Manufacturing facilities often used compressed air as means to power tools, conveyors, machines, presses, and other production equipment. Compressed air customarily was produced by a compressor or compressors at a central point, stored in a reservoir, and piped to the production equipment.

The demand for compressed air often varied greatly over a working day, with fluctuations reflecting rest breaks, lunch periods, and the start-up or shut-down of production lines.

Often in order to meet a fluctuating demand for compressed air, two or more compressors were used. A base compressor or compressors operated constantly. A trim compressor was activated when demand reached a preset level, as indicated by a decline in the air pressure in a reservoir to a minimum level. The number of base compressors varied from 1 to as many as 9 or more, depending on the level of demand for compressed air. In all plants, excess capacity was designed into the compressed air system, i. e. the compressed air production of the base and trim compressors combined exceeded the maximum demand for compressed air by the machinery.

Typically, centrifugal compressors operated on a curve defined by mass flow and pressure produced. This relationship was dependent upon the ambient temperature and relative humidity. The discharge pressure determined the optimum operating point on the curve and the curve characteristics were directly affected by temperature and relative humidity of the inlet air to the compressor. As a result, on-board controls provided by the original manufacturer attempted to maintain the compressor on the curve in some minimally acceptable operating range. In one situation, this was to be accomplished by throttling the inlet valve to the compressor in response to a drop in pressure and a flow exceeding the maximum point on the operating curve.

Alternatively, if the pressure rose to the minimum flow point on the operating curve, the release valve would impose an artificial flow by releasing air into the atmosphere from

the compressor discharge.

The minimally acceptable operating range for a centrifugal compressor was based on allowing adequate response time for the compressor's on-board controls and valves to respond to demand flow changes without the compressor riding up the compressor's curve to a surge point, which was potentially destructive to the compressor. This setup required that the compressor operate well below the optimum operating point.

By their nature, centrifugal compressors were very slow to deliver compressed air after a startup, with a typical response time of over a minute. Additionally, because of their potential to surge and shutdown, most users of centrifugal compressors always ran a back-up compressor just in case another one shuts down. The one-minute duration to deliver air from a start caused the user's production to stop. This was very wasteful in either that the back-up compressor is always releasing expensive compressed air to atmosphere or all of the compressors were throttling and/or releasing air to atmosphere.

One of the unique characteristics of centrifugal compressors was that typically at a 15% throttle setting, the power required is greater than 95% of the full load setting, thus throttling and venting are very wasteful situations. In addition, activation was expensive in terms of wear on the base compressors and associated maintenance and replacement costs.

Previous attempts to solve similar problems, but which do not achieve the function of the present invention, include U. S. Patent No. 2,512,043, titled"Fluid Pressure Control Apparatus,"issued to Stevens on May 26,1948. This patent discloses a system in which fluid under pressure is supplied from a reservoir, which is replenished by a compressor. If a momentary demand causes a drop of the fluid pressure in the reservoir, the system automatically draws fluid from an emergency reservoir, which is then replenished by the compressor.

U. S. Patent No. 5,632,146, titled"Load Shaping Compressed Air System," issued to Foss et al. on May 27,1997, discloses a load shaping system which increases the efficiency of a compressed air system by eliminating the activation of a trim compressor in response to a momentary increase in demand for compressed air. This patent is hereby incorporated by reference.

SUMMARY OF THE INVENTION The present invention is a system control designed to maintain the optimum operating conditions for multiple centrifugal air compressors operating in a compressed air system. The compressor system control includes a source of compressed air, a storage vessel, several valves, and at least one microprocessor for controlling the operation of the system. Additional components as described may be added to increase the efficiency and operation of the system.

The present invention increases the efficiency and operation of the overall compressor system by inclusion of a storage vessel and several strategically placed valves. Initially, the centrifugal compressors must compress the air to the maximum flow at the highest pressure as much of the time as possible. This is the optimum operating point of the compressors. By placing at least one adequately sized storage vessel between the centrifugal compressor discharge and at least one back pressure control valve, this will allow the compressor to operate at the optimum operating point without worry about the compressor surging. The back pressure control valve will provide a vent to the atmosphere prior to the compressor's on-board controls venting the compressed air.

By installing a fast acting modulating control valve directly to the first storage vessel, the back pressure control valve and the fast acting modulating control valve can then adjust the storage system to the compressor's optimum operating point based upon ambient conditions.

A third valve may also be included to store the compressed air at pressures up to the optimum operating point, and to release this air to users at a reduced and steady pressure on an as needed basis. Without this valve, the pressure will fluctuate as the user demand flows change. This fluctuation wastes considerable energy as the pressure rises above the user minimum requirement.

An additional storage vessel may be inserted between the centrifugal back pressure control valves and the valves controlling the user pressure. This stored air can be used to accommodate variations in user demand flows. The stored air will be built up from the releasing of the back pressure control valve as the first storage vessel pressure reaches a set point and the excess air spills over into the second storage vessel.

This stored air is then released to the users through the valve controlling the user

pressure.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a block diagram of the compressor system control of the present invention.

Figure 2 is a schematic diagram of the compressor system control.

Figure 3 is a graph depicting the pressure vs. mass flow relationship of a typical centrifugal compressor.

Figure 4 is a flow chart showing the operation of the system in response to fluctuations in user demand.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figure 1, there shown is a block diagram of the compressor control system, including the preferred embodiment, which is designed to maintain the optimum operating conditions for the multiple centrifugal compressors. The compressor system control 100 comprises a base compressor bank 104, microprocessor 166, piping means 109, and release valve extender 124. Additionally, in the preferred embodiment, a trim compressor bank 171, pressure control system 138, piping means 128, and demand shaping system 160 are also included.

Compressed air to the factory compressed air system is provided by base compressor bank 104 and trim compressor bank 171. The base compressor bank 104 is operated continuously during periods of compressed air demand, and the trim compressor bank 171 is activated, if necessary, to accommodate fluctuations in compressed air demand. Release valve extender 124 operates to store compressed air from the base compressor bank 104 via piping means 109, as well as prevent the discharge release valve 102 on the base compressor bank 104 from releasing valuable compressed air. Users are then provided with a constant regulated compressed air stream at output 148 from pressure control system 138, via piping means 145. In cases of instantaneous demand, the demand shaping system 160 is capable of supplying an additional burst of compressed air in order to prevent wasteful activation of an additional compressor. To control the operation of the various components previously described, microprocessor 166 monitors the conditions of the compressor system 100

and provides signals via connections 174-176 to control the operation of each component.

Referring now to Figure 2, there is shown a schematic diagram of the compressor system control as shown in Figure 1. This system control includes centrifugal compressors 105, trim compressors 170, storage vessel 110, back pressure control valves 125, fast acting modulating control valve 115, pressure control system 138, piping means 109 and 120, and demand shaping system 160. In the preferred embodiment, the base compressor bank 104 and the trim compressor bank 171 comprise at least one centrifugal compressor 105 and at least one trim compressor 170 respectively.

To increase the efficiency of the entire compressed air system, the centrifugal compressors 105 must operate at the maximum flow at the highest pressure as much of the time as possible. This is accomplished by installing at least one an adequately sized storage vessel 110 between the centrifugal compressor discharge and back pressure control valve 125. Storage vessel 110 is also connected to at least one fast acting modulating control valve 115 which can provide a vent to the atmosphere prior to the centrifugal compressor's 105 on-board control discharge release valve 102. In many cases, the on-board discharge release valves 102 of the centrifugal compressors 105 will all be preconfigured at different safety pressures directly from the factory. In the preferred embodiment, the fast acting modulating control valve 115 is set at a lower pressure than the lowest configured discharge release valve 102 of the centrifugal compressors, with the discharge release valve 102 of the centrifugal compressors acting as a secondary safety valve should the fast acting modulating control valve 115 fail.

Fast acting modulating control valve 115 must be chosen large enough to release the full output from all centrifugal compressors 105. The size of storage vessel 110 must also be sized appropriately to provide an adequate amount of time for pressure increases due to flow decreases to occur in a time appropriate for the fast acting modulating control valve 115 to respond. The size of the storage vessel 110 also depends on the system size, compressor sizes, and the size of the anticipated system events. In the preferred embodiment the size of storage vessel 110 is approximately 5,000 to 10,000 gallons can operate to store compressed air at a pressure matching the optimum operating pressure of the centrifugal compressors 105. Also in the preferred

embodiment, the fast acting modulating control valve 115 is a pneumatic, diaphragm actuated butterfly type valve such as the Premier Series manufactured by Cashco, Inc. of Decatur, Ill.. This operation allows the centrifugal compressors 105 to operate at a higher optimum operating point increasing the efficiency of the compressed air system without the compressor surging.

To adjust the system for variations in compressor performance and operation, the centrifugal back pressure controller 168 automatically adjusts the set point of the fast acting modulating control valve 115 and at least one back pressure control valve 125, via connection 174, based upon the ambient conditions as the centrifugal compressors 105 move up and down the operation curve as shown in Figure 3. In the preferred embodiment, the back pressure control valve 125 is a pneumatic, diaphragm actuated butterfly valve, sized appropriately to release the full output from the centrifugal compressors 105, and connection 174 is an electrical connection which passes an analog electrical signal from the centrifugal back pressure controller 168.

This analog electrical signal is then transduced into a pneumatic equivalent to the valve's positioner, which in turn applies or relieves air pressure from the diaphragm.

This process therefore does not require adjustment of the on-board discharge release valves 102 of the centrifugal compressors 105, and the compressors can always be operated at an optimum level regardless of ambient conditions.

To provide for a constant and stable pressure supply to the system users, the control system process includes the storage of compressed air at pressures up to the optimum operating point and the release of this air to the users at a reduced and steady selected pressure on an as needed basis. This is accomplished with at least one additional valve 140, which modulates to maintain a set point of downstream pressure.

Without valve 140, the user pressure at 148 will fluctuate as the centrifugal compressors 105 operate up and downward on their operating curve, as shown in Figure 3. This fluctuation causes considerable wasted energy as the pressure rises above the user minimum requirement. In the preferred embodiment, valve 140 is a pneumatic diaphragm actuated butterfly valve, sized large enough to release the maximum flow from both centrifugal compressors 105 and trim compressors 170.

As an additional embodiment, the compressed air may be stored at pressures greater than the optimum operating point of the centrifugal compressors 105 and above

the user required pressure at the user output 148. This pressure is the release set point of the fast acting modulating control valve 115. Although the centrifugal compressor efficiency lessens, it is more efficient to store this energy than to throttle the centrifugal compressors 105. This is accomplished by adding at least one additional storage vessel 130 located between the centrifugal back pressure control valve 125 and the valve or valves controlling user pressure 140. The air stored in this vessel 130 can be used to accommodate variations in user demand flows. The size of this storage vessel 130 should be appropriate to store quantities of air adequate to meet peaks in user demand flows without having to start more compressors. This stored air will be built up from the air released from the first centrifugal back pressure control valve 125 as the pressure in the first storage vessel 110 reaches the set point and the excess compressed air spills into the second storage vessel 130. Once back pressure control valve 125 opens, the pressure in all storage vessels can then continue to increase to the set point of the fast acting modulating control valve 115. This allows the centrifugal compressors 105 to ride up on their operating curve to a point of less flow. This condition during which this air is stored, even at less than optimum pressures, is much better than throttling or venting at a lower point on the operating curve. Storing this air can prevent other compressors from starting during events in user demand.

In the preferred embodiment, this second storage vessel 130 would be capable of holding at least 5,000 to 10,000 gallons in capacity at a pressure of approximately 85- 125 psi. The air stored in this second vessel 130 is released to the users through at least one valve 140 at output 148 via pipe 145. It may also be necessary to release air stored in the first storage vessel 110 and operate at a pressure which is less than the optimum pressure for a brief period of time. This procedure would involve control of the centrifugal back pressure control valve 125 automatically switching to forward pressure while there is an excess of user demand flow. When the user demand flow decreases and the pressure at the second storage vessel 130 rises, the centrifugal back pressure control valve 125 will automatically be returned to back pressure mode again and will maintain the optimum operating point for the centrifugal compressors 105.

To decrease the amount of energy required for optimum system operation, several trim compressors 171 may be combined to supplement the base or centrifugal compressors 105. These trim compressors 170 are generally smaller, quick response

compressors, which are capable of supplying a small amount of air in a very short amount of time. In the preferred embodiment, the centrifugal compressors 105 are normally greater than 350 horsepower, thus making them very slow (over 30 seconds) in the production of compressed air. The use of these smaller, quicker trim compressors 170 allows a completely optimized system in which the minimum horsepower is required and where the user flow is not wasted due to fluctuating pressure. These trim compressors 170 also allow the centrifugal compressors 105 to operate optimally without waste and energy is able to be stored to accommodate user flow events. This also allows the system to operate without having to have additional horsepower on-line to meet the fluctuations in demand. In the preferred embodiment the small trim compressors 170 are used prior to the addition of an additional centrifugal compressor 105, and any number of the centrifugal compressors 105 are automatically removed from service when adequate trim compressors 170 are available to cover their share of the user demand.

Additionally, in another embodiment, the demand shaping system 160 as set forth in U. S. Patent 5,632,146, referred to as a load shaping system and commonly assigned with the present invention, may also be used to increase the energy efficiency of the compressed air system and to prevent the need for operation of an additional trim compressor 170 during fluctuations in user demand. The pressure control system 138, which is composed of at lease one second storage vessel 130 and at least one centrifugal back pressure control valve 140, provides the user with a constant and relatively stable pressure as observed in the pipe 145 to the user output 148. The demand shaping system 160 will anticipate pressure changes in pressure of the second storage vessel 130 by closely monitoring any fluctuations in pressure at the second storage vessel 130. If the system cannot provide enough compressed air, the system may withdraw additional air from at least one additional storage vessel 158 located in the demand shaping system 160 through at least one valve 150. This event will typically occur when the first storage vessel 110 is depleted of air and there is a high demand for compressed air. The demand shaping compressor or compressors 155 then operate to replenish the demand shaping storage vessel 158 should additional compressed air be required in the third storage vessel 158. In the preferred embodiment, the demand shaping compressors 155 are approximately 15 to 25 horsepower in size, the additional storage vessel 158 has a

capacity of approximately 20,000 to 30,000 gallons and store compressed air at an approximate pressure of 200 psi.

The system is controlled by a microprocessor circuit 166 containing both a centrifugal back pressure controller 168 and a compressor automation controller 165 via electrical connections 174,175, and 176.

In the preferred embodiment, the compressor automation controller 165 is an XCEED Tm brand controller manufactured by Honeywell/APT, Minneapolis, Minnesota.

XCEED is a trademark owned by Honeywell Inc. The centrifugal back pressure controller 168 is an programmable logic controller (PLC), such as those manufactured by Allen-Bradley of Milwaukee, Wis.

The centrifugal back pressure controller 168 provides the logic for operation of the fast acting modulating control valve 115 and the first back pressure control valve 125. The centrifugal back pressure controller 168 also monitors the pressure at the first storage vessel 110 via a pressure sensor 118. Pressure sensor 118 provides a signal to the centrifugal back pressure controller 168 corresponding to the pressure in the first storage vessel 110 via connection 174. If the pressure in the first storage vessel 110 as monitored by pressure sensor 118 is greater than a predetermined pressure level in microprocessor 166, the centrifugal back pressure controller 168 initiates the release of compressed air through fast acting modulating control valve 115.

The compressor automation controller 165 is responsible for controlling the operation of the pressure control system 138, the centrifugal compressors 105, the trim compressors 170, and the demand shaping system 160. The pressure in the pressure control system 138 is monitored with a pressure sensors 132 at the second storage vessel 130, and with a pressure sensor 142 at the user's output 148. The centrifugal back pressure controller 168 may also operably connected to the compressor automation controller 165 via line 178 for sharing the current logic between each device. In an alternate embodiment, the centrifugal back pressure controller and the compressor automation controller may be combined so that one microprocessor controls all aspects of this system.

Figure 3 shows the operating curve of a typical centrifugal compressor 105. The horizontal axis represents mass flow and the vertical axis represents pressure. The operating curve 250 is developed under tests at controlled conditions, but the actual

compressor performance curve 255 is affected by discharge pressure, ambient temperature, and relative humidity. Discharge pressure in the system determines the operating point on the curve, and the curve characteristics are directly affected by temperature and relative humidity of the inlet air to the compressor. As a result, on board controls provided by the original manufacturer, in order to maintain the compressor on the curve in a proper operating range, throttle the inlet valve to the compressor or release air to the atmosphere from the compressor discharge. The minimally acceptable operating range is based on allowing adequate response time for the on-board controls and valves to respond to demand flow changes without the compressor riding up the curve to a surge point, 245 and 260, which is potentially destructive to the compressor. The optimum operation range is shown at points 235 and 270. To avoid surges, centrifugal compressors are typically operated well below optimum at points 230 and 280, and are then throttled before reaching optimum.

To detail the operation, as the pressure rises to the minimum flow point at maximum pressure on the curve, the compressor will first throttle the inlet valve to the compressor following the throttle line on the curve. As the flow decreases, the compressor will reach a Current Limit Low (CLL) 215 or minimum stable flow point 210, and the on-board controls will open the discharge release valve 102, releasing air to the atmosphere, and creating considerable waste. This typical throttle line 225 is well below the optimum operating point 235 and 270 of the compressor. Additionally, as pressure falls, the CLL may be reached at a lower pressure caused by excess user demand, yet the on-board controls may react by opening the discharge release valve 102 to atmosphere assuming the compressor has reached minimum stable flow. The present invention overcomes this limitation by operating the centrifugal compressors to compress the air to the maximum flow at the highest pressure as much of the time as possible. This operating point is the optimum operating point 235 and 270. This solution is accomplished by the control of pressure in the first storage vessel 110 of Figure 2 by using the fast acting modulating control valve 115 and the first back pressure control valve 125 to provide an environment at the compressor discharge that maintains the optimum pressure as much of the time as possible. Therefore, the compressor's on-board controls will not release compressed air to the atmosphere due to the prevention of a rise or fall in pressure.

Referring now to Figure 4, there shown is a flow chart depicting the operation of the system in response to fluctuations in user demand. This flow chart is circular in operation and will repeat throughout operation of the compressor system control.

Starting at step 300, the system proceeds to step 305. At step 305, the centrifugal back pressure controller 168, as shown in Figure 2, monitors the pressure at the first storage vessel 110 via an attached pressure sensor 118. If the centrifugal back pressure controller 168 determines that excess air is being produced by the compressors, it moves to step 320 and determines if all of the trim compressors 170 are unloaded or in an off state. If the controller 168 determines that excess air is not being produced at the first storage vessel 110, it moves to step 310.

At step 310, the compressor automation controller 165 determines if all of the trim compressors 170 are loaded via connection 175. If all are loaded, the system moves to step 315 and the compressor automation controller 165 starts the operation of an additional centrifugal compressor 105. If there are trim compressors 170 which are not loaded, the system will move to step 330.

Upon reaching step 320, if the compressor automation controller 165 determines that all of the trim compressors 170 are not unloaded nor in an off state, it proceeds to step 330. If the trim compressors 170 are all unloaded or off, the system proceeds to step 325 and removes on centrifugal compressor 105. After removing one centrifugal compressor 105, the system then reaches step 330.

When step 330 is reached, the centrifugal back pressure controller 168 will then monitor the pressure at the first storage vessel 110 via the attached pressure controller 118 to determine if the pressure continues to rise. If not, the system jumps to step 340.

If the pressure is still rising at the first storage vessel 110, the centrifugal back pressure control 168 proceeds to step 335 and opens the fast acting modulating control valve 115 to vent the compressed air to the atmosphere. The system then proceeds to step 340.

Should there be falling pressure at the second storage vessel 130, if included in the system, as determined by pressure sensor 132 and compressor automation controller 165, the system moves to step 345 and determines the rate of the pressure fall. If the pressure is not falling, the compressor automation controller 165 determines if there is excess pressure at the second storage vessel 130 at step 365. If there is excess pressure, the compressor automation controller 165 then unloads one trim compressor at step 370

and proceeds to step 385. If there is not excess pressure at step 365, the system proceeds directly to step 385.

If the pressure is falling at the second storage vessel 130, and the compressor automation controller 165 determines that this rate of fall is greater than the pressure set point of the system, it then determines if there is a demand shaping system and additional stored air as a solution available for the system at step 350. Should the rate of pressure fall be less than the set point, or should there be no demand shaping pressure available as a solution, at step 360 the controller will lower the setpoint at the first back pressure control valve 125 to spill compressed air from the first storage vessel 110 to the second storage vessel 130. The system then proceeds at step 375. If there is a demand shaping system available and if there is compressed air available in the demand shaping storage vessel 158, in step 355 the compressor automation controller 165 will open the demand shaping valve 150 to stop the pressure fall at the second storage vessel 130.

At step 375, the compressor automation controller 165 once again monitors the pressure at the second storage vessel 130 to determine if the pressure is rising or sufficient to meet demand. If the pressure is not adequate or falling, an additional trim compressor 170 will be loaded at step 390 and the system will proceed to step 400.

Alternatively, if there is enough pressure, then the controller refrains from starting an additional compressor at step 380 and returns the first backpressure control valve's 125 setpoint to optimum pressure at step 385.

Next, upon reaching step 400, the compressor automation controller 165 must determine if there is a proper pressure level to the users as measured by the pressure sensor 142 near the user output 148. If the pressure is inadequate, at step 395 the controller will open the pressure control valve 140 to return the pressure back to an acceptable level. If the pressure provided to the users is acceptable, the system is then at step 405 and operating at the proper system and pressure setpoint. The system then returns to step 300 and repeats itself as long as the compressor system control is operating.

In summary, the foregoing has been a description of a novel and unobvious multiple centrifugal air compressor system control. This description is meant to provide examples, not limitations. The applicants define their invention through the claims appended hereto.