In the good old days, most industrial plants ran their air compressors at levels below 100 psi (6.9 bar). The most common pressure rating of standard air compressors was 100 psi, so typical operation needed to be slightly lower. But over the years, plant pressures have crept up, now often reaching levels of 120 psi (8.3 bar) or higher. This has increased the compressed air specific power – the amount of power (kW) needed to produce a standard amount of compressed air (typically 100 cfm) – costing companies thousands of wasted dollars on their energy bills.
The physical properties of air dictate that to produce compressed air at 120 psi compared with 100 psi, you must input about 10% more power for the same volume of air. The relationship is about 1% additional power for every 2 psi of higher pressure. In addition to this, the higher pressure causes unregulated end uses in the plant to consume more compressed air – about 0.9% more per psi increase, increasing the energy cost an additional approximately 10% (not all demand is unregulated). Also, air compressors that output 120 psi must operate at reduced capacity as compared with lower-rated units, meaning a 200 hp (160 kW)-rated air compressor will not produce the same amount of compressed air as a 100 psi-rated model, so larger, more expensive capacity must be purchased for a given cfm output.
In industrial plants across the world, it is common to find some operating with low system pressures near 80 psi, while some are as high as 200 psi. When we investigate why there is such a wide variation, we often find that the increase in pressure has been caused by one or more critical pieces of production machinery that won’t operate correctly at the lower pressure. Investigating further, we see that the machinery has actually been designed for higher pressure.
Specifying a higher input pressure allows the designers of pneumatic machinery to reduce the one-time cost of the parts within the machine. Smaller versions of components such as tubing, regulators, actuators, and valves can be used because internal velocities are lower. This saves the manufacturer from having to purchase more-expensive components, but it leaves the user to pay higher ongoing operating costs related to the increased energy consumed not only by the offending machine, but also by all the other lower-pressure-rated machines and tools that might be connected to the same higher pressure.
Some knowledgeable companies have fought back and are specifying the supply of machines that are designed to use no more than 90 psi (6.2 bar). This sometimes requires the redesign of machinery, but it will allow them to keep their system pressure low, saving ongoing power costs year after year. This might be possible in your system if you care to ask.
Some other causes of artificially high system pressure settings:
- Poor pressure control of compressors, causing wide pressure variations, with the pressure increased to compensate for this problem
- Lack of adequate system storage receiver capacity, causing poor compressor control, with pressure increased to create artificial storage,
- Piping or pneumatic component restrictions that are causing excessive pressure restrictions, and the pressure must be raised to overcome them. This is a particular problem on end-use connectors, hoses, filters, regulators, lubricators right at the end use.
- Some end use that is consuming large amounts of compressed air for a short period of time, causing intermittent low-pressure problems, with extra pressure used to compensate.
- Unreliable compressors are shutting down intermittently, causing a lack of capacity and low-pressure events, with pressure increased as a result to give time to start another compressor,
- A general lack of capacity during peak demands, causing low pressure, more compressors are required,
- Sometimes, just because the compressors are rated at 125 psi (8.5 bar), the compressor service technicians will simply set the pressure at the higher range for no particular reason.
If these are your problems, you should realize these are preventable. Call in an experienced compressed air auditing professional to investigate.
Ron Marshall is a compressed air energy-efficiency expert and a compressed air trainer; learn more at www.compressedairaudit.com
Join Ron at the Compressed Air Efficiency LinkedIn discussion group at https://www.linkedin.com/groups/12065406