIANCE Magazine

Technology Report
Cool Compressor Efficiency

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New reciprocating compressors for R-404A and R-507 are engineered for improved EER and lower sound levels.

A range of NTZ compressors available from Danfoss Refrigeration & Air-Conditioning Division (Baltimore, MD, U.S.) covers capacities from 3400 to 24,000 Btuh and have EERs of up to 4.2. They are optimized for commercial freezers (including walk-ins), ice flakers, ice cream machines, medical and scientific equipment, and other low-temperature applications.

“NTZ compressors are equipped with newly developed and optimized electric motors having higher magnetic efficiency and reduced heat losses,” explains Danfoss compressor application engineer Thomas Tiedemann. “Both of these qualities result in a better EER for the compressor. In addition, the optimization point has been changed to –31°F (–35°C) evaporation and 105°F (41°C) condensation with a return gas temperature of 68°F (20°C). This optimization point is much closer to the operating conditions in which the majority of these compressors will be applied by our customers.”

Motors in the new compressors are 100% suction gas cooled, so no additional liquid injection or its associated components are needed, making the compressor an economical choice for OEMs. This also allows the compressor to be fully insulated to reduce operating sound levels when needed. Because there is no liquid cooling system, reliability is increased, and there is no danger of compressor damage due to a malfunctioning injection device.

“Liquid refrigerant injection is needed with some compressors to limit the discharge temperature and avoid oil cracking. With NTZ compressors, the whole application range is available without gas or liquid injection,” notes Tiedemann. “This was achieved by means of a high volumetric efficiency that maintains sufficient mass flow to transfer the motor losses even at very low evaporating temperatures, a high-efficiency motor with low internal heat losses, and enhanced heat transfer out of the cylinder head due to the crankcase being made of aluminum.”

Tiedemann notes that, with the constant evolution of design tools and techniques, more-advanced computer tools for flow optimization and thermal and mechanical simulations were available in the design of the new line than at the time the LTZ compressors were designed. These new tools were used to optimize all aspects of the design and helped reduce the noise of the compressor.

“With the latest computational fluid dynamics simulations, the built-in discharge muffler could be further optimized to reduce the noise level and the pulsations transmitted into the discharge line,” says Tiedemann.

Sound was just one of many characteristics the supplier sought to improve in the new compressor.

“The first and most important challenge in developing the NTZ compressor was to meet customers’ ever-increasing efficiency and noise requirements,” Tiedemann tells APPLIANCE. “At the same time, we had to achieve market-level pricing, maintaining competitive pricing while facing significant increases in raw-material prices over the past months and years.”

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