Centrifugal Gas Compressor Evaporative Cooling Injection Rate

[ReliableVibes]

I have a four stage centrifugal process gas compressor that has a wash liquid and a water injection used for anti-fouling and evaporative cooling of the process gas, respectively. I'm looking for guidance on how to best determine the maximum amount of water injection (for cooling purposes) that each stage can withstand without detrimental effects to the compressor (wash liquid injection rate will stay constant). I understand that the rule of thumb for wash liquid injection is no more than 3% of the mass flow rate, but does this the same for the cooling water injection? Is the 3% rule of thumb for total liquid injection (wash liquid plus water injection) or just wash liquid injection?

Any guidance that anyone may be able to give on this subject would be greatly appreciated.

Thanks,
ReliableVibes

 

[rotw]

The amount of water injection for cooling purpose depends on several parameters - procedure include:

- The heat capacity of the gas at constant pressure Cp
- The Mol. weight of the gas
- The total flow of the gas
- The pressure at the stage before injection
- The injection temperature of the cooling water
- The amount of water already contained in the gas
- The Delta T that is needed to be achieved at inlet of the stage from cooling

For instance suppose this is to avoid polymerization; you know that without cooling water the compression stage will produce a certain increase of temperature;
You also know the minimum temperature required at the outlet of the stage to avoid polymerization e.g 85 deg C -> your target temperature).

So basically you have to calculate what would be the inlet temperature (with water injection effect) that will lead to stay within 85 degC at outlet and you do this by adapting the amount of relative humidity required (can be by trial and error). Once you know the relative humidity you can estimate the water injection flow.