COMPRESSED AIR CONTROL SYSTEM

Abstract: This invention provides an air control system which is retrofittable in an intermediate position between upstream compressors (130) and air storage vessels and downstream useful production equipment (140) to provide benefits of greater air-utilization efficiency and a better air system balance between supply and demand, especially on an air-weight flow basis. A preferred embodiment employs a plurality of servo-driven flow controllers (30) in parallel between a pair of header tanks (22) and controls them in response to sensed downstream pressures and to the pressure differential across the control system. Another embodiment includes control means to manage the loading and unloading of compressors to and from the air system.

Description: COMPRESSED AIR CONTROL SYSTEM TECHNICAL FIELD The present invention relates to control systems for pneumatic plant supplies and is particularly, although not exclusively, concerned with an improved air control system for compressed air supplies used to power industrial or SUBSTITUTESHEET commercial equipment, and to a complete air system which integrates such air control system with air compressor means and useful air-powered equipment. BACKGROUND Many factories use compressed air as a major source of power for operating their production machinery. The air is typically supplied from on-site or nearby compressors and piped to a downstream use or demand area through a distribution system, with pressure-stabilizing storage means being provided upstream, in the form of tanks or receivers. Typical equipment includes pneumatically powered machine tools, drills, wrenches, presses and lathes, as well as mills and buffers, and low demand spray booths and instrumentation. Such equipment is commonly referred to in the art as the "article", a generic term connoting anything connected to the air system to make use of the supply. End users want the air system to maintain adequate pressure to run their air-powered equipment and few consider the energy costs of the air. Major changes in downstream demand create varying loads on the compressors. Air compressors are typically controlled SUBSTITUTESHEET according to system supply pressure, coming on-line as system pressure at the compressor, or nearby, drops below a threshold pressure and going off-line at a higher cutout pressure. This is necessarily a rather crude control system, especially as air compressors are slow to respond to changes. As factory production drops off, for example at the end of a shift, air demand declines, and this decline can be quite sudden. System pressure rises substantially to unload compressor horsepower until compressor output can be adjusted and this overpressure can increase the air consumption rate as still-operating machinery uses more air and also increases leakage. These overpressure losses constitute an artificial demand resulting in wasted energy. Being designed to maintain a threshold working pressure under conditions of heavy demand, most air systems use an excess of compressor capacity over the theoretical capacity needed to give satisfactory operational results, with consequent unnecessary capital expenditure and running costs. Prior art pneumatic control systems generally depend upon the above-described crude compressor switching together with pressure regulators operating at the article to protect individual pieces of equipment from pressure SUBSTITUTESHEET surges. Such regulators can be controlled from downstream pressure detectors with a feedback loop and typically discharge overpressures to atmosphere. In general, the prior art uses local control with little if any thought given to overall system control. In general, the prior art has paid little, if any attention to overall system efficiency. Indeed, it has been a surprising observation relating to the genesis of the present invention that most operators set their regulator valves to maximum, probably in the belief that they will get more performance out of their equipment. Such operators display little regard for system efficiency and equipment durability. These rather simple control devices merely provide a preliminary level of control and do little if anything to improve the overall system efficiency. Other substantial drawbacks associated with the use of such downstream flow controllers are that mechanical and pneumatic inertia, or hysteresis, slow the responsiveness of the system allowing, substantial abnormalities and consequent air, or energy, losses to develop. SUBSTITUTE SHEET SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved pneumatic control system for compressed air supplies which can reduce the energy consumption of a variable-demand compressed-air system. Another objective of this invention is to provide an improved pneumatic control system for compressed air supplies which control system is responsive and fast- acting. A further object of the invention is to provide an improved pneumatic control system for compressed air supplies which control system can increase the useful work loads that can be placed on a supply system without increasing compressor capacity. Yet another object of this invention is to provide an improved pneumatic control system for compressed air systems which can balance the air system by more closely matching supply to useful demand capacity than has heretofore been possible. A still further object of the present invention is to provide an improved pneumatic control system for compressed SUBSTITUTESHEET air supplies which can reduce wastage of air or compressor energy. Another object of the present invention is to provide an improved pneumatic control system for compressed air supplies which is adaptable to control high air flow ratings at peak demands yet capable of accurate control of much smaller off-peak loadings with greatly reduced flow rates. The invention provides a remedy for one or more of the problems encountered with prior art systems. Accordingly, the invention provides, in one aspect an air control system for a compressed air system for installation in a main air conduit disposed intermediately between air compressor means to supply compressed air to a main air supply conduit and air-operated equipment connected to receive pressurized air from said air supply conduit, the compressed air system having available air storage capacity upstream of said flow controller; which control system comprises: a) a flow controller for controlling air flow through said main air supply conduit; b) means to monitor air-weight flow downstream of the SUBSTITUTE SHEET flow controller and generate an air-weight flow related demand signal; and c) actuator means to operate the flow controller in response to said demand signal; thereby to adjust said flow controller to match the air flow through said main air conduit with said air-weight flow demand. The invention extends to such a compressed air system with the air control system installed in it, especially when the compressed air system is operating under the control of the installed air control system. In another aspect, the invention provides a pneumatic control system for compressed air supplies which comprises: (a) an upstream manifold connectable to a compressed-air supply conduit providing a source of compressed air; (b) a downstream manifold connectable to a variable compressed-air demand through a delivery conduit; (c) a plurality of airways extending between said manifolds to distribute compressed air from the source conduit to the demand conduit; (d) a plurality of flow controllers disposed one to each airway to control the air flow therein; (e) actuator means to actuate the air flow controllers in response to downstream demand to SUBSTITUTESHEET stabilize the downstream air pressure. In a broad sense, the invention also provides a management subsystem for an air control system having one or more air- flow controllers which sub-system comprises: pneumatically actuated servo means for said flow controllers; electropneumatic means to apply air to drive said servo means in response to an electric control signal; pressure-sensing means sensing downstream demand pressure in said air system and producing a pressure signal reporting pressure fluctuations with time; a logic controller coupled to said electropneumatic means and said pressure sensing means to receive said pressure signal and provide said control signal, said logic controller being capable of processing said pressure signal to provide an air-mass flow related signal as said electric control signal; whereby the management system can operate said flow controllers to stabilize downstream demand pressure. The air control system of this invention is thus positioned intermediately between the supply and the demand in a position where upstream storage capacity can be utilized for buffering purposes to smooth the balance between supply and demand. This SUB SHTUTH SHEET upstream storage capacity can be derived not just from receivers, but also from pipes, driers and any other volumetric capacity in the upstream system. In bigger air systems, there will be a plurality of compressors, which may number into the tens or more, and of course a considerable number and variety of downstream, demand equipment which can easily exceed tens and may number into hundreds of pieces of equipment supplied by a distribution network of branch lines. The downstream demand can be sensed by a pressure transmitter, said actuator means operating the flow controllers in response to said pressure fluctuations so as to smooth the demand on the compressed air supply. In this manner, the control unit of the invention is provided with a number of control sub-units each providing independent airways or circuits between the air source and the air demand, and each having its own flow controller. The air circuits should be parallel in a pneumatic sense and preferably are parallel in a mechanical sense. More than two circuits can be used and some or all of these can be coupled to be operated, or have their flow controllers driven, in unison or in harness. SUBSTITUTESHEET The use of a plurality of parallel-circuited flow controllers has distinct advantages over a single, more massive flow controller similarly placed. The size of equipment needed to manage air flow ratings of the order of tens of thousands of cubic feet per minute (cfm) introduces major hysteresis lags and inertia losses. Furthermore, such a unit lacks redundancy provisions and cannot be expected to control small off-peak flows in a sensitive manner. In a particularly advantageous embodiment, two or three or more control sub-units are thus coupled to handle peak demand, while a further, trim sub-unit is provided for effective control of much smaller off-peak loads, the coupled sub-units being shut- down during periods of very low demand, for example at night. The trim sub-unit can, of course also contribute to the control of peak loads. In addition to the control sub-units, the airways preferably also comprise a fail-open by-pass sub-unit which, in preferred embodiments of the invention, can be controlled to provide several useful functions distinct from those of the control sub-units, for instance, dampening surges and regulating back pressure. SUBSTITUTESHEET In the preferred practice of the invention the manifolds are designed to offer little or no resistance to air flow through them, providing free and easy distribution of air between the airways of the inventive control system and the supply and demand conduits. The manifolds can comprise header tanks extending transversely of the control airways and the supply and demand conduits. If the header tanks are generously proportioned, they not only will facilitate free air flow but can also contribute valuable air-storage capacity for buffering purposes. Assuming them to be elongated in a transverse direction and to have a substantially uniform transverse cross- section, by being, for example cylindrical, their individual cross-sectional areas can, with advantage, clearly exceed the cross-sectional areas of the respective supply and demand conduits with which they communicate, preferably by a margin of at least 20 percent. The use of header tanks in this manner facilitates a sturdy, integral construction which, if the sub-units are generally of self-supporting metal pipe construction, is braced by the sub- units extending between the header tanks. Although not the only possible configuration, this arrangement is advantageous in that it can be conveniently adapted for retro-fitting to existing air systems to be readily inserted as SUBSTITUTESHEET an intermediate unit in a section of trunk pipe in the air system. For this purpose the air control system can be provided with opposed, outwardly disposed and aligned supply and demand ports matched to the trunk pipe to be coupled thereto. A typical compressed air power system comprises air compressors constituting the compressed-air source, receiving vessels, heat exchangers and distribution piping as well as the useful equipment or machinery that is driven by the compressed air and constitutes the demand. One or more of the control units of this invention can be installed in such a system and is preferably located in a main feeder or trunk pipe as close, in terms of volumetric capacity, as practicable, to the demand, subject to adequate pipe diameters in the system. In most situations, only one control unit will be appropriate so that it will be located in a, or more probably the, main feeder or trunk. However, large air systems may benefit from a plurality of control units: one to each of a number of main branch conduits. By locating the control unit in a downstream position, unique characteristics of the invention provide special advantages which lead to a more balanced and economical air system. More SUBSTITUTESHEET specifically, the upstream volumetric capacity comprising besides the compressors, any receivers, upstream piping, heat exchangers and other upstream equipment, can be converted by the system of the invention into useful primary storage, providing valuable pressure-buffering capacity which contributes greatly to the overall balance of the system, under a diversity of conditions, improving its efficiency. Furthermore, the downstream distribution piping system, with the demand equipment, can be controlled to operate at a more constant pressure than was heretofore possible. Indeed, it is an important benefit of this invention that the demand pressure can for the most part, while ignoring strictly local transient effects as equipment is brought on and off line, be constrained within a surprisingly narrow range, greatly improving the operating efficiency and stability of the system to the satisfaction of users and management alike. BRIEF DESCRIPTION OF THE DRAWINGS One way of carrying out the invention is described in detail below with reference to the drawings which illustrate only one specific embodiment of the invention and in which:- Figure 1 is a schematic view, which may be read in elevation or in plan, of one embodiment of an air SUBSTITUTESHEET supply control system according to this invention and from which various electronic and sensor sub-systems have been omitted for clarity; Figure 2 is a view similar to Figure 1, of another embodiment of the invention, with some possible electronic and sensor sub-systems shown schematically, similar components using the same reference numerals as in the Figure 1 embodiment; and Figure 3 (added July 10, 1992) is a view similar to Figure 2 of a further embodiment of the invention in which multiple control units are employed in conjnuction with banks of compressors to manage their loading and unloading more efficiently. BEST MODE FOR CARRYING OUT THE INVENTION Referring to Figure 1, the air supply control unit is indicated generally at 10 and comprises three main air control sub-units 12, a trim air control sub-unit 14 and a by-pass sub-unit 16. The sub-units 12, 14 and 16 are each in the form of a pipe carrying various control devices, to be described, and each extends in a generally parallel manner between a pair of perpendicularly disposed header tanks 18 and 20 with which the sub-units 14 and 16 are in. free communication, providing controlled airways therebetween . SUBSTITUTE SHEET Header tanks 18 and "30 can be welded steel cylindrical tanks with domed end caps 22, and are provided with an array of flanged sub-unit ports 24 aligned along one side. On an opposite side, the tank 18 has a flanged inlet port 26 to mate with an air supply trunk conduit (not shown) and to receive compressed air. Similarly, tank 20 has, on a side opposite its array of ports 24, a flanged outlet port 28 ateable with an air delivery conduit (not shown) , which delivery conduit leads to the air-powered equipment or machinery served by the system. Tank 18 thus becomes an upstream manifold while tank 20 becomes a downstream manifold. Each air control sub-unit 12 and 14 is equipped in its mid- section with a flow controller 30 and at its ends with a pair of service valves 32 to isolate it from header tank 18 or 20. Upstream of the downstream service valve 32, each sub-unit 12 and 14 has a one-way check valve 34 to prevent back flow through the airway or circuit provided by the sub-unit, from the demand to the source. Each flow controller 30 has a pilot air port 36 for a pneumatic control line (not shown in Figure 1) linking the flow controller to a pneumatic sensing and actuating system SUBSTITUTESHEET which will be more fully described in connection with Figure 2. Pilot air ports 38, one to each, serve a similar role for the header tanks 18 and 20. The flow controllers 30 are preferably driven by servos indicated schematically (in keeping with other elements of the system) at 44. The servos 44 are preferably actuated and controlled by pneumatic signals received through the pilot ports 36 in the preferred management sub-system of the invention which employs electropneumatic transducers. With advantage, the flow controllers 30 have valve elements that are spring- biased to a closed position. However, in alternative embodiments providing some of the benefits of the invention, the pneumatic aspects of the management system can be substantially by electronic, fluidic or even manual elements, the servos 44 being actuated by an electronic signal. As indicated schematically by the common line 40, the three flow controllers 30 of the three main air control sub-units 12 are operated in unison, receiving similar control signals and opening and closing to a similar extent, as those signals are applied. The trim sub-unit 14 and its controller 30 operate independently of the main sub-units 12. These three main air sub-units 12 could with some advantage be operated in response to pressure SUBSTITUTE SHEET differentials or pressure levels by a more or less direct mechanical drive or even with some manual intervention, but such mechanical or manual management system would be far less efficient than the preferred electropneumatic management system to be described. By-pass sub-unit 16 is equipped with a motorized by-pass valve 46 having a further small servo air port 48 and is installed in an independent, but parallel, air circuit between the headers 18 and 20. It is preferably fail-safe, being biased to an open position in response to loss of servo pressure or other system failures. Motorization of the by-pass valve 46 enables it to fulfil a number of useful functions to be described. Each flow controller 30 is a slave unit in the overall control system and is preferably designed to have small or lightweight moving actuator elements to minimize internal mechanical inertia in the flow-controller's valve train and consequent hysteresis lags. The flow controllers 30 should also be designed to maximize the flow capacity of the controller 30 in relation to its size, thereby to minimize flow resistance, and pressure drops across the flow controller. They should be free-flow, without constriction in the wide-open position. Diaphragm-type gate regulators SUBSTITUTE SHEET with feedback control are suitable, for example the R18 series of pilot operated regulators supplied by C. A. Norgren Co., Littleton, CO. While stock flow controllers may be usable in practicing this invention, the preferred practice to meet the objectives disclosed herein is to modify such flow controllers or regulators to reduce their flow resistance and any pressure drop across them. This can be done by using lighter springs than stock, plugging any leaks or exhausts, polishing and other techniques familiar to those skilled in the art, including changing or modifying flow control elements to be lighter or to offer a fuller throat in the wide-open position. Also or alternatively, the logic circuits, to be described, can be adapted to compensate for pressure losses across the flow controllers, once their values have been determined. Such modifications can also be directed to reducing the mechanical inertia in the valve or regulator and to increasing the speed of response, all of which considerations affect overall efficiency. The issue of reduction of pressure losses is important because the control system is an introduced system that must justify its presence by appropriate savings. These SUBSTITUTESHEET will not be achieved if the control system introduces significant energy losses by way of pressure drops. Moreover, there is little pressure available for control purposes. A typical supply pressure may be in the range of 90 to 110 psig. If the regulator drops 20 lb. or so, as some do, it will impede equipment performance. The servos 44 preferably operate against spring means biasing the flow controller or valve 30 into a closed position. Suitable servo or pneumatic valve positioner units are supplied by Cashco, Inc. Elsworth KS, for example their #48 and #148 series actuators which have a rolling diaphragm, multi-spring construction. These are available in fail-open or fail-close constructions. Fail close is suitable for the flow controllers 30 while a fail-open model can be used as the motor unit for motorized by-pass valve 46. The check valves 34 are intended to prevent back flow and should be high-quality valves that are sensitive to small back pressures and seal efficiently, preferably bubble- tight . The service valves 32 should ensure minimum flow restriction, providing a full-bore flow path, and serve to SUBSTITUTESHEET isolate an individual air-control sub-unit 12 or 14 for maintenance or replacement. A full-port ball design is an example of a suitable design. With the multiple path arrangement shown, individual air control sub-units 12 or 14 can be taken down while the others are in service, avoiding shut-downs for maintenance. Service valve isolation is not normally necessary for the by-pass sub-unit 16, but can be supplied if desired. Preferably, the by-pass valve 46 is controlled to maintain a predetermined pressure difference across the air control system 10, to prevent an instantaneous flow rate increase and attendant depletion of upstream storage below desired pressure thresholds that may trigger premature compressor start-up. The arrangement shown in Figure 1 is, in the most preferred embodiment of this invention, constructed to be self- supporting and rigid. It will be appreciated that the generally recti-linear layout of a plurality of airways extending between cylindrical headers 18 and 20 facilitates this objective. The somewhat planar or laminar construc- tion with a modest depth and much greater width and height and an overall, more-or-less rectangular configuration is especially convenient for retrofitting in a variety of SUBSTITUTESHEET situations in a convenient and compact manner. The air control system can readily be installed as an integral unit in a horizontal or vertical position, with little, if any, additional support means such as brackets or struts, assuming the mating air supply and delivery lines are solidly supported at, or near the inlet and outlet ports 26 and 28. A sturdy or solid, self-supporting, integral structure of the air supply control unit 10 is promoted by the use of appropriately rated steel pipe for the pipe elements of the sub-units 12, 14 and 16, with flanged connections to and between the operative components of the sub-units 12, 14 and 16, and by a welded, flanged steel construction of the sub-unit ports 24 on the headers 18 and 20. These arrays of aligned ports 24 help resist rotational and twisting stresses. In one practical and exemplary embodiment, which should of course be understood to be non-limiting as to the scope of the invention, and which is suitable for managing a normal peak air flow capacity or rating of about 3,000 cfm, the air supply and delivery conduits of the air system, and therefore the inlet and outlet ports 26 and 28 of the air- control system of the invention, have internal diameters of SUBSTITUTESHEET 3 inches, 76 mm. while the sub-units 12, 14 and 16 have internal diameters of 2.5 inches, about 63 mm., each. The header tanks 18 and 20 suitably are about 48 inches, 122 mm., long or high and about 4 inches, 102 mm., in diameter with the width of the system 10 across the inlet and outlet ports 26 and 28 being about 54. inches, 137 mm. These dimensions give the header tanks 18 and 20 a volume of the order of about 580 cubic inches, 9.5 liters. With careful choice of the operative components, the depth can be constrained to about 18 inches, 46 mm. The embodiment of Figure 2 shows a modified air control unit 10 having three main air control sub-units 12, a by- pass sub-unit 16 disposed on the other side of the sub- units 12 from the Figure 1 position, but lacking any trim sub-unit 14. Shown schematically in line-and-box fashion is an electropneumatic management system employing air sensors and servos and digital or analog signal processing. This is a preferred regime which lends itself to computer interfacing, especially to a PC or a PC network and can function effectively with a modicum of human intervention. This management system, which will shortly be described in SUBSTITUTESHEET more detail, has its own novel and inventive characteristics and constitutes the best currently known means for managing the air control unit 10 or an air system in which an air control unit 10 is installed. However, it is not, of course, the only possible management system that can be used while still gaining benefits from the novel and inventive characteristics of the air-control unit 10. Simpler systems, perhaps employing more manual intervention, will occur to those skilled in the art. On the other hand, it can be expected that advances in the relevant arts of process management, signal processing and sensor design will yield improved management systems that better exploit the advantages of the invention or inventions disclosed herein. Referring now to the particular management sub-system depicted in Figure 2, this sub-system comprises a brain in the form of a small logic controller 100, an operator interface 102, sensors in the form of a pressure transmitter 104, a differential pressure transmitter 106 and a mass-flow meter 108, as well as four parallel- connected electropneumatic transducers 110. These components communicate through a bi-partite nervous system comprising on the pneumatic side, small gauge, high grade air lines, for example, 1/4 inch, 22 mm, reinforced plastic SUBSTITUTESHEET with brass terminations and on the electrical or electronic side by conductor wires of adequate gauge to carry, for example, 4 to 20 mA signal currents without noticeable attenuation. The nervous system includes a common air bleed line 112 which runs from port 38 in upstream header tank 18 and applies the pressure therein to the four electropneumatic transducers 110, one each of which is connected in independent parallel circuit by an air line 114 to the three flow controllers 30 in the main air-control sub- units 12, and to the motorized by-pass valve 116 in the by- pass sub-unit 16. The electropneumatic transducers 110 are separately actuated by signals received over individual conductors 116 and each has an air output line 118 coupled to port 36 of a flow controller 30 or port 48 of the motorized by-pass valve 46. In this manner, the electropneumatic transducers 110 can apply pressurized air from header tank 18 to drive or actuate the flow controller 30 or the by-pass valve 48 in accordance with electrical control signals received from the small logic controller 100. A suitable, sensitive electropneumatic transducer 110 designed for precision valve-actuation applications, is SUBSTITUTESHEET supplied by Fairchild Controls, for example their T6000 series of miniature transducers which transmit a pneumatic signal which is linearly proportional to a DC input, for example, a 4 to 20 mA control signal. These products are also field reversible permitting the output to be inversely proportional to the input signal. The pressure transmitter 104 reads the pressure in downstream header tank 20 from air port 38 via air line 118 and sends a corresponding output signal to the small logic controller 100 via a conductor 120. A suitable pressure transmitter is a variable capacitance sensor generating a 4 to 20 mA signal output with a sensitivity of at least 1 percent. Setra Systems, Inc., Acton MA supplies such a transducer in its model 206/207 line with an advertized sensitivity of about 0.13%. Differential pressure transmitter 106 receives air pressure inputs from the upstream header tank 18 via an air line 122 and from the downstream header tank 20 via an air line 124 and puts out a difference signal, reflecting the pressure difference between the tanks 18 and 20, to the small logic controller 100 via a conductor 126. The mass flow meter 108 preferably is connected just SUBSTITUTESHEET downstream of the intermediately mounted air control unit 10 as indicated by the line 129 and its output signal, preferably an analog current signal in the 4 to 20 mA range, goes to the small logic controller via a conductor 128. The mass flow meter can be any sensitive device capable of accurate flow measurements in a conduit without undue loss of pressure across it. A preferred mass flow meter produces a temperature- compensated output for which purpose it can include a temperature sensor to determine air temperature within the conduit and to track fluctuations in that temperature. A suitable mass flow meter is of the insertion type and comprises a heated probe residing in the air flow. The cooling effect of the air flow is measured to determine the rate of mass or weight flow. Sensitivities of 5 percent or less of full scale, preferably under 2 percent, with a flow time response under 5 seconds, preferably under 2 seconds, are desirable. Suitable such mass flow meters are supplied by Sierra Instruments under the trade mark "Accu-Flo". These instruments offer flow rate ranges up to 12,000 standard feet per minute with a full scale accuracy of about 1 percent and a flow time response of about 0.2 seconds. More accurate determinations of flow through a large duct subject to turbulence can be obtained by the use SUBSTITUTE SHEE^ of an array of mass flow meter probes disposed around the duct. Such a system is supplied by Sierra Instruments under the trade mark "Sting". Such an array system can accurately average the outputs of the probes. Bearing in mind the potential cash savings and return on investment the air control system of this invention can bring, the hardware used should be of high quality to be durable and reliable, and in the case of the sensors and controllers, sensitive and accurate, as well as fast acting. Further, it is desirable that the pressure transmitter 104 and the differential pressure transmitter 106 are able to put out real-time signals that are accurate reflections not just of static pressures at sample intervals, but are real- time reports of pressure fluctuations on a sub-second, better still, milli-second basis. Thus, a preferred output from the pressure transmitter 104 and the differential pressure transmitter 106 is dynamic and is at least a first order differential with respect to time of the static pressure, or pressure difference (transmitter 106) , to provide some forecast of the amplitude of pressure change. Further advantage will derive from a second differential pressure output which can be useful in forecasting the StJBST ^τ E S H speed at which a future pressure amplitude will be reached. Also shown schematically in Figure 2 are the connection of air compressors 130 and associated heat exchangers 132 and receivers 134 to the upstream header tank 18 through an air-supply trunk conduit 136. In a comparable manner, compressed air is delivered from the downstream header tank 20 through a delivery conduit 138 to multiple working equipment 140 via an air distribution system indicated schematically by lines 142. Clearly, the apparatus of the invention, notably the air control unit 10 can be operated in a number of different ways according to the management algorithms used in the small logic controller 100, if such is employed. Accordingly, the air control unit 10 can be operated in a simple or a more sophisticated manner. Broadly stated, the management sub-system described senses downstream air demand as reflected primarily by the pressure history at the downstream header tank 20, although further benefits may accrue from adding sensors in the distribution system downstream of the header tank 20, especially in the vicinity of critical or high-demand SUBSTITUTE SHEET equipment, and adjusts the flow controllers to stabilize downstream air pressure. This stabilization improves the efficiency of air use, leading to energy savings and better equipment performance and durability. By rapidly closing any open flow controller 30 as demand falls, for example as equipment is shut off, the compressor load that would otherwise surge downstream to be dissipated through air leaks and inefficient, overpressure operation of the remaining on-line equipment, is contained upstream, buffered in storage provided by the header tank 18, the receivers 134, heat exchangers 132 and upstream piping, including the supply condc:t 136. This isolation of the upstream surge quickly generates a back-pressure on the compressors leading to their early cutout as threshold pressure is sensed at the compressor. Such desirable energy-saving, early cutout is promoted not just by the fact of the closing of the flow controllers 30 but also by the swiftness with which they are closed, using the air control system of this invention. The reduced demand can now be supplied from the pressurized upstream storage capacity, augmented by storage in the air-control system of the invention, providing further savings as start up of the compressors is deferred. SUBSTI In a preferred embodiment, the small logic controller 100 employs a loop controller receiving constant input of the downstream demand pressure, and its time-based fluctuations, from the pressure transmitter 104 together with constant input from the differential pressure transmitter 106 which, by comparison monitoring yields an upstream storage pressure signal. These inputs are integrated over time to yield a mass-flow signal which serves as the basis for an output to the electropneumatic transducers 110. The output signal is processed to provide a derivative proportional to mass flow and to fluctuations therein, which is suitably biased to allow the electropneumatic transducers 110 to drive the flow controllers 30 against spring closing. The processing algorithms preferably also include input from temperature sensors to provide temperature compensation of the pressure signals and permit the computation of accurate air-weight flows. This processing system can also be a multiple loop system having individual loops, one for each flow controller 30, and a different loop for the by-pass valve 46. When employed with the embodiment of Figure 1, these process control loops can be operated in unison for the main sub- SUBSTITUTESHEET units 12 with a different algorithm for the trim sub-unit 14. This different algorithm will still have the trim sub-unit tracking weight-based air-flow demand, as accurately as possible, desirably in harness with the main sub-units 12 for maximal air demands, while including procedures to operate only the trim sub-unit at low demand levels, for example below 25 to 40% of maximum rating. Such a multi-loop primary logic control can be closely integrated with the complete air system, including the source air compressors, by including remote pressure or flow change inputs, for example from the compressor environs, which inputs can be used to introduce compensatory adjustments, for example to allow for a lagging control response imposed by the air compressor. This integrated system approach is of particular value when dynamic compressors are used for the base load. The small logic controller 100 can conveniently be incorporated in a console mounted separately from or on or nearby the air control unit 10 and could be incorporated with the operator interface 102 which can include a keyboard and monitor as well as other manual controls. This console can also include a selection of optional instrumentation, for example: an electronic pressure SUB dewpoint monitor with an adjustable alarm to warn of excess humidity; a digital flow rate display; calibrated pressure gauges; and a variety of other indicating and alerting means for high and low pressures, power on, bypass ready, excess hydrocarbon and so on. Especially in a busy environment with frequent changes in air demand, swift response is vital to the efficiency of this control system to ensure that the flow controllers 30 are in step with actual demand, not lagging it. Even quite small lags could leave a control system badly out of step with equipment such as a pneumatic ram operated in short bursts, leading to significant losses downstream if the flow controllers are open when the equipment is off line. The control system of this invention includes a number of features that promote response time and reduce lags. In particular, by means of the invention, it is possible to actuate the flow controllers 30, or selected ones thereof, to move with a speed reflecting the rate of change of air demand. This feature can be achieved using a sensed pressure signal which is at least a first order time differential, or using a direct mass flow sensor signal. - Preferably, the speed of the mechanical action of the flow controllers is directly proportional to the rate of change SUBSTITUTESHEET of demand so that there is little or no overshoot or undershoot of the set pressure. Contributing greatly to these ends is the use of a plurality of parallel air-control circuits provided by the sub-units 12 and 14. Where, as in preferred embodiments, the diameter of each sub-unit's flow path is from 10 to 33% less than that of the inlet port 26 and the outlet port 28, better still from 15 to 20% less, then any pair of sub- units will provide between them a greater flow section than the conduits 136 and 138, allowing full flow with the flow controllers 30 short of their wide-open positions, where they have better responsiveness. In contrast with a putative, single, more massive control sub-unit, the moving elements and valve trains of the smaller, parallel clus- tered controllers 30 of this invention have an individual lower inertial mass enabling them to respond faster. In addition the valve elements of the flow controllers 30, while spring-biased to a closed position are carefully adjusted and calibrated to a minimum effective closing pressure so as to minimize pressure build-ups across the flow controller 30 before it opens. To match falling system demand, the logic controller 100 reduces servo pressure to the individual main flow SUBSTITUTESHEET controllers 30 proportionately to the drop in air mass flow by an appropriate signal to the electropneumatic transducers 110. Ultimately, the main flow controllers are shut off as required to maintain stabilized control authority. To prevent a mass flow overload, the combined open gateways of the main flow controllers can be limited to a predetermined maximum. As an additional override feature, should the upstream pressure drop below a threshold value, the flow controllers can be temporarily driven in an opposite sense to close as pressure drops, so as to protect upstream storage from excessive depletion. As already implied and indicated, the by-pass valve 46 can protect against several eventualities. Firstly, it provides fail-safe protection to ensure that the control unit introduces no degradations into the air system, by failing open in the event of loss of primary control power. Further circumstances in which it can desirably be opened by an actuating signal from the small logic controller 100 or a remote host computer are: i) If the demand pressure falls below a field- adjustable threshold value while the supply SUBSTITUTESHEET pressure is adequate. ii) The differential pressure across the header tanks 18 and 20 exceeds a threshold. iϋ) Or for independent management reasons. Furthermore, the by-pass valve can be modulated to accommodate system anomalies, for example, it can be used to assist in back-pressure control to prevent a demand surge drawing down the system, by switching control authority for the valve from the downstream sensors to an upstream pressure signal. This facility is important for what is known as event management. An event is usually a known situation or set of circumstances that put what may be regarded as abnormal stresses on the air system because they are of short duration and attempts by the control logic to adjust to event conditions may be deleterious. An example is shift startup, such as the return from lunch: Everyone goes on line at the same time creating an abnormal demand surge. As described above, appropriate operation of the by-pass valve 46 can control this surge, temporarily rationing users as they build up local pressure while conserving upstream reserves and constraining active compressor capacity within stable demand limits. The invention can SUBSTITUTESHEET -36- thus smooth event curves. The invention provides, when properly practiced by following the teachings herein, an air control unit system which is capable of maintaining a substantially constant downstream pressure while managing a wide range of flow conditions within a specified design capacity. An example of achievable control levels is a flow constancy of 3% or better of total flow rating. A downstream pressure constancy within 1% is also achievable during wide, specified fluctuations in the upstream supply pressure. Source pressures of several hundred pounds, for example 225 psig, are suitable in practicing the invention with lower working pressures being field-adjustable within a range of for example from 50 to 150 psig. Under such pressure conditions, air flows can be balanced within a range of from 200 to 200 cfm. Desirably, for example, by using the configuration shown in Figure 1, adequate sub-unit capacity is provided for redundancy so that one sub-unit can be taken out of service while the others provide continual control at full air flow ratings. It will further be apparent from a consideration of the εUBSTITUTESHEET foregoing disclosure, that the invention provides a means of balancing supply and demand in a compressed air system, especially a larger system on an air-mass or air-weight flow basis, giving a more meaningful and more efficient control than simple pressure-related control. In achieving this, it is important to pay due regard to temperature effects by determining temperatures at flow- and pressure- measurement points and applying appropriate compensation to the values detected, to standardize them. Using accurate sensing and flow control equipment in conjunction with a logic controller, or computer device it becomes possible to determine not just the air-weight flow demands, but also their rates of change, which can be used to build some predictive capacity into the system. Further, comparison of demand patterns with historical records can improve the predictions to provide substantive probabilities of needed capacity in time for efficient management of the air compressors. Thus, the unique air-control system of this invention further provides the wherewithal to manage a compressor bank in a novel and surprisingly efficient manner. The air demand information provided by the control system and its sensors can be used to time or sequence switching of the SUBSTITUTESHEET compressors. For example, newer, more efficient compressors may be run continuously while older units are brought on only at tomes of peak load. Also compressor activity can be seguenced according to the controllability and responsiveness of the particular compressor as well as its part-load operating efficiency. Again in a large system, the demand area can be broken up into separate conduit zones for control purposes, the zones being demarcated by individual sensors or even by separate air control units 10. Then, data regarding storage pressure, mass flow and additional remote signals from the zones, preferably also with information as to the usage character of the zones, if they are so divided, and if such exists, can be interfaced with control means for multiple compressors to establish, with constant surveillance feedback, an optimal compressor sequence based upon individual compressor characteristics, including particularly their part-load efficiencies. Data from the zones can be weighted or adjusted to accommodate known, or predicted, air consumption characteristics. The following text is added July 10, 1992: Air compressors can be divided into several categories, SUBSTITUTE SHEET positive displacement in which a packet of air is compressed and discharged in a cyclic manner, being one major category. Dynamic compressors are another major category. Dynamic compressors continuously accelerate an air stream against an impact wall to compress it. Either or both of these types can be used as, or as components of air compressor 30 which in the latter event can also be a bank of air compressors. Rotary screw and reciprocating compressors are examples of positive compressors while dynamic compressors are usually centrifugal. Such compressors can be used with various management and control techniques, some of which will be described hereinbelow. Preferably, however the compressor bank 30 is operated in conjunction with the intermediate system control described above and with additional inventive compressor management techniques to be described hereinbelow. Such new compressor management systems which are advantageous in improving the efficiency of an air compressor system, especially one employing multiple compressors and more especially compressors having different performance characteristics, are also applicable to systems in which the intermediate control system of this invention is not applied, but in common with the principles used in said intermediate control system, an important SUBSTITUTESHEET principle of compressor management under the auspices of the present invention, is that control techniques are responsive to downstream pressure or demand pressure, and that compressor loading and loading changes are managed according to a predictive model. Such predictive models can be developed from first and second order time differentials of downstream or demand pressure. Preferably they are derived from air-mass or - weight flow determinations, and preferably also an air impedance is connected in air series between said compressors and said demand to improve said demand pressure determination. Also, in a preferred embodiment, said air impedance is variable, preferably between a wide open, full-flow non-resistive value and a fully closed value, and said impedance is adjusted to stabilize downstream pressure. Machines in current use in industry generally have the capability to run in what is known as an on-line/off-line mode, meaning that an air compressor can be unloaded, or uncoupled from any working load, while leaving its drive running, but eliminating air flow through the machine itself . SUBSTITUTE SHEET -41- Controls vary with different types of compressor. A rotary screw compressor or rotary positive machine generally uses what is known as a modulating control. A modulating control is an air throttle valve at the delivery system intake, operating on the air compressor output to throttle air flow through the compressor. The valve opens. and closes in approximately inverse proportion to the discharge air pressure seen by the compressor. As a result, when the compressor is operating, the inlet valve opens and closes, allowing more or less air to go through the machine to maintain what is intended to be a relatively constant pressure in the compressed air system. To unload this machine there is a problem in that at partial load, for example at 70% capacity, determined by the delivery system inlet valve setting, the restriction imposed by the inlet valve creates a vacuum upstream of the valve. This increases pumping losses which lower the inlet air pressure seen by the air compressor. As a result, even though there is a reduced volumetric through-put of compressed air or of Freon by increasing the number of compression ratios that this machine must see, the brake horsepower driving the air will tend to be relatively flat. SUBSTITUTE SHEET -42- Thus, using approximated round numbers to exemplify the point, a rotary screw machine that is running at 100% or lower loads such as 80% or 70% or lower, down to about a 40% load, may use about the same amount of horsepower as if it were operating at about a 90% load. There is a very flat degeneration of absorbed kilowatt input or absorbed horsepower. To unload this machine requires that the inlet valve be closed off totally and that the air/oil separator which is a basic component of a rotary positive machine be vented to atmosphere. When this occurs the compressor's horsepower loading can drop down into a range of about, 18 to 30% of the input brake horsepower. A further problem with a rotary screw compressor is known as rotor rattle. As the compressor is unloaded, a thrust instability develops which ultimately can wear and destroy the air end itself. Consequently, rotary screw compressors are usually run in a modulating mode, driving at least some air load. Most current rotary screw compressors can, judiciously, be operated in an on-line/off-line configuration but great care must be exercised to limit the number of on-line/off-line cycles, to avoid serious mechanical problems which may lead to compressor failures. SUBSTITUTESHEET Alternative designs to control rotary screw compressors have limited acceptance. One such design is a rotary turn valve mounted longitudinally within the air end housing. It runs on a helical screw and as a load is shed instead of throttling an inlet valve, the rotary valve is turned. It opens to by-pass some of the compressed air internally in the air end, relieving pressure back to an open section of the air end. This changes the displacement providing a better proportional control between the amount of air going through the compressor and the mode of driver input, which determines the horsepower consumption. This cam alleviates the pumping and throttling losses attributable to modulator valves. Another compressor regulator comprises a series of poppet valves, controlled through a simple microprocessor, and mounted longitudinally in the air end. These poppet valves are opened and closed to change the rate of by-pass at the air end and to effect a reduction in air capacity and a somewhat proportional reduction in horsepower. Other types of rotary machines, for example, non-lubricated rotary machines such as the Atlas Copco Z series are better adapted to run on-line/off-line. They have a variable load design that permits this, but again the reduced horsepower TESHEET -44 - at the no-load condition can still be in the 15 to 18 or 20% range. Reciprocating, or piston driven compressors are more controllable at part-load. There are several methods of unloading a reciprocating compressor. Inlet valve regulation, known as IVR, or as sweep control, means simply that the inlet valves are held open. Air compressor valving operates barometrically. The differential pressure across the valve can increase until it springs open in the manner of a heavily spring-loaded check valves. A single stage machine, in which all the compression from atmospheric to discharge is done in one compression stage, is usually limited to about 120 psig discharge, and requires inlet valve regulation or high pressure unloading. In the latter case a discharge valve is opened to isolate the compressor from the compressed air system. In either case horsepower is off-loaded to about 15% of the full load brake horsepower. A two-stage machine provides more flexibility with regard to control techniques, which can include inlet valve regulation, (holding the inlet valve wide open) . Another technique is known as total closure in which a suction ITUTESHEET - - valve is isolated by means of a secondary valve located upstream in the free air system. A further control technique comprises stepping the unloading to a combination of inlet valve regulation and volume bottles. In this technique, air is forced into a volume bottle about equal to the volumetric displacement of the cylinder, with zero compression. Using a series of controlled steps it is possible to establish a five-step control system. A performance curve of a five-step controlled machine, shows a significant degree of proportionality between volumetric through-put and the input brake horsepower. However, there are mechanical losses including inertial losses in the drive train; windage losses; lubrication losses; ancillary losses in driving oil pumps and other peripherals on the compressor, while still maintaining a reasonable degree of proportionality. Centrifugal compressors which are dynamic by design and are not positive displacement are the most difficult to control and understand. In a centrifugal compressor air is taken in at the eye of one or more impellers and is discharged tangentially from the impeller. Centrifugal compressors usually operate in a number of compression stages commonly SUBSTITUTE SHEET -46- two, three or four stages in industrial applications. One problem with controlling centrifugal compressors is a phenomenon of surge. If the compressor is starved for through-put, for any one of a variety of reasons such as limitations of the inlet system itself, the position of inlet control valves, changes in inlet temperature (which affect the weight of the inlet air) can all put a centrifugal compressor into a surge condition. Surge means that the impellor vanes are only partially filled, in which case the air tries to expand prematurely. In doing this the machine will undergo cyclic surging alternating between acting as a compressor and as an expander. Surging is usually preceded by an incipient surge in which operating temperature rises gradually leading to an actual surge failure where the machine will no longer operate as a compressor. This is a fairly common problem. Surging can be prevented by instantaneously isolating the machine from the downstream compressed load with a diverting valve that will forces the compressed air back to atmosphere. In a typical system, pressure modulating is sensed in an upstream valve as described for a screw machine. A flow sensor detects air flow and immediately opens a blow-off valve to blow air to atmosphere at the same time uncoupling the compressor aerodyna ically from the rest of the air system. SUBSTITUTESHEET - - The problem in terms of, horsepower conversion is that while modulating the machine down provides a reduction in input brake horsepower (which is more dramatic than with a comparable screw machine) until the blow-off valve opens, at this point, the brake horsepower is restored to almost a full load condition which is wasted to atmosphere. Other control techniques used with centrifugals include inlet guide vanes which divert the inlet air away from the eye of the impeller and you get a reduction, vecturally reduce volumetric throughput with a better reduction in horsepower. There are many ways of controlling centrifugal compressors. A preferred operating profile is to base-load a rotary machine be it rotary positive or dynamic and trim with a reciprocating machine which has better controllability. Compressor sequencers are known to the art, rotary, reciprocal and centrifugal for example under the trademark TENDA MATIL from Ingersoll-Rand Co. Such a sequencer senses pressure and sequences compressors loading and unloading according to their size and on-lσad/off-load characteristics. Selection has to be made both on the size of the machine and also on its ultimate controllability. SUBSTITUTESHEET -48- As a first major parameter or two parameters, a sequencer is programmed both on the size of the compressor and on the suitability of the compressor. Generally, a compressor sequencer works on air pressure. The pressure sensing from the compressor is transposed to the sequencer control. The sequencer control then matrixes which compressor will be appropriate for the need. The sequencer does one other thing too because it is a fairly simple matrix, it can even out the operating running time on the compressors by alternative . A compressor sequencer according to this invention, however, responds to changes of system pressure and equates them back through the PLC which from stored data will recompute which compressor should be operating both by size and suitability and control. The Ingersoll-Rand TENDA MATIL sequencer is a static pressure sensing device that transposes the pressure output to an electrical signal to output to a series of compressors which are then loaded and unloaded according to need. This is a reactive system. The present invention is practiced by sensing rate of change in system pressure, making the control function SUBSTITUTE SHEET - - proactive rather than reactive. Based on the rate of change of pressure, a compressor can be selected for bringing on- line according *-o the suitability of its characteristics. There is also an opportunity to cancel an order for a start signal for such a selected compressor, before it comes on. A drawback of prior art control systems is that when a transient event occurs, the next compressor in sequence is brought on line with attendant motor inrush current or imposition of elemental surge loads on the utility plus wear and tear on the compressor's mechanicals, even though the event that triggered the start up has passed. By sensing a rate of change of a pressure parameter, and changes in that rate of change, according to the invention, flexibility is gained. If the rate of change (pressure reduction) decreases, it is possible to cancel the start signal with attendant energy savings and reduced mechanical maintenance. Most industrial drive motors must be limited to four starts per hour or less. Once started, the motor must typically run for a minimum of 15 minutes before a safe shutdown can be accomplished. In general, unless the system is dynamically pressure-balanced, imposed changes are antagonistic to some SUBSTITUTE -50- system characteristic which resists the desired result. In typical industrial compressed air systems, system air pressure varies widely and sometimes wildly, and can cause the paradoxical situation that as system pressure tends to increase in response to reduced demand, overall.demand because of the artificial demand induced by the higher system pressure, increases. This is a further drawback of prior art seguencers. Increasing the demand for air and starting more compressors tends to force the system pressure up higher, forcing more air in back of this impedance, the impedance or this artificial demand requires the addition of more horsepower and so on. This problem can be overcome by integrating a sequencer with means to balance the system, on a mass flow basis, namely the intermediate control unit 10. The inventive intermediate control system described herein restructures the air system to a more efficient configuration. Air is metered out of the control unit at a constant pressure downstream thereof. The air impedance of the system is stabilized and surprisingly predictable at the constant distribution pressure. Primary storage is created by creating a controlled pressure peak across upstream storage volume which serves some of the transient SUBSTITUTESHEET events and surge loads that would normally would be seen as sudden demands requiring coverage by compressor horsepower. Effectively, two desirable ends are achieved at the same time. There is a predictable air impedance in the system, based on constant system pressure, the variable of artificial demand is substantially reduced or eliminated. If 100 CFM of demand at 80 lbs. is added, approximately that flow rate is what moves through the system. The inventive air system can be described as a positive displacement distribution chain. A particular weight pounds of air is used to operate a piece of equipment and is replaced directly with the same number of weight pounds of air at constant pressure, adjusted for constant temperature. The air in storage constitutes an important time buffer which permits tuning the sequencers to an algorithm. This algorithm can run in response to a rate of change of demand system pressure. In a system with multiple compressors, a determination has to be made, based on the aforementioned parameters, as to which compressors should be base loading and which should be used for load trimming. This is again a function of control by the size of the compressor and its controllability of horsepower rating at reduced demand. The intermediate control enables the air system to become SUBSTITUTE proactive, adjusting to predicted future demand, as opposed to reactive adjusting to historical demand. Peak horsepower can be supplemented with storage. For example suppose a pressure drop at the rate of 1 lb. per second is detected in the receiver tank, which then increases to 2 lb. per second, there is a clear indication of a growing demand to be met with additional horsepower. If circumstances now change through an equilibrium of balance within the system and instead of being 1 lb. per second it drops to 1/2 lb. per second, the differentiated pressure, or rate of change of pressure supply may start to get ahead of this new demand profile forestalling the need for addition of another compressor. With the control system of this invention that need can be cancelled before the compressor is added. The control system of the invention is carefully implemented to maintain constant downstream pressure. In doing this the sensed pressure changes or pressure differential are fed back through a proportional integral derivative loop and output the PLC through an IC back to the motor driving the valve. The motor driving the valve could either be a pneumatically controlled motor or it could be an electronic positioner. We intended to use a pneumatic control because you get a faster response with them and speed of response is still one of the major design parameters that we have to maintain in an intermediate control. The other problem however with a pneumatic control is that we still have the additional hysteresis introduced by the pneumatic circuit from the electronic to the pneumatic signal and then ultimately a whole new valve is going to respond and control the main flow. An electronic positioner could provide the advantage of a more direct or immediate translation and it makes a very convenient way of stepping the number of modules that are going to be using in the IC. You simply take the, pick an example, a 4 to 20 mA and divide the 4 to 20 into several difference amperages For example, the flow controllers 30, and possibly also the by-pass valve 48, may be operated in parallel within different ampere ranges, preferably from 1 to 6 milliamperes (mA) in each range. Thus, one flow controller may operate at from 4 to 8 mA, another at from 8 to 16 mA, and so on. Preferably these ranges are distinguished by a band gap of a milliampere or two. A further important and valuable capability of the use of the intermediate control unit 10 in combination with a compressor sequencer is an ability to help hold a compressor or compressors within their specified performance ranges, especially with regard to output SUBSTITUTESHEET pressure. Referring, for example, to a centrifugal compressor or compressor use as base load, the turned-on capability is limited and centrifugal compressors operate in a very narrowly defined performance band. If forced below or out of that band the compressor become what is termed as off- design. In the off-design state there are several undesired abnormalities. Flow and pressure become dramatically inverse as pressure increases in an off-design state, the flow rate drops and conversely if compressor discharge pressure is reduced, flow tends to increase, which leads to extreme anomalies in operation. Still referring, for example, to a centrii-gal compressor, but in an inventive system incorporating a control unit 10, downstream of the compressor to impose a selectable control delta P even though we are asking the compressor to climb up to a higher discharge pressure and imposing more brake? horsepower in this transaction, we are going force the compressor to operate in a much more efficient operating range probably back into an optimum design range. The aerodynamic efficiency of the machine is substantially reduced at lower output pressure. This is especially true SUBSTITUTESHEET with a three- or four-stage centrifugal compressor, with intercoolers. Every time another intercooler and another stage are added, this off-design efficiency can drop still further. By imposing a delta P through the use of a control unit 10, such a centrifugal can be moved into a better operating range, with an efficiency improvement that may be as high as 20 or 30 percent. Further control advantages can be obtained by adding trim compressors to take additional loads in response to real- load demand pressure drops. These are pressure changes which have been screened by the predictive process of the invention to eliminate transients. Such trim compressors are preferably positive displacement compressors, typically reciprocating or rotary screw compressors. The trim compressors can be brought on line by a compressor sequencer working through an intermediate control unit 10. In a preferred embodiment, this control unit 10 is a second unit additional .to a first unit controlling the output of one or more base load, typically centrifugal compressors. The intermediate control unit 10 allows air to flow based on this sequence and the compressors will start to load and SUBSTITUTE SHEET unload based on their own design capabilities and internal controls. An advantage of this control system is that it can be managed so that the compressors are no longer sequenced for the sake of the system, but through one or more. intermediate control units 10 controlling individual compressors or groups of compressors. In this manner, the intermediate controls 10, rather than the compressors can be sequenced. With this arrangement, in the event that a centrifugal compressor, used for base load, goes into drawdown, control units 10 other than that for the down-drawing compressor, will start to come on line and start to load the compressors they control. Conventionally, if one centrifugal compressor is added to a compressor in an off-design state, the horsepower absorbed by the centrifugal remains roughly constant, while the air through-put reduces. Referring to Figure 3 of the drawings, the air system shown schematically comprises base load bank of three centrifugal compressors 200, each outputting through a dryer 202 and three trim compressors 204, of positive displacement type, each also outputting through a dryer 202. The sum of storage upstream of a control unit, to be described is UTE SHEET indicated schematically as base load primary storage 210 and trim load primary storage 212. In the illustrative exemplary embodiment shown, two of the base load compressors 200 are rated at 3,000 horsepower (HP) and one is rated at 1,500 HP. Comparably, two of the trim compressors are rated at 1,000 CFM (cubic feet per minute) and one at is rated at 1,500 CFM. The different rating methods are customary for the different types of compressor. Each bank of compressors is supplied with its own intermediate control unit similar to the control unit 10 described above, namely a base load control unit 206 and a trim load control unit 208. A primary logic controller 214 receives pressure signal input, via line 216, from the upstream side of control unit 206 and via line 218 from the downstream or demand side of the control unit 208. The controller 214 outputs a control signal separately to each trim compressor 204 over a line 224. The downstream outputs of the control units 206 and 208 are fed to the working equipment over a common air delivery conduit 220. Examples of possible operating pressures are, for the demand side of the system, substantially as seen by the articles or working equipment, 80 psig indicated at 222, 90 psig in primary storage 210 for the base load output and 115-125 psig in primary storage 212 for the trim load output. The scenario depicted in Figure 3 is appropriate for centrifugal compressors rated at about 125 psig. Here, the base load control unit 206 imposes a pressure differential, a delta P, in a base load main air conduit 226 to raise the working pressure of the base load compressors to a .preferred range near their rating, and substantially above the working system pressure or demand pressure of about 80 PSIG. Under this control, the centrifugal compressors are operating substantially more efficiently. Control unit 206 and its associated logic controller, which may be a separate unit or incorporated with logic controller 214, runs the base load compressors at a substantially constant rating. Demand fluctuations are handled by the trim compressors 214 which are loaded on and off line by control unit 214 until such point as theer capacity is exceeded whereupon a base load compressor 200. This inventive system is valuable in employing one or more trim compressors 204, generally of a lower rating and with better unloading characteristics than the centrifugal SUBSTIT EE -59- compressors have. Logic controller 214 can effectively, with appropriate algorithms and the control network described and illustrated, fulfil the role of a compressor sequenced selecting appropriate compressors to load or unload. The selection algorithm or algorithms preferably takes account not only of capacity and manageability of the various compressors in the system, but also of their on- load efficiency, including temperature-related efficiency, some compressors being substantially less efficient at high inlet temperatures, and also their off-load horsepower savings. While an illustrative embodiment of the invention has been described above, it is, of course, understood that various modifications will be apparent to those of ordinary skill in the art. Such modifications are within the spirit and scope of the invention, which is limited and defined only by the appended claims.

Claims: Claims : 1. An air control system for a compressed air system comprising air compressor means (30) , air-operated equipment (140) , and a main air supply conduit connecting said air compressor means (30) with said air-operated equipment (140) pressure-sensing means (38) to sense system pressure and control means (100) to manage said compressors according to said system pressure characterized in that said pressure-sensing means (38) and said control means (100) determine pressure differentials and control said system in response to rates of pressure change, said control being managed to stabilize said system pressure. 2. An air control system for a compressed air system for installation in a main air conduit disposed intermediately between air compressor means (30) to supply compressed air to a main air supply conduit and air-operated equipment (140) connected to receive pressurized air from said air supply conduit, the compressed air system having available air storage capacity upstream of said flow controller; which control system comprises: a) a flow controller for controlling air flow through said main air supply conduit; b) means to monitor air-weight flow downstream of the SUBSTITUTESHEET flow controller and generate an air-weight flow related demand signal; and c) actuator means to operate the flow controller in response to said demand signal; thereby to adjust said flow controller to match the air flow through said main air conduit with said air-weight flow demand. 3. An air control system for compressed air supplies which comprises: a) an upstream manifold connectable to a compressed- air supply conduit providing a source of compressed air; b) a downstream manifold connectable to a variable compressed-air demand through a delivery conduit; c) a plurality of airways extending between said manifolds to distribute compressed air from the source conduit to the demand conduit; d) a plurality of flow controllers disposed one to each airway to control the air flow therein; e) actuator means to actuate the air flow controllers in response to downstream demand to stabilize the downstream air pressure. 5. A compressed-air system including: a) air compressor means (30) providing a compressed air supply; b) accessory generation equipment in pneumatic circuit with said air compressors; c) useful air-powered equipment connectable in pneumatic- circuit with said compressed air supply providing a variable demand for said compressed air supply; d) an air-control system pneumatically connected intermediately between said useful equipment on the one hand and said air-compressor and accessory generation equipment on the other hand which air control system comprises air flow controller means controlled in response to demand pressure variations to stabilize demand pressure. 4. A compressed-air system according to claim 3 wherein said air-control means can actuate said flow controller means according to integrated changes of demand pressure with time, thereby to provide air-mass flow related pressure stabilization. SUBSTITUTESHEET

Inventors: HUNT E CHARLES, MIREL ROBERT J, WILSON ROBERT E, HUNT, E., CHARLES, MIREL, ROBERT, J., WILSON, ROBERT, E.
Applicants: CONSERVAIR TECHNOLOGIES
Application year: 1992
Main classification: G05D16
IPC codes: G05D7, G05D16