Use of screw compressors and piston compressors
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In the carbon dioxide industry, screw compressors are widely used as refrigerators, while piston reciprocating compressors are used for compressing process gases. However, due to the short service life of the valves in piston compressors, which requires frequent replacement, and also because it is difficult to adjust the load on piston compressors, some people have proposed using screw compressors instead of piston compressors for compressing process gases. I believe there are several aspects that need attention: 1. Lubrication method for screw compressors: Since process gases are being compressed, dry, oil-free lubrication systems must be used for screw compressors. This raises questions regarding the requirements for moisture and dust in the gases, as well as how to pre-treat the gases； 2. Efficiency issues of compressors: Piston compressors are more efficient than screw compressors; how can compression efficiency be improved? ； 3. Compressor outlet pressure issue: The carbon dioxide process generally operates at pressures between 2.5 and 4.0 Mpa. A two-stage screw compressor can achieve 2.5 Mpa; to reach 4.0 Mpa, multiple stages are required. Will this affect efficiency? Please feel free to offer more suggestions.
Personally, I think: 1. Spraying water on a CO2 compressor shouldn’t work, as CO2 will dissolve in water, right? 2. There are hardly any dry screw compressors in high-pressure applications; almost all of them use oil injection. Is it acceptable to use an oil-injected screw compressor together with an oil removal device in the CO2 process? 3. It is difficult to reach 4.0 MPa with a screw compressor, and efficiency cannot be guaranteed; 3.0 MPa can be achieved through direct two-stage injection compression. Can the CO2 process pressure requirement be reduced to 3.0 MPa?
At present, piston engines certainly have many problems, but in high-pressure applications, they are necessary
This post was last edited by wumian on 2019-7-10 at 10:52. 1. CO2 has low solubility in water, and such a small amount of water cannot remove much CO2； 2. Screw with oil injection + oil removal is acceptable ； 3. For some gas sources with favorable conditions, liquefaction can be achieved at a pressure of around 2.5 Mpa; in such cases, can the efficiency of the screw compressor reach over 80%? Also, compared to screw compressors, piston compressors have an efficiency that is about 10% lower, right?
Regarding the efficiency of screw compressors and piston compressors, it is necessary to consider the actual operating conditions and configuration. Overall, well-designed piston engines are slightly more efficient, but the 10% figure is an estimate. If you truly have such a need, find a suitable manufacturer of piston pumps and screw compressors to develop a plan for you; this should enable you to obtain relatively accurate power consumption figures for comparison. However, the overall operating cost of screw compressors (maintenance cycle, difficulty of maintenance, cost of wear parts, etc.) should be an advantage over piston compressors.
Our company manufactures CO2 screw compressors, and we are the largest player in the commercial refrigeration sector – 90% of such compressors come from us. 1. For carbon dioxide process gas compressors, we use micro-oil screw compressors. 12% higher efficiency than Bigan screws. 2. When the exhaust pressure is greater than 25 bar, we use a two-stage configuration. Please take a look at the diagram; if you need a sample, just provide your email address and we’ll send one to you
The advantages of screw compressors are lower noise levels, a higher volume of exhaust gas compared to piston compressors of the same size, and easy maintenance. In the case of process gases, a post-treatment system can be installed directly to meet the required standards； The oil-free two-stage compression screw compressors have now been optimized in design; their efficiency is virtually on par with that of piston compressors. They are also easy to install. If higher exhaust pressure is desired, it is possible to consult the screw compressor manufacturer, who can then recommend a suitable model based on their technical expertise!
Screw compressors are very noisy; for example, York units: lol
1. This range of 2.5 to 4.0 MPa is beyond the capacity range of water spray screw compressors. The screw compressor uses synchronous gears, and essentially it has the same structure as an oil-free screw compressor; therefore, its exhaust pressure cannot be very high. 2. If a screw compressor is to be used, it must be an oil-injected screw compressor, with two stages of oil injection and an oil removal device in the final stage. There is no such thing as a minimal oil content; oil removal filters can achieve a level of 0.5 ppm. It depends on the requirements regarding oil content in the gases used in subsequent processes – if higher standards are required, then activated carbon is needed. If no oil can be contained, then a reciprocating machine is the only option. 3. The difference in efficiency between reciprocating machines and screw machines varies depending on the pressure model; at such high pressures, a 10% difference still matters. However, the operating and maintenance costs of reciprocating machines are high, and reflux adjustment leads to waste.