Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-09-14
2004-11-09
Bui, Bryan (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C700S275000, C700S282000, C700S301000
Reexamination Certificate
active
06816795
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to compressed gas systems and, more particularly, to a system for controlling and operating a plurality of compressors to efficiently supply compressed gas to a compressed gas distribution system.
Compressed gas is widely used in industry. For example, almost every industrial facility from a small machine shop to a large pulp and paper mill has some type of compressed air system. In many cases, compressed air is considered to be a “fourth utility” and is so vital that the facility cannot operate without it. Compressed air systems vary in size from a single five horsepower compressor to systems comprising many compressors and aggregating more than 50,000 hp. While compressed air is clean, readily available, and simple to use, a typical compressed air system operates with around 10% overall efficiency and compressed air is probably the most expensive form of energy in use in a facility. Improving system maintenance, reducing system pressure, and using more efficient compressors are recognized paths to improved efficiency for compressed gas systems.
For example, leakage can consume 20-30% of the compressor output. However, leakage can be reduced to less than 10% of compressor output by using good maintenance practices and reducing system pressure because leakage is proportional to the pressure drop across the leak. However, reducing the system pressure requires compressed air storage and an accurate control system to prevent the system pressure from dropping below an acceptable minimum level during periods of high or changing demand.
Referring to
FIG. 1
, a typical compressed gas system
20
comprises, generally, a supply side
22
(indicated by a dashed enclosure) and a demand side
24
(indicated by a dashed enclosure). The supply side
22
of the system typically includes one or more compressors
26
,
28
,
30
, and
32
, supply side piping
40
and air treatment equipment, such as an air dryer
38
. Gas compressors are commonly constructed as a compressor package including a drive motor, compressor controls, and a number of accessories. The compressors
26
,
28
,
30
, and
32
are each driven by a drive motor
34
and equipped with inlet filtration
48
to protect the compressors and downstream equipment from airborne particles in the inlet air, and a separator
50
to remove oil and moisture from the compressed gas leaving the compressor. Although not illustrated, gas compressor packages also commonly include intercoolers to cool the air between compression stages and aftercoolers to cool the compressed air discharged by the compressor. Typically, the compressor's controls (not illustrated) respond to a pressure signal, obtained from pressure sensors
54
, at the discharge of the compressor package to the supply piping
40
.
The demand side
24
of the system comprises distribution piping
43
, including a demand header
55
, leading to a plurality of end use equipment and processes
42
.
The supply
22
and demand
24
sides of the system are typically separated by an air receiver or accumulator
44
and, often, a flow control
46
. The accumulator
44
provides storage for a volume of compressed air and reduces compressor sequencing by permitting compressors to continue to operate when demand is momentarily reduced and providing a source of compressed air when demand momentarily increases. An appropriately sized accumulator can be used to protect end use equipment and processes having critical pressure requirements by controlling the amount and rate of system pressure change in response to demand events. The flow control
46
reduces pressure fluctuations in the demand side by adjusting the flow of compressed gas from the accumulator
44
to the demand side
24
of the system in response to changes in demand by the end uses
42
. However, the supply side
22
of a compressed gas system has, essentially, a fixed volume and the pressure in the supply side will decrease in response to consumption of gas by the demand side
24
unless and until there is sufficient flow from the compressors to replace the gas being transferred to the demand side of the system. Likewise, when demand is reduced, the supply pressure, typically measured in the supply side header
36
or the accumulator
44
, will rise until the compressor output is reduced to match the demand. If the system does not include a flow control
46
, changes in demand by the end uses
42
produce immediate changes in the pressure in the demand header
55
resulting in rapidly fluctuating pressure in the accumulator
44
.
Compressed gas systems are designed to operate within a fixed pressure range and to deliver to end uses
42
a volume of gas that varies with end use demand at a pressure in excess of a minimum pressure. Referring to
FIG. 2
, to compensate for pressure drops in the distribution piping
43
and to ensure that end uses
42
receive gas at a pressure in excess of the minimum, a minimum demand pressure
60
is maintained in the demand header
55
. Likewise, to compensate for pressure drops between the accumulator
44
and the demand header
55
, a minimum supply pressure
62
, in excess of the demand pressure
60
, is maintained in the accumulator
44
or supply header
36
. The compressor control system typically operates in a fixed pressure range
64
(indicated by a bracket) adding or removing compressor capacity from the system in response to pressure changes resulting from demand in an attempt to keep the supply pressure
66
above the minimum supply pressure
62
.
Air compressors operate most efficiently at full load and are, therefore, typically switched in and out of the system when the local pressure, typically measured at the outlet of the compressor package (sensor
54
) reaches the limits of a control range established for the compressor. The compressor may be sequenced by controlling the drive motor
34
with a simple start-stop pressure switch but frequent cycling can cause overheating and wear of the motor and compressor. To reduce cycling of the drive motor, many compressors include a load-unload control that enables disconnecting the output of the compressor from the system while the drive motor continues to drive the compressor at full speed. However, an unloaded compressor typically consumes 15-60% of full load power while delivering no useful work to the system and lengthy periods of unloaded operation are detrimental to system efficiency.
A modulating or throttling inlet control can be used on a rotary screw type compressor to adjust the compressor's output so that it more closely matches demand keeping the system pressure within the control range without unloading the compressor. However, when fully throttled, these compressors continue to operate against the system's pressure and typically consume approximately 70% of full power. A variable speed drive is a more energy efficient means of regulating the output of a compressor to more closely match the system demand, but variable speed drives are relatively expensive. In a typical compressor operating scenario, a plurality of compressors, each operating at full capacity, are sequenced in and out of the distribution system in an attempt to match the combined compressor output to the system demand. In the alternative, one or more compressors with variable output may be used to fine tune the total compressor output over some limited range to reduce sequencing of fixed capacity compressors.
Automatic compressor control systems comprise generally sequencing controls and network controls. Sequencing controls or sequencers are devices used to sequentially load and unload a plurality of compressors in response to changes in the local system pressure. Sequencers can provide a fairly tight control range for the compressor and can be arranged to alter the order in which compressors are sequenced to balance the duty cycle on a plurality of compressors. However, sequencers rely on a local pressure signal from the outl
Bui Bryan
Chernoff Vilhauer McClung & Stenzel LLP
Vo Hien
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