Ambient temperatures are a critical factor in compressed-air system design and performance.
“I don’t think my compressors are making the air they’re supposed to, especially number three,” said the plant maintenance manager. We were at a plastic packaging plant in Arkansas and spent the morning installing data-logging equipment on their compressed-air system.
It was early summer and the compressors were located above the production floor on a mezzanine. Outdoor and indoor temperatures were heating up. The compressed-air system consisted of three 500-hp Atlas Copco ZH400 centrifugal compressors and one 350-hp ZR315VSD variable-speed-drive-equipped, oil-free rotary screw. All of the compressor intakes were inside, so ambient air on the mezzanine was being compressed. The normal operational configuration was to base load two ZH400s, keep one in standby, and trim the plant with the ZR315VSD. There were times when all four compressors would have operated to meet plant demand, leaving the system without a backup.
While installing power-monitoring equipment and a combination pressure transducer/mass flow meter, we noticed it was quite warm on the mezzanine. Ambient temperature readings were also taken at the inlet of each compressor.
The ZR315VSD (Compressor #1) and two of the ZH400s (Compressors #2 & #3) were located together on one end of the mezzanine directly above the production floor. The third ZH400 (Compressor #4) was in a separate room above the maintenance shop, on the same level as the other three units, but in a slightly cooler location. Inlet ambient temperatures were:
#1: 111 F
#2: 110 F
#3: 119 F
#4: 107 F
The ambient temperatures were having a drastic effect on compressor performance.
Another performance factor was the operating pressure. The initial specification called for 110 psig discharge pressure. Plant pressure was being maintained at 118 psig. This also affected compressor output.
When the compressors were initially purchased, performance curves had been provided to the customer indicating that the compressors would supply about 2,300 scfm at 110 psig delivered discharge pressure during the summer months. Based upon the usage configuration of the then-current compressors at the facility, it was believed that two ZH400s would provide all of the air required for much of the time, with the ZR315VSD occasionally trimming the plant during the highest demand periods. Once the compressors were installed and operable, the ZR315VSD was required to operate nearly all the time. Often, the third ZH400 would also run. Figure 1 shows compressor performance at a 95 F and 110 psig discharge pressure.
The colored lines represent the operating curves of the unit at various inlet guide vane positions. The red line is the flow curve with the inlet valve 100% fully open. The compressor is rated at 2,313 scfm maximum delivery at these operating conditions. This curve was used to initially size the compressors and calculate their load.
Figure 2 shows the same compressor at 119 F operating temperature and 110 psig discharge pressure. The operating curves have all shifted to the left (lower flow). Maximum flow has been reduced from 2,313 scfm to 2,092 scfm, a loss of nearly 10%.
As mentioned, the customer was operating the compressors at 118 psig discharge pressure. Figure 3 shows performance at the actual 118-psig operating pressure. The delivered flow has been reduced to 2,028 scfm, a loss from the original curve of nearly 300 scfm. The rise to surge (the pressure difference between operating pressure and surge pressure) has been reduced from 28 psig to 10 psig. This leaves the compressor almost no room to turn down (the percentage that a centrifugal compressor can operate at partial load, accomplished by throttling the inlet valve), rendering it essentially a load/unload compressor. This caused further difficulties with air-system operations.
The ZH compressors (#2 and #3) have a safety feature designed to protect the drive motors. As operating temperature rises, the amperage rating of a motor will drop. On the ZH compressor, if ambient temperature exceeds the motor nameplate amperage temperature rating, a controller will limit the maximum inlet guide vane position to prevent overloading the motor. This further reduces maximum compressor flow output and pressure rise to surge.
After our investigation, it was discovered that Compressor #3 was functioning as a load/unload unit, with only a 3 psig rise from the load to the unload point. Compressor #3 spent much of the logged period in an unloaded condition while Compressors #2 and #4 were operating fully loaded, with Compressor #1 trimming.
Why didn’t Compressor #3 simply unload and shutdown after running for the minimum unloaded time?
Again, the drive-motor protection controller comes into play. In this case, it’s the minimum run time feature. Most of the wear on large motors occurs during start up. The heat built up from the inrush current needs to dissipate to the level of normal motor operational temperature before the motor shuts off. Once the motor shuts off, it needs to cool further to allow for the temperature spike that will occur the next time it starts. In this case, Compressor #3 needed to run in an unloaded state for a longer period of time than normal due to the high ambient temperatures.
While operating unloaded in “cool down,” a slight drop in pressure would cause it to load up. The sudden addition of 2,000 scfm to the system would cause an immediate pressure rise of 3 psig, which then unloaded the compressor. This restarted the countdown for the unloaded run time. This happened often enough that the compressor rarely got the chance to shut down. When it did shut down, it would restart shortly after, when another small decrease in plant pressure occurred.
The best way to increase compressor performance is to reduce temperatures at the compressor inlets. Ambient temperatures at the drive motors must also be reduced to improve motor reliability and increase compressor performance. The final recommendation was to reduce compressor discharge pressure to the minimum required for proper operation.
The first recommendation was to bring in fresh outside air and duct it directly to the compressor inlet filters. Temperatures in Arkansas regularly exceed 90 F. This would increase air compressor output capacity by 10%.
Next, the goal was to lower compressor room ambient conditions by increasing ventilation into the compressor area. The entire production facility is climate-controlled, though cooling in the compressor room was inadequate to control temperatures. Increasing AC ducting to the compressor room wasn’t a practical solution.
We recommended increasing air flow through the room by using forced-air ventilation into and out of the area from outside the building. Proper air turnover in the compressor room could reduce ambient temperatures around the drive motors by 10 F to 15 F, reducing motor operating temperatures as well as eliminating the controller’s inlet valve throttling to protect the drive motor. This would increase flow capacity by several more percent. It would also create more pressure rise to surge, allowing the compressors more turndown before unloading. This would eliminate the load/unload operating condition observed on Compressor #3.
This study illustrates just how critical inlet and ambient operating temperatures are to centrifugal compressor performance. When designing a compressed-air system, it is necessary to understand not only the pressure and flow requirements of the production facility, but the conditions under which the compressors will operate. Communication between customer, system designer, and supplier are critical to ensure proper system operation. EP
By Christopher Nacrelli, Atlas Copco
Christopher Nacrelli is the Business Development Manager at Atlas Copco Compressors, Rock Hill, SC (atlascopco.com). He specializes in optimizing centrifugal compressors for large air users in manufacturing and has authored several related whitepapers.