Energy Efficient Compressed Air Systems
IntroductionCompressed air systems generate, store and distribute energy in the form of compressed air for use throughout a plant. In a compressed air system, a single set of compressors can supply power to machines all over the plant, thus eliminating the need for numerous and dispersed electric motors. This advantage must be balanced against the relative poor energy efficiency of compressed air systems, which can be as low as 20% when leaks and part-load control losses are taken into account.
On a national scale, air compressors rank only behind pumps in terms of industrial motor drive electricity consumption. Thus, increasing the efficiency of compressed air systems can result in significant energy savings.
Principles of Energy-Efficient Compressed Air SystemsEnergy Balance ApproachTo compress air, the power delivered to the fluid (air) dWf is the integral of the product of the volume flow rate V and the pressure rise dP.
dWf = V dP
The electrical power supplied to an air compressor is:
dWe = V dP / (motor compressor control)
where motor is the motor efficiency, compressor is the compressor efficiency and control is the control efficiency.
Three types of compression are shown below. The right compression line represents isentropic compression, in which air is compressed adiabatically with no internal reversibilities. The left compression line represents isothermal compression, in which the air is cooled to keep the air temperature constant during compression. Isentropic compression has no cooling and isothermal compression has the maximum cooling possible. Actual compression processes lie somewhere in between isentropic and isothermal compression, and are called polytropic compression. The area to the left of the compression lines represents the fluid work dWf = V dP. Thus, isothermal compression requires less compressor work because the cooling is responsible for part of the decrease in volume.
Source: Cengal, Y. and Boles, M., Thermodynamics, 1998, WGB-McGraw-Hill.
Some air compressors utilize two stages of compression with intercooling between the stages to further reduce compressor power. The power savings from two-stage compression with intercooling are shown graphically below.
Source: Cengal, Y. and Boles, M., Thermodynamics, 1998, WGB-McGraw-Hill.
Assuming that air can be treated as an ideal gas, it can be shown that
Pvn = constant
during the compression process, where P = absolute pressure, v = specific volume, n = 1 for isothermal compression, n= k = Cp/Cv = 1.400 for isentropic compression of air and 1.0 < n < 1.400 for polytropic compression.
Substituting (Pvn = constant) into the equation for fluid work (dWf = V dP) and solving the differential equation yields the following results:
Wf = R T ln (P2/P1) for isothermal compressionWf = n R T1 [(P2/P1) (n-1)/n - 1] / (n - 1) for polytropic compressionRair = 0.06855 Btu/lbm-R
Example:Calculate specific capacities (cfm/hp) for isothermal and isentropic compression of 70 F air to 100 psig.
Actual compressors generate between 4 and 5 scfm/hp at 100 psig. The difference between the thermodynamic values of scfm/hp computed above and scfm/hp generated by actual compressors is due to the turbulence and friction generated within the compressor. Thus, this difference characterizes the efficiency of the compressor.
Example:Calculate the efficiency of a compressor with an actual specific capacity of 4.2 cfm/hp if the polytropic specific capacity is 6.0 cfm/hp.
dW = V dP / compressor compressor = V dP / dWcompressor = 4.2 scfm / 6.0 scfm = 70%
Motor efficiency is the efficiency of the motor at converting electrical power into shaft power. The efficiency of a premium-efficiency 100-hp motor is about 92%. Motor efficiency can be improved by specifying premium-efficiency motors.
Control efficiency is a measure of the losses incurred to vary compressed air output to match compressed air demand. In air compressors, control efficiency varies widely depending upon the type of part-load control employed.
Understood in this light, the energy balance equation serves as a useful guide for energy saving opportunities. Thus, primary energy savings opportunities are:
Reducing volume flow rate Reducing pressure rise Increasing control efficiency Increasing compressor efficiency Increasing motor efficiency.
Opportunities for Improving The Energy-Efficiency of Compressed Air SystemsThese principles can be organized using the inside-out approach, which sequentially reduces end-use energy, distribution energy, and primary conversion energy. Combining the energy balance and inside-out approach, common energy-efficiency opportunities in compressed air systems include:
End use Eliminate inappropriate uses of compressed air (reduce V) Install solenoid valves to shut off unnecessary air (reduce V) Install air saver nozzles (reduce V) Replace timed-solenoid with differential-pressure control (reduce V) Use blower instead of air compressor for low-pressure applications (reduce dP) Distribution Fix leaks (reduce V) Replace timed-solenoid drains with demand-control drains (reduce V) Decrease pressure drop in distribution system (reduce dP) Conversion Compress cooler outside air (increase compressor efficiency) Stage compressors with pressure settings or controller (increase control efficiency) Employ on/off, load/unload with auto shutoff, or variable-speed control for trim compressor (increase control efficiency) Add compressed air storage to decrease unload power and increase auto-shutoff (increase control efficiency) Replace desiccant with refrigerated dryer (reduce V) Use heat from compressors to heat building during winter
Recurring Energy-Efficiency ConceptsClose inspection of these energy-efficiency opportunities illustrates three important and recurring energy efficiency concepts.
The equation for air compressor energy use serves as a useful guide for comprehensively identifying energy saving opportunities.
Like most systems, compressed air systems are designed for peak conditions, but spend the vast majority of time operating at off-peak conditions. Thus, several energy efficiency opportunities result from improving control to reduce unnecessary compressed air use and power consumption during off peak conditions. Careful attention to control efficiency is vital to achieving energy efficiency.
To achieve energy savings, many end-use and distribution system savings opportunities must be coupled with modifications to the conversion equipment, which in this case is the air compressor plant. Thus, the whole-system inside-out approach is vital to maximizing energy-efficiency potential.
Air CompressorsThe three basic types of air compressors are reciprocating, rotary screw and centrifugal compressors.
Reciprocating Rotary Screw
Reciprocating compressors use pistons to compress air in cylinders. Single-acting compressors compress air on one-side of piston, and double acting compressors compress on both sides of piston. Large reciprocating compressors may employ multiple stages with intercoolers and double acting pistons to achieve high compression efficiencies. Single-stage compressors control compressed air output by stopping the pistons when compressed air is not needed. Multi-stage compressors control compressed air output by sequentially reducing the number of stages in use.
Rotary-screw compressors compress air by forcing air between rotating screws with decreasing volume between the screws. Most rotary-screw compressors control compressed air output by modulating the air intake valve, and or alternating between full open and fully closed operation.
Centrifugal compressors compress air by accelerating air from the tips of impellors rotating at high speeds into a volute. Centrifugal compressors are typically 250-hp or larger, and frequently employ multiple stages to achieve the desired compressed air output pressure. Centrifugal compressors control compressed air output by modulating an inlet valve or variable inlet vanes on the air intake, loading and unloading, or blowing off compressed air to atmosphere rather than into the compressed air system.
Compressor ControlsCompressor controls typically match compressed air output to compressed air demand by maintaining discharge air pressure within a specified range. There are five primary control strategies for maintaining the pressure within the desired range.
On/Off ControlIn on/off control, the compressor turns on and begins to add compressed air to the system when the system pressure falls to the lower activation pressure. The compressor continues to run and add compressed air to the system until the system pressure reaches the upper activation pressure when the compressor shuts off. Typical lower and upper activation pressures would be 90 psig and 100 psig. On/off control may also employ a timer to reduce short-cycling. Reciprocating compressors typically employ on/off control. On/off control is the most efficient type of part-load control, since the compressor draws no power when it is not producing compressed air.
Load/Unload ControlIn load/unload control, the compressor loads and begins to add compressed air to the system when the system pressure falls to the lower activation pressure. The compressor continues to run and add compressed air to the system until the system pressure reaches the upper activation pressure. It then unloads and does not add compressed air to the system until the system pressure drops to the lower activation pressure. Typical lower and up