To performance test or not performance test, that is the proverbial question. Jake often was asked this question. He knew the answer wasn’t easy or general enough to cover all situations.
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When he first began performance testing, a code for field performance testing existed, and instruments usually were calibrated pressure gauges and NBS [National Bureau of Standards] reference mercury-filled thermometers. Orifice plates were removed prior to testing and inspected, measured and replaced, if necessary. His mentors had drilled into him the need to assess measurement errors. In addition, there was the human element. To properly test in those days would require a cadre of personnel. This human factor then became part of the measurement error. Thermometers were selected so they could be read when they sat in the well. Unfortunately, sometimes the thermometer would need to be pulled part of the way out to read it.
Then, came temperature recording boxes. RTDs or thermistors were connected to the box and Jake read in one place all of the temperatures at one time. Jake took great care in calibrating the individual temperature elements before a test and then maintaining an ice bath during the test to do calibration checks. He built all the steps into his methodology.
Next, the manufacturer of his box developed an interface that would hook up to his portable computer. This allowed Jake to develop a program to monitor the test while he conducted it. The prospects elated Jake. What he found, though, gave him pause. When he had not been able to measure the discrepancies in data, he was oblivious to the accuracy of his tests. Now he found himself asking the question, “What are we getting when we field performance test a unit?” (In a future article, we will attempt to clarify and show examples of what this question raises.)
CFC Phase-Out Example
Jake and TJ had worked together for years when the chlorofluorocarbon (CFC) phase-out came around. As part of the corporate CFC phase-out program, they were charged with retrofitting, removing or replacing all of their CFC-containing refrigeration equipment. The task spanned several years. Each of the group’s engineers received a list of refrigeration equipment to evaluate.
TJ was responsible for a -40°C system with two large refrigeration machines and one small machine used primarily during plant startup, and then run sporadically as a topping unit when production was high. The nearly 30-year-old unit still ran, but it was questionable as to whether it would be retrofitted. All of the units operated on CFC-12. As part of the project, each machine had to be present-state performance-tested, meaning engineers tested the unit as it stood — with no tuneup or maintenance touchups. Also, each equipment manufacturer provided a reselection evaluation for HFC-134a operation. TJ had the reselection in hand when he set out to do the performance test.
During testing, TJ kept looking at the portable computer. Something did not seem right. He reran the test and the results came up the same. He recalibrated the RTDs and reran the test. TJ was stymied; the results were the same. He called Jake and said, “I need to talk about this test. It is a head scratcher!”
“Jake, can you tell me where I am going wrong on this test?” TJ asked when they met. They reviewed test data, design data and the reselection. It appeared the compressor was operating below 45% efficiency. Digging into interstage pressure and temperature details, they found the second stage of the three was at 20%. Jake wondered if there was an obstruction. They pulled out the reselection. Dimensions for each stage as built were included in the manufacturers report. Usually, in a multistage compressor, each higher stage has a smaller exit wheel width because the compressed gas has a lower specific volume at the higher pressure. They found the second stage’s width was nearly double in size, resulting in inefficient performance; it may have been operating in surge on occasion. TJ wondered, “How had this been missed for so many years?” Jake explained that improvements in the technology for performance testing now allows seeing so many things probably missed in the past. Because this was an auxiliary machine and operated sparingly, it just languished.
While this is just one example, it raises questions. Do you know how your equipment is operating? Have you done performance testing recently? Did you perform an acceptance test on your equipment when it was installed? In the next few columns, I will cover more on this topic, including performance testing reviews, performance test versus acceptance test, online testing, modern field testing and daily performance monitoring.
