Ashrae Journal - December 2008 - (Page 54) Tips for Controlling Noise Tips for Controlling Noise at Its Source • Substitute equipment or parts of equipment: use rotating machines rather than reciprocating machines, e.g., screw compressor instead of reciprocating compressor, semihermetic compressor instead of open-type compressor, modification of gear teeth, and screw profiles. • Maintenance: balancing and lubrication of moving parts, replacement or adjustment of worn or loose parts, modifying parts to prevent rattles and ringing. • Substitution of materials, e.g., replacing metal parts with plastic/rubber parts. • Change of work methods and layouts. Keeping noisy operations in the same area, separating noisy operations from non-noisy processes, selecting the slowest machine speed appropriate for a design duty conditions. • Reduction of resonance noise. Ensure that machine rotational speeds do not coincide with resonance frequencies of the supporting structure by changing the stiffness or mass of the supporting structure. • Reduction of acoustic radiation efficiency: e.g., replacement of a solid panel or machine guards with a mesh or perforated panel, or use of narrower belt drives. • Reduction of noise resulting from fluid flow. Provide machines with adequate cooling fins so that noisy fans are no longer needed. Use fan blades designed using CFD software to minimize turbulence, increasing the number and width of fan blades, reducing the thickness of fan blades. Use centrifugal, rather than propeller fans. Locate fans in smooth, undisturbed airflow. Use large low-speed fans rather than smaller faster ones. Minimize the velocity of fluid flow. Maximize the cross-section of fluid streams, Reduce pressure drop across any one vibrations. Proper surface finish, good workmanship, proper lubrication of moving parts, and use of rubber paints minimizes the vibration. Airborne Vibration (Noise). Noise is unpleasant or unwanted sound. The sources of noise in refrigeration and chiller packages are the motor, compressor, pumps and fans. It occurs as a by-product of vibration. Certain noises are irritating and can seriously affect efficiency and health. Noise also may be detected by an enemy. A navy will specify the acceptable noise levels for equipment. The airborne sound power level radiated by the auxiliary equipment is expected to comply with the levels stated in MILSTD-1474D.5 Airborne noise measurements are taken as sound pressure level reference of 1e-12 W at 1 m (3 ft) distances from equipment in one-third octave band over a frequency range of 10 Hz to 10 kHz. Noise levels are measured at 25%, 50%, 75%, and 100% rated output at six positions around the plant. Typically for a naval application, the airborne noise is measured as per MIL-STD-1474D. 54 ASHRAE Journal component in a fluid flow stream. Minimize fluid turbulence where possible (e.g., avoid obstructions in the flow). Choose quiet pumps in hydraulic systems avoid cavitations. Choose quiet nozzles for compressors. Isolate pipes carrying the fluid from support structures. Tips for Controlling Noise in Transmission Path • Use sound enclosures for noisy components on a machine/for a complete system. • Vibration isolation of machines from noise-radiating structures. • Vibration absorbers and dampers. • Addition of sound-absorbing material to reverberant spaces to reduce reflected noise fields. Tips for Control of Noise at Receiver • Use earmuffs. • Use active noise cancellation techniques. Noise Control for an Existing Facility • Undertake an assessment of the current environment where there appears to be a problem, including the preparation of noise control contours where required. • Establish the noise control objectives or criteria to be met. • Identify noise transmission paths and generation mechanisms. • Rank-order noise sources contributing to any excessive levels. • Formulate a noise control program and implementation schedule. • Carry out the program. • Verify the achievement of the objectives of the program. Any noise problem may be described in terms of a sound source and transmission path and noise control may take the form of altering any one or both the elements. When considered in terms of cost effectiveness and acceptability, experience puts modification of the source well ahead of modification of the transmission path. In existing facilities, however, the modification of the transmission path is the only feasible option. The application of the noise reduction techniques in the “Tips for Controlling Noise” sidebar may not result in significant noise reduction if the fundamental science of noise is not considered when applying these techniques. As a general rule, control of airborne noise is possible by optimizing radiation directivity by placing the radiating openings in remote areas and fitting them with silencers/screens and control of structure-borne noise is possible by reducing the surface of radiating parts and reducing the radiation efficiency (decrease the thickness of plates or use perforated plates). Minimum rate of change of force is associated with minimum noise. Since noise is a result of energy waste due to friction, ashrae.org December 2008 http://www.ashrae.org
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