"WHY SCREW COMPRESSORS?" - Part Two
J.BY P. F. MARAIS (Associate Member)
SPEED CONTROL AND SAFETY
The screw compressor has similar operating characteristics to a reciprocating compressor. Its rate of flow is almost proportional to the speed and almost independent of the discharge pressure. The torque to drive it is determined by the discharge pressure and remains constant as the speed changes.
Variation in flow is achieved most economically by controlling the speed. A drive must be chosen which is capable of supplying a constant torque over as large a range of speed as possible. The drive can be by d.c. motors, steam turbines, piston engines or constant speed a.c, motors operating through hydraulic torque converters. However, a continuous variation of the rate of flow is normally required only where it is necessary to maintain the discharge pressure constant or where it is necessary to vary the rate of flow to suit the programme selected for the discharge pressure.
In most cases, it will be satisfactory for the compressors to operate intermittently according to a permissible pressure variation. The compressor feeds into a grid system with or without a receiver in which the pressure may vary between a maximum and a minimum. The supply from the compressor is automatically shut off from the system when the maximum pressure is reached and is cut in as soon as the pressure drops to the minimum value. One of the following methods may be used for this purpose:
1. Full load-No load control
As soon as the pressure reaches the maximum value, the compressor changes over to operation under no-load conditions. When air is handled, the discharge port is connected to the atmosphere through a relief valve, the discharge pipe to the receiver is shut off by a non-return valve and the intake pipe is closed by a butterfly valve. The compressor operates under no-load conditions as a vacuum pump, consuming approximately 20 to 25 per cent of the nominal power. In the case of gas compressors, the relief is only at the discharge end, i.e. the discharge pipe is connected to the intake pipe by a relief valve and the non-return valve in the pipe to the receiver closes automatically.
To eliminate the leakage of air into the compression chamber under vacuum conditions, the intake is not throttled and the compressor operates on by-pass. In the case of double-stage compressors, the discharge of the L.P. unit is relieved directly by a by-pass (with non-return valve), there is no relief provided on the H.P. unit.
When full load - no load has been adopted, the compressor continues to run irrespective of the load conditions. This permits a relatively rapid change in operation so that the installation can readily follow the continuous variations of draw off from the grid system.
2. Shutdown control
In all cases where delivery is required only temporarily and idling considerably exceeds actual operation, it is advisable to adopt shut down control.
In this system, when the maximum pressure has been attained, the compressor is shut down. It is restarted as soon as the pressure in the grid system falls below the minimum value. To avoid overloading the power supply system and the starter of the electric motor, the change over the interval between successive starts should be as long as possible.
APPLICATIONS
A German firm was one of the first to construct these machines. At an early stage, they realized that this type of machine had special features making it suitable for the compression of various gases used in the chemical, mining, petroleum and allied industries. A closer investigation of some of these applications is of interest.
1. When compressing coke-oven gas, dirt in the gas leads to wear of the sliding elements, clogging of the valves and deposits on the dead spaces that could lead to blockage in the case of the piston-type compressor, and erosion and wear of the blades as well as to undesirable deposits thereon which can cause balancing problems in the case of turbo-compressors. With the screw compressor, deposits form on the rotors, but only to the extent that is compatible with the operating clearances. Any excess is removed by the self-cleaning effect of the screws and is carried with the gas into the discharge pipe. The output efficiency of such coated rotors is noticeably better than in new equipment. It is remarkable to observe how such machines, with the same power unit, progressively discharge greater volumes at lower discharge temperatures as the deposit on the rotors builds up. The routine opening of the machine for cleaning is also eliminated.
2. Screw compressors are also suitable for the compression of damp gases and vapours and were required for some specific reason, liquids may be sprayed into the machine in almost any desired quantity. The screw compressor can deal with fluid/gas mixtures even when they contain a considerable proportion of incompressible ingredients. One application is the compression of butadiene in synthetic rubber plants. The fluid sprayed into the machine can also be used for internal cooling. This feature is particularly valuable in the case of gases that must not exceed certain limits of temperature during compression. Further, high compression ratios are obtained when liquids are sprayed into the compressors and when compressing air a delivery pressure of 100 psig is easily obtained in this way from a single-stage machine. In certain cases spraying liquid into the compressor will prevent premature polymerisation.
Solvents can also be added, either continuously or at certain periods as required. In the case of a power failure, for example, the solvent can be sprayed in automatically before the machine stops so that sticky material is cleared out and the compressor can be restarted without trouble.
Modern chemistry is producing from natural gas, from mineral oil, from by-products of coal and an almost unlimited range of other substances, new materials for production purposes including plastics and synthetic fibres to an ever-increasing extent. In the process of manufacture of these products the compression of crude or purified gas, and the recycling of gases play an important role. Because of the particular features of the machine, these are suitable applications for the screw-type compressor. Further, the screw compressor need not be confined to having the usual two or at most three stages compressing up to 17 atm.g. Its exceptional qualities will permit it to be used for considerably higher pressures. The design of heavy robust units for pressures up to 50 atm.g. has been initiated for use in new factories producing synthetic material.
3. Experience has led to the decision to limit the compression ratio of anyone stage to 4/1 because the heat of compression then generated is approximately 200°C. Up to this temperature, the expansion of the rotors due to the heat generated can still be controlled satisfactorily, despite the small clearances present. This temperature limitation also applies to each stage of a two-stage machine, that can compress air to a maximum delivery pressure of 11 atm. When designing the two-stage machine, the compression ratio for each stage is selected by taking the square root of the overall compression ratio. If the overall compression ratio is 11/1 then the ratio per stage should be √11/1 = 3.32/1. Thus the first stage will have an end' pressure of 3.32 atm and the second stage an end pressure of 11 atm. By selecting standard casings and components, a three-stage machine can be built for end pressures up to 18 atm. In this case, in the third stage a smaller compression of the ratio is used, being only 1.64/1. This limit is governed by such factors as the load on the bearings and the bending of the rotors. Delivery pressure of 18 atm, however, does not satisfy today's demands by industry. The construction of machines for even higher duties had to be considered. This has led to the development of machines with overall compression ratios of 40/1 using three stages of compression. It required to be more robustly constructed, as can be seen from Fig. 8.
The compressor casing has to withstand a test pressure of 60 atm. Although the rotors appear to be similar, that for the machine with the higher duty has heavier bearings and better arrangement is made for sealing without changing the basic design. In this design a compression ratio of 12/1 per stage is possible. Provision is also made to recover any gas leaking past the seals or to provide sealing using an inert gas or liquid, as the application may demand.
The pressure-volume curve, Fig. 9, is for a machine designed and built for an end pressure of 41 atm. An interesting feature of this machine is how the compressor stages are coupled to the gearbox. This is done using a torsion bar, which, apart from being flexible, also offers another advantage. The torsion bar is attached inside the hollow pinion shaft to enable the bar to be as long and flexible as possible without increasing the length of the machine. Small misalignments are taken care of in a satisfactory manner. Moreover, this type of coupling offers a further advantage: In addition to being flexible, it can transmit axial thrust forces. This feature is shown in Fig. 10.
The axial thrust on the compressor rotor acts from the pressure side towards the suction side. The transmission pinion has no thrust bearing but has simple helical teeth. Consequently, an axial thrust is exerted on the pinion in a direction away from the compressor. Using the inter-connected torsion bar, this axial thrust is transmitted to the compressor rotor and counteracts the thrust produced in the rotor. The thrust produced by the pinion is approximately 40 per cent of that produced by the compressor rotor. The thrust bearing of the compressor is relieved of this proportion of the thrust load.
4. Another interesting application is the provision of compressed air for pneumatic transport systems. The screw compressor has proved to be most suitable and has been used for this purpose for more than 12 years. The main feature is that the machine does not require lubrication. For this reason, it is ideally suited for bulk transport of foodstuffs of all kinds and of chemical powders where the slightest trace of oil or oil vapour would be deleterious. Even for the bulk transport of material such as cement this type of machine has proved superior to other types because no danger exists of clogging the aerating boards and the pipelines from oil vapour after a lengthy period in use. Besides being a compact as well as a robust machine, it meets all conditions prescribed for the transport of materials in bulk. It can be mounted in any suitable position on a truck and can be driven from the card and shaft of the truck or by its petrol engine. The machine developed for this purpose delivers approximately 300-500 ft3/min at a pressure of about 28-30 psig. Users of these machines have recorded the following times for discharging one ton of material:
Cement: 1.0 minute.
Lime: 1.3 minutes.
Sand: 1.6 minutes.
Flour: 1.6 - 2.0 minutes.
Soda: 1.7 - 2.1 minutes.
With these short unloading times, the waiting of trucks is reduced to a minimum. Because of the lightweight of the compressor, the little payload is forfeited. Almost no maintenance will be required on these machines. They have roller bearings and are air-cooled. Even dust particles that may enter the machine cannot do any harm. In special applications, such as at central loading or unloading stations, the machines may be mounted on fixed foundations and may be driven by electric motors. If higher delivery pressures are required, the same machine can compress to 7 atm by a slight injection of water.
Where a vacuum conveying system is used, this machine is again most suitable, because vacuums as high as 90 per cent of a perfect vacuum can be attained. This machine can also be used for inverse operation, i.e. for the expansion of compressed air. It can be used as an air motor with an output higher than that of a conventional air motor. Even at its lower range, the screw compressor proves to be a most versatile, reliable and simple machine.
5. The feature of being able to compress air free of oil and its reliability has made this machine most suitable for the automobile industry, where it is used in paint shops, and for control equipment. One German automobile factory alone has 28 of these machines in operation.
CONCLUSION
This brief account illustrates the use and application of the screw-type compressor. This machine has proved to be most useful in filling the gap between the reciprocating compressor and the turbo-compressor. In fact, in many instances, it has taken the place of the reciprocating machine. Bearing in mind that this machine is still in a rather early stage of development compared with some of the other types, it will no doubt be used for more applications in future.
The reasons for the use of a screw compressor have been given and the question forming the title of this paper has been answered.
ACKNOWLEDGEMENT
The author wishes to thank the Gutehoffnunghuette Sterkrade A.G. for permission to use photographs, curves and portions of articles appearing in their catalogues and technical reports listed hereunder:
GHH Catalogue: Screw Compressors.
GHH Technical Report No. 2/61.
GHH Technical Report No. 4/62.
GHH Technical Report No. 1/65.
DISCUSSION
Mr W. Rodenbeck (Visitor): In his paper, Mr Marais has given an excellent description of the screw compressor and has explained the technical features of this machine as well as its typical application in various industries.
This contribution briefly supplements the paper concerning two main points, namely:
1. An additional special application of the screw compressor;
2. Borderline cases between the application of a screw compressor and a turbo-compressor.
1. An additional special application of the screw compressor
In some cases, it is desirable to use the hot compressed air without after-cooling so that considerable savings in energy are achieved. A striking example of this is the use of compressed air for forging hammers or for certain chemical processes when not only the pressure but also the high temperature of the air are desirable features.
In these cases, the application of a compressor with non-lubricated working chambers is to be strongly recommended. This eliminates the danger of formation in the pipe system of self-igniting deposits due to the presence of oil in the hot air.
It is interesting to note that, in some countries, safety regulations go so far as to require after-coolers and separators for all compressors with lubricated working chambers.
Therefore, it is hardly necessary to emphasize that the screw compressor is the most suitable machine for such an application.
2. Borderline cases between the application of a crew compressor and a turbo-compressor
As stated by Mr Marais, the screw compressor has proved most useful in filling the gap between the reciprocating compressor and the turbo-compressor.
In comparing the performance curves (output against delivery pressure) of a reciprocating, a screw and a turbo compressor it can be seen that the steep performance curve, i.e. where there is almost the same output against different delivery pressures, is typical for the compressors with positive displacement, namely the reciprocating and screw compressors, whereas the turbo-compressor has a flat performance curve, i.e. where the output varies against different pressures. These differing performance curves are the decisive factors in establishing the most suitable controls for the different types of compressor.
In most cases, the reciprocating or screw compressor, according to the air demand, runs either at full-load or idles at no-load, with the suction valve closed. However, on a turbo-compressor, within a certain range, flexible adjustment of the output can be effected automatically at high part-load efficiency to meet the varying air demands at pre-set constant delivery pressures. The range of this flexibility is determined by the existing surge point on the one hand and the maximum overload point on the other.
Therefore, in a borderline case, where the application of either a screw compressor or a turbo-compressor is envisaged, it can be said that the different methods of controlling the machines must be taken into consideration. The most economical proposition can be established only after careful study of all fluctuations in air demand which the proposed unit would meet during operation. Depending on these fluctuations, which means depending on the time per shift for which full-load and part-load are required, either the full-load-no-load operation of the screw compressor or the more flexible operation of the turbo-compressor would determine the most efficient operation.
The disadvantage of the full-load-no-load regulation of the screw compressor, namely, the low power factor of the drive motor at no-load, would have to be weighed against the disadvantage of the turbo-compressor, namely the range of control, as pointed out above.
It appears usual for technical discussions to hinge to a great extent on the full-load conditions, whereas part-load conditions are often not considered.
Dr E. C. H. Becker (Member): Mr Marais has displayed most commendable timing in the selection of his subject. Now that the major functional difficulties, like an early bearing failure, have been largely designed out, screw compressors are becoming available for a large variety of applications. Indeed, so striking are the advantages that at least from a technical viewpoint we can expect an increasing and wider usage of screw compressors.
Among the features touched on in the paper is that loading and off-loading can be so effective as to eliminate the need for a discharge receiver. An indication of the limitations here and capacities involved would be of interest. Also, it is understood that in some designs of screw compressors the timing gears are not used, but presumably, some form of lubrication for the rotors is required. Are the sizes and efficiencies of screw compressors such as to render them competitive with centrifugal compressors for the bulk supply of compressed air at 100 p.s.i.g.? With a machine so versatile that it can effectively pass high concentrations of bulk solids, wide application in the refrigeration field is expected.
Perhaps Mr Marais would like to enlarge on these points.
For a machine to appear with such advancement on current practice necessarily involves extensive and ingenious design. Those interested in an excellent description of several of the basic design problems and also in the intricate rotor geometry and corresponding machining may refer with profit to the excellent papers by Professor A. J. R. Lysholm (J. Insn. Mech. Engrs., Vol. 150, 1943, page 11; Vol. 151, 1944, page 179).
AUTHOR'S REPLY TO DISCUSSION
Mr P. F. Marais. The contributions by Mr Rodenbeck and Dr Becker have ably supplemented the paper.
Mr Rodenbeck refers to an interesting application of screw compressors. In instances where these machines are used for pneumatic transport of food-stuffs such as flour, the high temperature of the air from the compressor aid in destroying harmful bacteria which may be present.
Borderline cases as outlined by Mr Rodenbeck may arise and these require careful examination. It would, of course, be unwise to I t a compressor size which will be so large in capacity compared with the normal duty that it will be running for Lengthy periods under idling, or no-load conditions. If the machine size is select d to match its duty, then this method of operation would hardly occur and the higher efficiency of the screw compressor, compared with the turbo compressor, will be a factor to be considered seriously.
Dr Becker has mentioned two interesting points.
The one is the elimination of a timing gear, which has proved to be successful on machines where oil is injected into the compression chamber. On such machines, it is also possible to increase the pressure ratio per stage because the oil acts in two ways. Firstly the oil fulfils the function of a sealing medium thus reducing the internal losses, and secondly, it acts as a coolant thus permitting higher ratios per stage. Although oil is a lubricant it does not fulfil the function of a lubricant in these instances as no lubrication is required, because the moving rotors do not come in contact with one another or with the casing.
Dr Becker also quite correctly stated that this type of machine is most suitable for compressing gases with a fairly high content of solids. He mentioned using these machines in the refrigeration plant. In my paper, I also mentioned the use of these machines in chemical plants, where the application is the same.
In reply to Mr Low's question relating to the noise made by these machines, it is correct that these machines are noisy. The reason for the noise is because the method of operation is that of a reciprocating machine, with the result that the air is delivered in pulsations. In screw compressors these pulsations have a high frequency, thus producing a high pitched noise. This undesirable feature was recognised by manufacturers at an early stage and much research has been put into damping the noise. By the development of high-efficiency silencers, satisfactory damping of the noise in these machines has been achieved. It has been found that machines, where oil is injected into the compression chamber, are even quieter than the oil-free type and are operating with an acceptable noise level even without silencers. If such a compressor is driven by a diesel engine, normally it is found that the diesel engine is noisier than the compressor. Some chemical plants, and in particular pharmaceutical laboratories, objected strongly against the noise level initially. Due to effective damping, it has been possible to satisfy the requirements of these industries, and screw compressors have also now been introduced to a large extent for these applications.
In reply to Mr Kent regarding an off-hand rule to determine whether reciprocating, screw or rotary compressors should be used, there is no such simple rule of thumb. It is necessary to investigate each case and to consider its particular requirements. When carrying out such an investigation the following factors must be borne in mind:
(1) Screw compressors have been built and are used successfully for delivering volumes ranging from approximately 300 ft3/min. When considering screw compressors for a particular application, features such as the low maintenance requirements, relatively small space taken up, low weight, and high efficiency, as well as reliability, must be considered.
(2) It has been found that at the lower ranges in certain instances reciprocating compressors may have higher efficiency and may require a smaller capital outlay. Against this, of course, the relative advantages of the screw compressor must be considered. On the higher range, turbo-compressors may require a smaller capital outlay and may have more desirable operational characteristics. In practice, it has been found that within the range of about 1 000 ft3/min to approximately 10 000 fr3/min, screw compressors are most suitable and competitive to the other types of machines available.
In reply to Dr Grant, it may be stated that these machines have been used for various applications in a chemical plant, where corrosive gases had to be compressed. The corrosive action of the gases has been counteracted by sleeting special materials such as special stainless steels for the components of the compressor, and also by special coating the rotors.
The application of the screw compressor for the compression of corrosive gases is more a problem of material than a problem of the compressor and equally applies to vessels and other apparatus used in a chemical plant. Many corrosive gases become more aggressive when moisture is present and provided a guarantee can be given that the gas is dry, this machine can be u ed for very corrosive gases by selecting suitable material for their construction.
This problem also relates to the life of these machines. In the paper, the author endeavoured to explain that these machines incorporate many features of the turbo-compressor and that the undesirable wearing properties of the reciprocating compressor had been eliminated. Therefore, the limitation to the life of a screw compressor is the same as that for a turbo-compressor, and if corrosive gases are to be compressed the life of the machine is determined by the corrosiveness of the gas.
P. F. Marais has been associated with compressor plant and equipment for many years. After he graduated from the University of the Witwatersrand in 1950 he started at the Randfontein Estates Gold Mining Company Limited as a pupil engineer. In 1953 he joined Rolfes Limited at Elandsfontein as an Assistant Engineer, and after some time with the Gutehoffnungshuette Sterkrade A.G. in Germany. returned to Rolfes Limited. Subsequently, he was appointed Manager of the Mechanical Engineering Department, and he is at present a Director of this Company.