Compressed air system design, layout and installation for optimum efficiency

Design Considerations 

There are 5 basic elements to consider when designing a compressed air system; 

1. Compressed Air Demand and Supply
   Determining the compressed air demand is crucial in selecting a correctly sized compressed air system. Under- or over- estimating the compressed air demand can lead to an extremely inefficient system.
   
   The demand for an existing compressed air system can be determined by using a number of methods. This includes monitoring the air flow using a flow metre to implementing a data logger to track the compressors activity over a set period of time.
   
   For smaller and less complex systems, looking at the ratio between unloaded and loaded compressor running times can highlight the average demand over a set period of time.
   
   If you are wanting to determine the compressed air demand for a new system then you should consider the operating pressure requirements and the duty cycle of each piece of equipment.
   
   It is also worth considering leakage and artificial demand, and how they can be reduced, as they will both represent a significant portion of your overall demand.
   
   Compressors are typically controlled by line pressure. Here, a drop in pressure will indicate an increase in demand. This will be corrected by increasing the compressor output. A rise in pressure suggests a drop in demand and will cause a reduction in compressor output. Compressors have various capacity control systems to monitor the changes in pressure and adapt the air supply to the changing air demand. One of the most efficient capacity control systems is a load/no load control. This runs the compressor at full load or idle, adapting to demand variations.
   
   When designing the compressed air system, remember that total plant supply may be delivered by either a single compressor or a multiple compressor installation. This can be centralised or decentralised. Single compressor installations are usually better suited to small systems or those systems which operate almost wholly at full load. There are a number of advantages of operating a multiple compressor installation such as; the option of centralised or decentralised operation, maintenance flexibility and backup capability. 
   
   
2. Compressed Air Quality
   The compressed air quality you require will depend on the application. The cost of producing compressed air usually increases as the level of air quality required increases. It is therefore very important to meet – but not exceed – your required level of compressed air quality. If higher quality compressed air is only required at certain points in your compressed air system, rather than treating the entire system, consider only producing higher quality compressed air at these specific areas. 
   
   
3. Compressed Air Storage
   Compressed air is stored in an air receiver tank. It is important to consider the size of the air receiver in designing your compressed air system.
   
   A properly sized air receiver tank will;
   
   • Prevent excessive cycling; some compressor controls – start/stop and on-line/off-line – rely on storage to limit the maximum cycling frequency at demands less than 100% of supply. A properly sized air receiver tank will prevent excessive cycling.
   • Provide sufficient storage capacity for any peaks in demand; stored compressed air can be used to eliminate any pressure drops in the compressed air system during the time required to bring additional compressor capacity online. The storage capacity required will depend on a number of factors such as the amount of excess demand in cubic metres, start-up time of the compressor, the available pressure differential between the compressor and point of use, and the time available to replace stored compressed air.
   Having a flow controller installed after the air receiver tank is also very important. By increasing or decreasing the flow, a flow controller maintains a constant line pressure. This ensures that additional compressed air is provided when needed without causing downstream pressure fluctuations. In addition, it provides the required pressure differential between the compressed air receiver tank and the system to create storage without changing system pressure downstream. 
   
   
4. Compressed Air Distribution
   The compressed air distribution piping is the link between demand, supply and storage. Responsible for transporting the compressed air, due consideration should be given to the distribution system you select in designing a compressed air system.
   The perfect distribution system will provide a sufficient supply of compressed air at the required pressure to all locations where compressed air is required.
   The flow of compressed air in pipelines will create friction which will results in pressure drop. Ideally the pressure drop in the pipelines should be no more than 68 to 137 mbar.
   To reduce the pressure drop it is worth considering the following points in designing the compressed air distribution system;
   
   
   • Reduce the distance that the air must be transported
   • Reduce the friction through the pipes by increasing pipe size and removing avoidable elbows, valves and other flow restrictions
   • Eliminate leaks
   • Consider selecting smooth bore piping
   • Reduce the flow rate of the air through the system
   • Minimise the drop in pressure across the system components
   • Carefully consider the system components you select as they can also be responsible for pressure drop- Ensure all compressed air equipment is properly maintained
5. Compressed Air Energy Efficiency
   The energy efficiency of each piece of compressed air equipment will impact on the all up efficiency, reliability and lifetime costs of your compressed air system.
   
   The energy costs of a typical compressed air system can account for almost three quarters of its lifetime costs. In addition, depending on utilisation, electrical power can account for up to 90% of the total costs of compressed air production.
   
   In designing your upgraded or new compressed air system, you will no doubt investigate a number of products before selecting the right compressed air equipment for your installation. Bear in mind that the energy costs taken over the lifetime of any compressor add up to many times that of the initial capital cost, which can make any purchase price difference a false economy.
   
   If you want to operate a compressed air system that performs at its optimum - efficiently and reliably - it would therefore be prudent to compare the lifetime costs and energy efficiency of these products before making your final decision.

Don’t forget to visit the KAESER Know How Blog next month for part 2 of this blog post which will discuss the layout and installation considerations for designing a compressed air system. 

Reference: KAESER Compressors, Inc. (June 2007): Designing your compressed air system: How to determine the system you need

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