Q the cost of providing compressed air (including maintenance and repair of equipment) continues to escalate. What can we do to help control this cost?

Background: Metalcasting facilities typically have many compressed air applications. Core machines require clean, dry air, table lift cylinders in molding machines must receive a minimum amount of stable pressure to provide consistent quality, and compressed air is used for sand blasting and conveying tasks.

To support these production processes, compressed air systems must be properly sized and configured and adequately serviced. Maintaining a stable, consistent flow of clean air is a critical factor to greater reliability, lower energy consumption and better productivity.

Recommendation: There are four major areas that determine the cost and efficiency of the compressed air system: unstable head pressure; high levels of moisture and oil; size and configuration of discharge piping; and leakage and filtration. Managing these four areas can reduce air demand, improve efficiency and reduce costs. Results from one metalcasting facility's experience are shown in Table 1.

Unstable Head Pressure--Unstable pressure can be caused by intermittent air demand from heavy compressed air uses, such as in large molding machines. Excess demand can cause pressure loss and lead to production shutdowns. Utilizing a multiple compressor operating scheme can contribute to this behavior if each compressor is controlled individually.

Operation of multiple compressors simultaneously is inefficient because none will operate at full load. Pressure stabilization can be accomplished by using adequately sized pressure/flow controllers and dedicated storage tanks to serve as reservoirs. Setting the system pressure at a level more closely aligned with the minimum operating pressure of the end-use application can increase efficiency.

Moisture &Lubricant Carryover--The appropriate operating dew point of compressed air is essential for reliable production. The pressure dew point is the temperature at which water vapor in the compressed air begins to condense. The lower the pressure dew point, the drier the compressed air. It is important to determine the degree of dryness required by production parameters. Drying air beyond the minimum required level is costly and wastes energy.

After the required degree of dryness is determined, it is important to know both the volume of air and its pressure before selecting a specific dryer. In order for a dryer to provide its rated pressure dew point, the inlet air temperature and the air at the dryer location must meet the levels specified for the equipment.

Size & Configuration of Discharge Piping--Compare the pressure reading at each machine with the pressure reading at the compressor to measure the pressure loss. A 10-psi pressure drop will cause a 13% loss of efficiency, and a 30-psi pressure drop causes a 55% efficiency loss. The size of the compressor discharge piping can lead to these pressure drops if it is too small for the volume of air that was being sent through it.

The configuration of the compressor discharge piping also can create pressure drops if the discharge piping is connected to a crossing header near a 90-degree turn. These piping details will cause pressure fluctuations because the compressor fights against the resistance of the discharge piping, which creates a back pressure. A strong back pressure gives false signals to the compressor controls to cause premature compressor unloading.

To correct this situation, replace the existing discharge header piping with a larger diameter pipe and replace the existing 90-degree crossing header with a 30-degree directional entry pipe.

Leakage & Filtration--Total air demand can be reduced by replacing worn, leaking hoses and shutting down unneeded compressed air applications. Replacing old pre-filters and after-filters on the dryers and swapping manual condensate drains for a level-actuated electronic drain also will reduce artificial demand. Point-of-use filters, regulators and lubricators (FRLs) ensure that compressed air applications receive a clean, lubricated supply of air at the appropriate pressure level.

FRLs must be properly sized to satisfy the peak flow requirements. Improperly sized FRLs lead to pressure drops, shorter end-use equipment life and inconsistent air quality. Oversized filters may not develop the centrifugal action to separate liquids, and undersized filters may induce too large a pressure drop.

FRLs should be positioned as close as possible to a compressed air application with as few elbows and bends between them and the application as possible. The combined total pressure drop in the filter and lubricator should not exceed 3 psi.

Table 1. Improved Air Compressor Operating Profile

Before Improvement--Air Demand of 12,000-15,000 scfm

3,000-hp centrifugal compressor   90% loaded
1,500-hp centrifugal compressor   88% loaded
800-hp reciprocating compressor   65% loaded
800-hp reciprocating compressor   standby, up to 50% loaded

After Improvement-Air Demand 20,000 scfm (adequately served with
3,000-3,800 hp)

3,000-hp centrifugal compressor   100% loaded
1,500-hp centrifugal compressor   off
800-hp reciprocating compressor   off
800-hp reciprocating compressor   standby, up to 50% loaded