|
|
|
|
 |
|||||||||
|
|
|
|
|
|
|
|
|
|
Netscanner™ System: Application Notes and Technical Papers
McQuay International Demonstrates a New Application for
Intelligent Pressure Scanners: |
| ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– |
by Earl Campaigne Jr., Senior Development Engineer, McQuay International Introduction One of the most significant advances in critical pressure measurement for industrial, commercial and military applications has been the development of “intelligent” pressure sensing and data acquisition systems. Such systems make it possible to gather critical data more accurately, reliably and cost-effectively than with previous generations of measuring equipment. The heart of intelligent pressure sensing systems is the intelligent pressure scanner, a microprocessor-based, multichannel device capable of multiple pressure measurements in conjunction with a direct PC interface. Primary features are engineering unit outputs, ease of re-calibration, high accuracy and digital temperature compensation. In addition to providing high-accuracy, low-cost measurement of multiple pressure points, intelligent pressure scanners can also be daisy-chained and networked to a host PC or laptop computer to form a low-cost distributed data acquisition system. A New Application for Intelligent Pressure Scanners Electronic pressure scanners have been successfully deployed in a variety of pressure sensing applications. Originally, they were used exclusively in applications involving dry, non-corrosive gases. They were originally designed for aerospace applications by Pressure Systems, Inc., who introduced the first electronic pressure scanner in 1978. More recently, however, intelligent pressure scanning systems have found use in a variety of industrial and commercial test and measurement applications. McQuay International, a manufacturer of chillers that use centrifugal compressors, recently conducted a test which utilized intelligent pressure scanners from Pressure Systems, Inc., (PSI). The pressure scanners we used are part of PSI's System 9000, a line of intelligent instrumentation and data acquisition products that feature digital temperature correction and linearization, built-in calibration and a networkable serial output. System 9000 intelligent pressure scanners provide high accuracy measurements in engineering units directly to the host computer. PSI's scanners were used alongside conventional measuring devices during routine testing of one of the company’s centrifugal compressors. During these tests, the PSI scanners were exposed to a refrigerant, HFC-134a (a beneficial, 0 ODP replacement for CFCs), in both gaseous and liquid form for a total of more than 4,000 hours during both operational and non-operational periods. The trial was conceived by our engineering team, one of whose members had previously worked for a firm where electronic pressure scanners were used to evaluate gas turbine engines. At the time, McQuay was completing construction of a 1,500-ton test stand for use in evaluating the company’s centrifugal compressors, with an eye toward improving their efficiency. During such evaluations, a compressor was fully instrumented with static pressure taps, total pressure probes and total temperature probes, as a way of gauging the thermodynamic state of the HFC-134a refrigerant at various stages throughout the compression process. The data gathered would play a critical role in helping McQuay meet the growing demand for higher efficiency compressors dictated by environmental regulations, utility incentive programs and the desire of companies to minimize operating costs to stay competitive. Test data acquired from instrumentation is used to determine overall compressor efficiency, as well as individual compressor component performance. From this information, compressor computer models are generated and used to develop more efficient centrifual compressors. For McQuay, acquiring the necessary pressure measurements using conventional equipment was difficult and expensive. It was in the hope of improving the testing process that we initially approached PSI about incorporating their intelligent pressure scanning equipment into our test stand. Designing the Test Our engineering team had not counted on becoming pioneers, yet PSI informed us that electronic pressure scanners had never been used in an application where they were directly exposed to a refrigerant like HFC-134a. A field trial was thus proposed and agreed upon -- the results of which have extended the range of proven applications for intelligent pressure scanning and data acquisition systems. In designing the trial, two hurdles had to be overcome. First, the engineering teams were concerned about the effect HFC-134a might have on a dynamic "O” ring that was part of the basic design of the electronic pressure scanners. This was addressed by PSI, which produced a version of its 9010 scanner in which the “O” ring was eliminated. Second, the question arose as to whether the scanners’ performance might be compromised by exposure to what, for them, would be an entirely new medium, HFC-134a. Although PSI had designed the 9010 scanners for on-line calibration, McQuay wanted to be certain of their accuracy during the trial. To address this second concern, we decided that an old measuring system -- consisting of conventional transducers, gauges and LED readouts -- would be partially left in place. Thus, the engineers would have a reference by which to compare the electronic pressure scanners. The resulting setup for the trial was somewhat more complicated than would normally be required in a non-test situation: of the 16 pressure channels available on the PSI 9010 scanner, nine were fed into the existing transducer lines to allow for two independent measurements of the same pressure. The remaining seven were connected normally, without any attachment to the older lines, so that their pressure data was output directly to a laptop computer in the control room. Following initial installation, the test configuration was evacuated and charged with HFC-134a refrigerant. This should have cleared the way for the trial to begin. However, the start was delayed due to difficulties encountered during the shake-down process. Though entirely unrelated to the PSI instruments, the delay was a cause for concern, since it meant the scanners had been subjected to several successive evacuations even before the trial began. The big question for the engineering team was: had the sensors sustained any damage or become miscalibrated during the shake-down period? Fortunately, such fears proved unfounded. The 9010 intelligent pressure scanners, with their self-contained transducer and microprocessor, proved to be a very powerful, cost effective and highly accurate means of measuring pressure even under extreme conditions. During the tests on our centrifugal compressors, McQuay test technicians and engineers were impressed with the performance of the intelligent pressure scanners. Their ability to be daisy-chained and connected to a laptop computer was an ideal configuration for this type of testing, in which a large number of pressures had to be recorded. Each scanner combines 16 silicon piezoresistive pressure transducers with an on-board 32-bit microprocessor. Factory calibration data is stored within the pressure transducer, which also contains a temperature sensor. The sensor uses this calibration data to make automatic corrections and output high accuracy engineering unit data directly to our computer. Despite the extreme test conditions, the piezoresistive sensors offered highly reliable performance, delivering accurate pressure measurements continuously throughout the trial. The integral microprocessor performed digital temperature compensation and linearization of transducer outputs without fail. The Benefits of Intelligent Pressure Scanning Systems Intelligent pressure scanning systems have revolutionized the data acquisition process. Before the advent of electronic pressure sensors, multiple pressure measurements were made using either discrete transducers or mechanical pressure scanners. Mechanical pressure scanners offer reasonable accuracy at low cost, but are limited to low scan rates -- typically less than 10 readings per second. Discrete transducers, on the other hand, provide higher scan rates but at much higher initial costs. They are also more expensive to maintain than electronic pressure sensors, since discrete transducers must be recalibrated periodically to assure their accuracy. Intelligent pressure scanners, with their high scan rates and relative low initial and maintenance costs, combine the benefits of both these alternatives while overcoming their inherent limitations. The successful integration of electronic pressure scanners in our centrifugal compressor test stand demonstrates the versatility of these scanners and extends their use for critical measurements in a wide variety of applications. No doubt there are many other applications where mechanical pressure scanners or discrete transducer measurement equipment are still being used that could be replaced with intelligent pressure scanning systems to achieve a more efficient, reliable and easier way of gathering critical data. |
|
