A small sand blasting company was incurring very large electrical bills totaling $48,000 per month, even though they ran their compressors on average only 8 hours per day.
A look at their compressor hour meters showed the compressor was only running at 50% load. The company had been sold on a large 250-hp compressor, due to their high peak demands. This compressor was a premium efficient, two-stage unit with spiral valve control. However, due to its low duty cycle, it appeared that using one large compressor was not the best idea in terms of energy costs.
The company’s bill was made up of two components, an energy charge and a demand charge. The energy charges were in blocks, with the first blocks of energy costing more than the final blocks. The demand charges were calculated on the highest 15 minute peak of the month. So, if the compressor consumed 200 kVa only once, the customer had to pay for that peak for the whole month. Demand costs accounted for more than 50% of the bill. In an area where most customers pay about 8 cents per kWh, the demand charges were making the overall cost per kWh about 18 cents per kWh.
The company asked a compressed air auditor to assess their system. Data loggers were placed to determine the load profile. The data showed an interesting thing — the compressor unload power was rated at only 48 kW, yet the measurements showed the actual power was more than double that, about the same as a 125-hp fully loaded compressor. A malfunction within the compressor was causing abnormally high energy consumption.
The auditor recommended considering the purchase of smaller compressors in future. Even when working correctly, the 48 kW of unloaded power is considerable; using a compressor half the size would reduce power consumption to about 24 kW, or, if a VSD compressor was installed, unloaded power consumption might be eliminated altogether.
Some tips for this customer:
• Consider a different utility rate structure. This customer had the choice of paying more for energy and less for peak — this would save $1,000 per month.
• Correct the power factor. Running at a power factor near unity would reduce demand charges by 15% compared to the present 84% level.
• Repair the malfunction causing high unloaded power consumption.
• Consider running multiple smaller compressors rather than one big unit.
• Consider using a properly sized VSD as one of the units.