Performance Comparison: Centrifugal Compressor VS Oil-free Screw Compressors

With the development of research and application, the effect of nitrogen in oilfield development has been gradually recognized by the petroleum engineering community, and therefore it is more and more widely used. In the nitrogen production process, we often need large-displacement air compressor as the key air supply equipment for the raw material gas of the production process, and the large-displacement air compressor (displacement of 42.5Nm³/min or more) mainly has centrifugal compressor and Oil-free Screw Compressors two product categories, each compressor has its own specialties, and also has its own scope of use, which can not be replaced by each other and will coexist for a long time. Therefore, it is very necessary to the above two kinds of compressor performance comparison, so that you can choose the compressor to provide a certain reference, so as to avoid selection errors.

1. Comparison of the operating principles of screw compressors and centrifugal compressors

Screw compressor  is a volumetric compressor, it uses the engagement of the yin and yang rotor to form a multi-stage compression, allowing a certain amount of air leakage, that is, to reach the maximum exhaust pressure and will not continue to rise, so the safety is guaranteed, while the piston compressor uses the reciprocating movement of the piston in the cylinder gas compression, generally not allowing leakage, which will lead to a continuous rise in the exhaust pressure, the general exhaust pressure can reach higher, but the gas compression is not continuous, so the gas has a pulsation; the discharge channel pipeline blockage will cause safety problems.  And because the screw machine is a volumetric compressor, so the discharge pressure will be higher than the centrifugal compressor, can meet certain equipment high-pressure gas supply requirements; centrifugal compressor uses the impeller rotation to achieve air compression, its discharge pressure is the smallest of the three kinds of compressors, but the flow of the continuity is the best, and will not overload the phenomenon.

2. Performance comparison between screw and centrifugal compressor

2.1 Basic composition

Conclusion: The level of general centrifugal compressor is much lower than that of oil-free screw, and the workload and cost of on-site installation is large. The basic composition of the centrifugal compressor and oil-free screw compressor is almost the same, they are composed of the driver, drive system, rotor, cooling system, lubrication system, sealing system, regulating system and control system. However, the connotation of some of its sub-systems and the typical scope of supply is very different.

2.2 Transmission system

Conclusion:It is the high speed of the centrifugal compressor that requires a more complex design in the bearing and lubrication system, thus contributing to its characteristic of being unsuitable for frequent start-ups and stops.

2.3  Comparison of lubrication systems

Conclusion: The lubrication system of centrifugal compressors is much more complex than that of oil-free screw compressors, and is relatively less reliable and requires more maintenance.

2.4 Sealing systems

General centrifugal compressor mainly uses two types of seals; labyrinth type and carbon ring type . Oil-free screws also use the same principle of labyrinth seals, only in a slightly different form.

Conclusion: Due to the use of the same principle and the same nature of sealing, centrifugal and screw compressors have similar sealing life, air sealing and oil sealing ability, and both can guarantee 100% oil-free air quality.

2.5 Comparison of regulation systems

Due to the existence of surge phenomenon , in the realization of flow regulation at the same time to completely avoid the surge, so the regulation of centrifugal compressors and oil-free screw compressors are very different. Currently the regulation used for centrifugal air compressor there are two main ways: automatic dual mode and constant pressure control. These two ways are through the control system on the inlet throttle valve and venting (bypass) valve for coordinated control.

(1) Automatic dual mode

Automatic dual mode is the standard mode of regulation, it can make the exhaust pressure control in the range of 0.1bar, for centrifugal compressors it is also the most energy-saving mode of regulation.

(2) Constant pressure control

Constant pressure control is an optional adjustment method, which can stabilize the exhaust pressure within the range of 0.03 bar, but if the supply volume is often lower than the gas volume of the wheeze, there will be a lot of outgassing, which is not energy-saving. Therefore, it is often used in the occasion that the exhaust pressure has strict requirements, and the air supply is often stabilized above the surge flow rate.

2.6 Control system

Conclusion: only centrifugal compressors have some special requirements for lubrication, transmission and regulation system, so compared with the oil-free screw, its control system is slightly more complex, slightly more control points. But generally the control systems of standard supply of centrifugal compressors are very simple, many are optional content.

3. Maintenance

As can be seen from Table 8: Centrifugal compressors have more maintenance points than oil-free screw compressors.

4. Summary and conclusion of performance comparison

4.1 Comparison of the advantages and disadvantages of screw and centrifugal compressors

4.1.1 Characteristics of screw compressors

As far as the principle of gas pressure increase is concerned, screw compressors and piston compressors are the same, they belong to the volumetric compressor. In terms of the main moving parts, and centrifugal compressors are similar, so the screw compressor has the same characteristics of the above two models.

Advantages of screw compressors.

(1) High reliability. Screw compressor parts less, no wearing parts, so the operation is reliable with a long service life, overhaul intervals can be in the 4 ~ 80,000 hours.

(2) Easy to operate and maintain. Screw compressor has a high degree of automation, the operator does not need long-term professional training to operate, it can realize the unattended operation.

(3) Good power balance. Screw compressor can be smooth and high-speed operation, especially suitable for use as a mobile compressor with small size, light weight and less floor space.

(4)Strong adaptability. Screw-in compressor has the characteristics of forced gas transmission, volumetric flow is almost independent of the impact of exhaust pressure, in a wide range higher efficiency can be maintained. In the structure of the compressor does not make any changes in the conditions, it can be applied to a variety of media.

(5) A number of mixed transport. The rotor teeth of the screw compressor actually leave a gap between the surface, so it can withstand the liquid impact, which can transport liquid gases, gases containing dust and gas that easy to polymerize and so on.

Disadvantages of screw compressors.

(1) High cost. Because the teeth of the rotor screw compressor is a space surface, you need to use a special tool in the special equipment for processing, in addition, the cylinder of the screw compressor has a high degree of processing accuracy requirements.

(2) Can not be used for high-pressure occasions. Due to the restrictions on the rotor stiffness and bearing life, screw compressors can only be applied to medium and low-pressure range, the discharge pressure can not exceed 3MPa.

(3) Can not be used for micro-occasions. Screw compressors depend on the gap to seal the gas, only when the volume flow rate is greater than 0.2m³/min,  they have the performance advantage.

4.1.2 Performance characteristics of centrifugal compressors

Advantages of centrifugal compressor.

(1) large production capacity, uniform gas supply, compact structure, less floor space.

(2) simple structure, less wearing parts, convenient maintenance, reliable operation, long continuous operation cycle, generally can run continuously for 1 to 2 years, so do not need to prepare the machine.

(3) between the rotor and stator, in addition to bearings and shaft seals, there is no contact friction part , in the cylinder there is no need to add lubricating oil, eliminating the shortcomings of the gas with oil.

(4) Centrifugal compressor is a high-speed machine, suitable for use of turbine or gas engine direct drive.

Disadvantages of centrifugal compressors

(1) The efficiency of centrifugal compressors is generally low, this is because of the higher velocity of the air stream in a centrifuge and the higher energy losses.

(2) Centrifugal compressor can only work in the design working conditions to obtain the highest efficiency, in the point of leaving the design working conditions for operation, the efficiency will decline. More prominently, when the energy is reduced to a certain extent when the compressor will appear “wheezing”, if not dealt with in a timely manner, it can lead to damage to the machine.

(3) centrifugal compressor is not easy to get a small flow rate at the same time as the high pressure ratio. The single-stage pressure ratio of the centrifugal compressor is rarely more than 3.

(4) For high pressure ratio, small flow rate of the centrifugal compressor, due to the small flow rate, the air flow channel becomes narrower, so it is difficult to manufacture and process and the flow loss is large, the efficiency of the compressor is reduced.

(5) poor adaptability of operation, the nature of the gas has a greater impact on the operational performance. When driving, stopping and normal operation the media change is large, the load change is also large, the driver should leave a large power margin, but in normal operation, no-load consumption is large.

5. Conclusion

(1) In the membrane nitrogen industry, it is possible to use either centrifugal or screw machines as the air source.

(2) The efficiency of centrifugal compressors is lower than that of screw compressors. Centrifugal compressor due to the fast gas flow, energy loss, the current production of centrifugal compressor efficiency is generally only 60%; while the efficiency of the screw compressor can reach 70% ~ 80%, in terms of energy saving considerations, the use of screw compressor than the use of centrifugal compressor is more effective.

(3) The displacement of a single centrifuge is much larger than that of a screw compressor. According to market research, the current volume of a smaller single centrifuge displacement can reach 80 ~ 120m³/ min; and exhaust volume of a single high-pressure screw unit is up to 42m³ / min; i.e., a centrifuge gas production is basically the same as the gas production of three screw compressors even greater.

(4) The exhaust pressure of single-stage centrifuge is much lower than that of screw compressor. For the membrane separation of nitrogen, screw compressor is more suitable for high-pressure resistant PAKER membrane tube; while centrifugal machine, oil-free screw compressor is more suitable for lower pressure PAKER membrane tube.

(5) the difference between the oil content of the gas is large, centrifuges belong to the oil-free lubrication, the oil content in the gas is very low, the membrane tube on the gas filtration requirements are low; the service life of the membrane tube has a small impact; and as a result of the widespread use of oil lubrication cooling, the problem of filtration system of the screw compressor once occurs, the service life of the membrane tube will have an impact.

(6) gas volume adjustment. Due to the centrifuge in the reduction of flow to a certain extent will produce “wheezing” phenomenon, so the centrifuge can only be adjusted between 70% to 100% of the exhaust; and screw compressor due to the non-existence of “wheezing” phenomenon, the use of volumetric adjustment, its exhaust can be adjusted between 0 ~ 100%. In 0~100% of any adjustment.

From the above conclusions, it can be seen that it is possible to use centrifugal machine and screw machine as the gas source of membrane nitrogen production equipment, both of them have their own advantages and disadvantages, and each of them has its own advantages and disadvantages, so they should be analyzed in accordance with the specific situation and make a reasonable choice of use.