Hitting the compressor ‘sweet-spot’

Hitachi Global Air Power
Friday, 01 January, 2010

Achieving energy efficiency with compressors

Properly matching an air compressor to the application is the first step towards improving energy efficiency. Rather like a tennis racquet, each compressor variant has a 'sweet-spot' - defining the duty zone at which the compressor operates with the highest efficiency. Represented graphically, the sweet-spot is a distinct zone within a given range of operating hours per year, and percentage loads.

Different compressors have varying sweet-spot characteristics, and this is what makes certain compressors more suitable to specific applications. For example, Figure 1 shows a conventional load/unload compressor - a basic model against which others can be compared - which has a sweet-spot at the 90% load demand.

Figure 1.

A variable speed drive (VSD) compressor has a sweet-spot from 30 to 70% (when used in excess of 2000 hours per year), as shown in Figure 2. Outside this zone it will not be an efficient option, putting paid to the myth that a VSD compressor is suited to all applications. While capable of working outside its sweet-spot, it will not be an efficient or cost-effective choice if the bulk of its duty lies outside the 30 to 70% optimum operating range.

Figure 2.

By comparison, Figure 3 shows that a variable air-end compressor has a sweet-spot from 50 to 90% duty (when used in excess of 1000 hours per year). Furthermore, a hybrid compressor - effectively a variable air-end, variable speed drive combination unit - takes on the best characteristics of both models, as shown in Figure 4, giving it an extended sweet-spot from 30 to 90% load. This can make a hybrid unit an ideal choice for applications where the load percentage might fluctuate widely during the normal course of operation. Where the compressed air demand is more stable, however, a more basic compressor choice might present a more cost-efficient option.

Figure 3.

Figure 4.

Key.

Once the compressor variant that most closely correlates with the likely duty has been identified, significant additional energy and cost savings can be realised by considering 'secondary' options, applicable to the specific environment in which the compressor will be operating.

A dirty-environment pack, comprising additional or upgraded filter systems, can help prevent a compressor from running at a higher - 'false' - load outside the sweet-spot. Regenerative-air systems that recover waste heat from the compressed air process for re-use by the compressor itself can produce a 10% energy saving.

Similarly, heat-recovery systems recover waste heat from the compressor for use in external applications such as ambient, process, or water heating which will contribute to energy efficiencies of the facility as a whole.

A soft-start feature can moderate energy demand peaks and can help keep a facility within a lower electricity tariff level.