Air Compressor with Adjustable Speed Drive
Air Compressor: Adjustable Speed vs. Constant Speed
Adjustable Speed Drive (ASD) control of oil-lubricated or oil-free rotary screw air compressors is more efficient than load/unload, modulating, or modulating with unloading flow control.
Item ID: 90
Process Loads & Appliances--Industrial Processes
Variable speed compressors have been available since the 1990s in sizes up to 700 hp and have achieved significant market penetration.
Depending upon the air flow profile, ASD compressors can provide significant energy savings when compared with rotary screw compressors with either load/unload or modulating controls. Very small compressors equipped with on-off controls tend to be very efficient. Adjustable speed compressors are ideal for service as trim compressors in multiple compressor installations. In this scenario, conventional fixed speed compressors are operated efficiently at full-rated output to serve base loads while fluctuating air demands are efficiently met by the variable speed compressor.
Energy savings are extremely application dependent and vary with a plant's air flow profile. Compressors that supply less than rated load much of the time offer significant savings. ASD compressors also allow for the air pressure to be reduced as you are not charging a receiver to 110-psig or more when 100-psig is required. In this case, the receiver is not being used to store air so your compressor can be unloaded. Benefits may arise due to pressure stabilization in the plant. Additional savings occur as compressed air storage can be reduced when adjustable speed compressors are placed in the engine room. Power factor is improved when pulse width modulated drives are used.
Baseline Description: Oil Lubricated Rotary Screw Compressor with Modulation Flow Control
Baseline Energy Use: 4758 kWh per year per hp
The baseline energy use is for a small industrial plant with 24 hours per day operation. Their peak air demand of about 230 cfm is met with a 60 hp oil lubricated rotary screw compressor with modulation controls. The air profile for the plant calls for lower consumption during the night shift, with a dip around lunch time, and continuous but lower consumption during the week ends. Atlas Copco claims that about 64% of all industrial plants have a similar profile.
Manufacturer's Energy Savings Claims:
Savings Range: From 35% to 40%
Manufacturers claim energy savings of 35% to 40% (according to Atlas Copco).
Best Estimate of Energy Savings:
"Typical" Savings: 38%
Energy Savings Reliability: 6 - Approved Measure
Installation of an ASD compressor would reduce energy consumption of 285,526 kWh/year by 38% resulting in an annual energy savings of 108,500 kWh/year. This is equivalent to a savings of 1,808 kWh/year per hp. Savings would be less for one or two shift operations, but much greater if a larger ASD compressor (up to 350 hp) was put into trim service at an industrial facility with a larger air load.
Note that this is not 38% of a facility’s total compressed air system energy use, but reflects only the energy use of the trim compressor that is unloading or modulating to meet variable air demands. Compressors that are base loaded would continue to operate in a base loaded manner.
This is a deemed measure under the October 1, 2014 BPA "Energy Efficiency Implementation Manual." The deemed amount is described in a projects payment table.
Energy Use of Emerging Technology:
2,950 kWh per hp per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
Air compressors greater than 20 hp and less than or equal to 500 hp account for about 39,885 GWh of electrical energy consumption in the U.S. (From the USDOE's "United States Industrial Electric Motor Systems Market Opportunities Assessment"). The number of motors by air compressor size range are:
- 21—50 hp, 138,743 motors;
- 51--100 hp, 36,450 motors;
- 101--200 hp, 27,288 motors;
- 201--500 hp, 16,275 motors.
A conservative estimate of the total compressor drive motor horsepower is made by multiplying the number of motors times the lower value of the motor horsepower range. This yields: 2,913,603 + 1,858,950 + 2,756,088 + 3,271,275 = 10,799,916 drive motor hp in the U.S. that are candidates for VSD retrofits.
The U.S. total compressor motor hp will be pro-rated by population to estimate the total compressor hp in the Northwest region, which has about 4% of the national population. 0.04 x 10,799,916 hp = 431,997 total motor hp.
This number will be reduced by 40% to reflect the number of compressors in trim versus base load service. Without market survey data, it will be assumed that 25% of all trim compressors are currently VSD units. Incorporation of these assumptions yields a total compressor hp of:
0.6 x 0.75 x 431,997 hp x = 194,398 compressor hp.
Regional Technical Potential:
0.35 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: hp
Emerging Technology Unit Cost (Equipment Only): $902.00
Baseline Technology Unit Cost (Equipment Only): $653.00
The equipment cost for a 20 hp Atlas Copco VSD rotary screw compressor is $15,895 (2014 quote). Freight and installation costs amount to an additional $2,150 for a total installed cost of $18,045 ($902/hp). The RS Means 2015 Facilities Construction Cost Data publications lists a 25 hp 2-stage reciprocating constant-speed compressor as having an equipment (including a 250 gal tank) plus labor cost of $16,325 ($653/hp).
Simple payback, new construction (years): 1.5
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.