Compressors are the workhorses of a refrigeration system, and the component that produces the most noise. Appliance engineers and appliance marketers pay much attention to noise factors when developing an appliance, knowing full well that a noisy appliance is the last thing consumers want to place in (or around) their homes. Compressor suppliers also seek to limit vibration, as noise and vibration are often closely linked, while increasing efficiency and reliability.
Bristol Compressors International Inc., known for reciprocating compressors that boast the same energy efficiencies as scroll compressors, teamed up with a long-time supplier partner to incorporate an external muffler to make their refrigeration workhorses quiet as a mouse.
When Bristol needed to lower the noise volume on its new Benchmark compressor family while meeting the 13-SEER energy efficiency requirement at a minimal cost, it turned to Kraftube Inc.
“The general problem that we solve over and over again with each new compressor design is to prevent pressure pulsation energy from reaching the world outside the compressor, where it can cause problems such as high sound and vibration in the condenser tubing, or high airborne sound levels from the compressor shell,” Scott Hix, vice president of product engineering for Bristol, tells APPLIANCE magazine.
Bristol and Kraftube have had a 20-year-long relationship; Bristol often includes Kraftube early on in the compressor design process, and this project was no different. The two companies quickly collaborated to review design concepts for the new component and test to ensure the concepts would be feasible from a production standpoint and reliable. The team, comprising engineering and quality assurance groups from both Bristol and Kraftube, worked with a failure modes and effects analysis (FMEA) process to analyze function, dimensions, and product pilots prior to launch.
The new component from Kraftube was a steel-body external muffler, replacing an internal-discharge muffler also developed by Kraftube for earlier compressors. The external muffler, according to Hix, reduces the compressor’s discharge pulsation energy before it enters the condensing unit, resulting in lower noise levels.
“By placing the muffler in an optimum location outside the compressor, based on refrigerant being used and compressor pulsation frequency, the overall effectiveness of the muffler system is improved,” Hix says. “By optimizing the position and size of this external muffler, a simple expansion device is able to handle the discharge pulsation and overall system cost is reduced.”
Kevin Kinnally, vice president and general manager of Kraftube, explains that several variables contribute to noise reduction, including material thickness, internal hole configuration in the standpipe tube, and internal baffles. It was these variables that the two companies worked to fine-tune to the compressor application.
“This muffler is different from our usual muffler products in that it must be painted to withstand operating conditions and be UL approved,” explains Hix. “The usual muffler is positioned inside the compressor and does not require paint or UL approval. In the case of the latest development, the external muffler, the muffler is a separate component of the condensing unit, is exposed to the elements, and is a pressure vessel. Therefore, it must be qualified on the additional requirements and obtain approval as a ‘stand-alone’ component.”
The end result was an external muffler design that maintained a low cost despite the rising price of copper, and met the higher pressure requirement associated with using the new HFC-410A refrigerant.
The external muffler works to essentially cancel the noise from the compressor pulsation by creating its own acoustic wave. This is all dependent on muffler placement, compressor frequency, refrigerant type, and component type.
Bristol incorporated the new design into its latest compressor family, which is used by several leading HVAC manufacturers, including York’s LX-series air-conditioners and heat pumps (pictured).
Hix describes the Bristol/Kraftube relationship as open and friendly. “Our engineers work together through simple phone calls, sharing data, and working together from models created in SolidWorks. We have found that the closer we work together and the earlier in the process we start the collaboration, the better the design will be.”
As evident in their collaboration with design after design, the partnership will continue. Bristol and Kraftube already have plans to review Bristol’s current programs, develop ideas for new products and look into the next-generation technology, including the requirements for pulsation attenuation.
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