5 replies to this topic

[Mirzazada]

Hello Everyone,

I am currently working on an assignment in which it is required to design a compressor. While doing my research on the topic, I saw multi-stage compressors where multiple (around 6) impellers were a mounted inside the same casing. My question is, do such compressors need inter-cooling like multiple-stage compression where the fluid passes to cooler before entering to the following compression stage cascade?

[Pilesar]

Multiple impeller wheels often fit inside a centrifugal compressor casing. Even though each wheel does a portion of the compression, it is the grouping of wheels that is usually called a stage. Designing and rating the wheels is very specialized. Process engineers usually don't go into more detail than the stage level so often do not care about the number of wheels or the wheel design. Cooling between stages is a way to improve compression efficiency and avoid process and mechanical problems that might occur at high temperatures. The main drawback for interstage coolers is the pressure drop through the exchanger. Compressor design should consider mechanical and process constraints when optimizing the configuration of compression stages and coolers. Some cooling may take place within a stage by spraying a mist within the casing but the main cooling is usually external to the case between stages.

[horatorres]

Process engineer set the following parameters

 

- Gas flow rate

- inlet pressure, and temperature

- Normal process conditions fluctuations

- Physical properties of the gas flow

- Initial Estimation of HP, BHP , discharge temperature, # stages required to avoid temperatures higher than 146 °C.

 

The compressors manufacturers use this information to detailed compressors calculations and send a proposal for the client. They MUST do that.

 

Cooling inter - stages is a normal process to avoid high temperatures over 146 °C, because oils will loss ose lubricant properties at temperatures over 140 - 150 °C. 

 

You will find a general procedure to calculate a centrifugal compressor  in this link

 

 

https://www.linkedin...le_article_view

 

Horacio

[breizh]

Hi,

To support your work a few document I've been collecting over the years .

A lot of stuff available on internet using your favorite search engine .

 

https://convertf.com...pressor-sizing/

 

Centrifugal Compressor Performance Calculator (elliott-turbo.com)

 

Good luck

Breizh

[Pilesar]

As gas is compressed in a centrifugal compressor, it also increases in temperature. It is not practical to cool the gas within the compressor case, so as soon as the gas exits the case it is routed through a heat exchanger where it is cooled. The cooled gas is denser (takes up less volume) so cooling is a form of compression even though it does not increase the gas pressure! Sometimes the gas between stages is cooled so much that liquid condenses. The liquid is separated and removed before the next stage of compression to protect the compressor and also to reduce the quantity of fluid to be compressed in the next stage.

  Theoretically, as the number of compressor stages with intercoolers increase, the average gas temperature becomes lower. So why limit the number of stages? One reason is that the pressure of the gas drops between stages as it travels through the piping,exchanger, and separator vessel. This pressure drop offsets some of the advantages of cooling the gas and has to be considered during design optimization. An extra stage means extra equipment which adds capital cost to the design. The optimum number of compression stages can be different depending on the fluid composition. For example, a nitrogen compressor can be designed for a hotter outlet temperature than can be tolerated by olefins where components tend to react at high temperatures. The main process gas compressor in an olefins plant might be designed with four stages or more usually with five stages depending on the optimization criteria used.

  The compressor manufacturers have standard case sizes and they are best suited to calculate the performance of new machines. So the process engineer designs the plant around a good guess of the interstage conditions considering that the stage pressures and temperatures may be slightly different after the compressor vendor tweaks the design to fit the equipment they offer. 

[SilverShaded]

It's amazing these days where i'm seeing final refinery designs and compressor efficiencies have simply been left at the default value in the simulator...