Compressed air systems: Calculate the costs
August 12th 2011

Air Society (BCAS) has recognised that many users of compressed air do not know how much their system costs to operate on an annual basis, or how to calculate the cost. Therefore they are unaware of how much money could be saved by improving the performance of their system – even though the delivery of quality compressed air (in terms of pressure, flow and purity) is essential to their production facility. Chris Dee, executive director of BCAS offers help with calculating the electricity cost Electricity costs are by far the largest single expense in operating any compressed air system with additional costs for maintenance – including parts and labour – also needing to be included.

The BCAS view, over a typical 10-year period, is that the breakdown of costs is as follows:

• 73% energy costs
• 18% capital equipment purchase costs
• 7% maintenance costs
• 2% installation costs.

The BCAS Installation Guide refers to three groups of air compressors:

• Small – Up to 40L/s
• Medium – 40 to 300L/s
• Large – Above 300L/s

Although this introduction focuses on medium machines, the principles also apply to the smaller and larger machines.

Simple calculation

The following information is needed for a quick and simple calculation of your electricity costs to run a single fixed-speed compressor operating at full load:

• Compressor motor nameplate rating – kW
• Compressor motor nameplate efficience – % (see note that follows)
• Operating hours per year – hrs
• Cost you are paying per kWh – £/kWh from your electricity bill

Note: Use 90% if there is no figure given and it is a motor less than 10 years old and has not been rewound. If it is an older motor or one that has been rewound use a figure of 80% as older motors (and motors that have been rewound) may be several percentage points less efficient even than standard-efficiency motors, therefore it makes sense always to specify higher-efficiency models wherever possible.

The annual electricity costs can then be calculated by inserting your information in the formula:

Electricity costs = (kW x annual hours of operation x cost of electricity) / motor efficiency

For example, if you are using a compressor with a motor nameplate rating of 75kW that has an efficiency of 90% and it is running on full load for 2000h/year and the cost of electricity is £0.07/kWh then the annual electricity cost for the compressor will be:

Annual Electricity costs = (75 x 2000 x 0.07) / 0.90 = £11,666 per annum This, remember, is the electricity cost only for the air compressor and for a more

accurate assessment of the electricity costs for the system the same formula can be used for all additional compressors and other electrical motors used in the system.

More accurate

A more accurate method to calculate electricity consumption and hence costs involves taking electrical measurements of both full-load amps and volts. Motor fullload kW and efficiency are not required for this calculation, although you will need a full-load power factor, which can be obtained from compressor/motor manufacturers. The first calculation takes full-load amps, converts to full-load kW, and then multiplies by hours of operation and electricity costs and is shown below:

• Annual electricity costs = (Full-load amps x volts x 1.732 x power factor)/1000 x (annual hours of operation) x (electricity cost in £/kWh)

For example, if you are using a compressor that has a full-load rating of 115 amps and running at 460 volts for 2000 hours per year and the cost of electricity is £0.07/kWh, with a full-load power factor of 0.85 and a correction factor for 3 phase power of 1.732, then the annual electricity cost for the compressor will be:

(115 amps x 460 volts x 1.732 x 0.85) /1000 x 2000 hours x £0.07/kWh = £10,903 per year

If the air compressor often operates below full-load, and has a good control system, electricity costs will clearly be less than if the air compressor only ran at full-load during all hours of operation. Estimate the percentage of time the compressor is running at full-load, and add the percentage as another multiplier in the equation shown previously. Repeat the calculation for the percentage of time the compressor is running unloaded (or at partload) and include a factor to compensate for the reduced load on the motor (0.20 to 0.30 is a good estimate for unloaded operation for rotary screw compressors; 0.30 is used in the equation below. Finally, add the two results for total energy costs. The following shows an example calculation taking into account part load operation:

Annual electricity costs = [(motor full-load power) x (annual hours of operation) x (electricity cost in £/kWh)] x [(percent of time running fully loaded) + (0.30) x (percent of time running unloaded)].

For example, for an air compressor with a motor full-load power rating of 75kW, running for 2000 hours per year of which 65% of the time it is fully loaded and 35% of the time unloaded, and the unloaded operation consume 30% of the electricity of fully loaded. Assume a full load motor efficiency of 90% and the cost of electricity is £0.07/kWh Annual electricity costs = [(75kW) x (2000 hours) x (£0.07/kWh)/0.9] x [0.65 + (0.30) x (0.35)] = £8808 per year.

For an even more accurate calculation of energy costs for compressors running at part-load, repeat the calculation using various percentages of time running at different percentages of load. Manufacturers’ data on energy consumption for the different percentages of load will be needed.

Remember, the calculations shown will only provide a good estimate of energy consumption, not an exact number. As Lord Kelvin said in 1883: “If you cannot measure it, you cannot improve it”

For further information and advice on the economics of a compressed air system, please visit the BCAS website at www.bcas.org.uk or contact the technical department at BCAS.

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