Are centrifugal air compressors more energy efficient?
With the continuous development of my country’s industry, enterprises themselves are not only facing fierce competition in the market, but also putting forward strict requirements on their own production and operation costs. “Throttling” means “opening up”. Centrifugal air compressors (hereinafter referred to as centrifugal air compressors) As a general-purpose air compression equipment, it is increasingly favored by users due to its oil-free compressed air and high operating efficiency.
However, most users only have a conceptual understanding of “centrifuges are very energy-saving”. They know that centrifuges are more energy-saving than other compression forms such as oil-free screw compressors, but they do not systematically consider this from the product itself to actual use. question.
Therefore, we will briefly explain the impact of these four factors on “whether a centrifuge is energy-saving” from four perspectives: comparison of commonly used compression forms, differences in centrifuge brands on the market, design of centrifuge air compressor stations, and daily maintenance.
1. Comparison of different compression forms
In the oil-free compressed air market, there are two main categories: screw machines and centrifuges.
1) Analysis from the perspective of air compression principle
Regardless of factors such as screw rotor profile design and internal pressure ratio design of each brand, screw rotor clearance is a key factor affecting efficiency. The higher the ratio of rotor diameter to clearance, the higher the compression efficiency. Similarly, the centrifuge impeller diameter and The greater the gap ratio between the impeller and the volute, the higher the compression efficiency.
3) Comparison of comprehensive efficiency between theory and practice
A simple comparison of machine efficiency cannot reflect the results of actual use. From the perspective of actual use, 80% of users have fluctuations in actual gas consumption. See Table 4 for a typical user gas demand fluctuation diagram, but the safety adjustment range of the centrifuge is only 70%~100%. When the air consumption exceeds the adjustment range, a large amount of venting will occur. Venting is a waste of energy, and the overall efficiency of this centrifuge will not be high.
If the user fully understands the fluctuation of his own gas consumption, the combination of multiple screw machines, especially the solution of N+1, that is, N fixed-frequency screws + 1 frequency converter, can produce as much gas as needed, and the variable frequency screw can adjust the gas volume in real time. The overall efficiency is higher than that of the centrifuge.
Therefore, the bottom section of a centrifuge is not energy-saving. We cannot simply consider the fluctuation of actual gas consumption from the perspective of equipment. If you want to use a 50~70m³/min centrifuge, you need to ensure that the fluctuation of gas consumption is within 15~21m³/min. range, that is, try to ensure that the centrifuge is not vented. If the user predicts that his gas consumption fluctuation will exceed 21m³/min, the screw machine solution will be more energy-saving.
2. Different configurations of centrifuges
The centrifuge market is mainly occupied by several major international brands, such as Atlas Copco of Sweden, IHI-Sullair of Japan, Ingersoll Rand of the United States, etc. According to the author’s understanding, each brand basically only produces the impeller part of the centrifuge with core technology. , other parts adopt a global supplier procurement model. Therefore, the quality of parts also has an important impact on the efficiency of the entire machine.
1) High-voltage motor driving the centrifuge head
Motor efficiency has a great impact on the overall efficiency of the centrifuge, and motors with different efficiencies are configured.
In GB 30254-2013 “Energy Efficiency Limits and Energy Efficiency Levels of High-Voltage Three-Phase Cage Asynchronous Motors” promulgated by the National Standards Committee, each motor level is divided in detail. Motors with energy efficiency greater than or equal to Level 2 are defined as energy-saving motors. , I believe that with the continuous improvement and promotion of this standard, the motor will be used as an important criterion for judging whether the centrifuge is energy-saving.
2) Transmission mechanism—coupling and gearbox
The centrifuge impeller is driven by a gear speed increase. Therefore, factors such as the transmission efficiency of the coupling, the transmission efficiency of the high and low speed gear systems, and the form of the bearings will further affect the efficiency of the centrifuge. However, the design parameters of these parts have been As the confidential data of each manufacturer is not disclosed to the public, therefore, we can only make simple judgments from the actual use process.
a. Coupling: From the perspective of long-term operation, the transmission efficiency of dry laminated coupling is higher than that of gear coupling, and the transmission efficiency of gear coupling decreases quickly.
b. Gear speed-increasing system: If the transmission efficiency decreases, the machine will have high noise and vibration. The vibration value of the impeller will increase in a short period of time, and the transmission efficiency will decrease.
c. Bearings: Multi-piece sliding bearings are used, which can effectively protect the high-speed shaft driving the impeller and stabilize the oil film, and will not cause wear to the bearing bush when starting and stopping the machine.
3) Cooling system
The impeller of each stage of the centrifuge needs to be cooled after compression before entering the next stage for compression.
a. Cooling: The design of the cooler should fully consider the impact of the inlet air temperature and cooling water temperature on the cooling effect in different seasons.
b. Pressure drop: When the gas passes through the cooler, the gas pressure drop should be minimized.
c. Precipitation of condensate water: The more condensation water precipitates during the cooling process, the greater the proportion of work done by the next-stage impeller on the gas.
The higher the volume compression efficiency
d. Drain the condensed water: quickly discharge the condensed water from the cooler without causing leakage of compressed air.
The cooling effect of the cooler has a great impact on the efficiency of the entire machine, and it also tests the technical strength of each centrifuge manufacturer.
4) Other factors affecting centrifuge efficiency
a. The form of the air inlet adjustment valve: the multi-piece air inlet guide vane valve can pre-rotate the gas during adjustment, reduce the rectification of the first-level impeller, and reduce the pressure ratio of the first-level impeller, thereby improving the efficiency of the centrifuge.
b. Interstage piping: The compact design of the interstage piping system can effectively reduce the pressure loss during the compression process.
c. Adjustment range: A wider adjustment range means less risk of venting and is also an important indicator for testing whether a centrifuge has energy-saving capabilities.
d. Inner surface coating: The exhaust temperature of each stage of compression of the centrifuge is 90~110°C. The good internal temperature-resistant coating is also a guarantee for long-term and efficient operation.
3. Air compressor station design stage
The system design of centrifugal air compressor stations is still at a relatively extensive stage, mainly reflected in:
1) Gas production does not match demand
The gas volume of an air compressor station will be calculated at the design stage by counting the gas consumption points and multiplying by simultaneous usage coefficients. There is already sufficient margin, but the actual purchase must meet the maximum and most unfavorable working conditions. In addition to the factors of centrifuge selection, from the actual results, the actual gas consumption is mostly less than the gas production of the purchased compressor. Coupled with the fluctuation of actual gas consumption and the difference in the adjustment capabilities of various brands of centrifuges, the centrifuge will undergo periodic venting.
2) The exhaust pressure does not match the air pressure
Many centrifuge air compressor stations only have 1 or 2 pressure pipe networks, and centrifuges are selected based on meeting the highest pressure point. However, in fact, the highest pressure point accounts for a small proportion of the gas demand, or there are more low-pressure gas needs. At this point, it is necessary to reduce the pressure through a downstream pressure reducing valve. According to authoritative data, every time the centrifuge exhaust pressure is reduced by 1 barg, the total operating energy consumption can be reduced by 8%.
3) The impact of pressure mismatch on the machine
A centrifuge is most efficient only when it operates at the design point. For example, if a machine is designed with a discharge pressure of 8barg and the actual discharge pressure is 5.5barg, the actual operating power consumption of 6.5barg should be referred to.
4) Insufficient management of air compressor stations
Users believe that as long as the gas supply is stable to ensure production, everything else can be put aside first. The above-mentioned issues, or energy-saving points, will be ignored. Then, the actual energy consumption in operation will be much higher than the ideal state, and this This ideal state could have been achieved through more detailed calculations in the early stage, simulation of actual gas fluctuations, more detailed gas volume and pressure divisions, and more accurate selection and matching.
4. The impact of daily maintenance on efficiency
Routine maintenance also plays an important role in whether the centrifuge can operate efficiently. In addition to the conventional three filters and one oil for mechanical equipment, and the replacement of valve body seals, centrifuges also need to pay attention to the following points:
1) Dust particles in the air
After the gas is filtered by the air inlet filter, fine dust will still enter. After a long time, it will be deposited on the impeller, diffuser, and cooler fins, affecting the air intake volume and thus the overall machine efficiency.
2) Gas characteristics during compression
During the compression process, the gas is in a state of supersaturation, high temperature and high humidity. The liquid water in the compressed air will combine with the acidic gas in the air, causing corrosion to the inner wall of the gas, impeller, diffuser, etc., affecting the air intake volume and reducing efficiency. .
3) Quality of cooling water
Differences in carbonate hardness and total suspended particulate matter concentration in the cooling water lead to fouling and scaling on the water side of the cooler, affecting the heat exchange efficiency and thus affecting the operating efficiency of the entire machine.
Centrifuges are currently the most efficient type of air compressor on the market. In actual use, in order to truly “make the most of everything and enjoy its effects”, not only do centrifuge manufacturers need to continuously develop more efficient products; at the same time, accurate It is also particularly important to make a selection plan that is close to the actual gas demand and achieves “how much gas is used to produce as much gas, and how high pressure is used to produce as high pressure”. In addition, strengthening the maintenance of centrifuges is also a reliable guarantee for the long-term stable and efficient operation of centrifuges.
As centrifuges are used more and more widely, we hope that more and more users will not only know that “centrifuges are very energy-saving”, but also be able to achieve energy-saving goals from the perspective of the design, operation and maintenance of the entire system, and improve the company’s own efficiency. Competitiveness, make your own contribution to reducing carbon emissions and maintaining a green earth!
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