Compressed air as a process variable? It could lead to boosted productivity

Flatlining header pressure can help achieve productivity gains.

By Chris E. Beals

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Companies continually search for ways to increase productivity and profitability. Most gains come from:

  • Improved quality (process repeatability, product consistency).
  • Increased capacity (production, throughput).
  • Reduced costs (energy, material, capital investment).

Many plants use advanced control and automation systems to minimize process variability and achieve productivity gains. Unfortunately, few treat compressed air as a process variable, so they fail to reap the benefits that these tools can offer in managing this critical "fourth utility."

Controlling air pressure variation can stabilize a production process that depends on compressed air, increasing uptime, reducing waste and providing the foundation for increased throughput. With stable air pressure, header pressure can be reduced, reducing air demand and saving energy. Pressure/flow controllers can flatten air pressure variations. The unit limits pressure variations to the compressor room and stabilizes header pressure.

What are they?

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.

System configuration

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.

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.

Reducing 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.

Eliminating artificial demand

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.

Reducing energy 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.

Minimize the risk of pressure loss

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 shown in Figure 3 will support the system at full header pressure during the time it takes to start a backup compressor when the largest compressor fails.

Lowering the dew point

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 degrees Fahrenheit dew point to 80 psi, the pressure dew point drops to 30 degrees fahrenheit. 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.

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