[pullquote] Refrigeration machine No. 13 was jinxed, people at the plant said. Soon after the unit was installed, a serious explosion on the site caused major damage to No. 13, primarily to its steam turbine. A specification error on the replacement turbine only was discovered as it was being placed on the foundation: the turbine spun one way and the compressor the other way. A reversing gearbox and foundation redesign to accommodate the new drive train finally allowed No. 13 to get back online. However, its performance never quite returned to normal.
The condensers were cooled with river water and routinely cleaned to remove sediment that eluded the river-water screens. When river-water contamination was high, No. 13 suffered first because the header orientation made it the “filter” for the system.
In one instance, No. 13 had shut down overnight due to high condensing pressure. The next morning, it started up easily, came online and ran for four hours before tripping again on high condensing pressure. Zebra mussels were discovered inside. And so the jinxed reputation grew.
When the plant began to phase out CFCs, No. 13, as one of the newer machines, was picked for retrofitting to HFC 134a. This first required running a performance test on the unit. This showed No. 13 was below original specifications but that was attributed to age and possible fouling of the heat exchangers.
The conversion to HFC 134a went smoothly and then the startup began. No. 13 did well at low capacity, so the load gradually was increased. The unit ran for a while and then tripped on high condenser pressure and low evaporator pressure.
The first assumption was the condenser had fouled. Inspection found no major fouling but the tubes were brush-cleaned and the condenser buttoned up. The machine was restarted and came online easily but tripped out again when load was increased.
HFC 134a requires higher compressor lift, so the speed was increased within the limits of both the compressor and the turbine. This was rechecked after the second trip and still was within specifications.
Next, the evaporator was opened. The tubes didn’t seem that dirty but were cleaned and the unit buttoned up. The restart duplicated the previous attempts. The crowd calling No. 13 jinxed was growing louder.
The young engineer on the retrofit project then analyzed the data collected and found the pressure spikes coincided with a drop in evaporator level. This explained why the unit was going out on both high condenser pressure and low evaporator pressure. But why?
The plant routinely had problems with the float valves that controlled refrigerant level on other machines and so assumed this was the culprit. No. 13 was shut down, its charge removed, and then opened up for inspection.
The float valves were found to be operating normally and within tolerances for range. The internals on the heat exchangers were clean and free from any damage. So what was causing the problem? It’s a jinxed machine, counseled the crowd. “Why don’t you just replace it?”
The young engineer thought the indication of restricted flow warranted more investigation. He had the piping removed from the float valves to the bottom of the heat exchanger. There was no blockage! He then took off the insulation from a service valve at the bottom entrance to the evaporator. The arrow on the valve showed it had been installed backward. As flow increased, the valve would choke flow and act as another restriction in the line.
The unit was put back together, refrigerant recharged, and started up. Load was applied and the unit came up to full capacity. The performance test showed a vast improvement from the CFC-12 test. The backward valve likely had been restricting capacity for a long time.
So, as we begin the next round of refrigerant retrofits (see “Montreal Protocol Addresses HFCs” and “Refrigerant Replacement Heats Up"), always check and recheck your assumptions and machine condition. After doing the retrofit analysis, run a performance check on the existing refrigerant. Then, follow up on any questionable performance parameters before changing the refrigerant. You may discover problems you didn’t know existed.
Active in both, the American Society of Mechanical Engineers and the American Society of Heating, Ventilating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), Clark was chairman of ASHRAE's task group on Halocarbon Emissions and served on the committee that created ASHRAE SPG3 - Guideline for Reducing Halocarbon Emissions. He has written numerous papers on CFC alternatives and retrofitting CFC chillers. He was awarded a U.S. patent on a method for reducing emissions from refrigeration equipment. He has served as technical resource for several others.
You can email him at [email protected]