Under pressure
15 Jan 2000
Compressed air is one of the most basic and most frequently overlooked resources for any process plant. Yet the complex systems of multiple compressors required by many facilities can incur hidden costs. If more compressors are running than are needed to meet the process's air requirements, or if air escapes through the various parts of the compressor system, then the system is consuming more power than necessary and reduction of power consumption is one of the industry's hottest topics (see p7).
The compressor manufacturers' answer to this problem is to develop centralised control systems to operate multiple compressor units. Two of the largest suppliers Atlas Copco and Ingersoll Rand offer such systems, but take different tacks on the problem.
Most multiple compressor installations work on a 'pressure cascade' principle. The first, or base load, compressor is designed to meet the plant's maximum pressure needs that is, the minimum amount of compressed air. If the pressure falls below the base load compressor's operating limits, a second compressor is automatically started up.
However, because the cascade effect is triggered by a fall in the output pressure, the base load compressor has to be turned on at a higher pressure than its maximum, then reduced to bring the next one on line. This means that the compressors are always running at a higher pressure than is necessary, leading to significant wastage of power (see diagram).
Ingersoll-Rand's approach to this problem involves a control system known as Intellysis. Capable of controlling a network using up to eight individual compressors, this system matches the pressure delivered against a variety of pre-programmed 'targets'.
The system stores a seven-day, 24-hour programme of pressure demand, and monitors the pressure delivered via a transducer within the distribution network, placed downstream of the system's filters, dryers and receivers. The Intellysis system switches in and out the optimum combination of units to deliver the required pressure.
The Atlas Copco system designated ES300 also works by optimising the performance of multi-compressor installations. The system looks at the capacities of each compressor in the network and automatically brings on-line the best combination of units to meet the compressed air demand. This is far more efficient than cascade control at keeping air pressure within the rigidly-defined bands demanded by such applications as drugs and bottle-making.
Not only does this technique avoid the power wastage associated with cascade systems, says Atlas Copco, it also ensures that all the compressors run for a similar period of time, incurring roughly the same amounts of wear on the moving parts.
