26 Compressors
Turn up the pressure on compressor testing
MODERN refrigeration processes are comprised of a wide variety of highly advanced techniques to remove heat from an enclosed space.
These methods range from vapour compression and vapour absorption refrigeration to thermo-electric and magnetic refrigeration. Vapour-compression cyclical refrigeration is the most widespread form of refrigeration for household and commercial refrigerators, air-conditioning installations, the food processing industry, refrigerated trucks and other large industrial refrigeration systems.
With this particular refrigeration technique, the liquid refrigerant evaporates by absorbing the heat contained in the space to be cooled down, therefore lowering its temperature. Later in the circuit, the gaseous refrigerant is compressed and condensed again into a liquid state, releasing the heat.
The cycle
The vapour-compression refrigeration system consists of four main components: a compressor, a condenser, an expansion valve and an evaporator. Different compressor types can be used in refrigeration systems depending on the particular application as well as on size, noise, efficiency and pressure issues.
The most commonly used refrigeration compressors are reciprocating and rotary screw compressors.
As with any mechanical device, compressors are subjected to noise and vibration issues which might compromise the manufacturer’s quality standards and customer satisfaction. When vibro-acoustic problems occur, it is crucial to determine the acoustic hotspots and detect structural resonances. Extensive experimental testing with dedicated measurement systems can help discover where these issues originate and
provide valuable information on how to optimise the design to prevent these issues from resurfacing.
Bad vibrations
When GEA Grasso, a Dutch component manufacturer for industrial refrigeration installations, considered improving its testing systems for compressor measurements, it turned to LMS for a more integrated and flexible data acquisition and analyser system. Headquartered in The Netherlands, GEA Grasso has more than a century of experience in producing compressors for industrial refrigeration and is a leading manufacturer of reciprocating compressors and packages. “When developing compressors, we have to take into account important design criteria, such as reliability, durability, energy efficiency, maintenance and depreciation costs, flexibility in capacity control, as well as noise and vibration,” explained Hans Vermeer, manager of the Testing Department at GEA Grasso. “Especially vibrations will become increasingly important in the future, due to new building designs – for instance, buildings with flexible steel constructions or machine rooms that are situated above an office can easily radiate noise – so keeping compressor vibrations to a minimum is crucial for us.” The refrigeration compressors that GEA Grasso produces are low speed piston compressors (from 500rpm to 1500rpm) with a relatively large piston that generate relatively high compression forces between 8Hz to 50Hz. At these low frequencies, building foundations and steel construction parts as
well as piping and other large components are easily excited and might cause problems ranging from simple annoyances to severe situations, such as tearing of piping welds and refrigerant leaking. Until recently, to deal with compressor vibration issues, GEA Grasso relied on three different measurement systems: one system to carry out sound intensity measurements, identify acoustic hotspots and sound power levels; a second system to do impact testing and discover structural resonances and a third one to perform valve positioning measurements for compressors that need charge conditioning.
Recently GEA Grasso invested in the LMS Test.Xpress package to improve efficiency and decrease testing time.
Hydra ACR News March 2014 Visit ACR News online at
www.acr-news.com
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