FEATURE COMPRESSED AIR
VARIABLE SPEED LIMITS W
Mark Whitmore, general manager of BOGE explains how to get the best out of variable speed compressor installations in order to significantly reduce energy consumption
hen companies try to reduce energy consumption in their compressed air
systems they often consider the use of variable speed drives (VSD), either installed on new dedicated units or as a retrofit to existing machines. This makes a lot of sense: A VSD allows a compressor to adjust its operating speed to keep system pressure within a set band, matching energy consumption closely with real demand. VSD installations need to be made with care. The wrong equipment or settings can result in a system that is just as expensive to run as the fixed speed alternative. Sometimes VSD installations can even increase costs by harming reliability and shortening the life of key components. Here’s how to get it right.
RIGHT SIZE If the loads are high relative to the output of the compressor, requiring operation at more than 80% of its capacity then variable speed operation won’t offer significant savings. A more common situation that can cause problems is where there is relatively low demand for compressed air most of the time interspersed with infrequent peaks. Operating a compressor at very low speeds can create problems including poor efficiency, moisture accumulation in the compressor lubricant and motor overheating. As a result, compressor manufacturers set a minimum turn-down speed for their variable speed machines, typically 30% of their maximum output. If demand falls below this threshold the
machine will revert to stop-start operation. In this situation it is often
preferable to install two compressors, either of which may be fitted with a VSD. One unit will be sized to meet base demand, the other to provide the capacity needed at peak times.
RIGHT EQUIPMENT The performance of even a properly sized and specified VSD compressor depends on the characteristics of the overall installation. Any company seeking efficiency improvements should consider the use of VSD compressors alongside a range of further possible efficiency measures including heat recovery, leak detection and improvements to air distribution pipework. One key consideration is the installation of adequate air storage to slow and smooth pressure variations as demand changes, helping the compressor to keep up. Leaks and pressure differentials across
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Installing VSD in a compressed air system must be done with care in order to fully benefit from energy savings
system components including pipework and filters can mean that pressure at the point of use falls below the specified range, especially during periods of peak use. The installation of remote pressure sensors close to the point of demand can allow the system to compensate for these issues without requiring excessive pressure at the compressor when it is not required. Retrofitting a VSD to an existing fixed speed machine is sometimes seen as a money saving option but is not without risk. To improve motor cooling at low speeds for example, dedicated variable speed compressors are usually equipped with a larger motor than fixed speed machines of the same capacity. In a retrofit installation users may have to accept a higher minimum speed to prevent the compressor overheating and shutting down. Moreover, the motors in fixed speed compressors are not usually fitted with insulated bearings to prevent damage caused by the stray currents generated by VSDs.
RIGHT CONFIGURATION Most VSD compressors have a target pressure setting, upper and lower acceptable pressure limits and separate limits that determine whether the system should be unloaded or shut down if it reaches its minimum variable operating speed. The system will behave differently depending on how these parameters are set. If the target pressure is close to the upper limit the compressor will run faster after start up, reaching the required pressure more quickly to meet demand peaks but potentially consuming more energy in the process. Setting the target close to the lower
limit has the opposite effect but can mean more frequent stops/starts during periods of low demand. The width of the acceptable pressure band will affect the frequency of stops/starts, although this may be determined by the specification of the load. In some circumstances, optimal performance may require adjustments to the PID parameters used in the compressor’s speed control program. Compressor manufacturers can provide assistance where this is required.
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BOGE Compressors T: 01484 719921
www.boge.co.uk
10 SEPTEMBER 2016 | FACTORY EQUIPMENT / FACTORYEQUIPMENT
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