Industrial Utility Efficiency    

Compressed Air System Upgrades Need Follow-up Attention at Food Operation

There is usually a deep feeling of pride welling up inside the designers and installers after completing the installation of a new compressed air system, especially if it starts up and works perfectly. There is great satisfaction in seeing the machines working perfectly, producing a reliable, clean, dry flow of compressed air at the lowest possible cost. It puts smiles on the faces of all involved and pleases power authorities and energy organizations who might then write some very significant checks to partially fund these efficient installations.

But what happens after a few years, are things as perfect as at the start? This is a question with an answer that very few people know for their system. This article describes some interesting experiences with a food products company at two plants where compressed air assessments of optimized systems done a few years after the system upgrades showed problems.

 

Plant No. 1: System Upgrade Saves Energy, Delivers Quality Air

The first large plant, which was built about 20 years ago, featured a compressed air system consisting of three 300-horsepower (hp), air-cooled lubricated air compressors running on a central controller in the main boiler room. Parts of the plant are cooled to an ambient temperature near 40 oF to prevent product spoilage, so desiccant drying is used to maintain air quality. A heated blower style dryer was installed to condition the compressed air, along with an onboard pressure dewpoint-dependent switching control feature that saves energy by reducing regeneration power consumption and desiccant cooling purge when moisture loads are low.

The system has two very large storage receivers in the main air compressor room to assist with air compressor control, slowing down changes to pressure to allow the air compressors to shut down and to start up again without any low-pressure events. A pressure/flow controller was installed to regulate plant pressure to the lowest possible pressure to reduce artificial demand caused by supplying production machinery with higher than required pressure.

All the air compressors were originally fixed-speed units, but one was upgraded to Variable Frequency Drive (VFD) about five years into the life of the system to save energy. This system upgrade was supported with an energy incentive when it was installed, so verification readings were done a few months after the project with impressive savings numbers found.

When left conditions were excellent. The air compressor controller maintained very good compressed air system efficiency by keeping the air compressors’ discharge pressure as low as possible within a single pressure band. The VSD retrofit allowed the modified air compressor to be used as a trim unit, speeding up and slowing down to vary its flow output to achieve more constant pressure within its control range. This operation minimized wasteful unloaded runtime. Typically, in unloaded operation fixed-speed air compressors consume 30% of the rated full load power, in this case about 80 kW, while running unloaded and producing no air. System design should always focus on minimizing this runtime or the power within it.

Compressed air quality was excellent and the air compressors and dryers were operating as efficiently as possible. The large blower-style heated desiccant dryer maintained compressed air pressure dewpoints of -40 oF and below during all conditions. The dryer was well sized, being rated slightly larger than the capacity of the three air compressors. Only two units were originally required for peak flows, leaving one for backup duty. Since the air dryer was only loaded to a fraction of its capacity its onboard pressure dewpoint controller delayed the regeneration cycles until they were needed, reducing the power consumption of the dryer and the necessary cooling purge that flows to cool the desiccant after a heating cycle.

 

Plant No. 1: Compressed Air System Performance Falters

As the system aged various changes affected the installed equipment. A compressed air scoping assessment was recently done that showed some major problems that have affected system efficiency and the quality of air the system produced. As with any system, as the plant ages more and more system leakage develops, loading the system to higher levels. Also, additional devices are added to the compressed air system as production processes are upgraded. This has increased the compressed airflow, in this case the additional flow has pushed the compressed air demand higher than the capacity of the two main air compressors, requiring a third unit to run. This is a reliability problem, because the failure of any one air compressor will now cause pressure-related production outages during peak demands, affecting the product throughput of the plant.

A primary problem currently is heat. Higher system load means more heat is produced by the air compressors. This system is located in the main boiler room of the plant, so ambient temperatures are always high. Since the air compressors are air-cooled, this high temperature negatively affects the system and its various components. The first component to feel the effects was the VFD unit, this air-cooled device ran successfully for a number of years, but finally suffered premature heat-related failure. The plant struggled with this component for quite a number of months, repairing it, and then having the drive fail over and over again until the maintenance staff gave up trying. The removal of this device reduces the overall efficiency of the system by bringing back inefficient unloaded runtime.

In addition, after a few more years, the central air compressor controller, also located in the boiler room failed due to excessive ambient temperature (Figure 1). This controller had become obsolete so, parts are not available. This failure meant the air compressors now had to run in a cascaded pressure band control strategy, which caused higher than desired average air compressor discharge pressures. The higher the discharge pressure, the more energy an air compressor consumes per unit output, making the system less efficient still. The controller replacement was deemed too expensive for the plant’s tight budget so it was never done.

Blank air compressor controller

The air compressor controller stares blankly out at the operators. This heat-related problem is more of an issue because the controller is obsolete with no parts available.

The central controller also controlled the pressure/flow regulator. With the failure of the main controller also came the failure of the pressure/flow controller, which increased system pressure substantially. This caused extra compressed airflow, which in turn caused the air compressors to consume even more energy, creating even more heat.

And finally, due to high ambient temperatures, and the occasional failure of the air compressor cooling air ventilation dampers, the air compressors were regularly subject to overheating. As these machines aged, the lubricant coolers within the enclosure became less and less able to remove the heat of compression, causing higher than desired air compressor discharge temperatures. Overheated air compressors often have this problem because the heat will cause the air compressor lubricant to break down at an accelerated rate, causing varnish to form on internal heat transfer surfaces. This varnish and the resulting degradation of the cooler performance causes high discharge temperatures, even during normal ambient conditions.

The high discharge temperatures caused problems with air drying equipment. A rule of thumb states the moisture content of undried air compressed air doubles for every 20 oF increase in temperature. Therefore, overheated compressed air hitting the air dryer overwhelms the unit due to excessive water vapor content. In the overloaded condition the dryer cannot maintain rated pressure dewpoint, and during mid-shift peak production the pressure dewpoint of the compressed air going into the refrigerated plant reached unacceptable levels for this plant (Figure 2).

Dew point controller

High inlet temperatures cause poor pressure dewpoint in Plant 1 (levels should be below -40 oF). The pressure dewpoint control has been turned off due to this problem, causing wasted energy during light loading.

The excessive water in the compressed air causes stress on the desiccant and at times free water is present in the output of the dryer. Some of this water fouled the onboard pressure dewpoint probe used to control the dryer causing it to fail. After quite a few expensive replacements local staff gave up with operating the pressure dewpoint-dependent switching feature of the dryer, the unit was switched to fixed-cycle mode, causing the system to become even more inefficient due to higher than required heater and cooling air duty.

As is typical with many companies these days, the operational and maintenance staff are overloaded, and budgets are tight. This is preventing the much-needed repair and replacement of the older malfunctioning compressed air equipment and is leading to reduced reliability, inefficient system operation and poor air quality.

 

Plant No. 2: Multiple Issues Drive Compressed Air System Upgrades

This processing plant is actually two plants in one. The building has been in place for over 50 years but has undergone various extensive renovations, the last being an expansion for a new product. For some reason the company chose to install a separate independently controlled compressed air system, rather than connecting the new line to the old main system.

The main compressed air system was retrofitted about 15 years ago to add a water-cooled VSD air compressor to the plant’s two fixed-speed air compressors. A second air-cooled VSD air compressor was installed after one of the older fixed-speed units came to the end of its useful life. A third air-cooled remote VSD air compressor was installed near a critical production line a few years ago because the line was having low-pressure events.

Since the production areas are where refrigerated desiccant style dryers are used, a main heatless desiccant dryer with pressure dewpoint control was installed for the original air compressors, then when the second air compressor was installed a separate heated style unit was purchased. When the third VSD air compressor was installed the company purchased a surplus heatless desiccant dryer with pressure dewpoint control for the remote system air demand, isolating the new air compressor from the main plant system with a check valve so it could run independently from the main plant system in the event of trouble.

For the new expansion two 75-hp VSD air compressors were installed with separate heatless desiccant dryers. Large storage and flow control was designed into the system to help with air compressor control and lower plant pressure. Early in the production of the plant it was found the two air compressors were not large enough to maintain pressure during peak flows so a third 100-hp air compressor was added. The system was originally verified by the local power utility and shown to be operating at peak efficiency but degradation in the system characteristics occurred when the larger air compressor was installed.

 

Plant No. 2: Assessments Point to System Efficiency and Air Quality Issues

Recent plant assessments on both systems found less than desirable system efficiency and some air quality problems were occurring.

For the main system, the air compressors are all run independently with no central control system, the local controls were set to make the VSD air compressors all share the load. This led to undesirable operation where the variable air compressors all run near minimum speed, the least efficient point for this style of air compressor. One of the air compressors was found to have internal problems that caused it to run at very poor specific power of over 35 kW per 100 cfm when it is running at the bottom end of its curve, yet the unit continued to run in this condition due to the compressor settings. The remote air compressor, being separated by a check valve, continued to run at night and on weekends, even during light loads where only one main air compressor is needed for the whole system.

Malfunctions with the air dryer controllers due to age and lack of maintenance caused two of the three dryers to continue to cycle at full purge throughout non-production periods. The dryer purge represents most of the plant load during low flow periods, wasting significant power. One of the dryers was found to have a failure preventing it from purging but the unit remained stuck partway through its operating cycle (Figure 3). This dryer was allowing saturated air into the dry side of the system which negatively affected air quality. The operating staff were unaware of this problem as no secondary pressure dewpoint measurement devices monitor the system pressure dewpoint.

Air System Monitor

All looked normal with this air dryer control, but careful monitoring showed it was stuck on one part of its cycle.

With the new expansion, control problems were being experienced with the system because of control gap and the existence of check valves within the air dryers. Since the 100-hp base air compressor is larger than then individual VSD air compressors there were times where the air compressors were fighting for control, with the lead VSD speeding up and slowing down, but at the same time with the fixed-speed rapidly loading and unloading (Figure 4). Problems with system control were also caused by the check valves. These valves prevented system pressure from passing back to the individual air compressor controllers. When an air compressor would unload and shut off, the air dryer purge would cause the pressure at the air compressor discharge to fall rapidly causing the air compressor to start back up again, even though it was not required.

Load Graph

Mismatch of air compressor sizes and location of check valves caused control gap problems when a large base unit (orange trace) was running. It is seen loading and unloading, fighting the control of the VSD air compressors (dark green and light blue). Click here to enlarge.

A central air compressor controller was installed later to attempt to correct the control problems, but this unit, for some reason that was never determined, was incapable of properly controlling multiple VSD air compressors. Even under central control the air compressors would fight due to a control gap caused by the mismatch in size of the 100-hp, fixed-speed unit and either one of the two 75-hp variable air compressors. A general sizing rule is to have the main VSD air compressor slightly larger than base fixed-speed units to avoid control problems, although this rule was not followed when the base air compressor was purchased, with negative consequences.

When the compressed air assessment was done an air quality problem was found with the dryer for the fixed-speed air compressor. This dryer had developed a condition where it would freeze for long periods of time in the same position during the repressurization part of its operating cycle. This caused the side drying the airflow to become saturated, passing wet air into the dry side of the system, increasing the pressure dewpoint to unacceptable levels for a refrigerated plant. The problem was diagnosed as a sensor calibration issue, the dryer was designed to wait until the side being depressurized reached a low pressure. Due to a pressure transducer calibration error the pressure signal to the dryer control never reached the required setting, therefore the dryer froze midway through its regeneration cycle and stayed that way until a random pressure fluctuation allowed it to continue. This problem was only identified during the assessment, the operating staff were unaware of these problems, again no remote secondary pressure dewpoint measurement was in place to ensure the compressed air output remained dry.

 

Compressed Air Leaks Addressed

The compressed air assessment in both plants turned up a significant number of leaks, over 200 in total, by far the most leakage occurred in the older plant. A database of leakage locations, including photographs, was created and passed to the maintenance department for repair. This plant, as with the other, is staffed to a minimum level and the maintenance department finds it difficult to remedy the leakage flow. Most of the repair requires work to be done during midnight shift, weekends or holidays. After much trouble in arranging the work, a leak repair blitz was initiated by plat staff, resulting in a significant reduction in leakage flow.

 

Assessments, Monitoring Recommended

The results of the assessments of these systems showed that despite the fact that the systems were all set up with efficient equipment, and should have run very well, changes to the plants, control settings, and aging or failure of components caused inefficiencies and air quality problems. In most cases plant operators were unaware of the problems. This shows the value of regular compressed air assessments by a third party.

Also missing on these systems is any working air quality measuring and alarming system. Air dryers often fail, allowing wet air to pass into the plants. It can be many hours, days or even months before the operating personnel realize the problem and make corrections. It is best to have secondary air quality monitoring instruments installed downstream in a location that can best detect negative conditions and allow quick notification of issues to repair personnel.

These systems had air compressors and air dryers that ran inefficiently for many years, feeding excessive leaks, consuming much more power than required. A good monitoring system for pressure, flow, power, specific power, pressure dewpoint and leakage could have gone a long way in allowing plant managers to see the problems as they developed and make timely repairs.

It is nice to be able to report the plant management has decided to upgrade the systems in all of these plants in the near future to renew the equipment and improve efficiency, reliability and air quality. More happy days are coming!

 

For more information about this article, contact Ron Marshall, Marshall Compressed Air Consulting, tel: 204-806-2085, email: ronm@mts.net.

To read more air compressor control technology articles, please visit www.airbestpractices.com/technology/air-compressors.