Treating compressed air as a process variable can reap big rewards

Figure 1. The pressure variation upstream and downstream of a pressure/flow controller.

How it's done

Pressure/flow controllers are precision control valves that vary the airflow to maintain discharge pressure within approximately one percent of setpoint without introducing additional pressure loss. Multi-valve pressure/flow controllers are also available for systems with critical redundancy requirements or unusually large variations in compressed air demand.

Figure 2 shows a typical system using a pressure/flow controller. Normally, only one of two trim compressors operates upstream of the pressure/flow controller, which minimizes the energy consumed. The remaining compressors operate downstream of the pressure/flow controller at a pressure equal to the header pressure plus the differential across the filter and dryer. Operating at this lower pressure reduces power to these base-loaded compressors by 10 to 20%, depending on individual pressure requirements.

Figure 2. A typical system using a pressure/flow controller.

Eliminate system dynamics

Variations in system demand, compressor operating mode, setpoints and pressure drops across cleanup equipment cause pressure swings. Pressure/flow controllers eliminate pressure swings by separating the air supply from the air demand, which stabilizes header pressure.

The pressure differential produced by reducing the pressure in the distribution header is useful storage that supports intermittent demand events without starting another compressor or affecting header pressure.

In addition, reducing the header pressure reduces artificial demand in unregulated uses. Artificial demand occurs in unregulated uses when plant personnel increase header pressure to resolve system problems. It also can arise when plant personnel overcome pressure drop by adjusting a regulator to increase the pressure. As a rule of thumb, we estimate artificial demand by assuming that air demand increases one percent for all unregulated uses, for each psi increase in header pressure. Pressure/flow controllers reduce artificial demand, in turn, reducing power requirements.

Reduce costs

In addition to reducing artificial demand, pressure/flow controllers allow base-loaded compressors to operate at a lower discharge pressure, thus reducing power requirements. For example, for positive displacement compressors, power decreases one percent for every two-psi reduction in discharge pressure. The additional storage required by the pressure/flow controller to maintain a flat header pressure improves the efficiency of compressor operating modes, such as load/unload, which depend on the amount of storage.

Most compressed air systems offer efficiencies between 3.2 and 4.0 scfm/ibhp. Therefore, reducing demand by 3.2 to 4.0 scfm reduces input power by one bhp. The corresponding saving would be $1.00/hp/day, or $365 annually, if the energy rate is $0.05 per kWh.

In a typical compressor system, operators must elevate the pressure to keep it from dropping below a minimum acceptable pressure when demand increases or a compressor fails. By installing a pressure/flow controller with control storage, a plant can operate its header pressure at the minimum acceptable pressure, which eliminates artificial demand and reduces power without risking production interruptions. For example, given sufficient control storage and a backup compressor, the configuration will support the system at full header pressure during the time it takes to start a backup compressor when the largest compressor fails.

A lower dew point helps ensure that liquid water won't appear in the compressed air system, which may avoid the need to install expensive and complex desiccant dryers. A pressure/flow controller maintains the lower header pressure setpoint by expanding stored air. The expansion process increases the capacity of air to hold moisture, just as compression reduces it. For example, when a pressure/flow controller expands air from 125 psi 38 F dew point to 80 psi, the pressure dew point drops to 30 F. However, the exact magnitude of the dew point reduction is also a function of the system configuration.

Storage needed

To maintain a constant system pressure, the pressure/flow controller requires sufficient upstream and downstream storage. The three common types of compressed air storage are summarized below:

Control storage: Upstream or control storage is a form of potential energy that the pressure/flow controller releases to maintain header pressure. Proper control storage volume depends on the trim compressor's capacity and operating mode, the magnitude of the largest event (normally the failure of the largest compressor), and intermittent, coincidental events.

For example, the part-load efficiency of many rotary screw compressor operating modes relates directly to the amount of system storage. In addition, compressor manufacturers impose a minimum load/unload cycle time to prevent mechanical failure. For compressors operating in load/unload mode, a good guideline is a minimum of 10 gallons of storage for every cfm of capacity in the largest trim compressor. Assuming a 10-psi control band, using this approach to size the control storage provides reasonable compressor efficiency and a minimum load/unload cycle time of four minutes. Additionally, the control storage should have the capacity, within the available differential, to support the system through the largest event (normally the failure of the largest compressor) without loss of header pressure. In systems with large events, this may require supplementing the control storage with high-pressure offline storage.

General storage: Downstream, also called general or header storage, controls the rate of pressure change in the distribution header, which gives the pressure/flow controller and trim compressors time to respond to an event. To be effective, general storage must be located strategically so that high flow rate applications can access it during each cycle. Otherwise, the event will draw down the volume in the header, producing unacceptable pressure variations.

Secondary storage: Secondary storage is dedicated storage, with or without metered recovery, that's located at an end-user. While secondary storage isn't required for a pressure/flow controller to function properly, its use reduces critical pressures, which allows plant personnel to reduce header pressure and save energy. For example, installing secondary storage at baghouses and dust collectors allows them to operate at 80 psi instead of 100 psi or more.

In addition, secondary storage improves equipment performance, protects critical equipment and processes during power outages, and averages the demand from intermittent events over time. For example, in paper mills, secondary storage protects paper machine felts and wires while the paper machine coasts to a stop during a power outage. Also, secondary storage can support air padding from storage rather than from online horsepower or by starting another compressor. Using secondary storage to overcome pressure differentials on production equipment often increases productivity and reduces waste.

When and how to spec

Exceeding a pressure/flow controller's maximum capacity introduces a pressure drop that consumes useful storage differential pressure. Using an oversized unit, on the other hand, results in header pressure variations at minimum airflow. To size a pressure/flow controller properly, you'll need the following information:

Minimum, normal and maximum airflow.

Inlet temperature.

Maximum and minimum setpoint pressure.

Maximum and minimum upstream pressure.

While every system can benefit from a pressure/flow controller, not every system needs one. The arguments against them are threefold. The first is that pressure/flow controllers increase power consumption because the compressors must operate at higher pressures to fill storage. When a plant reconfigures its system to resemble Figure 2, most of the horsepower is consumed downstream of the pressure/flow controller, and the system actually operates at a lower average power consumption.

The second argument is that installing a pressure/flow controller minimizes the value of the existing header storage and requires adding additional control storage. While it's true that other control schemes can capitalize on the existing capacitance, they can't exploit the contained air without letting the header pressure drop, which means they're normally operating at a higher pressure than is necessary.

Thirdly, some argue that pressure/flow controllers add unnecessary cost to compressed air system retrofit projects. The truth is that no one can make this claim without first analyzing the system in question. Also, the value of the storage in terms of lower, more accurate header pressures usually far exceeds the costs.

The systems that won't benefit from a pressure/flow controller must contain most, if not all, of the following:

Large system capacitance.

No production benefits from a more accurate header pressure.

Little, if any, artificial demand.

Centrifugal compressors operating in a pressure range where performance doesn't change significantly.

In the final analysis, plant personnel should base the decision to install a pressure/flow controller upon individual priorities and lowest operating cost. Because each system is unique, the solutions are always different. Don't force a solution to fit any preconceived plan. Let the analysis dictate the solution.

Regulators vs. controllers

Standard regulators are unacceptable as pressure/flow controllers because they have both a control band and a differential pressure that adds unnecessary pressure drop. Additionally, the standard regulator's slow response rate produces pressure variations in the header, harming the intended benefit of a pressure/flow controller.

While precision-piloted regulators still have a three- to seven-psi control band, the better units have a minimal differential pressure and the response rate is satisfactoryif sufficient storage is present on both sides of the regulator.

A precision regulator can be used on a smaller system successfully. However, the additional pressure drop requires either raising the trim compressor operating pressure or increasing the size of the control storage receiver.

Increasing the trim compressor discharge pressure to overcome these additional losses increases operating cost by 1.25 to 3.5%, while installing a larger receiver increases capital cost. Either way, the additional cost is unnecessary, because an appropriate pressure/flow controller has a control band of less than one psi. In larger systems, the additional operating cost for a precision regulator will exceed any first cost savings.

Last word

Compressed air audits sometimes reveal a bypassed pressure/flow controller. In most cases, we find the plant took it offline because it was improperly sized or the system had insufficient control storage, excessive pressure differentials or users that require high pressure.

Further investigation usually reveals the plant purchased and installed the equipment on the basis of a sales brochure or a recommendation in a "free" or a "supply-side" audit performed by an equipment supplier. While some pressure/flow controllers deployed this way have run successfully, the systems on which they're installed don't have major air demand variations. In addition, these systems often contain missed opportunities to reduce energy costs that would have more than paid for a complete audit and the equipment required for the system to function properly. The best way to ensure a successful installation and to maximize your energy savings is to perform a thorough plant-wide compressed air audit with a critical focus on production and demand-side opportunities.

Chris Beals is Senior Auditor with Air Science Engineering, Inc. He can be reached at [email protected] and 303-771-4839.

Figures: Air Science Engineering, Inc.