WO1995010078A1

WO1995010078A1 - Method and apparatus for supplying a pressurized compressible medium to a network

Info

Publication number: WO1995010078A1
Application number: PCT/NL1994/000247
Authority: WO
Inventors: Antonius Albertus Leenhoven
Original assignee: Zeevenhooven Acat B.V.
Priority date: 1993-10-07
Filing date: 1994-10-07
Publication date: 1995-04-13
Also published as: EP0775338A1; AU1078595A; NL9301729A

Abstract

The invention relates to a method for controlling the supply of a pressurized compressible medium to a network (3) debouching in at least one take-off point (2) by continuously measuring the amount of the compressible medium flowing to the at least one take-off point and pressurizing and supplying to the network an amount of fresh compressible medium substantially corresponding with the measured amount. The compressible medium is herein pressurized by at least one compressor (4, 5, 6, 7), the rotation speed of which is controlled subject to the amount of fresh compressible medium for pressurizing. The invention further relates to a controllable apparatus for supplying a pressurized compressible medium to a network debouching in at least one take-off point (2), with at least one compressor (4, 5, 6, 7) connected in series to the network, control means (8) connected for signal-generation to the compressor and means (10, 11) connected for signal-generation to the control means for continuous measurement of the amount of the compressible medium flowing to the at least one take-off point.

Description

METHOD AND APPARATUS FOR SUPPLYING A PRESSURIZED COMPRESSIBLE MEDIUM TO A NETWORK

The present invention relates to a method for con¬ trolling the supply of a pressurized compressible medium to a network debouching in at least one take-off point.

Different control methods are known for systems with which a pressurized compressible medium, for example compressed air, can be supplied to a large number of consumers. In the known control methods the pressure of the compressible medium in a network is generally controlled by switching on an apparatus for pressurizing a compressible medium such as a compressor when the pressure in the network falls below a determined set value and by switching it off again when this pressure approaches or reaches a determined maximum permissible value. In such control methods a buffer is generally connected to the network in order to prevent too large pressure differences in the network. A drawback to this known control method is that regular switching on and off of a compressor results in a relatively high level of wear, while moreover at switch-on a load peak occurs, which results in relatively high energy costs. In order to reduce the regularity with which the compressor has to be switched on and off a very large buffer can be used, but this entails comparatively high installation costs. Bringing the buffer to and holding it at pressure moreover requires a relatively large amount of energy. Control methods are also known wherein the compressor is not switched off when the maximum pressure is reached but remains in no-load operation. This reduces the wear of the compressor and fewer peak loads occur. The drawback to this known control method however is that the energy consumption of a compressor in no-load operation is still considerable, whereby relatively large losses occur. In yet another control method the compressor is operated practically uninterruptedly at full load and a suction opening to the network is opened or closed in continuously variable manner subject to the pressure fall or pressure rise occurring in the network. The energy consumption of this control is also comparatively high. The invention therefore has for its object to pro¬ vide a method for controlling the supply of a pressure- compressible medium to a network of the above described type, wherein these drawbacks do not occur. This is achieved according to the invention by continuously measuring the amount of the compressible medium flowing to the at least one take-off point and pressurizing and supplying to the network an amount of fresh compressible medium substantially corresponding with the measured amount. By selecting not the pressure but the volume flow as the control quantity a marked saving on energy consumption is achieved, while there will also be relatively little wear of the compressor. In addition the consumption can thus easily be made visible, which may encourage more conscious use of the compressible medium and thus lead to savings. The measured amount is preferably converted on the basis of at least one measured condition parameter of the compressible medium to an equivalent amount at a particular defined standard condition. A control quantity is thus obtained which can serve without further processing as input signal for a compressor.

When the pressure of the compressible medium in the network is measured and the amount of fresh compressible medium is adapted on the basis of the measured pressure in order to obtain a substantially constant pressure in the network, considerable savings are achieved, as the minimum pressure required in the network can then be chosen as constant operating pressure.

The compressible medium is preferably pressurized by at least one compressor, the rotation speed of which is controlled subject to the amount of fresh compressible medium for pressurizing. By making use of a rotation speed- controlled compressor a practically linear adaptation of the delivered amount of pressurized compressible medium to the consumption is possible.

In a preferably applied control method according to the invention the compressible medium is pressurized by at least two compressors connected in parallel, the rotation speed of one of which is controlled and the other of which is only switchable on and off, wherein the rotation speed- controlled compressor is continuously switched on and controlled subject to the compressible medium for pressurizing and wherein, when the amount of compressible medium for pressurizing exceeds the maximum capacity of the rotation speed-controlled compressor, the other compressor is switched on while the rotation speed-controlled compressor is simultaneously regulated downward such that the amount of compressible medium pressurized by the compressors increases uniformly and, when the amount of compressible medium for pressurizing falls below the maximum capacity of the rotation speed-controlled compressor, the other compressor is switched off while the rotation speed- controlled compressor is simultaneously regulated upward such that the amount of compressible medium pressurized by the compressors decreases uniformly. Therefore, controlling the speed of rotation of only a part of the installed compressor power will suffice, thus resulting in a comparatively inexpensive installation, as the costs of rotation speed controllers are virtually proportional to the power of the compressor controlled thereby.

The invention further relates to a controllable supply apparatus for pressurized compressible medium with which the above described method can be performed. To this end the invention provides an apparatus as described in the preamble to claim 6, provided with control means connected for signal-generation to the compressor and means connected for signal-generation to the control means for continuous measurement of the amount of the compressible medium flowing to the at least one take-off point.

Preferred embodiments of the controllable supply apparatus according to the invention form the subject-matter of the dependent claims 7-11. The invention is elucidated on the basis of an embodiment, wherein reference is made to the annexed drawing, wherein: fig. 1 shows a schematic diagram of a controllable supply apparatus according to the invention; fig. 2 shows a more detailed flow diagram of the apparatus shown schematically in fig. 1; fig. 3 is a diagram to elucidate the control method according to the invention; and fig. 4 is a logic circuit in which the control method according to the invention is embodied.

A controllable apparatus 1 for supplying a pressurized compressible medium to a network 3 which leads to at least one take-off point 2 comprises four compressors 4,5,6,7 mutually connected in parallel and connected in series to the network 3 (fig. 1) . The apparatus further comprises control means 8 which are connected for signal- generation to the compressors 4-7. Further connected to control means 8 in signal-generating manner are means 9 which continuously measure the amount of compressible medium flowing through network 3 to the take-off point 2. The apparatus 1 further comprises means 10,11 for measuring a number of condition parameters. The measuring means 10 can for instance be formed by a temperature meter and the measuring means 11 can comprise a pressure gauge. The condition parameter-measuring means 10,11 are likewise connected for signal generation to the control means 8 where the measured condition parameters are used to convert the measured amount of consumed compressible medium to an equivalent amount of compressible medium at particular defined standard conditions, for instance a pressure of 1,013.25 millibar and a temperature of 0 or 20°C. For this conversion use can be made of the formula:

Qnorm = Z X Q X P X (293 / (273 + T) ) , wherein Q_norm is the converted amount of used compressible medium, Q is the measured amount of compressible medium, P is the pressure and T the temperature of the compressible medium. The factor Z is a correction factor for the fact that the compressible medium will generally not be an ideal gas but, for instance in the case of compressed air, a combination of a number of gases. For most practical applications however, Z will be practically equal to 1, so that this can be further disregarded here. On the basis of the converted volume flow of the pressurized compressible medium the compressors 4,5,6,7, the capacity of which will be known under the above stated standard condition, can be controlled by the control means 8. A (small) buffer 12 is further present between compressors 4-7 and network 3, whereby small fluctuations in the volume flow delivered by the compressors 4-7 resulting from switch-on and switch-off transient phenomena can be compensated.

Each compressor 4,5,6,7 comprises a motor 13 which drives a fan 14 which draws in ambient air (fig. 2) . Motors 13 are each controlled by the control means 8. The indrawn air is eventually compressed and fed through a narrowing outflow pipe 25 to the network 3. Before the compressed compressible medium, in the example shown compressed air, reaches the end user it is conditioned. For this purpose the compressible medium is guided via a series of filters 20,21,22 past one of the drying installations 23,24 by causing one or more of the controllable valves 15,16,17,18,19 to be opened and/or closed under the influence of the control means 8. Instead of being guided past the shown drying installation 23,24 the compressible medium can also be guided past a refrigeration drier 26 or an absorption drier (not shown here) to be connected to the valves 27,28. The series of filters 20,21,22 is connected through a separate pipe system to a waste treatment plant 29. The conditioned, compressed air is then fed via two buffers 12 connected in parallel to two pipe systems connected in parallel, for instance a pipe system for compressed air for machine tools and one for compressed air for general use. The volume of the pressurized compressible medium supplied to each of the pipe systems is measured in flow meters 9 of the so-called "vortex shedding" type. The regulation of the supply of the pressurized air to the network proceeds as follows:

The compressor 4 is rotation-speed controlled, whereby the amount of air to be pressurized by this compressor can be controlled in practically linear manner from zero to the maximum capacity of compressor 4. When, assuming a situation in which no compressed air is being used, compressed air must be supplied to a take-off point, the control means 8 will first increase the rotation speed of compressor 4 such that the demand for compressed air is fulfilled. However, when the amount of compressed air extracted from network 3 exceeds the maximum capacity of the rotation speed-controlled compressor 4 (which in the example shown amounts to 25% of the nominal capacity of the total apparatus) , a first auxiliary compressor 5 is switched on (fig. 3) . The rotation speed-controlled compressor 4 is then simultaneously regulated downward by control means 8 such that the total amount of pressurized air delivered by both compressors 4,5 does not increase shock-wise but proceeds uniformly and smoothly. If the consumption of compressed air now increases further, the rotation speed- controlled compressor 4 is then once again regulated upward in order to be able to satisfy the increased demand.

If the demand for compressed air were then to exceed the total capacity of both compressors 4,5 (which, assuming that compressor 5 likewise has a capacity of 25% of the nominal capacity of the total apparatus, together have 50% of the nominal capacity) , the small auxiliary compressor 5 would then be switched off, a large auxiliary compressor 7 (with a capacity amounting to 50% of the nominal capacity of the apparatus) would be simultaneously switched on and the rotation speed-controlled compressor 4 again regulated downward to a capacity such that the total capacity delivered by the now switched-on compressors 4,7 does not change in shock-wise manner. Should the amount of compressed air demanded increase still further, the first auxiliary compressor 5 will, when this becomes greater than 75% of the nominally installed capacity, then be switched on again and the rotation speed-controlled compressor 4 will be regulated downward again.

The apparatus further comprises a third auxiliary compressor 6, the capacity of which likewise amounts to 50% of the nominal compressor capacity and which is only switched on in the case of a peak load (thus when more than 100% of the nominal capacity of compressed air is demanded) or in the case of failure of one of the other compressors 4,5,7. Because the total capacity of the four compressors 4,5,6,7 amounts to 150% of the nominal required compressor capacity, at least 100% and in some cases even 125% of the nominal required capacity will always be available even in the case of failure of one of the compressors. Because only the rotation speed of the (relatively small) compressor 4 is controlled, a relatively small frequency controller will suffice for control. Since the frequency controller represents a relatively costly part of the total installation and the price of a frequency control increases in almost direct proportion to the power of the machine for regulation thereby, this results in a considerable cost-saving.

The above described control of the compressors is carried out by the circuit in the control .means 8 shown in detail in fig. 4. The quantities of the pressurized compressible medium measured by the volume flow meter 9, the temperature meter 10 and the pressure gauge 11 are each processed via galvanic separating means 30 and an A/D converter 31 into digital signals which are supplied to the control means 8. The temperature T of the compressible medium measured by the temperature meter 10 is then increased in an adding block 33 with a Kelvin value of 0°C (273 Kelvin) from a reference block 32, and in a block 35 a reference temperature (for instance 293 K = 0°C) from a block 34 is divided by the result coming from block 33 and the thus obtained quotient is fed to a multiplying block 39. The digitized volume flow Q and pressure P of the compressible medium coming from the measuring means 9,11 are multiplied by each other in block 36 and subsequently divided in block 37 by a reference pressure P_ref originating from block 38. The thus obtained quotient is likewise fed to the multiplying block 39. The output signal of block 39 is the volume flow Q "^•n-,-o,-r-m-, converted in accordance with the above stated relation. This operation is performed for both the pipe systems shown in fig. 2. Both the thus obtained converted volume flows are added up in block 41 and the output signal of block 41 thus indicates the amount of com¬ pressed air demanded at any moment. The digitized pressure signal is further fed as input signal to a PID controller 42, the output signal of which is fed to a frequency controller (not shown) which directly controls the rotation speed of the first compressor 4. The PID controller 42 adjusts to an internally set value of the desired pressure. As long as the requested amount of compressed air is smaller than the maximum capacity of the first compressor 4 only this control will take place. If however the compressor 4 is running to maximum capacity and the measured pressure is nevertheless falling, one of the auxiliary compressors 5,6,7 will have to be additionally switched on. Engaging of the auxiliary compressors is carried out by a flow rate limit value circuit 43 or by a pressure difference limit value circuit 44. The latter receives a value of the difference between the adjusted pressure from block 45 and the measured pressure from pressure gauge 11, which difference value is calculated in block 46. This calculated pressure difference is transmitted via an adding block 47 to two comparing blocks 48,49. In block 48 is determined whether the difference between the adjusted and the measured pressure exceeds a determined value, in the embodiment shown 40 millibar. If this is the case, a so- called "soft-starter" 50 is then actuated by a flip-flop circuit 60 and a further logic circuit connected therebehind. A check is made in block 49 as to whether the difference between the adjusted pressure and the actual pressure falls below a determined lower value. In that case a so-called "soft-stop" 51 is directly actuated by the flip- flop 60 whereby the associated compressor is stopped.

When the compressor is started by the flow rate limit value circuit 43, the total demanded amount of compressed air from block 41 is compared in a block 52 with a first limit value. If this limit value is not exceeded, then none of the compressors is additionally switched on. If however this limit value is exceeded, no action is as yet undertaken but the signal from the comparing block 52 is first placed in a wait block (designated with 53 in each case throughout the circuit) , whereafter it is determined in a second comparing block 54 whether a second limit value of the flow rate is also exceeded. If this is the case, the larger auxiliary compressor 7 is additionally switched on and not the small auxiliary compressor 5. In similar manner comparisons are made with a third limit value in block 55 and a fourth limit value in block 56. Thus is determined whether the compressors 5 or 7 separately, 5 and 7 together or even 5,7 and 6, have to additionally switched on. Placed prior to the comparing block 56 is a further block 57 in which a progressive average of the output value of block 41 is determined in order to prevent all compressors 4-7 being started up immediately at incidental load peaks. The output signal of the flow rate limit value circuit 43 sends a pulse to the adding block 47 and/or adding block 58 whereby the pressure difference signal from block 46 is apparently increased and an exceeding of the set limit value is forced. The signal indicating the apparent pressure difference is then fed again to the above described circuit 44 and/or a corresponding circuit 59, whereby eventually the necessary compressors are additionally switched on.

The "soft-start" circuit 44 not only has a starting function 50, which as indicated above is controlled from the flip-flop circuit 60 and the subsequent function blocks, and a stop function 51 actuated directly by the flip-flop 60 and thus having priority over the "soft-start", but also a so-called "disable/enable" gate 61 which ensures by means of a time delay and a reset that when a "soft-stop" is carried out this does not continue as far as the maximum ignition angle of the "soft-starter" but only until the screw-type compressor and separator are vented, and is then de-activated.

As is apparent, when one or more auxiliary compressors 5,6,7 are additionally switched on, the pressure of the compressible medium in the network 3 will increase, whereby the PID controller 42 will detect exceeding of the set pressure value and regulate downward the rotation speed- controlled compressor 4 until this detected pressure difference has disappeared.

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Claims

1. Method for controlling the supply of a pressur¬ ized compressible medium to a network debouching in at least one take-off point, characterized by continuously measuring the amount of the compressible medium flowing to the at least one take-off point and pressurizing and supplying to the network an amount of fresh compressible medium substantially corresponding with the measured amount.

2. Method as claimed in claim 1, characterized in that the measured amount is converted on the basis of at least one measured condition parameter of the compressible medium to an equivalent amount at a particular defined standard condition.

3. Method as claimed in claim 2, characterized in that the pressure of the compressible medium in the network is measured and the amount of fresh compressible medium is adapted on the basis of the measured pressure in order to obtain a substantially constant pressure in the network.

4. Method as claimed in any of the foregoing claims, characterized in that the compressible medium is pressurized by at least one compressor, the rotation speed of which is controlled subject to the amount of fresh com¬ pressible medium for pressurizing.

5. Method as claimed in claim 4, characterized in that the compressible medium is pressurized by at least two compressors connected in parallel, the rotation speed of one of which is controlled and the other of which is only switchable on and off, wherein the rotation speed-controlled compressor is continuously switched on and controlled subject to the compressible medium for pressurizing and wherein, when the amount of compressible medium for pressurizing exceeds the maximum capacity of the rotation speed-controlled compressor, the other compressor is switched on while the rotation speed-controlled compressor is simultaneously regulated downward such that the amount of compressible medium pressurized by the compressors increases uniformly and, when the amount of compressible medium for pressurizing falls below the maximum capacity of the rotation speed-controlled compressor, the other compressor is switched off while the rotation speed-controlled compressor is simultaneously regulated upward such that the amount of compressible medium pressurized by the compressors decreases uniformly.

6. Controllable apparatus for supplying a pressur¬ ized compressible medium to a network debouching in at least one take-off point, with at least one compressor connected in series to the network, characterized by control means connected for signal-generation to the compressor and means connected for signal-generation to the control means for continuous measurement of the amount of the compressible medium flowing to the at least one take-off point.

7. Controllable supply apparatus as claimed in claim 6, characterized in that means connected for signal- generation to the control means are present for measuring at least one condition parameter of the compressible medium and the control means are adapted to convert the measured amount of compressible medium on the basis of the received signal of the measured condition parameter to an equivalent amount at a particular defined standard condition.

8. Controllable supply apparatus as claimed in claim 6 or 7, characterized in that means connected for signal-generation to the control means are arranged on the network for measuring the pressure of the compressible medium present therein, and the control means are adapted to generate and transmit to the compressor a pressure correction signal on the basis of the received signal of the measured pressure.

9. Controllable supply apparatus as claimed in any of the claims 6-8, characterized in that the control means comprise a frequency controller.

10. Controllable supply apparatus as claimed in claim 9, characterized by at least two compressors connected in parallel, one of which is connected to the frequency controller and the other of which is connected to an on/off switch accommodated in the control means.

11. Controllable apparatus as claimed in claim 10, characterized in that a plurality of compressors connected in parallel are connected to the on/off switch of the control means and the mutual difference in capacity of these compressors is at most e-qual to the maximum capacity of the compressor connected to the frequency control.

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