Innovation in Air Compressor Controls
Implementing more efficient controls in compressed air generation sparks significant energy savings at several U.S. Navy installations.
By Ned Dempsey, P.E., and Rear Adm. Benjamin F. Montoya, F.SAME, USN (RET).
New compressed air controls are driving significant energy savings at Puget Sound Naval Shipyard. PHOTO BY EWING CREATIVE INC.
The Department of Defense (DOD) continues to be a leader in looking for ways to decrease energy consumption and reduce its carbon footprint, both to cut costs and to align with directives such as Executive Order 13514. In fact, the U.S. military is actually more aggressive in pursuing energy reduction, given these mandates, than many areas in the private sector. In pursuit of DOD’s goals, its service branches have been sponsoring energy studies and soliciting recommendations from consulting firms—and taking steps to fulfill them.
These energy audits show that one area in particular where large amounts of energy are used, and where operations can benefit from more efficient controls, is in compressed air generation.
The U.S. Navy especially uses compressed air for a multitude of functions: sand blasting, painting, operating air-actuated tools for grinding, cutting and fastening devices, as well as portable air ventilators, breathing air, and for freeze seals. When the fleet is in port, land-based compressor capacity needs to be available to serve shipboard needs and pressurize ship systems. This is especially important on nuclear-powered vessels.
ROOTING OUT THE WASTE
The Navy has mounted compressed air efficiency improvement programs in several locations, including: Puget Sound Naval Shipyard (PSNS), Bremerton, Wash.; Naval Base Coronado, Calif.; and Joint Base Pearl Harbor-Hickam, Hawaii. At each of these installations, audits analyzed airflow, system pressure and energy usage to power compressors over a period of time. The audits uncovered wasteful situations and made recommendations for either reducing the waste or eliminating it altogether.
PSNS, for example, used eight large and aging centrifugal compressors for industrial air supply, some of which were in need of major servicing. One of the compressors was a 2,500-hp 11,500-V device that was difficult to start. The shipyard was considering replacing this and one other compressor with new 1,250-hp units, and then begin a cycle that would replace the other units in subsequent years. But PSNS wanted to ensure that as part of the overall process of making changes they also considered the energy efficiency of the new solutions in order to qualify for financial incentives from the regional power supplier, the Bonneville Power Administration.
CONTROLLING COMPRESSED AIR
At the time of the PSNS audit, the Navy had the policy of always running one more compressor than was necessary. This was a result of needing to provide an adequate air supply under all conditions, while not being able to control the air supply system to any degree of accuracy. As a result, there was a significant amount of energy wasted. Further, the centrifugal compressors were often operated in “blow-off” mode. Unneeded compressed air was vented to the atmosphere. This was extremely inefficient as compressor power consumption remained constant even when airflow requirements decreased.
One of the major recommendations of the audit was to bring all PSNS compressor resources under central control by a microprocessor-based system. This setup can monitor demand for compressed air flow and pressure and adjust the supply accordingly. The audit further recommended that each compressor at PSNS should be run in an operating range where it is most efficient, without blow-off.
In the case of the centrifugal compressors, ideally that meant base loading them, operating them either at full output or turned off. The auditors recommended that PSNS add three new 400-hp rotary screw compressors to support variable air supply needs beyond that. These are more efficient than the centrifugal compressors when operating at partial load. When system airflow needs exceed the capacity of these trim compressors, another centrifugal compressor should be turned on and operated at full output. In this manner, unneeded compressors are kept shut off on standby and wasteful air blow-off would be virtually eliminated.
Control schematic of the air compressor system at Puget Sound Naval Shipyard. After the Pneu-Logic controls were installed—eliminating the old, inefficient blow-off controls—a re-audit showed that the specific compressed air delivery efficiency increased by 50 percent. IMAGE COURTESY PNEU-LOGIC CORP.
INSTALLING NEW CONTROLS
To implement these controls strategies, PSNS installed a PL4000 compressor controller manufactured by Pneu-Logic Corp. of Portland, Ore., and networked it with all the compressors.
The Pneu-Logic system can be retrofit to accommodate future growth. To provide air flow and pressure feedback to the controller, eight flow meters were installed—one was placed at the discharge of each centrifugal compressor and one was placed downstream of two new 5,000-gal air receivers and a new pressure/flow controller. Remote pressure sensors also were installed at various locations in the shipyard with feedback provided to the controller.
When it is time to bring a new compressor online, the controller decides which one to run by consulting the “readiness scores” of available compressors. The controller assigns each compressor a run-time score that changes with time as the compressor operates. It takes into account factors such as how many hours each compressor has run, and when maintenance is scheduled for a particular compressor. When a compressor hits a threshold, the control system gives it a break and turns on another from the same group, with a similar capability, to replace it.
The evaluation process is complex because of the number of combinations that are possible in the typical multi-compressor plant. The controller maintains a database of tables of compressor combinations that provide optimal solutions for different conditions. In general, to fulfill the audit recommendation and minimize energy waste, the controller will run, at most, one trim compressor at part output. All others are either running at full performance or turned off. An exception to this rule is when load sharing among multiple centrifugal compressors will either prevent or minimize blow-off.
The operation of the compressor control system, as well as the status of individual compressors, can be monitored and controlled using the system’s graphical control panel. Using the controls, it is possible to tune the system’s operation as it runs. The controller also can export operational data to a distributed control system for remote access.
After the Pneu-Logic controls were installed at PSNS—thereby eliminating the old, inefficient blow-off controls, a re-audit showed that the specific compressed air delivery efficiency increased by 50 percent. This translated to a per year savings of more than 10 million-kWh of electricity.
Since the initial control system installation at PSNS, Pneu-Logic has performed a second project at the shipyard, providing additional savings of approximately 200,000-KWh per year. PL4000 controllers also are being installed at Coronado and at Pearl Harbor. The results of these new controls can be measured by several million dollars of energy savings, and through significant equipment cost savings derived from refurbishing existing compressors rather than buying new ones.
These savings validate that this program is precisely aligned with the Navy’s directive that its shore operations adopt innovative facility technologies such as advanced metering and energy management systems to improve energy efficiency.
As centralized air compressor control systems are deployed to other Navy facilities, the dividends seen at PSNS and elsewhere will be multiplied—and the Navy’s experience will serve as a growing example of sustainability and energy reduction to the other military branches.