Implement load sharing in a multiple compressor installation

If you’re a plant facility manager tasked with reducing energy costs and minimizing the facil­ity’s carbon footprint, the compressed air system likely offers the greatest opportunity to address both tasks. Does your compressed air system consist partially or entirely of centrifugal air compressors?

Then implementing a load sharing control system may substantially reduce energy use and carbon footprint. To maximize potential Return on Investment (ROI) of a load sharing control system, it is critical to address both the supply side and demand side of the com­pressed air system. You’ll want to begin with an air system audit. The project manager should compile a list of audit objectives and expectations to share with an independent auditor that specializes in compressed air systems.

There are many firms that offer energy audits for an entire facility. While these firms may do an acceptable job on the supply side of the system, a compressed air audit specialist typically provides more in-depth demand side analysis and also likely knows about the latest advances in supply side controls. These additional insights matter because controls are the most critical part of the plan in terms of performance and energy savings.

This article outlines and explains the steps neces­sary to implement an intelligent load sharing system controller applicable to facilities with multiple cen­trifugal compressors.

Recommended guidelines

1. Set objectives and expectations for compressed air system audit.

a. Collect data and provide in Excel format with final report for internal review. Data readings should be collected at least every 15 seconds, averaged into one-minute intervals.
b. For larger systems, particularly with multiple cen­trifugal compressors, the use of insertion-type flow meters in the air piping and power meters on the compressor is recommended.
c. Request histograms showing percentage of time at flow and at power, with at least one full week of data.
d. Focus on clean-up of equipment, including pres­sure drops across dryers and filters. If desiccant dryers are in use, confirm purge pressure is set at factory recommendation. If purge pressure is not at factory recommendation, confirm purge pres­sure. Verify desired operating pressure dewpoint is achieved, either from the dryer control panel or with a portable dewpoint monitor installed immediately downstream of the dryer.
e. Another supply side focus is pneumatic equipment that creates dynamic pressure drop in the system, such as fast acting solenoid valves. Point of use pres­sure transmitters can log data showing the impacts of these production components on the system.
f. Identify opportunities to increase system perfor­mance by installing storage tanks, whether they are in the compressor room or in the facility on the demand side.
g. Test for leaks facility wide, showing location and volume of leaks. Estimate ROI based on cost of repairs vs potential savings.
h. Identify applications where compressed air is misused. For example, blowing with full pressure compressed air is a common misuse. Using low pressure nozzles with amplifiers and low-pressure blowers are more efficient solutions.

2. Become familiar with the various types of compres­sor controls.

a. Start with the compressors currently operating in the plant.
b. In many cases, audit report recommendations may include a new compressor or compressors, so take time to understand control methods that differ from what you have experience with.
c. Request data on unloaded power for rotary screw compressors and blowoff points for centrifugal units. Control curves for various concepts show best case power vs load.

3. Understand flow gaps. This requires knowledge of existing or future compressor controls and how they will impact opportunities to reduce or eliminate wasted energy based on flow profile (histogram time at flow).

4. Evaluate load sharing designs for multiple centrifugal installations or systems with a mix of centrifugal units and rotary screw compressors.

a. Pressure bumping strategies – Individual centrifu­gal compressor set points are bumped in order to keep them operating in turndown range. Bump intervals are usually below 1 psig, typically in the 0.25 to 0.50 psig range. This control design keeps all units operating in turndown before any com­pressors begin to blowoff.
b. Inlet guide vane (IGV) and blowoff valve (BOV) control – A local compressor controller operates both valves keeping the compressor in turndown range, again turning down all units before any units go into blowoff. Some companies offering this design require that their compressor control­ler be installed on the units. Other suppliers may provide a separate, wall-mounted interface box, integrating all compressor operations together.
c. Load shaping – Large volumes of storage and, in many cases, boosting pressure in one or more of the tanks, provides significantly higher pressure than plant operating pressure. The design scheme includes small horsepower boosters and flow con­trol valves to reduce pressure out of the high pres­sure tanks. Typically there is a trim compressor (or compressors). The control logic is to bump centrifu­gal operating pressures to keep them operating in turndown range when necessary.

5. Properly size the header pipe to minimize pressure drop.

6. Consider installing permanent flow and power moni­toring equipment after completion of the audit.

7. Plan for the future. Is system demand expected to increase or decrease in the foreseeable future?

Centrifugal control technologies

Centrifugal compressors utilize dynamic compression, converting kinetic energy to pressure energy. Ambient conditions, as well as cooling water temperature, impact flow and power performance. If the final audit report recommendations include performance curves, always request that the curves reflect performance in standard cubic feet per minute (scfm), which is 14.7-psia, 60°F and Dry Air (0% RH). This is important when looking for control gaps.

Eliminating blowoff is critical to maximize centrifugal compressor system performance. Often it is the single greatest opportunity to reduce power costs.

Figure 1 depicts a standard performance curve with pressure line, surge line, requested flow, maximum flow, Inlet Guide Vane (IGV) positions and turndown. Pres­sure line is the operating pressure, requested flow is anticipated required flow, maximum flow is peak com­pressor output at operating pressure and surge line is the natural surge of the unit at operating pressure. Natural surge – the point on the curve where the surge line crosses the pressure line – is to be avoided, as it is the point where flow reverses, possibly damaging the com­pressor. The curved colored lines show IGV positions by angle, with 90° being fully shut, 45° being half open, and 0° fully open. Minimum flow is the point where the natural surge line crosses the pressure line, in this case 2,500 scfm @ 120 psig.

Figure 1: Typical Centrifugal Performance Curve with Surge Line and Turndown. Courtesy: Atlas Copco Compressors

During commissioning, a control line will be set up to protect the compressor from natural surge. The IGV will throttle shut as pressure rises (flow drops). Once the compressor reaches maximum turndown, the blowoff valve will begin to open in order to prevent the com­pressor from surging.

In Figure 2, the point where the red dotted line (con­trol line, just below the natural surge line) crosses the pressure line illustrates the new, site-adjusted blowoff point. It’s common to set the control line 5-8% below the natural surge line. Job site turndown percentage and blowoff flow point will be less than what is shown on a standard performance curve. Jobsite blowoff point is now approximately 2,650-scfm @ 120-psig.

Figure 2: Centrifugal Performance Curve Showing Control Line and Turndown Flow (red dotted lines). Courtesy: Atlas Copco Compressors

Positive displacement controls

Lubricated rotary screw air compressors offer four types of control: inlet modulation, geometry (also known as variable displacement), load/no load and VFD (variable frequency drive, aka variable speed drive or VSD). Oil-free rotary screw compressors offer load/ no load or VFD (VSD). Note that the VSD curve is based on utilizing a variable frequency drive motor (in lieu of induction motor with VFD drive).

Figure 3 shows the various controls in percent load (horizontal axis) versus percent power (vertical axis) under ideal conditions.

Figure 3: Compressor Control Curve Showing Geometry, Load/No Load, Modulation & VSD. Courtesy: Atlas Copco Compressors

Every air system has unique performance charac­teristics, therefore the curve data shown above may not reflect actual, real world control performance. System design characteristics influence compressor performance. Factors such as storage tank volume, header volume, operating pressure band, number of compressors and flow events suddenly reducing system pressure can all affect system efficiency. The VSD curve shows a linear relationship between per­cent load and percent power, which is based on the unit shutting off and remaining in standby at loads below maximum turndown/minimum flow rating of the compressor.

Control gaps

Control gaps exist in nearly all compressed air systems. A control gap is a flow window where demand is too low for the trim compressor to run without blowing off, but too high for it to unload and shut down. Eliminat­ing control gaps from your system can be difficult. It’s important to understand where they exist, and why. Understanding control gaps will enable plant personnel to choose replacement equipment and system improve­ments that may eliminate gaps.

Control gaps for centrifugal compressors are sometimes more difficult to eliminate than they are in rotary screw systems, especially when all the compressors are rated for similar flows. Utilizing compressors of various capacities and operating pressures can reduce or eliminate gaps.

Fully understanding the plant flow and pressure profile is key to unlocking solutions. Figure 4 shows flow gaps in a system with three centrifugal compressors. One unit is new with an inlet valve utilizing IGV’s, and two are older units using butterfly-style inlet valves. The turndown range of a centrifugal compressor utilizing IGV’s on the inlet is considerably wider than on a unit with a butterfly inlet. A and B show the flow gap where #3 Turbo would be running in blowoff, and the tank volume required to store the flow gap cfm for one minute. Pressure bandwidth on turbo compressors is typically low, therefore the storage volume required to cover a flow gap for only one minute is generally large, as illustrated.

Figure 4: Compressor Control Gaps with 3 Centrifugal Compressors. Courtesy: Atlas Copco Compressors

Figure 5 is a histogram illustrating the system demand profile as percentage of time in various flow windows. Histograms are useful for identifying flow gaps, where system compressors and local controllers are not capable of efficiently meeting system demand. This can waste a tremendous amount of energy. Thorough analysis of the flow profile allows for correctly sizing new or replace­ment compressors. Figure 5 shows minimum through maximum flow, in 300 cfm windows.

Figure 5: Histogram Showing Percentage of Time in Various Flow Ranges. Courtesy: Atlas Copco Compressors

Figure 5 illustrates how often the equipment listed in Figure 4 is in blowoff (red bars). In flow gap A, #2 turbo is blowing off 12.8% of the time. In flow gap B, #3 turbo is blowing off 30.3% of the time. This system, as currently configured, is blowing off and wasting energy 43.1% of the time.

Figure 6 displays control range in the same system with two centrifugal compressors and one load/no load rotary screw. The graph shows no flow gaps, however that can be somewhat misleading. The screw compressor could have a high cycle interval or be running in unloaded condition. The histogram in Figure 5 will help determine if the fixed-speed screw is the right choice for the system.

Figure 6: Compressor Control Gaps with Two Centrifugal Compressors & 1 Load/No Load Screw. Courtesy: Atlas Copco Compressors

Figure 7 shows control range in the same system with two centrifugal compressors and one VSD screw. The graph shows no flow gaps, but that also can be misleading. The VSD screw compressor could be running in start/stop if trim demand is below mini­mum flow (speed). Compressor control design could impact performance near the minimum flow range of the compressor. Some VSD screws operate in load/ no load below minimum flow, therefore providing a wider range of capacity resulting in increased effi­ciency. Again, using the histogram showing time at flow will help determine which type of VSD screw is the right choice for the system.

Figure 7: Compressor Control Gaps with Two Centrifugal Compressors & One VSD Screw. Courtesy: Atlas Copco Compressors

System flow profile has a significant influence in selecting the correct equipment to reduce or even eliminate control gaps.

Subhead storage

Storage tanks are often the most economical perfor­mance investment to make in a compressed air system. Header size and length can be misleading with respect to volume in gallons. For example, 500’ of 8” header may sound like a lot of storage, but it is only 1,305 gallons by volume. Every air system is unique, so no rules of thumb consistently apply here. Figure 8 shows equivalent gallons of storage versus 1,000’ of various pipe sizes. From a value standpoint, tanks provide a much better performance value improvement compared to designing a system that relies solely on header capacity for storage.

Figure 8: Header Capacities per 1,000’ of Pipe in Gallons. Courtesy: Atlas Copco Compressors

Final words

Implementing a load sharing control system can drasti­cally improve operational efficiency and reliability in a multiple compressor system. Permanent metering equipment with data logging capabilities will assist in assessing system performance as plant conditions change. Setting an efficiency metric will allow for inves­tigating negative changes in efficiency. With experi­ence, operators will be able to target supply side or demand side inefficiencies.

The realized return on investment is in the hands of the project manager or project team. Developing an action list, plus a list of requirements for implemen­tation, will allow for setting realistic expectations. A well thought out plan will eliminate guessing and give the manager needed data to make the best investment for the facility.

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