Energy Efficient Compressed Air Systems

Energy Efficient Compressed Air Systems

Introduction

Compressed 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 Systems

Energy Balance Approach

To 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 / (hmotor hcompressor hcontrol)

where hmotor is the motor efficiency, hcompressor is the compressor efficiency and hcontrol 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 compression

Wf = n R T1 [(P2/P1) (n-1)/n - 1] / (n - 1) for polytropic compression

Rair = 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 / hcompressor

hcompressor = ∫ V dP / dW

hcompressor = 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 Systems

These 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 Concepts

Close 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 Compressors

The three basic types of air compressors are reciprocating, rotary screw and centrifugal compressors.

Reciprocating / Rotary Screw / Centrifugal

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 Controls

Compressor 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 Control

In 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 Control

In 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 upper activation pressures would be 90 psig and 100 psig. When unloaded, rotary screw compressors typically partially close the air inlet valve and bleed the remaining compressed air in the sump to atmosphere.

Power draw when fully unloaded varies from about 60% of full load power to about 30% of full-load power, depending on compressor design and on the length of time the compressor runs unloaded. To fully unload, the load/unload cycle time must be long enough to allow the compressed air in the sump to bleed to atmosphere when the compressor unloads. Thus, load/unload control works best when coupled with adequate compressed air storage, which lengthens load/unload cycles while modulating pressure variation to end uses.

Most compressors with load/unload control also have an “automatic shutoff” option, in which the compressor shuts itself off if it runs unloaded for about 5 to 10 minutes. The compressor will remain off for a specified period of time before restarting to avoid short-cycling. Running the compressor in “automatic shutoff” mode can result in significant energy savings during periods of low compressed air demand. In addition, adequate compressed air storage increases load/unload cycle time, and the likelihood that the compressor shuts off after running unloaded for a few minutes.

Modulation Control

In modulation control, the position of the inlet air valve is modulated from full open to full closed in response to compressor output pressure. Modulation control typically employs PID control with a narrow control range about + 2 psig. Inlet modulation is a relatively inefficient method of controlling compressed air output.

Variable-Speed Control

Rotary-screw air compressors can be equipped with variable frequency drives to vary the speed of the screws and the corresponding compressed air output. As in other fluid flow applications, the variation of speed to vary output is extremely energy efficient.

Blow-off Control

In centrifugal compressors, the quantity of air flow through the compressor can only be controlled by modulating the inlet air valve over a relatively small range. When flow is reduced below this range, the flow becomes unstable in a “surge” condition. To avoid surge, centrifugal compressors may discharge compressed air to the atmosphere to control compressed air output to the system. Blow-off control is the least efficient method of controlling compressed air output, since input power remains constant as the supply compressed air to the system decreases.

Power / Output Relationships by Control Type

The following figure shows typical relationships between fraction input power to the compressor (FP) and fraction compressed air output (FC) for various types of control. At full output capacity (FC = 1.0), compressors draw full power (FP = 1.0). The power draw at less than full output capacity is a function of the type of part-load control. The figure shows that at part load, most energy efficient control mode is on/off, followed by variable speed, load/unload, modulation and blow-off control.

Assuming linearity, fraction power, FP, can be calculated from fraction capacity, FC, and fraction power at no load, FP0, according to the following relationship:

FP = FP0 + (1 – FP0) FC

Some compressors use a combination of basic control modes described above. For example, the figure below shows the relationship between fraction of full-load power and fraction of full-load output capacity for a compressor using a combination of modulation and load/unload control. The top line shows full modulation control, in which the compressor continues to draw 70% of full load power even when producing no compressed air. The bottom line shows a combination modulation and load/unload control, in which compressed air output is modulated by the inlet valve down to 40% of total capacity. Below 40% of full output capacity, the compressor loads and unloads to vary compressed air output. In this example, the compressor draws 25% of full-load power when fully unloaded.

Centrifugal compressors typically employ three primary methods to control compressed air output to meet demand: inlet modulation by a flow control valve or variable inlet vanes, load/unload and blow off. Inlet modulation by a flow control valve or variable inlet vanes varies the quantity or rotation of inlet air to the compressor, which reduces compressed air output and input power. In most cases, however, the control range using inlet modulation is limited to between about 70% and 100% of compressed air output. If flow is reduced below about 70% of full output capacity, an unstable flow condition called “surge’ may result. To control flow below about 70% of full output capacity, some compressors can load and unload to match compressed air demand. When fully unloaded, the compressor generates no compressed air and can draw as little as 15% of full load power. Centrifugal compressors without load/unload capability continue to generate compressed air, but blow off the excess compressed air to the atmosphere. Because compressor power remains constant while compressed air output falls, blow-off control is the least efficient method of controlling compressed air output.

Many centrifugal compressors employ some combination of these basic control options. For example, the figure below shows the fraction power to fraction capacity curves for a centrifugal compressor with “Constant Pressure” and “Auto-dual” control modes. In “Constant Pressure” mode, variable inlet vanes modulate inlet air to the compressors down to about 70% of full load capacity, and compressor power draw follows linearly. If compressed air demand falls below 70%, blow off valves discharge compressed air to the atmosphere and power draw remains constant. Alternately, the compressor could be set to run in “Auto-dual” mode. In Auto-dual mode, the variable inlet vanes modulate inlet air to the compressors down to about 70% of full load capacity, just as in Constant Pressure mode. However, in Auto-dual mode, the compressor will unload when compressed air demand falls below 70% of full-load capacity and compressor power draw will be reduced to about 15% of full load power. The plot below shows fraction of full load power draw (kW) on the vertical axis and fraction of full load capacity (cfm) on the horizontal axis for Constant Pressure and Auto-dual modes.