Feasibility and necessity of converting a centrifugal air compressor from three-stage compression to two-stage compression
Thread Content
This post was last edited by hzggp on 2016-2-25 at 19:51. Our company has two centrifugal air compressors in parallel operation. The main technical parameters of one of these units are as follows: the rated flow rate is 300 NM3/min, the rated discharge pressure is 0.50 MPa(G), the power of the motor associated with it is 2200 HP (i.e., 1641 Kw), and the voltage is 10 KV. Key features of the equipment’s structure: it has a top-and-bottom split design, with three stages of open impeller compression; the first stage uses a single shaft (low-speed shaft), while the second and third stages each use a single shaft (high-speed shafts). Low-speed shaft speed: 20,995 rpm, high-speed shaft: 29,796 rpm. Origin of the problem: In the actual system, only 0.42 MPa (G) is required to operate this air compressor; as for the flow rate, the use of two compressors in parallel ensures that the production requirements are met even during summer. The outlet pressure of 0.42 MPa (G) is now automatically adjusted using the inlet guide valve (the relief valve on the machine remains closed at all times); the opening degree of the guide valve is 50%–60% in winter and 90%–100% in summer. Current of the high-voltage motor: around 85 amperes in winter (i.e., 1251 Kw), and around 96 amperes in summer (i.e., 1413 Kw). Since the actual pressure of the compressed air is nearly 0.1 MPa lower than the rated design pressure of the air compressor (0.50 – 0.42 = 0.08 MPa), the outlet pressure is adjusted through throttling at the inlet. Some believe that this approach results in significant energy waste; it is recommended to remove the third compression stage of the compressor and adopt a two-stage compression system, which could save 500 HP (375 Kw) in energy consumption while still meeting the current requirements for pressure and flow rate. It saves 375 Kw of electricity per hour – a truly inspiring and admirable energy-saving technology upgrade solution. To be honest, even half or even one-third of this figure would already be very satisfactory; saving 100 Kw in energy consumption is indeed impressive. I am very puzzled by such claims. As my knowledge of centrifugal air compressors is quite limited, and I know even less about gas dynamics, the compressor supplier also did not provide the original performance curves such as P-Q and N-Q. I wonder whether it’s possible to achieve such significant energy savings after the modification, given that there is already throttling involved in the current operation On the other hand, what adverse effects will removing the third-stage compression component have on the operation of the equipment? I think there are many experts in the power machinery section, so I’m asking everyone for their opinions. To use Mr. Jack Ma’s words: One should still have dreams, in case they come true.
The compression ratio is determined during the manufacturing process of the original equipment; in other words, the compressor is designed and manufactured with a specific compression ratio. If you can find a way to change it from three-stage compression to two-stage compression, please let me know – I’d really like to see that! It would be something truly unusual to see
I find it a bit odd to propose such a modification plan to the manufacturer (which is actually the repair service). But the problem is that they have indeed had such cases; I went to the site and saw it myself. The third-stage impeller was indeed removed, but the energy savings were not noticeable.
I think it’s pretty ridiculous. Especially when modifying the original model, manufacturers may seem to agree to it readily, but once problems arise after the modification, warranty and maintenance issues come into play ； On the other hand, it’s very difficult to assess the performance after the modification; aspects such as dynamic balance and vibration deviate significantly from the original design and monitoring data, to the point where they are no longer comparable. Taking our unit as an example, although it is not a compressor, the exhaust fan has been changed from using two stages of expansion to recover thermal energy to just one stage; as a result, vibration has increased, and there are significant temperature/differential pressure differences as well as airflow disturbances, which has made the situation worse. Regarding the energy-saving issue of compressors you mentioned, I haven’t worked with models of the same type, but I can try to understand it through analogy. On one hand, the outlet pressure of the compressor is determined by the system/backpressure; under the same operating conditions after the modification, the compression ratio remains unchanged ； On the other hand, similar to reciprocating compressors, using Holberg regulation for energy savings essentially involves adjusting the air intake volume, which is similar to the principle of adjusting the inlet guide vanes in rotary compressors. I also think it’s weird to modify the original model.
A friend said that it might be possible to save a lot of electricity by modifying the second-stage impeller instead of simply removing the third-stage impeller. In other words, the compression capacity of the second stage is increased.
This post was last edited by hzggp on 2016-2-29 at 14:55. Some also say: It is possible. Because the pressure inside the high-pressure shell of the third-stage compression is very high, it has been reduced after passing through the third stage (from 0.75 MPa to 0.42 MPa). I don’t know where this 0.75 figure comes from? Also, why doesn’t the air temperature drop after throttling and expansion?
Who here has the performance curves for the Fusheng P700 with three-stage compression? This would be very helpful for a rational analysis of this issue! !
To be honest, this renovation project has been completed and has been in operation for nearly 20 days now. I won’t comment on how it’s performing for the time being, as I’m afraid it might be seen as advertising or speaking ill of others. The main purpose of my post is to facilitate thorough discussion among everyone on the management, maintenance, and renovation of relatively complex moving equipment such as centrifugal air compressors. Revisit and review some principles of compression to apply theoretical knowledge more effectively in practice.
Has it been changed already? How is it performing?
Has it been changed already? How is it performing?
It can save some electricity, but its gas production capacity is greatly reduced; the exhaust temperature exceeds the limit, requiring an increased flow rate of cooling water