Process and a Device for Increasing the Efficiency of Compression Devices

This invention relates to a process for increasing the efficiency of a compression device and to a device for performing the process.

It is known how to alter the conveyed volume flow of helical compressors of the middle and upper power class by means of an axial power valve spool. The purpose of the power valve spool is to assist and to enable the start-up of the helical compressor during the start phase. During the start phase, the power valve spool is opened, and the compressor only conveys a reduced volume flow. After start-up, the power valve spool is closed; and, during the following operational phase, the volume flow is 100%. In the field of cryogenics, helical compressors are used for the compression of helium, for example. As published in the art "The Linde-Turborefrigerator for MR-Tomographs, J. Clausen et al., Advances in Cryogenic Engineering, Vol. 35, pp. 949-955, Ed. R. W. Fast, Plenum Press, New York, 1990", the control of the suction pressure is known by determining the suction pressure with a pressure measuring device and influencing the rate of mass flow by a bypass valve switched parallel to the helical compressor so that the suction pressure is maintained at constant values.

If this control concept is used in cryogenic installations for load cases which are hard to predetermine, such as, for example, in research centers in the cooling of superconductive magnets, then in partial load operation, which normally lies between 50% and 100% of the maximum conveying capacity, the result is a considerably reduced efficiency of the compression device.

The objection of the present invention is therefore to improve the efficiency of the compression device for cases with partial loads.

Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.

According to the invention, this object is achieved by a process with which the control device influences the power valve spool position of the compressor and the position of the bypass valve switched parallel so that the mass flow or the volume flow between the suction side and the pressure side is continuously adapted to the requirements of the cooling process while maintaining the suction pressure constant as far as possible.

In cryogenic applications a compression device circulates a fluid, such as helium, for example, in a closed cooling circuit. The compression device consists of at least one compressor, preferably a helical compressor, and also a bypass valve switched in parallel thereto, which is used to decompress the compressed fluid. The suction pressure is supplied to a control device, which controls the position of the power valve spool of the compressor and also the position of the bypass valve. So as to attain a high control performance with small bypass losses for load cases which are hard to predetermine, the positions of bypass valve and power valve spool are controlled so that a fluid flow flows as a control reserve through bypass valve and is in the order of a fraction of the total fluid flow.

The axially adjustable power valve spool of the helical compressor enables the conveyed volume flow to vary in the range from normally roughly 15% to 100%. One advantage of the invention when compared with known solutions is regarded as being that in partial load operation the mass flow flowing via the bypass valve is reduced or even completely interrupted by controlling the position of the power valve spool, as a result of which there is a substantially greater level of efficiency for the compression device in partial load operation. No power loss occurs when the bypass valve is closed and the volume flow on the suction side determined by the position of the power valve spool brings about the preset pressure. An essential criterium of the compression device is the control performance, in particular the rate of response and the control accuracy with which the suction pressure can be brought into agreement with the preset desired value. The two actuators, i.e., the power valve spool and the bypass valve, have different control characteristics. The power valve spool behaves sluggishly. For a displacement of from 0 to 100% volume flow an execution time of circa 1 minute is required. In addition, its control characteristic is not linear and does not have the same percentage and also cannot be structurally adapted to the requirements of the user. However, the bypass valve has a low delay time and a flow characteristic which can be optimized and thus also has the same percentage, for example. A partial load operation without bypass losses is suitable for stationary processes. If fast pressure changes and small pressure fluctuations have to be controlled, the non-linear control characteristic and also the inertia of the power valve spool have a very negative effect.

In the case of the load which is hard to predetermine with fast pressure changes and small pressure fluctuations, the bypass valve advantageously always stays open to a certain extent. Thus, the bypass valve permits fast control and good control accuracy of the suction pressure. The power valve spool is adjusted more slowly until the mass flow through the bypass valve attains a predetermined desired value range. This bypass flow corresponds to a control reserve which can quickly be controlled. In this case the requirements on the control quality mainly determine the size of the losses in partial load cases. The position of the power valve spool is advantageously not permanently altered for mechanical reasons. The control of the power valve spool may occur with hysteresis. Alterations in the region of the control reserve can also be controlled without adjusting the power valve spool, so that the size of the losses in partial load cases can also be chosen so that movements in the power valve spool are avoided as far as possible.

Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:

FIG. 1 shows the diagrammatic construction of an installation in which the new process comes to be used;

FIG. 2 shows the diagrammatic construction of a controlled compression device for performing the new process;

FIG. 3 shows a further diagrammatic construction of a controlled compression device for performing the new process;

FIG. 4 shows a further diagrammatic control concept of a compression device for performing the new process;

FIG. 5 diagrammatically shows a further control concept of a compression device for performing the new process; and

FIG. 6 is a top elevational view of a typical prior art helical compressor with which the process of the present invention is used.

FIG. 1 shows a cryogenic cooling device 1 for the production of liquid helium, consisting of a controlled compression device 2 having a compressor 21, the cooling circuit of which is connected to a cooler 3 via connecting lines 22, 23. The cooler 3, which consists of two heat exchangers 31, 32, an expansion machine 33 and also a valve 34, is connected to the heat exchanger 4 contains gaseous helium 41, liquid helium 42, and inside, a condensing coil 43 with connecting lines 44, 45, for example.

FIG. 2 shows the controlled compression device 2 with an external control device 28a. The helical compressor 21 is provided with an axially displaceable power valve spool 24b and a corresponding drive device 24a, which is triggered with the error signal Y.sub.LS. The powered valve spools are used for controlling conventional valves which influence counterpressure, and/or suction pressure and/or volume flow. The pressure control of a compressor is normally performed by varying the size of the opening of a counterpressure valve. The bypass valve 25, which is regulated via a valve drive 26 by error signal Y.sub.Bp, is disposed parallel to the helical compressor 21. A pressure measuring device 27 registers the suction pressure and conveys the actual value X1.sub.actual to the control device 28a, which produces the error signals Y.sub.LS and also Y.sub.Bp after comparison with the desired value X1.sub.desired. Various strategies are advantageous when controlling the positions of power valve spool 24b and bypass valve 25, according to the demands on the compression device and on the consumer. For example, the suction pressure is controlled by the bypass valve 25, as long as the fluid flow of the connected consumer is smaller than the minimum fluid flow which can be conveyed through the compression device 2. When the fluid consumption of the consumer is greater, the bypass valve 25 is closed and the suction pressure is only controlled via the position of the power valve spool 24b.

The controlled compression device 2 shown in FIG. 3 has a different control concept when compared with FIG. 2. The position of bypass valve 25 is determined by control device 28a on the basis of the preset desired value X1.sub.desired and of the measured suction pressure X1.sub.actual. A fluid flow measuring device 29 continually determines the flow through the bypass valve 25 and supplies these values X2.sub.actual to a control device 28b, which after comparing them with a desired valve X2.sub.desired transmits the error signal Y.sub.LS to the driving device 24a of power valve spool 24b. Fast suction pressure changes are controlled by bypass valve 25 provided with a short delay time, which brings about a high control performance, short rate of response and high control accuracy. The control reserve of the fluid flow through bypass valve 25, which can be preset via the desired valve X2.sub.desired of control device 28b, is adjusted by the sluggish power valve spool 24b. The control reserve of a fraction of the total fluid flow, which flows via bypass valve 25, enables a reduction in the bypass losses to a tolerable range with a high control performance. By driving the power valve spool 24b with hysteresis, gradually or in stages, the number of movements of the power valve spool 24b can be reduced.

FIG. 4 shows a further control concept of a controlled compression device 2. The position of the bypass valve 25 is again determined on the basis of the suction pressure of the pressure measuring device 27. The mass flow through the bypass valve 25 is determined with a valve lift measuring device 20 and this value X2.sub.actual is supplied to a control device 28b, which, after comparing it with the preset value X2.sub.desired for the mass flow through the bypass valve 25, supplies an error signal Y.sub.LS for the drive device 24a of the power valve spool 24b. Apart from a continual drive of the bypass valve 25 and also of the power valve spool 24b, other drive forms are also conceivable, such as gradual or stepwise drives.

FIG. 5 shows a further control concept of a controlled compression device 2. The suction pressure determined by the pressure measuring device 27 is supplied with the actual variable X1.sub.actual to the controller 28a, which after comparing it with the desired variable X1.sub.desired places the error signal Y.sub.Bp at the valve drive 26. The error signal Y.sub.Bp is supplied to a subordinated controller 28b as actual value X2.sub.actual, which after being compared with the desired value X2.sub.desired emits a correcting variable H.sub.La and thus controls the drive device 24a of the power valve spool 24b. The solution represented with FIG. 5 of a controlled compression device 2 has the advantage that existing compression devices can be operated without any hardware alterations with the control concept specified by the invention.

Referring now to FIG. 6, there is shown a typical single stage, conventional compressor 21 which is of the type used as a compressor in FIGS. 1-5. The compressor 21 is illustrated and discussed in Mark's Standard Handbook for Mechanical Engineers, Ninth Edition, Avalone el al., sec. 14-38 (1987). The compressor 21 is connected to an input line 22 and an output line 23 (see also FIGS. 1-5). The air is compressed by helixes or screws 25 disposed between the input and output lines 22 and 23, respectfully.

Exemplary of a screw or helix compressor 21 of this type is set forth in J. Clausen et al., supra., which includes the following description of a compressor with which the process of this invention is used.

The single stage screw compressor with a maximum capacity of appr. 7.4 g/s at p=8.4 bar is operated with oil injection and air cooling. Oil separation is performed in five stages with the charcoal/molecular sieve adsorber being installed separately from the compressor unit. Special emphasis has been given to maintain the cleanliness of the cycle gas. Due to the extended periods of operation and the small flow passages within the coldbox even smallest amounts of contaminations may accumulate and result in intolerable pressure drops. Therefore, only special grade synthetic oil with a very small content of volatile condensible material is used. Additionally, the adsorbens is baked out before use to remove carbon dioxide and other gaseous impurities. Arrangement of the complete unit within a sound absorbing casing reduces the noise level of 70 dB(A) and at the same time allows installation both in- and outdoors. The air cooling with internal bypass control keeps the compressor module in operation at ambient temperatures between -20.degree. C. and +40.degree. C. The use of a belt drive and a multi-range motor serve for easy adaption to different electrical power standards (50/60 Hz) by simply exchanging the belt drive wheels. The connection between the compressor and the coldbox is performed by flexible tubes and can vary between 20 m (standard) and 100 m (option).

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.