Hitting the compressor ‘sweet-spot’

Selecting the right air compressor can be confusing. There are numerous compressor types, variants and options to consider — each of which can impact energy efficiency and operating costs. Identifying a compressor’s ‘sweet-spot’ can help determine if it is right for the application.

Industry is under increasing pressure to improve its environmental performance, and during these times of global financial uncertainty, businesses are escalating their efforts to reduce costs. Typically, a compressor system will be one of the largest sources of energy consumption in a factory, and overall inefficiencies of 30 to 50% are not uncommon. For plant managers looking to cut production costs, optimising the compressed air system can be a good place to start.

Businesses are often under the impression that the measures needed to improve environmental performance are essentially unrecoverable costs. While many such measures have little or no financial return, this is not necessarily true when they improve energy efficiency. Projects that reduce energy consumption will help decrease running costs, and will consequently minimise the plant’s carbon emissions to the environment.

Many businesses consider optimising environmental performance a nebulous concept that has been forced upon them; something that will offset their efforts to run a profitable business. Plant managers are all too frequently under pressure to minimise operating costs and at the same time ‘tick the environmental box’, and will often take the least action necessary to appear to be improving environmental performance. It is often a pleasant surprise when a company realises it can take real action to reduce its carbon footprint, and actually reduce costs significantly at the same time.

Key for figures 1 through 4: Showing the sweet-spot zone, and zones of questionable efficiency and low efficiency

Application matching

It is common for a factory to have an air compressor chugging away in the background. Here, there is a misconception that this compressed air is ‘energy for free’ - this could not be further from the truth. As 10% of the world’s industrial energy is used to generate compressed air, any efficiency improvements made can have a significant impact on a factory’s overall energy consumption.

Properly matching an air compressor to the application is the first step towards improving energy efficiency.

For new installations, this can be done theoretically based on the performance characteristics of each compressor model. When replacing a compressor, however, an ‘air audit’ is an essential tool to properly assess existing conditions and determine what improvements can be made. A plant with air compression systems running at 50 to 70% efficiency - a fairly typical finding of an air audit - can be improved to 90% efficiency with the installation of the right equipment. An air audit will help determine the number of hours an air compressor is expected to work per annum, and assess its normal operational load-percentage range. This data can be used to determine the most suitable compressed air solution for the application. Every application is different, so the best-suited compressor will vary from case to case.

Figure 1: An on/off load compressor has a sweet-spot at the 90% load demand, at which point it will work most efficiently.

Maximum output for minimum input

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 (See above for a key to the colours used in Figures 1 through 4).

Figure 2: A variable drive compressor has a sweet-spot from 30 to 70% when used in excess of 2000 hours per year.

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, exploding 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.

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: The sweet-spot for a variable air-end compressor lies between 50 and 90% load, with operating hours of over 1000 hours per year.

Payback time

The zone at which a compressor runs at the highest efficiency usually represents the period of shortest payback. Payback (expressed in years) is a measure of how long it will take the reduced running costs to offset the capital investment of a replacement compressor. Generally, this is measured as the cost differential between an ideal compressor and an existing or base model, divided by the cost savings per year.

For an existing installation, the payback concept allows various new compressor options to be compared against the in-situ system - effectively measuring the time it will take to recover the cost of an upgrade against doing nothing. For a new installation, the payback will measure the time it will take to offset the additional expense of a more efficient compressor over a basic on/off load model. Payback is a good measure of operating efficiency, although the correlation is not absolute. For example, a highly efficient compressor might not give a good payback figure if it is prohibitively expensive to purchase. Generally, however, a short payback provides a good indication that a compressor is well matched to its application, and has a good balance between capital cost and running cost.

Figure 4: Hybrid compressors effectively take on the characteristics of both variable air-end and variable speed drive models, giving it a sweet-spot from 30 to 90% load, when used in excess of 2000 hours per year.

Extra efficiency

Once the compressor variant has been identified that most closely correlates with the likely duty, 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. A compressor running with clogged filters will consume more power to generate the same amount of air and therefore will be wasting energy. Improved filter systems can help keep a compressor running in its sweet-spot and hence prevent unnecessary energy use.

Regenerative air systems recover waste heat from the compressed air process for re-use by the compressor itself. Compressors will typically produce air at 18 °C. For every three degrees the compressed air can be heated up, there is a corresponding increase in power efficiency of 1%. Using waste heat generated by the compressor to reheat the compressed air from 18 to 50 °C 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. While not improving the efficiency of the compressed air process itself, these 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. This will not reduce energy usage per se, but can reduce energy costs. Typically, if a facility exceeds a predetermined threshold, it will then pay a ‘penalty’ tariff rate - often double the price of the normal tariffs. The demand spike caused by compressors starting up - although only momentary - can often be enough to elevate a site above the penalty tariff threshold for the rest of that day.

A reputable compressed air solutions provider will not simply try to sell a user the compressor variant they have in stock, but rather will advise on the most energy- and cost-efficient compressor for the application. An air audit will help determine the compressor variant with a sweet-spot matched to the application. This will allow for the ongoing provision of cost-effective and environmentally friendly compressed air into the future.

Champion Compressors Ltd
www.championcompressors.com.au