Keeping the pressure off: compressed air is integral to many manufacturing applications but must be used at the optimum pressure for each application to achieve best return on investment.
If the compressed air is stored at this higher pressure and only used at exactly the level required for the application, the storage vessel or receiver needs only to be topped up to the full capacity, which is more efficient.
In order to achieve this optimum usage, the compressor usually operates between two pressure levels, with the receiver fitted with a pressure switch set to give compressor cut-off at the required storage pressure (usually the highest achievable for filtration efficiency) and a lower level usually about 10-20% lower.
This set-up means the compressor is not running continually, increasing component wear and associated maintenance costs. Operating the compressor at too high a pressure will also generate more heat and thus more water, which must be removed, increasing air processing costs and still supplying a system requirement at too high a pressure for no increase in output.
Cylinders rely on inlet pressure to generate the required force. If the generated air from the compressor is simply supplied directly into the mains, pressure will vary both from the compressor loading (cut-in and cut-off settings), but also as other parts of the plant demand air, creating a pressure drop in the distribution ring main. This pressure variation will result in varying force from the cylinder, impacting on process quality.
The solution is a pressure reducing valve which can rapidly pay for itself and indeed is mandatory in applications like blow guns and cooling nozzles where high-pressure compressed air use is potentially hazardous.
Pressure reducing valves or regulators are designed to maintain constant outlet pressure irrespective of inlet pressure (the regulation characteristic) and irrespective of the outlet flow (flow characteristic). Correct product selection and deployment in the relevant part of the air system will help optimise energy efficiency.
The penalty for poor regulation characteristics is varying outlet pressure, while poor flow characteristics can result in pressure drop which is directly reflected in energy costs. Every regulator suffers from some amount of pressure drop so system design must address this.
To work consistently within their design limits, all pressure regulators will require a supply pressure at least 1 bar higher than the required outlet pressure. They will work with a lower differential but performance can be impaired.
Regulators can be broadly broken into four types: general purpose, pilot operated, precision and special purpose. The regulator chosen must exactly fit application performance requirements. A regulator which controls the pressure to a distribution main is usually of the general purpose type, or pilot-operated for large volume/flow applications. Once it has been established whether a standard or precision regulator is needed, the suitability of the regulator flow capacity for pipe size needs must be decided. If there is no variation in the inlet pressure then the regulation characteristic of the regulator is unimportant but the flow characteristic will be. If the inlet pressure is exposed to variations then the regulation characteristics of the chosen regulator must also be considered. A variety of spring ranges are offered with most regulators. Ideally the regulators should be operated inside the middle third of their range, since at the lower end of their range the spring loses some sensitivity and at the higher end may suffer in linearity. Low rate springs can also help reduce pressure drop, so springs can be selected to best-fit system requirements.
If a precision regulator is required, the level of sensitivity, flow and regulation characteristics, and, if necessary, relief capacity and temperature sensitivity must be established.
Most precision regulators employ a constant bleed of air to atmosphere to aid in their response, so the penalty for employing them is a constant air demand, even under 'no flow' conditions, meaning an energy loss. Correct selection could see a general purpose regulator with ordinary performance characteristics fulfilling what may be considered a precision regulator's function without system degradation at a lower installed cost and with a reduced air demand therefore more cost-effectively and energy efficiently.
Filter/regulators both clean the air to the application and control the pressure in one compact unit. For general-purpose applications, filter/regulators are usually more cost-effective than two separate units. The filter/regulator also ensures the regulator is optimally protected. Some specialist filter/regulators are available for instrument applications with fine particle removal or even oil removal properties with precision regulator characteristics, as are others with special material compatibility.
Where cylinders are supplied directly from a compressor, not only will product quality be optimised by controlling the inlet pressure for the application with a general-purpose pressure regulator, but energy costs can be reduced by up to 30 per cent by using double-acting cylinders where a reduced pressure can be achieved on the non-working return stroke.
Taking time to understand each application in detail and then selecting and installing the correct type and size of pressure regulator will ensure optimal performance and energy efficiency.
For further information visit www.norgren.com/uk/industrialautomation
Ben Rickards, product technical manager for Norgren, reports.
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|Title Annotation:||Focus on: Compressed Air|
|Publication:||Plant & Works Engineering|
|Date:||Jun 1, 2013|
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